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Ⅰ IntroductionFlyback Diodes, which are also known as freewheeling diodes, generally refer to diodes that are inversely paralleled across the ends of energy storage elements such as inductors, relays, and thyristors. When a voltage or current changes suddenly in a circuit, it protects other components in the circuit. When using a flyback diode, the circuit current can be changed more gently to avoid the occurrence of voltage spike. This article will introduce in detail what is flyback diode, how freewheeling diode works, flyback diode selection and the flyback diode function.How Freewheeling Diode WorksCatalogⅠ IntroductionⅡ DesignⅢ How It Works?Ⅳ SelectionⅤ Applications5.1 Summary5.2 In Forward Switching Power Supply5.3 In Converter Technology5.4 In Unidirectional Half Wave Silicon Control Rectifier Circuit5.5 In BUCK CircuitⅥ Something Has to CareIn electronics, a flyback voltage or an inductive flyback is a voltage spike created by an Inductor when its power supply is removed abruptly. The reason for this voltage spike is the fact that there cannot be an instant change to the current flowing through an Inductor.In addition, time constant of the inductor determines the rate at which the current can change through an inductor. This is similar to the time constant of a capacitor, which determines the rate at which its voltage can change.The freewheeling diode is named because it plays the role of freewheeling in the circuit. It is generally used in the circuit to protect components from being damaged or burned out by voltage breakdown, connected in parallel to both ends of the elements that generate the induced electromotive force(EMF), and form a loop with them, so that the high electromotive force generated in the loop is consumed by the continuous current method, thereby protecting the components in the circuits.Flyback diodes are connected in parallel at both ends of the coil. When the current passes through the coil, it will generate induced electromotive force at both ends. When the current disappears, its induced electromotive force generates a reverse voltage to the components in the circuit. When the reverse voltage is higher than the reverse breakdown voltage of the elements, it will cause damage to the elements such as triode and thyristor. When the current flowing through the coil disappears, the induced electromotive force generated by the coil is consumed by the work formed by the diode and the coil, thereby protecting the other elements in the circuit.Ⅱ DesignIn the following figure, it is showed that a flyback diode is placed across the inductor. An ideal flyback diode will have a very large peak forward current; capacity which helps in handling the voltage transients from damaging the diode, and inductor’s power supply is suited for reverse breakdown voltage and low forward voltage drop. Voltage spike can be 10times to the voltage of power supply which depends on the equipment involved and the application. So it is understood that not to underestimate the energy which contain within an energized inductor. Figure 1. Flyback DiodeFor an ideal flyback diode selection, a diode which has very large peak forward current capacity (to handle voltage transients without burning out the diode) should be selected, moreover, low forward voltage drop, and a reverse breakdown voltage fitted the inductor's power supply. Depending on the application and equipment in real requirement, some voltage surges can be upwards of 10 times the voltage of the power source, so it is critical not to underestimate the energy contained within an energized inductor.Flyback Diode Selection Note You Should KnowWhen used with a DC coil relay, a flyback diode can cause delayed drop-out of the contacts when power is turned off, due to the continued circulation of current in the relay coil and diode. When rapid opening of the contacts is important, a small value resistor can be placed in series with the diode to help dissipate the coil energy faster, at the expense of higher voltage at the switch.Schottky diodes are preferred in flyback diode applications as switching power converters, because they have the lowest forward drop (~0.2V rather than >0.7V for low currents) and are able to quickly respond to reverse bias (when the inductor is being re-energized). They therefore dissipate less energy while transferring energy from the inductor to a capacitor.When the flyback diode is used to simply dissipate the inductive energy, as with a solenoid or electric motor, cheap 1N540x and 1N400x general-purpose diodes are used instead. Ⅲ How It Works?Flyback diodes are often used with energy storage elements to prevent sudden changes in voltage and current to provide a pathway. The inductor can provide continuous current to the load through it to avoid sudden changes in load current and smooth the current. In the switching power supply, you can see a freewheeling circuit composed of a diode and a resistor connected in series, which is connected in parallel with the primary side of the transformer. When the switch is turned off, the freewheeling circuit can release the energy stored in the transformer coil to prevent the induced voltage from being too large and breakdown the switch. Generally, it is often to choose the fast recovery diode or the Schottky diode as flyback diode.Circuit Expressions Figure 2. Flyback Diode in Switching Power Supply CircuitIn Figure 2(c), when KR is turned on, the upper is positive voltage and the lower is negative voltage,  and the current direction is from top to bottom. When the VT is turned off, the current in the KR is suddenly interrupted and an induced potential is generated. The current direction is kept constant, that is, keeping the KR current direction from the top to bottom, which based on the Lenz's law. The induced potential and the power supply voltage are superimposed and applied across the VT, making it easy for the VT to breakdown. To avoid it, VD is used to short-circuit the induced potential generated by KR, that is, The current flows clockwise in the small circuits of the diodes and relays to protect the VT. R and C in Figure 2(b) also use the principle that the voltage on C cannot be abruptly changed to absorb the induced potential.In short, the flyback diode is connected in parallel to the relay or the inductor at both ends of the circuit. When the inductor is powered off, the electromotive force at both ends does not disappear immediately. At this time, the residual electromotive force is released through a freewheeling diode to reverse the reverse generated by the coil (the EMF is consumed in the form of current). It can be seen that the freewheeling diode is not a substantial component, but plays a "freewheeling" role in the circuit.For example, reversely connect a flyback diode at both ends of a relay coil or at both ends of a unidirectional thyristor. In practice, electromagnetic relays are usually controlled by triodes or MOS tubes to achieve automatic control of electrical loads (such as through a single-chip microcomputer), and the coil of the relay is a large inductance, which can store electrical energy in the form of a magnetic field. So when it pulls in, it stores a lot of magnetic field. When the triode controlling the relay changes from on to off, the coil is powered off, but there is a magnetic field in the coil. At this time, the back electromotive voltage can be as high as 1000v to destroy other circuit components. This is because the access of the diode is exactly the same as the direction of the reverse electromotive force. So that the reverse potential is neutralized by the freewheeling diode in the form of current to protect other circuit components. In addition, it is generally a diode with a fast switching speed.  Figure 3. Freewheeling Diode CircuitBecause the relay coil exists inductive load, which will absorb the self-inductive voltage of the relay coil when the triode is turned off. According to Lenz's law, when the current on the inductor decreases, a self-inductive voltage is generated. The direction of this voltage is that the forward terminal is negative and the collector of the driving tube is positive. This voltage will break through the triode, so an freewheeling diode is connected in parallel with the relay to absorb this self-inductive voltage.1) The influence of the time parameter of the circuit below the ms level on the mechanical contact is ignored.2) Even the 1N4000 reverse recovery time is far below the ms level, and the forward conduction time is shorter.3) Capacitance between the driving tubes and parasitic capacitance of the relay is enough to disable the high-speed diode.4) The consumption of inductive energy storage mainly depends on the winding resistance, which is generally in an overdamped state.It is general to use transistors as switches. As shown in Figure, a transistor TR1 is used to control the conduction of the relay coil, and the relay contact is used to control the load circuit.In a thyristor circuit, the thyristor is generally used as a contact switch, if a large inductive load is controlled, a high-voltage back electromotive force will be generated, and the principle is the same as that of a relay.Flyback diode also used on displays coils commonly used in relays. It is often used with energy storage elements to prevent sudden changes in voltage and current and provide a path. The inductor can provide continuous current to the load to avoid sudden changes in load current and smooth the current. In the switching power supply, it is common to see a freewheeling circuit composed of a diode and a resistor connected in series. The following circuit is connected in parallel with the primary side of the transformer. Figure 4. Flyback Diode in Relay CircuitThe freewheeling diode is added to both ends of the inductive load, and the inductive here is to have an inductive characteristic. The characteristic of the inductive load is that the current cannot be abruptly changed, in other words, it can't be all of a sudden. Common inductive loads include relay coils and solenoid valves.Figure 5.  Typical Freewheeling CircuitThe Figure 5 shows the typical application circuit of the flyback diode, where the resistor R determined whether it is needed or not. When the energy storage element VT is turned on, the upper voltage is positive, and the lower voltage is negative, and the current direction is from top to bottom. When the VT is turned off, the current in the energy storage element is suddenly interrupted, and an induced potential is generated at this time. This induced potential and the power supply voltage are superimposed and applied to both ends of the VT, which can easily cause VT to break down. VD can be added for this purpose, so that the induced potential generated by the energy storage element can be short-circuited to achieve the purpose of protecting the VT. Ⅳ Selection1) Based on working voltage2) Based on working current1N4007 is a not bad choice but not the best, because the PLC may be damaged before the diodes have time to play the freewheeling effect. Therefore, it is best to use FR107 to protect the freewheeling circuit, which can better protect the PLC output interface, and the cost will not rise too much. It is also possible to choose IN5819 or IN5817, which has better performance than FR107, but the cost is a little higher. Ⅴ Applications5.1 SummaryFlyback diodes are usually used with energy storage elements, and their role is to prevent sudden changes in voltage and current in the circuit and provide a power-consuming path for reverse electromotive force. The inductive coil can provide continuous current to the load through EMF, so as not to change the load current and smooth the current. In the switching power supply, a freewheeling circuit always composed of a diode and a resistor connected in series. This circuit is connected in parallel with the primary side of the transformer. When the switch is turned off, the freewheeling circuit can release the energy stored in the transformer coil to prevent the induced voltage from being too large and breakdown the switch.5.2 In Forward Switching Power SupplyIn the forward switching power supply, when the MOS is turned off, the secondary side of the transformer provides current to the outside by the energy stored in the inductor. In order to make the inductor play this role under load, a freewheeling diode is added on the secondary side of the transformer. The inductor, load, and freewheeling diodes create paths to transfer the energy in the inductor to the outside.5.3 In Converter TechnologyIn the electronic converter circuit, the single-phase bridge rectifier in the rectification section is the single-phase rectifier circuit with the most practical applications. And three-phase bridge rectification is the most widely used method for power systems, especially generator excitation systems. Both of these circuits must be connected to a flyback diode. Its function is almost the same. Take a single-phase bridge circuit as an example: When the rectifier bridge is connected to an inductive load, because the inductor current cannot be abruptly changed, during the thyristor off time, it must connect freewheeling diode at both ends of the load to provide a smoothing path  to prevent dangerous overvoltages across the inductive load, and also the thyristor can be commutated to conduct.The three-phase bridge rectifier circuits used in generator excitation systems are divided into three-phase half-control bridges and three-phase full-control bridge circuits. Therefore, in order to ensure reliable commutation of the rectifier components, the half-control bridge needs to connect flyback diodes in parallel at both ends of the inductive load, while the full-control bridge does not need to do so. In addition, when the conduction angle is changed, the average voltage and line current of the half-controlled bridge change more slowly than the full-controlled bridge.At present, current converters such as rectifiers and inverters are now used in a large number of devices, in which flyback diodes are typically added to the internal DC bus of the converter. Because if the load is an inductive element, when a large-capacity inverter on the bus fails, the DC bus will generate huge reverse surge energy. At this time, it is necessary to provide a discharge channel for this energy, otherwise it will break down or burn the converter. This channel needs a diode to form, that is a flyback diode.5.4 In Unidirectional Half Wave Silicon Control Rectifier CircuitFor unidirectional half-wave silicon control rectifier circuit with large inductive load, when the  silicon control is turned off in the negative half cycle, the inductive load will generate a high reverse induced electromotive force. This reverse electromotive force is sufficient to cause the silicon control to break down and burn. After that, the reverse electromotive force can be discharged into the forward voltage drop of the diode (about 0.7V), thereby effectively protecting the circuit components. 5.5 In BUCK Circuit Figure 6. BUCK CircuitIn the BUCK circuit, fast recovery diodes or Schottky diodes are generally selected as freewheeling diodes. It is generally used in the circuit to protect components from being broken down or burned by induced voltage. The two ends of the element form a loop with it, so that the high electromotive force generated in the loop is consumed in a continuous current manner, thereby protecting the elements in the circuit.In theory, the diode is selected at least 2 times the maximum current. In actual use, due to the strong transient overload resistance of the diode, an ultra-fast diode with a maximum current of 50A can also be used. In addition, a reasonable heat sink generally has little damage in actual use. The total impedance when conducting is the internal resistance of the motor plus the equivalent internal resistance of the drive tube. And the total impedance during freewheeling is the internal resistance of the motor plus the equivalent internal resistance of the freewheeling diode. In general, the AC equivalent internal resistance of the freewheeling diode is smaller than the AC equivalent internal resistance of the driving transistor. Therefore, in conventional design, the maximum current of the freewheeling diode is generally doubled to the maximum current of the motor.The transient current is only a moment, and the anti-overload capability of the surface-contact diode is enough, as long as it is not used in overvoltage, if necessary, a small resistor can be connected in series to limit the current. The flyback diode is to protect the switching device. The transient current during freewheeling is related to the working voltage of the motor and the internal resistance of the winding, and has nothing to do with the power of the motor. If necessary, the peak value of the transient current is the reverse self-inductance voltage minus diode junction voltage drop and then divided by the loop resistance. The reason why a diode with a certain current used is because the internal resistance of the winding of the low-voltage high-power motor is low, so the transient current will be relatively large. A series of small resistors can suppress the peak current, the transient voltage of the switch tube rises slightly because the operating voltage is not high, and now the current withstand voltage of transistors is at least 50V or more. Ⅵ Something Has to CareFreewheeling diodes are commonly used in switching power supplies, relay circuits, thyristor circuits, IGBTs, and other circuits. They are widely used, so it is necessary to pay attention to the following points when using them: 1) Fylback diode is an effective method to prevent the high voltage generated by self-inductive potential from causing damage to related components when the DC coil is powered off.2) The polarity of the flyback diode must not be connected wrongly, otherwise a short circuit situation will be caused.3) The flyback diode is always reversed to the DC voltage, that is, the negative pole of the diode is connected to the positive pole of the DC power supply.4) The flyback diode works in the forward conduction state, not in the breakdown state or the high-speed switching state, that is, the flyback diode does not used in electrical breakdown, recoverable situation, but its unidirectional conduction effect is the key point.5) Zener diodes can't be regarded as flyback diode. Because the zener diodes use reverse characteristics, and the flyback diodes use forward characteristics. Frequently Asked Questions about Flyback Diode or Freewheeling Diode1. What is a flyback diode?A flyback diode is a diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. 2. What is the role of freewheeling diode?A Flyback diode is also called as freewheeling diode. ... Here catch diode is used to eliminate flyback, when the abrupt voltage spike is witnessed across the inductive load when the supply current abruptly reduced. It helps the circuit from damaging. 3. What is a flyback diode used for?A flyback diode is a diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. 4. How does a flyback diode work?The Flyback diode makes inductor to draw current from itself in a loop until the energy is dissipated in diode and wires. When the current flow to an AC induction motor is suddenly interrupted, then the inductor tries to maintain increasing the voltage and the current by reversing polarity. 5. How do you choose a freewheeling diode?The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating. A diode in your application having 50 volts would be more than adequate. Again 1N4001 will do the job. 6. How do I choose a flyback diode for a relay?Specify a diode for at least 79.4 mA current. In your case, a 1N4001 current rating far exceeds the requirement. The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating. 7. What are the advantages of freewheeling diode?What are the advantage of free wheeling diode in a Full Wave rectifier? It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load. 8. Why freewheeling diode is used in controlled rectifier?When the inductive circuit is switched off, this diode gives a short circuit path for the flow of inductor decay current and hence dissipation of stored energy in the inductor. This diode is also called Flywheel or Fly-back diode. circuits, inverter circuits, and chopper circuits by making it continuous. 9. What is the effect of adding free wheeling diode?It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load. 10. What is the use of freewheeling diode in converter circuit?A free wheeling diode is used in converter circuits . It is connected across the load. During positive cycle of input it is reverse biased. During negative cycle of input the diode conducts and the energy stored in the circuit inductor during the previous half cycle is delivered to the load itself.

CategoryⅠ IntroductionⅡ Electronic Ballast Circuit Diagram Research Application 2.1 Overview 2.2 Circuit Structure of High-Performance Electronic Ballast       2.2.1 Power Factor Correction Circuit       2.2.2 Inverter Circuit       2.2.3 Lamp Circuit Network       2.2.4 Control Circuit2.3 High-Performance Electronic Ballast Dedicated Integrated Controller of ML4830 Series       2.3.1 Introduction to ML4831/32 Function       2.3.2 The Improvement of the Internal Function of ML48332.4 High-performance Electronic Ballast Built by ML4833Ⅲ FAQ Ⅰ IntroductionIn the 1970s, a worldwide energy crisis emerged. The urgency of energy conservation has led many companies to focus on energy-saving light sources and electronic ballasts for fluorescent lamps. With the rapid development of semiconductor technology, various high-return power switching devices are emerging, which provide conditions for the development of electronic ballasts. In the late 1970s, foreign manufacturers took the lead in launching the first generation of electronic ballasts, which was a major innovation in the history of lighting development. Because it has many advantages such as energy-saving, it has aroused great concern and interest around the world. It is considered to be an ideal product to replace the inductance ballast. Later, some well-known enterprises have invested considerable manpower and material resources to carry out higher-level research and development. Due to the rapid advancement of microelectronics technology, the development of electronic ballasts to high performance and high reliability has been promoted. Many semiconductor companies have introduced a series of products for dedicated power switching devices and control ICs. In 1984, Siemens developed an active power factor correction IC such as the TPA4812 with a power factor of 0.99. Subsequently, some companies have successively launched integrated electronic ballasts. In 1989, Finland's Hell Valley Company successfully launched electronically adjustable ballast monolithic integrated circuit ballasts. Electronic ballasts have been promoted and applied throughout the world, especially in developed countries. Figure 1. BallastChina's research and development of electronic ballasts started late, the technology is not advanced, early understanding of the difficulty and complexity of this product is insufficient, the development of special semiconductor devices has not kept up, the quality of products has not passed, and the market is extremely irregular. A large number of low-priced inferior goods were thrown to the market, causing losses to consumers and seriously damaging the image of electronic ballasts.  In the late 1990s, due to the rapid development and improvement of production levels, from circuit design to electronic components, the products entered a relatively mature stage, and high-quality products entered the construction project. The implementation of China's green lighting project paved the way for the promotion and application of electronic ballasts. Knowledge of Electronic Ballast for Fluorescent Lamps and Germicidal Lamps The electronic ballast is an electronic control device that uses a semiconductor electronic component to convert a direct current or low frequency alternating current voltage into a high frequency alternating current voltage, and drives a light source such as a low pressure gas discharge lamp (sterilization lamp) or a tungsten halogen lamp. The most widely used is the electronic ballast for fluorescent lamps. Due to the adoption of modern soft-switching inverter technology and advanced active power factor correction technology and electronic filtering measures, the electronic ballast has good electromagnetic compatibility and reduces the self-loss of the ballast. Ⅱ Electronic Ballast Circuit Diagram Research Application2.1 OverviewOn October 1, 1997, China's "Green Lighting Project" was officially launched. This is a major decision and measure in the field of lighting technology, which has a huge impact on China's energy, electric light source and lighting technology, and even environmental protection. As an important target of the "green lighting project", China will replace the incandescent lamp with an integrated energy-saving lamp composed of electronic ballasts and compact fluorescent lamps and promote more than 300 million energy-saving lamp, forming the terminal's ability to save 22 billion kWh, which is equivalent to saving about 49-63 billion yuan electricity construction funds. In addition to saving electricity, it can actually reduce social expenditures by 30-40 billion yuan. According to relevant experts from the Ministry of Information Industry, under the same luminous flux conditions, energy-saving lamps can save 80% of energy compared with incandescent lamps, and the cost of purchasing energy-saving lamps can be recovered in the 8-10 months of electricity savings. The use of electronic energy-saving lamps in ordinary households, enterprises and institutions, hotels, restaurants, and commercial systems is more cost-effective than incandescent lamps. However, the old-fashioned inductance ballasts currently working at the industrial frequency generally have the disadvantages of high energy consumption, low efficiency, large volume, and large amount of copper needed. Therefore, the state has set a policy which is to replace traditional inductance ballasts with high frequency electronic ballasts. Currently, some electronic ballasts have appeared on the market, and Table 1 lists the performance comparison of these electronic ballasts. According to the International Electrotechnical Commission standard IEC929 and China's professional standard ZBK74012-90, the electronic ballast should be used in "normal conditions, the lamp should be activated, but it does not cause damage to the lamp performance"; "The shortest time to apply the cathode preheating voltage should not be less than 0.4s" and "the crest factor of the open circuit voltage shall not exceed 1.8; during the minimum warm-up period, no extremely narrow voltage peaks that do not affect the rms value shall be generated", etc.  As listed in table 1, except for high grade electronic ballasts, they are unqualified products. In particular, as early as 1982, the International Electrotechnical Commission (IEC) developed a standard called “interference of household equipment and similar electrical equipment to the power supply system”, namely the IEC555-2 standard. In 1987, Europe also developed a similar EN60555-2 standard. Both standards strictly limit the power factor of the equipment to be close to 1, and it also clearly stated that, all products that do not meet the standards are not allowed to be sold. In view of the great harm caused by the low power factor, it is very important and necessary to impose regulations on the power factor of electronic equipment and products that must be close to 1. Figure 2. Brief Comparison of Low, Medium and High Grade Electronic Ballasts We believe that the high-performance electronic ballast should be a product that has both power factor correction and lamp filament preheating, lighting adjustment and lamp circuit protection, and is fully compliant with IEC555-2 and similar standards. The basic principles of the circuit structure and power factor correction circuit that must be provided for high-performance electronic ballasts are briefly discussed in this article. The integrated controllers for electronic ballasts ML4831, ML4832, ML4833 and high-performance electronic ballast circuits composed of them are highlighted. 2.2 Circuit Structure of High Performance Electronic BallastThe RFI and EMI filters in the figure filter out conducted RF interference and electromagnetic interference from the grid, while obstructing the conducted RF and electromagnetic interference generated by the ballast circuit from entering the grid. The bridge rectifier circuit converts the input AC to DC. The power factor correction circuit acts to improve the input AC current waveform, ensuring that the input current is sinusoidal and in phase with the input voltage, achieving a power factor close to or equal to one.  The inverter circuit completes the conversion of the DC high voltage to the high-frequency AC, and finally transmits the input power to the fluorescent tube through the lamp circuit network. In addition to transmitting electrical power, the lamp network will also perform preheating of the fluorescent filament, sampling and feedback of the lamp operating state signal. The feedback signal of the working state of the lamp is taken from the power factor correction circuit and the dimming signal, and processed by the control circuit to obtain the driving pulse of the switching device in the correct inverter circuit. 2.2.1 Power Factor Correction CircuitThe power factor of the system is defined as PF=γcosφ1 In the formula, γ=I1/IRMS, which is the ratio of the fundamental rms value of the input current to the rms value of the input total current and is also called the distortion factor of the current. φ1 is the phase shift angle of the fundamental current and voltage. If the input voltage of the system has no phase shift (ie, the system is purely resistive) and there is no harmonic component (ie DF=1), the PF of the system must be one. Unfortunately, the input rectification filter units that most of the current devices connect with the power frequency grid are composed of uncontrolled diodes and large-capacity electrolytic capacitors. The instantaneous value of the current on the grid side is quite high (generally about 2 to 3 times that of IRMS), the duration is very short (usually no more than 4ms), and it is severely non-sinusoidal, so the PF of the system is much lower than 1.  The power factor correction is aimed at the drawbacks of the traditional uncontrolled rectifier circuit, and adopts corresponding circuit measures. While increasing the DF value of the system, the phase shift of the input fundamental current and voltage is minimized, and finally the target with the PF value equal to 1 is achieved. As a boost-type active power factor correction circuit commonly used in electronic ballasts, the control circuit uses the input voltage signal as a reference, and the product of the input current and the output voltage signal is used as a modulation source to obtain a sinusoidal pulse width modulation (SPWM) signal to the step-up DC/DC power conversion circuit to adjust the on/off time ratio of the power switch. In the end, a stable DC high voltage is obtained.  The power switching device in the step-up power conversion circuit is driven by the SPWM signal outputted by the control circuit to turn on and off at a high speed, thereby ensuring that the current waveform flowing through the inductor connected in series with the rectifier bridge is a sine wave, and is in phase with the input voltage. Thus, the distortion factors γ=1 and φ1=0 of the system input current are obtained, that is, cosφ1=1, and the system power factor is 1. 2.2.2 Inverter CircuitThe most important function of the inverter circuit is to convert the high-voltage direct current outputted by the power factor correction circuit into a high-frequency alternating current for the fluorescent lamp. The power MOSFET push-pull tubes (V1 and V2) are alternately turned on and off under the driving pulse with a duty cycle of 50%, and is commutated when the current crosses zero in the parallel resonant loop of the power transformer primary inductance and capacitance thus to realize zero voltage switching(ZVS) and perform chopping on high voltage DC. The zero-voltage switching eliminates switching losses associated with output capacitance and parasitic capacitance charging of MOSFET tube, and the gate drive charge is minimal, which helps reduce gate losses.  Since the high frequency AC obtained by the secondary coupling of the power transformer is directly fed to the lamp network, there is no phase shift between the lamp current (ie, secondary current of the power transformer) and the output current of the inverter circuit (ie, primary current of the power transformer). Considering that the total impedance of the lamp network is reduced at high frequencies, and the negative resistance characteristic of the fluorescent lamp itself, it can be found that as the lamp current decreases (corresponding to the weakening of the light intensity of the lamp), the output current of the inverter circuit will increase. 2.2.3 Lamp Circuit NetworkThe lamp circuit network not only needs to deliver the high-frequency AC power to the lamp tube to complete the efficient conversion of electricity and light, but it also needs to implement functions such as filament warm-up, lamp current detection feedback, and auxiliary power supply for the entire electronic ballast system.  The power transformer primary T is connected to the inverter circuit, and the lamp current is directly transmitted to the lamp through the capacitor, and the secondary winding supplies the lamp with filament current for preheating and maintaining the operation. The current transformer TA performs detection and sensing of the lamp current, and sends a signal about the operation of the lamp to the control circuit at any time by the change of the lamp current. The control circuit can judge the light intensity of the lamp according to the magnitude of the lamp current (even including the disconnection and short circuit of the lamp), and then send corresponding control signals to the inverter circuit. 2.2.4 Control CircuitThe control circuit for high-performance electronic ballasts should have a series of functions including power factor correction, lighting adjustment, light-on preheating, lamp disconnection alarm, and lamp restart program control. At present, some integrated circuit controllers for electronic ballasts appearing in the domestic and international device market are mostly based on PFC control, with appropriate addition of lamp control functions, or implementation of lamp control by external circuits. It is worth mentioning that the ML4830/31/32/33 series products can be said to be integrated controllers for high-performance electronic ballasts. 2.3 High-Performance Electronic Ballast Dedicated Integrated Controller of ML4830 SeriesML4830/31/32/33 are integrated circuit controllers developed by American Micro Linear Corporation for high-performance electronic ballasts. The first generation ML4830 has been eliminated; the second generation ML4831 is manufactured by bipolar integrated circuit technology; the third generation ML4832 uses Bicmos process to replace the original bipolar process, the circuit bias current is greatly reduced, and the consumption is greatly reduced. The fourth-generation ML4833 not only adopts the Bicmos process but also has a major improvement in the internal structure, so the function is enhanced and the performance is better. Although these devices can use the functional block diagram of figure 3, the internal structure of ML4831 and ML4832 and the internal structure of ML4833 are respectively shown in figure 4 and figure 5. Figure 3. Functional Block Diagram of ML4831, 32, 33  Figure 4. Internal Block Diagram of ML4831, 32  Figure 5. Internal Structure Block Diagram of ML4833 2.3.1 Introduction to ML4831/32 FunctionThe ML4831/32 is composed of a continuous current type boosting power factor correction stage controlled by an average current. It has a dedicated control circuit for electronic ballasts with various ballast control links. Lamp start-up and restart timing can be achieved by using external circuit components to provide a wide range of control over different types of lamps. The ballast link uses an additional programmable method of frequency modulation and adjustment of the frequency range of the voltage-controlled oscillator to control the lamp power, so it is suitable for various types of output networks. The gain modulator in the ML4831/32 is highly immune to interference caused by switching high-power switching devices. The output of the gain modulator appears as a reference to the current error amplifier at the inverting input of the amplifier. Isine is the current drawn from the AC input; UEA is the output of the error amplifier (pin 1). The output of the gain modulator is limited to 1V. The PWM regulator in the PFC control section compensates for the positive voltage generated by the multiplier output through the negative voltage developed across the pin 4 sense resistor. At the same time, the power MOSFET is protected against high-speed current transients by weekly current limiting. Once the voltage at pin 4 is below 1V, the PWM cycle is terminated immediately. The overvoltage protection (OVP) terminal (pin 18) of the ML4831/32 is used to protect the power circuit from high voltage damage when the lamp is suddenly disconnected. The OVP take-off point can be set by directly tapping the voltage divider resistor to the high-voltage DC bus. As long as the voltage at pin 18 exceeds 2.75V, the power factor correction (PFC) transistor will be turned off and the ballast operation can continue.  The threshold of the OVP should be set to a value that the power device can operate safely, but is not too low to affect the operation of the boost power conversion link. The internal operational transconductance amplifier performs PFC voltage feedback, current sensing and loop amplification. The transconductance amplifier is designed with a low signal forward transconductance so that a large value resistor can be used as a load and a small (<1μF) ceramic capacitor for AC coupling in the compensation network. The compensation network can take the form of figure 6, introducing a zero point and a pole at frequencies fz and fP, respectively: fZ=1/2πR1C1fP=1/2πR1C2 It is noted that the DC-to-ground path and the output of the transconductance amplifier may be out of tune, and the offset error voltage value reflected at the input is determined by uos=iO/gm. Capacitor C1 in figure 6 is used to block DC and minimize the adverse effects of offset. All of the operational transconductance amplifiers in the ML4831/32 incorporate a Slew Rate enhancement to improve recovery under circuit power-up and transient response conditions because the transconductance amplifier changes from a small transconductance state to a large transconductance state. The response to large signals is essentially non-linear. Figure 6. Compensation Network for Transconductance Amplifier The ML4831/32 controls the output power of the lamp by frequency modulation of the non-overlapping conduction of the power switch tube in the inverter part of the ballast circuit. That is to say, during the discharge of oscillation timing capacitor CT, the output of both ballast power tubes is low. The frequency range of the voltage controlled oscillator (VCO) in the device is controlled by the output of the LFB amplifier (pin 6). As the lamp current decreases, the voltage at pin 6 rises, causing the CT charging current to drop, thus causing the oscillation frequency of the oscillator to become lower. Because the ballast output network attenuates high frequencies, the power fed to the lamp increases accordingly. In general, the frequency of the oscillator can be calculated as follows: fosc=1/（tchg＋tdis） Attention: A zero charge current occurs when LFBOUT (pin 6) is high level. Typically, the charge current varies with the two inputs to the oscillator: One is the output of the warm-up timer, and the other is the output of the lamp feedback amplifier (pin 6). During the warm-up phase, the charging current is fixed at a value of Ichg (preheat) = 2.5 / Rset (3). During normal operation, the charging current varies with the voltage of pin 6 from 0 to UOH. When the voltage at pin 6 is zero, the oscillator frequency is lowest and the lamp power is maximum. The discharge current is much larger than the current flowing through the timing resistor RT. For example, when the discharge current is 5 mA, the discharge time is：  tdis ≈ 490 × CT. The ML4831/32 also includes a parallel regulator that limits the UCC voltage to 13.5V. When the UCC is 0.7V lower than 13.5V, the quiescent current of the device will be less than 1.7mA, and the output will be turned off, allowing the device to be started directly using the resistor attached to the rectified AC bus. In addition, because the ML4831/32 contains a temperature sensing function, the ballast operation is stopped as soon as the junction temperature of the device exceeds 120 °C. In order to better utilize the internal sensing function without using an external sensor, the position of the ML4831/32 must be carefully determined on the ballast's circuit board to ensure that the device can properly transfer the operating temperature of the ballast. The chip temperature of ML4831/32 can usually be estimated by the following formula:  Tj=65TA/PD(°C/W) It is worth noting that fully and reasonably using the sensing function inside the device is useful for reducing the total cost of the ballast. The starting scheme of the device is specifically designed for the ML4831/32 in accordance with the principle of ensuring the longest lamp life and minimizing the ballast heating. Figure 7(a) contains a starting scheme including preheating of the filament and sudden breaking of the lamp. When the ballast is energized, the time that the voltage on the CX rises from 0.7V to 3.4V is called the warm-up time of the filament.  During this time, the oscillator's charging current Ichg = 2.5/Rset, the oscillator produces a very high frequency, but does not produce a voltage sufficient to start the lamp. After the filament is preheated, the frequency of the inverter circuit drops to a minimum, and a high voltage is generated to start the lamp. If the voltage of the inverter circuit does not jump when the lamp should start to work, the lamp feedback voltage entering pin 9 will rise above Uref, the CX charging current will be bypassed, and the inverter circuit will stop working until CX drops to a 1.2V threshold by RX discharge. Stopping the inverter circuit in this way can avoid the failure of the lamp to start or the inverter circuit to overheat when it is disconnected from the socket.  In general, it is better to choose a large resistance RX to make this period longer. When CX reaches the 6.8V threshold, the oscillator will turn off LFBOUT, so the lamp will be driven to full power, then dimmed, and the potential of the CX pin is clamped at approximately 7.5V. The whole process is shown in the waveform of figure 7(b). Figure 7. Lamp Start Preheat and Interrupt Timing Scheme and Its Waveform 2.3.2 The Improvement of the Internal Function of ML4833The ML4833 is a modified version of the ML4831/32. In addition to the full functionality of the ML4831/32 described above, the most prominent improvement is in the power factor correction section. The power factor correction part of the ML4833 is a step-up type PFC control circuit for peak current sensing. This form of circuit only requires voltage loop compensation, which is simpler than the ML4831/32 with average current control mode circuit. It consists of a voltage error amplifier, a current sense amplifier without compensation, an integrator, a comparator, and a logic control circuit.  In the boost type power conversion part, the correction of the power factor is performed by the current sensing resistor to output the sensing voltage and the current flowing through, and the duty ratio is adjusted by comparing the integrated voltage signal of the error amplifier with the voltage across the Rsense. The control timing of the duty ratio is as shown in figure 8. Considering that all of the high-performance electronic ballast integrated control chips of Micro-Linearity are packaged in 18-pin DIP or SOIC packages, the improvement of the device structure will inevitably bring about changes in the internal functional frame and external pin functions. Figure 8. PEC Link and Duty Cycle Control of ML4833 2.4 High-performance Electronic Ballast Built by ML4833Figure 9 shows the complete circuit diagram of a high-performance electronic ballast built by ML4833. The circuit is a typical AC/DC/AC structure: the RFI suppression filter circuit is added to the input terminal, the booster active power factor correction circuit is composed of AC/DC in the front stage, and the high-frequency inverter circuit is composed of DC/AC in the rear stage. A closed-loop is formed through T5, VD11, R23 and pin 8 of the control to make the system works stably. Figure 9. Complete Circuit Diagram of High-performance Eectronic Ballast Built with ML4833  Ⅲ FAQ1. What is the use of electronic ballast?An electronic ballast will convert power frequency to a very high frequency to initialize the gas discharge process in Fluorescent Lamps – by controlling the voltage across the lamp and current through the lamp. 2. What is the output voltage of an electronic ballast?This unit operates off the AC mains with a voltage of 230 Volts and voltages generated within the unit can reach 600 to 800 Volts. 3. What is inside an electronic ballast?Lighting ballasts generate an initial high voltage to start the arc that excites the gases in fluorescent and HID lamps and makes them shine. ... Lighting ballasts for fluorescent light bulbs and HID lamps made before 1980 may contain polychlorinated biphenyls (PCBs). 4. How do you make an electronic ballast for tube light?An electrical ballast is nothing but a simple high current, mains voltage inductor made by winding number of turns of copper wire over the laminated iron core. Basically, as we all know a fluorescent tube requires a high initial current thrust to ignite and make the electrons flow connect in between its end filaments. 5. How do you wire an electronic ballast?Connect the ballast to the power from the breaker panel by wiring the black wire from the breaker panel to the black wire on the ballast, using a wire nut. Connect the white wire from the breaker to the white wire from the ballast. 6. What's the difference between electronic and magnetic ballast?A magnetic ballast uses coiled wire and creates magnetic fields to transform voltage. ... An electronic ballast uses solid-state components to transform voltage. It also changes the frequency of the power from 60 HZ to 20,000 HZ or higher depending on the ballast. 7. How do you test an electronic ballast with a multimeter?Insert one probe of the multimeter into the wire connector holding the white wires together. Touch the remaining probe to the ends of the blue, red and yellow wires leading from the ballast. Depending on the ballast, you may have only red and blue wires. 8. Are electronic ballasts non-linear loads?Rectified input, switching power supplies and electronic lighting ballasts are the most common single-phase non-linear loads. 9. Which is not the advantage of electronic ballast?Electronic ballasts are more efficient and more compact in size and weight. They also provide the ability for continuous power adjustment in different settings. A disadvantage is that power fluctuations may cause a failure but this can be offset by adding a buffer capacitor. The operation of the ballasts generates heat. 10. Can you repair an electronic ballast?I eventually replaced the 2 switching transistors in this ballast as well and it worked. So the next time you have a problem with an electronic ballast from a fluorescent fitting open it and check before buying a new one. They can be expensive and more often than not they can be repaired. 

Ⅰ IntroductionIn electronics, the transient interference of voltage and current source is the main cause of damage to circuits and equipment, and it often causes immeasurable losses to the society. These disturbances usually come from the  the starting and stopping operation of power equipment, instability of the AC grid, lightning interference and electrostatic discharge, etc. They are almost ubiquitous and always present. Therefore, scientists have developed a high-efficiency circuit protection device TVS to effectively suppress transient interference.TVS (transient voltage suppressor) is a new product developed on the basis of Zener diode technology. Its circuit symbol is the same as that of ordinary Zener diode. As a common circuit protection component, it is widely used in various fields: automotive electronics, consumer electronics, power drives, industrial power distribution, renewable energy, telecommunications, home appliances, measuring instruments, medical electronics, industrial control, lighting, security systems, building control and automation, audio / video equipment, computers, etc. Learn more about tvs diode, let's check the following transient voltage suppressor tutorial. TVS Diode Tutorial: Transient Voltage SuppressorCatalogⅠ IntroductionⅡ Terminology2.1 Basic Characteristics2.2 Electrical Characteristics2.3 Main ParametersⅢ TVS SelectionⅣ TVS vs Varistor, CapacitorⅤ Application Examples5.1 Lighting Protection5.2 Transistor Protection5.3 Electric Relay Protection5.4 Silicon Control Protection5.5 Integrated Op Amp Protection5.6 Integrated Circuit (IC) Protection5.7 Microcomputer System Protection5.8 DC Regulated Power Supply Protection5.9 Suppression of Electromagnetic Pulse Interference Ⅱ Terminology2.1 Basic CharacteristicsTVS diode, under the specified reverse application conditions, when subjected to a high-energy transient overvoltage pulse, due to it has a very fast response time (sub-nanosecond) and a very high  surge absorption ability, its working impedance can be immediately reduced to a very low on value, allowing large currents to pass, and clamping the voltage to a predetermined level, thereby effectively protecting precision components in electronic circuits from damage. TVS can withstand instantaneous pulse power up to kilowatts, and its clamp response time is only 1ps (10-12S). The forward surge current allowed by TVS can reach 50 ~ 200A under the conditions of TA = 250C and T = 10ms.TVS diodes work similarly to common Zener diodes. If the breakdown voltage is higher than the mark, the TVS diode will conduct. Compared with the Zener diode, the TVS diode has a higher current conduction capability. When the two poles of a TVS diode are subjected to reverse transient high-energy shocks, the high impedance between the two poles of the TVS diode becomes low at a speed of the order of 10 ^ -12S, while absorbing surge power of up to several kilowatts. The clamped voltage between the two poles is at a safe value, which effectively protects precision components in electronic circuits from being damaged by surge pulses. Figure 1. Working Characteristic Curve of TVS DiodeWhen the reverse voltage of the two poles of the TVS is greater than the maximum reverse voltage, it starts to conduct reversely; after the reverse voltage is greater than the breakdown voltage, it begins to be broken down, while the current starts to change suddenly; when the reverse voltage is greater than the maximum clamping voltage, the tube is in an avalanche breakdown state. At this time, the current flowing through the tube increases sharply, and the voltage difference across the tube does not change much (the voltage is clamped).Under specified reverse application conditions, the TVS diode will provide a low-impedance path, and the instantaneous current flowing to the protected component will be shunted to the TVS diode through a large current method, while the voltage across the protected component will be limited to the clamping voltage of TVS. When the overvoltage condition disappears, the TVS diode returns to a high impedance state.  Note: Unidirectional and Bidirectional TVS DiodesUnidirectional TVS SymbolBidirectional TVS Symbol1) Look at the signs: for unidirectional tvs diode, there is a thin color ring, connected to the positive electrode, and for bidirectional tvs diode, there are two rings in the middle, or there is no sign, no polarity.2) Look at the specifications: bidirectional tvs is bidirectional conduction, and unidirectional tvs is unidirectional conduction.3) Look at the model: The model name of the tvs tube is regular, and most of the tvs diode models can see the parameters on the case. For details, it is necessary to consult the manufacturer.4) Using multimeter tool: the unidirectional has voltage, while avalanche breakdown characteristics are available on the DC side; voltage is on both sides, and the DC side is symmetrical on both sides.Bidirectional tvs diodes can absorb instantaneous large pulse power in both forward and reverse directions and clamp the voltage to a predetermined level. In addition, bidirectional TVS is suitable for AC circuits, and unidirectional TVS is generally used for DC circuits. 2.2 Electrical Characteristicsa. Unidirectional TVS (V-I characteristic) Figure 2. Unidirectional TVS DiodeThe unidirectional tvs diode has the same forward characteristics as ordinary Zener diodes, and the reverse breakdown inflection point is approximately “right angle” as a hard breakdown and is a typical PN junction avalanche device. The curve segment from the breakdown point to the VC value indicates that when there is a transient overvoltage pulse, the current of the device increases sharply while the reverse voltage rises to the clamped voltage value and remains at this level.b. Bidirectional TVS (V-I characteristic) Figure 3. Bidirectional TVS DiodeThe V-I characteristic curve of the bidirectional tvs diode is similar to the two back-to-back unidirectional tvs diodes combination. Its forward and reverse directions have the same avalanche breakdown characteristics and clamping characteristics. The symmetry relation of the breakdown voltage on both sides of the positive and negative is as follows: 0.9≤ VBR(positive)/(inverse) ≤1.1, once the interference voltage at both ends of it exceeds the clamping voltage will be immediately suppressed, thus the bidirectional tvs are very convenient for ac loop application.  2.3 Main Parameters1) breakdown voltage V(BR)In the region where the device breaks down, the voltage across the device is measured at the specified test current I (BR), which is called the breakdown voltage. In this area, the tvs diode becomes a low impedance path.2) maximum reverse pulse peak current IPPIn reverse operation, IPP refers to the maximum pulse peak current allowed by the device under specified pulse conditions. The product of IPP and the maximum clamping voltage VC (max) is the maximum value of the transient pulse power.The TVS should be properly selected during use, so that the rated transient pulse power PPR is greater than the maximum transient surge power that may occur in the protected device or wires.When the instantaneous pulse peak current appears, the TVS is broken down and its breakdown voltage value rises to the maximum clamping voltage value. As the pulse current decreases exponentially, the clamping voltage also decreases and returns to the original state. Therefore, TVS diode can suppress the impact of possible pulse power to effectively protect the electronic circuits.The test waveform of the TVS peak current uses a standard wave (exponential waveform), which is determined by TR / TP.Peak current rise time TR: The time from when the current reaches 0.9 IPP from 0.1 IPP.Half-peak current time TP: The time after the current passes through the maximum peak from zero and then drops to 0.5 IPP.The TR / TP values of typical test waveforms are listed below:A. EMP wave: 10ns / 1000nsB. Lightning wave: 8μs / 20μsC. Standard wave: 10μs / 1000μs3) Maximum reverse working voltage VRWMWhen the device operates in reverse, the voltage across the device is called the maximum reverse operating voltage VRWM under the specified IR, usually VRWM = (0.8 ~ 0.9) V(BR). At this voltage, the power consumption of the device is small. When used, VRWM should not be lower than the normal working voltage of the protected device or circuits.4) Maximum clamping current VC(max)The maximum voltage value across the device under the peak pulse current IPP is called the maximum clamping voltage. When used, VC (max) should not be higher than the maximum allowable safe voltage of the protected device. And the ratio of the maximum clamping voltage to the breakdown voltage is called the clamping coefficient.Clamping coefficient = VC(max) / V(BR), the general clamping coefficient is about 1.3.5) Reverse pulse peak power PPRThe PPR of TVS depends on the pulse peak current IPP and the maximum clamping voltage VC (max). In addition, it is also related to the pulse waveform, pulse time and ambient temperature.When the pulse time Tp is constant, PPR = K1‧K2‧VC (max) ‧IPP(K1 is the power coefficient, and K2 is the temperature coefficient of the power) The typical pulse duration tp is 1MS. When the pulse time tp applied to the transient voltage suppression diode is shorter than the standard pulse time, its peak pulse power will increase as tp is shortened.Figure 4. Peak Pluse Power vs Pluse TimeTVS reverse pulse peak power PPR is related to the pulse waveform subjected to surge, expressed by the power coefficient K1: E=∫i(t)‧V(t)dt     i (t) is the pulse current waveform, and V (t) is the clamping voltage waveform.This rated energy value is not reproducible to TVS in a very short time. However, in practical applications, surges often occur repeatedly. In this case, even if the single pulse energy is much smaller than the pulse energy that the TVS device can withstand, if repeat, these single pulse energy will accumulated, in some cases, it will exceed the pulse energy that the TVS device can withstand. Therefore, the circuit design must carefully consider and select the TVS device, so that the accumulation of pulse energy repeatedly applied within the specified interval does not exceed the pulse energy rating of the TVS device.6) Capacitance CPP  Figure 6. The Capacitance of TVS CircuitThe capacitance of TVS is determined by the area of the silicon sheet and the bias voltage. In the case of zero bias, the capacitance value decreases with the increase of the bias voltage. The value of the capacitance will affect the response time of the TVS device.7) Leakage current IRWhen the maximum reverse working voltage is applied to the TVS, the TVS tube has a leakage current IR. When the TVS is used in a high impedance circuit, the leakage current is an important parameter. In practice, especially in automotive electronics, this parameter affects static current.  Ⅲ TVS SelectionWhen selecting tvs diode, the specific conditions of the circuit must be considered, and generally the following principles should be followed:1) The clamping voltage VC (max) is not greater than the maximum allowable safe voltage of the circuit.2) The maximum reverse working voltage VRWM is not lower than the maximum working voltage of the circuit. Generally, VRWM can be selected to be equal to or slightly higher than the maximum working voltage of the circuit.3) The rated maximum pulse power must be greater than the maximum transient surge power present in the circuit. Ⅳ TVS vs Varistor, Capacitor1) TVS diode and varistor do not have switching characteristics like switching elements, but have voltage regulation characteristics like zener diodes.2) The varistor can withstand a larger surge current, and the larger the varistor can withstand the larger surge current, which can reach tens of kA to hundreds of kA at the maximum; but the non-linear characteristics of the varistor are poor and the limiting voltage is higher at large current, and the leakage current is larger at low voltage.3) The non-linear characteristics of TVS diodes are the same as those of Zener tubes. Leakage current before breakdown is very small. After the breakdown, it is in a standard voltage stabilization. Compared with varistors, the maximum clamping voltage of TVS diode is smaller, but its current capability is poor than a varistor. Since the breakdown voltage VBR and the clamping voltage VC of the varistors are relatively high, the current flow capability is relatively strong, and the surge pulse absorption capability is stronger, so it is more suitable for ESD or surge protection of the power interface.4) For the reaction speed, the response speed of the TVS is fast (ps level), while the varistor’s is slow ( ns level). In addition, the capacitance of both is large (ps: TVS also have low capacitance products).5) The TVS tube has high reliability, and a long service life, while the varistor has poor reliability, easy aging and short service life.Other ViewCompared with ceramic capacitors, TVS diodes can withstand a voltage of 15 kV, but ceramic capacitors have a lower ability to withstand high voltages. A 5 kV shock will cause about 10% of the ceramic capacitor to fail, and by 10 kV, its damage rate will reach 60%. Ⅴ Application Examples5.1 Lighting ProtectionIn thunderstorm-prone areas, lightning-induced voltage often breaks down some of the integrated circuits in a computer network. The reason is that cables are damaged due to transient high voltage caused by lightning induction, by installing tvs diodes in the microcomputer, it is useful to reduce damages and commercial loss. And the result shows that it is very practical, and it can improve the reliability of the whole machine application.TVS also have many other applications, for example, for VMOS high power transistors, the tvs diodes between the gate and the source and the machine can prevent gate breakdown and improve the reliability of the VMOS power tube application. 5.2 Transistor ProtectionVarious transient voltages can cause damage to the EB junction or CE junction of the transistor. Especially when the collector of the transistor has an inductive load (coil, transformer, motor), a high-voltage back-EMF can be generated, which often causes the transistor to be damaged. It is necessary to use a tvs diode as a protector. 5.3 Electric Relay ProtectionRelay contacts often use large currents to switch on and off high-current inductive loads such as motors, and the inductor has a high back electromotive force when switching, and has a large amount of energy. What’s more, the contacts are burned or broken to produce an arc, and the surge current generated by the arc is very large. To protect the contacts by suppressing the occurrence of arcs to protect the relays, adding a tvs diode is more effective. In the past, a capacitor or a capacitor series resistor, a diode or a diode series resistor and other suppression methods were used. 5.4 Silicon Control ProtectionThe thyristor may has wrong trigger and cause malfunction. The control electrode current cannot be too large and the voltage cannot be too high, in order to do it, TVS can be used for protection. 5.5 Integrated Op Amp ProtectionIntegrated op amps are very sensitive to external electrical stress. In the process of using op amps, if having excessive voltage or current due to operating errors or abnormal working conditions, especially surges and electrostatic pulses, it is easy to damage the op amp. In the integrating circuit, if the capacitor is charged and discharged to a high potential, and then the power supply voltage is cut off, a transient voltage will be generated at the input terminal, and a large discharge current will occur, resulting in damage to the operational amplifier. At this time, tvs protection method adopted at the input terminal of op amp to avoid device damage. If the capacitance value is large (such as greater than 0.1μF), the protecting effect will be very significant. 5.6 Integrated Circuit (IC) ProtectionAs integrated circuits become more integrated, their withstand voltages are getting lower and lower, and they are easily damaged by transient voltages. Protective measures must be taken, for example, adding tvs diode in the circuit, the CMOS circuit has a protection network at its input and output ends. 5.7 Microcomputer System ProtectionIn a typical microcomputer system, various interference or transient voltages entering through the power line, input line, and output line may cause the microcomputer to malfunction and fail, especially from the switching power supply. The on-off motor near the microcomputer, voltage surges and transients of AC power, electrostatic discharges, etc. may cause the system to fail, and in severe cases may damage the device. Connecting the tvs diode to the input and output lines of the power supply of the microcomputer can prevent the transient voltage from entering the “microcomputer” bus, strengthen the microcomputer's resistance to external interference, ensure the normal operation, and improve its reliability. 5.8 DC Regulated Power Supply ProtectionA DC regulated power supply with a transistor that expands the current output, adding a tvs diode to its regulated output can protect the equipment, and can also absorb peak voltage from the collector to the emitter in the circuit to protect the transistor. In a word, adding a tvs diode at the output end of each voltage stabilization source can greatly improve the reliability of the whole operation. 5.9 Suppression of Electromagnetic Pulse InterferenceA nuclear explosion will cause a strong electromagnetic pulse, which causes induced voltage in the wire. If the induced voltage exceeds the breakdown voltage of the device, it may cause the breakdown of the component, especially for long-term transmission, it is more easily to cause high voltage.TVS diodes are connected in parallel to the signal and power lines, which can absorb the induced voltage caused by electromagnetic pulses, ensure the reliability of the system, and avoid radiation damage to components. Frequently Asked Questions about Transient Voltage Suppression Diode1. How does a transient voltage suppressor work?Transient Voltage Suppressor Diode is a clamping device, so whenever the induced voltage exceeds the avalanche breakdown voltage, it absorbs the excess energy of the overvoltage event, and then it automatically resets after overvoltage condition. 2. What does a transient suppressor do?A transient voltage suppressor or TVS is a general classification of an array of devices that are designed to react to sudden or momentary overvoltage conditions. ... This makes TVS devices or components useful for protection against very fast and often damaging voltage spikes. 3. What is a transient voltage surge suppressor?A transient voltage surge suppressor is a device which is installed on an AC or DC power line to act as a cutoff if there is a momentary surge of electrical power, also known as a “transient.” TVSS devices are considered crucial to the protection of sensitive equipment which would result in circuitry damage or data. 4. What is a voltage transient?A transient voltage is a temporary unwanted voltage in an electrical circuit that range from a few volts to several thousand volts and last micro seconds up to a few milliseconds. ... Faulty contactors and lightning are the most common source of transients. 5. How does a transient voltage suppressor diode work?Transient Voltage Suppressor Diode is a clamping device, so whenever the induced voltage exceeds the avalanche breakdown voltage, it absorbs the excess energy of the overvoltage event, and then it automatically resets after overvoltage condition. 6. Which device can be used as a transient suppressor?Transient voltage suppression diodeOne such common device used for this purpose is known as the transient voltage suppression diode that is simply a Zener diode designed to protect electronics device against overvoltages. 7. What does a suppression diode do?A transient-voltage-suppression (TVS) diode, also transil or thyrector, is an electronic component used to protect electronics from voltage spikes induced on connected wires. 8. What is the difference between Zener and TVS diode?Zener diodes are used to make the voltage more stable. They act as a regulator as well as a protective device. TVS diode is intended to prevent high voltage transients such as Surge and ESD damaging. 9. Where are transient voltage most dangerous?Transient voltages are most dangerous while taking measurements on equipment. Should someone turn something off it could cause a transient voltage spike. 10. What is transient protection?Transients (momentary spikes in voltage or current) can disrupt or damage the products connected to signal or power lines. The most common transient protection schemes limit the voltage amplitude, current amplitude or transition times on the circuit they are protecting.

IntroductionThyristor, commonly known as silicon controlled rectifier(SCR), its normative term is reverse blocking three-terminal thyristor. Thyristors are high-power semiconductor devices that have both switching and rectifying functions, and are used in various circuits such as controllable rectification and frequency conversion, inverters, and non-contact switches. As long as it is provided with a weak point trigger signal, it can control the strong electric output. So it is a bridge for semiconductor devices to enter the field of strong electricity from the field of weak electricity. So far, thyristors are the most widely used semiconductor devices in the electronics industry. Despite the continuous emergence of various new semiconductor materials, 98% of semiconductor materials are still silicon materials, which are still the basis of the integrated circuit industry. It is widely used due to its small size, light weight, high power and long life.Intro to Thyristors: the SCRCatalogIntroductionⅠ Thyristor Basics1.1 Brief Introduction of Thyristor1.2 Working Principle of  ThyristorⅡ The Main Characteristics of Thyristors2.1 Basic Structure of Thyristor2.2 Volt-ampere Characteristics of Thyristors2.3 Static Characteristics of Thyristors2.4 Characteristic Equation of ThyristorⅢ The Main Parameters of Thyristor3.1 Main Parameters of Unidirectional Thyristors3.2 Main Parameters of TRIACⅣ Main Function of ThyristorⅠ Thyristor Basics1.1 Brief Introduction of ThyristorThyristor, also called silicon controlled rectifier, is an abbreviation of semiconductor thyristor. It is a high-current switching semiconductor device that uses small currents to control. There are two commonly used types: ordinary thyristors (also called unidirectional thyristors) and TRIAC(triode for alternating current). Because of its small size, light weight, high efficiency, long life, vibration resistance and because it is noiseless, easy to use, it has attracted great attention from domestic, foreign, industrial and agricultural production departments in a short period of time and has been widely used in various production equipment and household appliances. According to its working principle, it can be roughly divided into four categories: f— Rectification: change AC power into adjustable DC power. — Inverter: converts DC power to AC power with a certain frequency. — DC switch: used for DC loop switch or DC voltage regulation. — AC switch: used for AC loop switch or AC voltage regulation. According to its service objects, it can be used in industries, agriculture, national defense, transportation, mining, metallurgy, light industry, chemical industry and other departments.In performance, thyristors not only have unidirectional conductivity, but also have more valuable controllability than silicon rectifier elements (commonly known as "dead silicon"). It has only two states: on and off.Thyristors can control high-power electromechanical equipment with milliamp currents. If the frequency exceeds this value, the average switching current allowed to pass will decrease due to the significant increase in the switching losses of the components. At this time, the nominal current should be degraded.Thyristors have many advantages, such as: controlling high power with low power, power amplification multiples up to several hundred thousand times; extremely fast response, turn on and off in microseconds; non-contact operation, no spark, no noise; high efficiency, low cost and so on.Disadvantages of thyristors: poor static and dynamic overload capacity; easy to be misguided due to interference.The two types of thyristors, unidirectional thyristors and three-terminal TRIAC, are briefly introduced below.1.2 Working Principle of  Thyristora. Unidirectional ThyristorThe internal structure of the unidirectional thyristor is shown in figure 1 (a). It can be seen from figure 1 (a) that the unidirectional thyristor is composed of four layers semiconductors P1N1P2N2. There are three PN junctions in the middle: the junction J1, J2, and J3. The anode A is drawn from P1, the cathode K is drawn from N2, and the control electrode (or gate) G is drawn from the middle P2. The circuit symbol of the unidirectional thyristor is shown in figure 1 (b). Figure 1. Schematic Diagram and Circuit Symbol of Unidirectional ThyristorIn order to understand the working principle of the unidirectional thyristor, the unidirectional thyristor can be equivalently regarded as a combination of a PNP transistor T1 and an NPN transistor T2. The middle layer P2 and layer N1 are shared by two transistors. The anode A is equivalent to the emitter of T1, and the cathode K is equivalent to the emitter of T2, as shown in figure 2. Figure 2. Working Principle of Unidirectional ThyristorThe key to understanding how unidirectional thyristors work is to understand the role of the control electrode.(1) No voltage or reverse voltage is applied to the control electrodeWhen the control electrode is left floating or a reverse voltage is applied between the control electrode and the cathode, that is, UGK<0, there must be IG=0. If a reverse voltage is applied between the anode and the cathode, that is, UAK＜0. Due to J, and J2, the transmitting junctions of T1, T2, are both reverse biased and T1 and T2 are in the off state, at this time, the current flowing through the unidirectional thyristor is only the reverse saturation current of the J1 and J3, IA≈0, and the unidirectional thyristor is in the blocking state; if a forward voltage is applied between the anode and the cathode, that is, UAK>0, J2 is in a reverse biased state, because IG=0, T2 must be in the off state. and the current in the unidirectional thyristor is only the reverse of J2. At this time, the current in the unidirectional thyristor is just the reverse saturation current of J2, IA≈0, and the unidirectional thyristor is still in the blocking state. Therefore, when no voltage is applied to the control pole or reverse voltage is applied, IG = 0, the unidirectional thyristor is in a blocking state, and has positive and negative blocking capabilities.(2) Apply forward voltage to the control electrodeWhen a forward voltage is applied between the control electrode and the cathode, that is, UGK> 0, the emitter junction J3 of T2 is in a forward bias, and IG≠0. If a reverse voltage is applied between the anode and the cathode, that is, UAK <0, because the emission junction J1 of T1 is reverse biased and T1 is in the off state, the unidirectional thyristor is in the blocking state, IA≈0; If a forward voltage is applied between the anode and the cathode, that is, UAK> 0, because the emission junctions J1, J3 of T1, T2 are forward biased, and the collector junction J2 is reverse biased, T1, T2 will be in an amplified state. After IG is amplified by T2, the collector current of T2 is IC2 = β2IG. The collector current of T2 is the base current of T1, after being amplified by T1, the collector current of T1 is IC1 = β1β2IG. This current flows into the base of T2 for amplification, and in this cycle, a strong positive feedback is formed, which makes T1, T2 quickly enter the saturation state, and the unidirectional thyristor is in the on state. After the unidirectional thyristor is turned on, UAK, the value of the voltage between the anode and the cathode is very small, and the external power supply voltage is almost completely dropped on the load.(3) Turn-off of the unidirectional thyristorFrom the above analysis, it can be seen that after the unidirectional thyristor is turned on, the base of T2 always has the collector current IC1 of T1 flowing, and the value of IC1 is much larger than the IG applied at the beginning. So even if the control electrode voltage disappears and IG = 0, it can still rely on the positive feedback of the tube itself to maintain conduction. Therefore, once the unidirectional thyristor is turned on, the control electrode will lose the function of controlling. After the unidirectional thyristor is turned on, if you want it to turn off again, the anode current IA must be reduced so that it cannot maintain positive feedback. To this end, the anode can be disconnected or a reverse voltage can be applied between the anode and the cathode.To sum up, under the condition that a forward voltage is applied between the anode and the cathode of the unidirectional thyristor, if a forward voltage is added between the control electrode and the cathode at a certain time, the unidirectional thyristor will change from the blocking state to the conducting state. This is triggered into conduction. After the unidirectional thyristor is turned on, the control electrode will lose the function of controlling. If you want to turn off the unidirectional thyristor again, you must make its anode current less than a certain value IH (called the holding current) or reduce the voltage UAK between anode and cathode to zero. b. TRIACA TRIAC is a three-terminal element with a five-layer structure of N1P1N2P2N3. It has three electrodes: a main electrode A1, a main electrode A2, and a control electrode (or gate) G. It is also a gate control switch. Regardless of its structure or characteristics, it can be regarded as a pair of anti-parallel ordinary thyristors. Its structure, equivalent circuit and symbols are shown in figure 3. Figure 3. Symbol, Structure and Equivalent Circuit of the TRIACThe main electrodes A2 and A1 of the triac are connected in series with the control object (load) RL, which is equivalent to a non-contact switch. The "on" or "off" of this switch is controlled by a signal uG (called a trigger signal) on the control electrode G. When there is a voltage (u ≠ 0) between the main electrodes A2 and A1, the moment the trigger signal uG appears, it will be conductive between A2 and A1 of the TRIAC, which is equivalent to the closed state of the switch. And once it is turned on, even if uG disappears, it can be kept on until u = 0 or the current in the series circuit of the main electrode and the load is reduced to a certain value, then it is turned off. After the cutoff, it is equivalent to the off state of the switch. In this way, the small current signal on the control electrode can be used to control the large current in the main electrode circuit. Figure 4. Volt-ampere Characteristic Curve of TRIACGenerally speaking, regardless of the voltage polarity between the two main electrodes A2 and A1 of TRIAC, as long as a certain amplitude of positive and negative pulses is applied to the control electrode, it can be turned on. So i represents the current in the main electrode and u represents the voltage between A2 and A1. The functional relationship between the two (called the volt-ampere characteristic curve) is shown in figure 4. It can be seen from the curve that the TRIAC has basically the same symmetrical performance in the first quadrant and the third quadrant.According to the voltage u on the main electrode and the polarity of the trigger pulse voltage uG on the control electrode, combined with the volt-ampere characteristic curve, the TRIAC can be divided into four trigger modes, which are defined as follows:(1) I+trigger: In the first quadrant of the characteristic curve (A2 is positive), the control electrode is a positive trigger relative to A1.(2) I-trigger: In the first quadrant of the characteristic curve (A2 is positive), the control electrode is a negative trigger relative to A1.(3) Ⅲ+trigger: In the third quadrant of the characteristic curve (A2 is negative), the control electrode is a positive trigger relative to A1.(4) Ⅲ-trigger: In the third quadrant of the characteristic curve (A2 is negative), the control electrode is a negative trigger relative to A1.Among these four trigger modes, I+ and III- have higher sensitivity, and are two commonly used trigger modes.In the control circuit of the new type electric heating electric appliance, the trigger signal applied to the control electrode of TRIAC is output by a single chip microcomputer or an integrated circuit. Some output a continuous positive (or negative) voltage signal, and some output a series of zero-crossing trigger pulses synchronized with a 50Hz sinusoidal AC power supply. The former is called a potential trigger, while the latter is called a pulse trigger. Their waveforms are shown in figure 5 and figure 6, respectively. Figure 5. Figure 6. Ⅱ The Main Characteristics of Thyristors2.1 Basic Structure of ThyristorA thyristor (also known as semiconductor controlled rectifier) is a high-power semiconductor device with a four-layer structure (PNPN). It has three lead-out electrodes, namely anode (A), cathode (K) and gate (G). Its symbolic representation and device cross-section are shown in figure 7. Figure 7. Symbol Representation and Device Cross-sectionOrdinary thyristors bidirectionally diffuse P-type impurities (aluminum or boron) in an N-type silicon wafer to form a P1N1P2 structure, and then diffuse N-type impurities (phosphorus or antimony) to form a cathode in most regions of P2, and at the same time lead out a gate electrode on P2 and form an ohmic contact is formed in the P1 as the anode.2.2 Volt-ampere Characteristics of ThyristorsThe on and off states of the thyristor are determined by the anode voltage, anode current and gate current. Volt-ampere characteristic curves are usually used to describe the relationship between them, as shown in figure 8. Figure 8. Volt-ampere Characteristic Curve of ThyristorWhen the thyristor VAK applies a forward voltage, J1 and J3 are forward biased, and J2 is reverse biased. The applied voltage almost falls on J2, and J2 plays a role of blocking the current. With the increase of VAK, as long as VAK <VBO, the passing anode current IA is small, so this region is called a forward blocking state. When VAK increases beyond VBO, the anode current suddenly increases, and it will be in a low voltage and high current state at the moment the characteristic curve passes the negative resistance. The on-state current IT determined by the load flows through the thyristor, the device voltage drop is about 1V, and the state corresponding to the CD section of the characteristic curve is called the on-state. VBO and its corresponding IBO are usually referred to as forward breakover voltage and breakover current. After the thyristor is turned on, it can maintain the on-state by itself. The transition from the on-state to the off-state is usually controlled by an external circuit without using a gate signal, that is, the device can be turned off only when the current is below a certain threshold value called the holding current IH.When the thyristor is in the off-state (VAK <VBO), if the gate electrode is made positive with respect to the cathode and the gate electrode is supplied with current IG, the thyristor will breakover at a lower voltage. The breakover voltage VBO and the breakover current IBO are both functions of IG. The larger the IG, the smaller the VBO. As shown in figure 3, once the thyristor is turned on, the device is turned on even if the gate signal is removed.When the anode of the thyristor is negative with respect to the cathode, as long as VAK <VBO, IA is small and has nothing to do with IG. However, when the reverse voltage is large (VAK≈VBO), the reverse leakage current through the thyristor increases sharply, showing thyristor breakdown. Therefore, VBO is called the reverse breakover voltage and breakover current.2.3 Static Characteristics of ThyristorsThe thyristor has 3 PN junctions, and the characteristic curve can be divided into (0 ~ 1) blocking area, (1 ~ 2) breakover area, (2 ~ 3) negative resistance area and (3 ~ 4) conducting area. a. Forward Working Area— Forward blocking (0 ~ 1) areaWhen a forward voltage is applied between AK, J1 and J3 bear the forward voltage, while J2 bears the reverse voltage, and the applied voltage falls almost entirely on J2. The reverse-biased J2 acts to block the current, and the thyristor is not conducting at this time.— Avalanche area (1 ~ 2 is also called breakover area)When the applied voltage rises close to the avalanche breakdown voltage VBJ2 of J2, the width of the space charge region of the reverse-biased J2 expands, and the internal electric field is greatly enhanced, which causes the multiplication effect to be strengthened. As a result, the current through J2 suddenly increases, and the current flowing through the device also increases. At this time, the current passing through J2 is transformed from the original reverse current to the current which is mainly attenuated by J1 and J3 through the base region and multiplied in the space charge region of J2. This is the avalanche area where the voltage increases and the current increases sharply. Therefore, the characteristic curve turns in the area, so it is called the breakover area.— Load area (2 ~ 3)When the applied voltage is greater than the breakover voltage, a large number of electron-hole pairs generated by the avalanche doubling of the space charge region of J2 are extracted by the reverse electric field. The electrons enter the region N1 and the holes enter the region P2. Due to the inability to recombine quickly, carrier accumulation occurs near both sides of J2: holes in region P2 and electrons in region N1, compensating for the charge of the ionized impurities and narrowing the space charge region. As a result, the potential in region P2 increases and the potential in the region N1 decreases, which acts to offset the external electric field. As the applied voltage at J2 decreases, the avalanche multiplication effect also weakens. On the other hand, the forward voltage of J1 and J3 has been enhanced, and the injection has increased, causing the current through J2 to increase, so a negative resistance phenomenon has occurred in which the current increases and the voltage decreases.— Low resistance on-state region (3 ~ 4)As mentioned above, the multiplication effect causes the accumulation of electrons and holes on both sides of J2, causing the reverse bias voltage of J2 to decrease; at the same time, the injection of J1 and J3 is enhanced, and the circuit is increased, so that charges continue to accumulate on both sides of J2, and the junction voltage continues to decrease. When the voltage drops to the point where the avalanche multiplication stops and all the junction voltages are cancelled, holes and electrons still accumulate on both sides of J2, and J2 becomes forward biased. At this time, J1, J2, and J3 are all forward biased, and large currents can pass through the device  because it is in a low-resistance on-state region. When fully conducting, its volt-ampere characteristic is similar to that of a rectifier element.b. Reverse Working Area (0 ~ 5)When the device is operating in reverse, J1 and J3 are reverse biased. Due to the very low breakdown voltage of the heavily doped J3, J1 withstands almost all of the applied voltage. The volt-ampere characteristic of the device is the volt-ampere characteristic curve of the reverse bias diode. Therefore, the PNPN thyristor has a reverse blocking region, and when the voltage increases above the J1 breakdown voltage, the current increases sharply due to the avalanche multiplication effect, at which time the thyristor is broken down. 2.4 Characteristic Equation of ThyristorA two-terminal device of a PNPN four-layer structure can be regarded as P1N1P2 and N1P2N2 transistors with current amplification coefficients of α1 and α2, respectively, where J2 is a common collector junction. When a forward voltage is applied to the device, the forward-biased J1 injects holes and passes through region N1 to reach the collector junction (J2). The hole current is α1IA; while the forward-biased J3 injects electrons and passes through region P2. The current carried to J2 is α2IK. Because J2 is in the reverse direction, the current through J2 also includes its own reverse saturation current, ICO.The current through J2 is the sum of the above three, that is,（1）Assuming the emission efficiency γ1 = γ2 = 1, according to the principle of current continuity IJ2 = IA = IK, so formula (1) becomes:（2）The formula shows that when the forward voltage is less than the avalanche breakdown voltage VB of J2, the multiplication effect is small and the injection current is also small. So α1 and α2 are also very small, thus（3）The ICO at this time was also small. Therefore, J1 and J3 are forward biased, so increasing VAK can only increase the reverse bias of J2. It cannot increase the ICO and IA a lot, so the device is always in the blocking state, and the current flowing through the device is the same order of magnitude as the ICO. Therefore, formula (3) is called a blocking condition.When the increase in VAK causes the reverse bias of J2 to increase and avalanche multiplication occurs, assuming multiplication factor Mn = Mp = M, then ICO, α1, and α2 will all increase by M times, so (2) becomes（4）At this time, the denominator becomes smaller, and IA will increase rapidly with the growth of VAK, so when（5）The avalanche steady-state limit is reached (VAK = VBO), and the current will tend to infinity, so equation (5) is called the forward breakover condition., , Using this feature, the breakover point conditions are derived from the characteristic curve equation (4). Because α1 and α2 are functions of current, M is a function of VJ2, which can be approximated with M(VJ2)=M(VAK), ICO is a constant and derive  with respect to (4). The outcome is（6）Since the breakover voltage is lower than the breakdown voltage,  must be a constant value. Because , the numerator must also be zero and obtain （7）According to the definition of transistor DC voltage amplification factor,                      （8）We can get the small signal current amplification factor                      （9）Using formula (9), formula (7) can be changed to                        （10）That is, at the breakover point, the product of the multiplication factor and the sum of the small signal is exactly 1. As long as the PNPN structure satisfies the above formula, it has switching characteristics, that is, it can be switched from an off-state to an on-state.Because α changes with the current IE, when IA increases, both α1 and α2 increase. It can be seen that, when the current is large, the value of M satisfying (6) can be reduced instead. This shows that IA increases and VAK decreases accordingly.α is both a function name of the current and a function of the collector junction voltage. When the current increases as α is constant, the corresponding reverse bias of the collector junction decreases. When the current is large,                             （11）According to equation (2), J2 provides an on-state current (ICO <0). Therefore, J2 must be forward biased, so J1, J2, and J3 are all forward biased, and the device is conducting. The off-state of the device changes to the on-state. The key is that J2 junction must be changed from reverse-biased to forward-biased. The condition for J2 to reverse to the forward direction is that holes and electrons should accumulate in regions P2 and N1, respectively. The condition for the accumulation of holes in region P2 is that the amount of holes α1IA injected by the J1 and collected by J2 into region P2 is greater than the amount of holes that disappear by recombination with (1-α2) IK, that is                           （12）Since IA=IK, α1+α2＞1 is obtained. As long as the conditions are true, the hole accumulation in region P2 is the same, and the region electron accumulation condition is（13）Thus （14）It can be seen that when the condition of α1+α2＞1 is satisfied, the potential of region P2 is positive, and the potential of region N1 is negative. J2 becomes forward-biased and the device is in a conducting state, so α1+α2＞1 is called a conducting condition.Figure 9. SCR (Silicon Controlled Rectifier) Symbol Ⅲ The Main Parameters of Thyristor3.1 Main Parameters of Unidirectional ThyristorsIn order to correctly use a unidirectional thyristor, it is necessary not only to understand its working principle, but also to master its main parameters.(1) Forward repetitive peak voltage UFRMUnder the condition that the control electrode is disconnected and the unidirectional thyristor is in the forward blocking state, when the junction temperature of the unidirectional thyristor is the rated value, it is allowed 50 times per second, and the duration should not exceed 10 ms. The forward peak voltage that can be repeatedly applied to the unidirectional thyristor is called the forward repetitive peak voltage, which is expressed by UFRM. Generally, the secondary voltage is specified as 80% of the forward breakover voltage.(2) Reverse repetitive peak voltage URRMUnder the same conditions as the forward repetitive peak voltage, the reverse peak voltage that can be repeatedly applied to the unidirectional thyristor is called the reverse repetitive peak voltage, which is expressed by URRM and is generally 80% of the reverse breakover voltage.(3) Rated voltage UNUsually, the smaller one of UFRM and URRM is used as the rated voltage of the unidirectional thyristor. This is because the voltage added to the tube in practice is generally a positive and negative symmetrical voltage, so the voltage with a smaller value shall prevail. But because the transient over-voltage will also damage the tube, when selecting the tube, for safety reasons, the rated voltage of the tube is required to be greater than 2 to 3 times the actual peak voltage.(4) Rated forward average current IFThe average value of the power frequency sinusoidal half-wave current allowed to pass through the unidirectional thyristor under the ambient temperature of 40°C and specified heat dissipation conditions is called the rated forward average current IF. How many amp of the unidirectional thyristors we generally say refers to this current value. The amount of IF is related to factors such as the ambient temperature, heat dissipation conditions, and the conduction angle of the component. The rated current of the unidirectional thyristor is calibrated by the power frequency sinusoidal half-wave average current under certain conditions. This is because the load connected to the rectifier output often requires the average current to measure its performance. However, from the perspective of the unidirectional thyristor heating, regardless of the current waveform flowing through the unidirectional thyristor and the conduction angle of the unidirectional thyristor, as long as the effective value of the designed current is equal to the effective value of the rated current IF, then the heating of the unidirectional thyristor is equivalent and allowed.(5) Holding current IHAt room temperature, under the condition of the control electrode short circuit, the minimum anode current required to maintain the unidirectional thyristor to continue conducting is called the holding current IH. If the anode current of the unidirectional thyristor is less than this value, the unidirectional thyristor will change from the conducting state to the blocking state.(6) Control electrode trigger voltage UGK and trigger current IGAt room temperature, under the condition that the voltage between the anode and cathode of the unidirectional thyristor is 6V, the minimum DC current value of the control electrode required to change the unidirectional thyristor from the blocking state to the conducting state is called the trigger current IG. The DC voltage UGK between the control electrode and the cathode corresponding to the trigger current IG is called a trigger voltage. Generally, UGK is about 1 to 5V, and IG is tens to hundreds of mA.3.2 Main Parameters of TRIACIn various control circuits, the TRIAC is a relatively easy-to-damage component. Once the TRIAC is found to be damaged, you just need to replace the TRIAC with the same parameters. There are many characteristic parameters of TRIAC, and the following are the main parameters that should be considered during maintenance.— Off-state repetitive peak voltage-rated voltage VDRMWhen the control electrode is disconnected and the component is at the rated junction temperature, the voltage corresponding to the sharp bending point of the forward and reverse volt-ampere characteristics is called the off-state non-repeating peak voltage. 80% of it is called the off-state repetitive peak voltage. It is also called rated voltage, which is expressed by VDRM.When the TRIAC works, the peak value of the applied voltage momentarily exceeds the reverse non-repetitive peak voltage, which can cause permanent damage to the TRIAC. Moreover, due to the increase in ambient temperature or poor heat dissipation, the reverse non-repetitive peak voltage value may decrease. Therefore, when a TRIAC is selected, its rated voltage value should be 2 to 3 times the possible maximum voltage in actual operation. If the power supply voltage is 220V, a TRIAC with a rated voltage above 500V should be selected so that the selected components can withstand the surge voltage.— Rated on-state average current—rated current IT(AV)Under the specified conditions, the maximum average on-state current allowed when the TRIAC is on is called the rated on-state average current. According to the standard series of TRIAC, this current is taken to the corresponding current level, which is often referred to as the rated current for short and represented by IT(AV).Because the current overload capacity of the TRIAC is much smaller than that of ordinary motors and electrical appliances, the rated current of the TRIAC should be 1.5 to 2 times the maximum current in actual operation when selected.— Gate trigger current IGT (voltage UGT)This refers to the minimum trigger signal current (voltage) value that can make the TRIAC conduct reliably and add to the control electrode. If the trigger current (voltage) obtained by the TRIAC control electrode is less than the number of times, the TRIAC may not be turned on.— On-state average voltage UT(AV)Once the TRIAC is turned on, it is equivalent to the closed switch. Because the TRIAC is connected in series with the load, the smaller the voltage between the two main electrodes, the better. After the TRIAC is turned on, the average value of the voltage between the two main electrodes is called the on-state average voltage, which is usually referred to as the tube voltage drop. If the tube pressure drop of the TRIAC is too large, the motors and solenoid valves it controls may not work properly because they cannot get the full voltage.— Holding currentWhen the control electrode is disconnected at room temperature, the TRIAC is reduced from a large on-state current to a minimum main electrode current that is just necessary to maintain conduction, which is called a holding current. The TRIAC is turned off only when the main electrode current decreases below the holding current. Ⅳ Main Function of ThyristorThe functions of thyristors are as follows: first, converter rectification; second, voltage regulation; third, frequency conversion; fourth, switch (contactless switch). The most basic use of ordinary thyristors is controlled rectification. The diode rectifier circuit we are familiar with is an uncontrollable rectifier circuit. If the diode is replaced by a thyristor, it can constitute a controllable rectifier circuit, inverter, non-contact switch, achieve motor speed control, motor excitation, automatic control and so on. In electrical technology, the half cycle of alternating current is often defined as 180°, which is called the electrical angle. In this way, in each positive half cycle of U2, the electrical angle experienced from the beginning of the zero value to the moment when the trigger pulse arrives is called the control angle α; the electrical angle at which the thyristor conducts in each positive half cycle is called the conduction angle θ. Obviously, both α and θ are used to indicate the on or off range of the thyristor during the half cycle of the forward voltage. Controllable rectification is achieved by changing the control angle α or the conduction angle θ, and changing the average value UL of the pulsed DC voltage on the load. The function of a thyristor is not only rectification, it can also be used as a non-contact switch to quickly turn on or off the circuit, to achieve the inverter that converts DC power to AC power, to change AC power of one frequency to AC power of another frequency, etc. This article mainly introduces the basic principle, characteristics and main parameters of thyristors. Frequently Asked Questions about Thyristors (SCR)1. What are the characteristics of SCR?Characteristics of Thyristor or Characteristics of SCRReverse Blocking Mode of Thyristor. Initially for the reverse blocking mode of the thyristor, the cathode is made positive with respect to anode by supplying voltage E and the gate to cathode supply voltage Es is detached initially by keeping switch S open.Forward Blocking ModeForward Conduction Mode 2. Why SCR is called as thyristor?Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor device made of silicon. This device is the solid state equivalent of thyratron and hence it is also referred to as thyristor or thyroid transistor. 3. Is SCR and thyristor are same?Thyristor is a 4 layer device formed by alternate combination of p and n type semiconductor materials. It is a device used for rectification and switching purpose. SCR is the mostly used member of thyristor family and it is the name commonly used when we talk about thyristors. 4. What is a thyristor used for?Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to switch off automatically, referred to as "zero cross" operation. 5. How does a SCR thyristor work?So how does it work? With no current flowing into the gate, the thyristor is switched off and no current flows between the anode and the cathode. When a current flows into the gate, it effectively flows into the base (input) of the lower (n-p-n) transistor, turning it on.

I IntroductionLaser sensor is a kind of sensor which uses laser technology to measure. It is generally composed of laser, optical parts and photoelectric devices. It can convert the measured physical parameters (such as length, flow, speed, etc.) into optical signals, and then use photoelectric converter to convert the optical signals into electrical signals. Through the filtering, amplification and rectification of corresponding circuits, the output signals can be obtained, so as to calculate the measured quantity. Laser technology has the characteristics of strong direction, high brightness and good monochromaticity. It is widely used in industrial and agricultural production, national defense and military, medical and health, scientific research and other aspects, such as distance measurement, precision detection, positioning, etc., as well as length benchmark and optical frequency benchmark.Laser Distance Sensor OverviewCatalogI IntroductionII What is Laser? 2.1 The Concept of Laser 2.2 Important Characteristics of Laser 2.3 Types of Laser 2.4 What can Laser Sensor Detect?III Laser Displacement Sensor 3.1 What is Laser Displacement Sensor 3.2 How Does Laser Displacement Sensor Work? 3.3 Application of Laser Displacement Sensor 3.4 What are the Parameters to Know When Choosing a Laser Displacement Sensor?IV Laser Distance Sensor 4.1 Classification of Laser Distance Sensors  4.2 Measuring Principle of Different Laser Distance Sensors 4.3 Application of Laser Distance SensorV Laser Sensor Application CaseVI FAQII What is Laser?2.1 The Concept of LaserLaser light is different from ordinary light. (See more about light and photoelectric effect in the article introducing light sensor and photoresistor)We need to use laser to produce laser light. In the normal state, most of the atoms in the laser are in stable low energy level E1. Under the action of appropriate frequency of external light, the atoms in low energy level absorb photon energy to excite and transition to high energy level E2. The photon energy E = e2-e1 = h V, where h is the Planck constant and V is the photon frequency.  On the contrary, when the frequency of light is V, the atom in level E2 will jump to the low energy level to release energy and emit light, which is called stimulated radiation. First of all, the laser makes the atoms of the working materials abnormally in the high-energy level (i.e. inversion distribution of the particle number ), which can make the stimulated radiation process dominant, so that the induced light with the frequency of V can be enhanced, and the large stimulated radiation light can be produced through the avalanche amplification of the parallel reflector, which is called laser light for short.Figure1. Laser2.2 Important Characteristics of Laser（1）High directivity, small divergence angle of light speed, the laser beam extends only a few centimeters from a few kilometers away.（2）High monochromaticity, the frequency width of laser light is more than 10 times smaller than that of ordinary light.（3）High brightness, laser beam convergence can produce temperatures up to several million degrees.2.3 Types of LaserLaser can be divided into four types according to working substance:（1）Solid state laserIts working substance is solid. Ruby laser, neodymium doped yttrium aluminum garnet laser (i.e. YAG laser) and neodymium glass laser are commonly used. Their structures are basically the same, characterized by small and solid, high power. At present, neodymium glass laser is the device with the highest pulse output power, which has reached tens of megawatts.（2）Gas laserIts working substance is gas. Now there are various kinds of gas atoms, ions, metal vapor, gas molecular lasers. Commonly used are carbon dioxide laser, helium neon laser and carbon monoxide laser, whose shape is like a common discharge tube, characterized by stable output, good monochromaticity, long life, but small power, low conversion efficiency.（3）Liquid laserIt can be divided into chelate laser, inorganic liquid laser and organic dye laser, the most important of which is organic dye laser. Its main feature is that the wavelength is continuously adjustable.（4）Semiconductor laserIt is a younger laser, and the more mature one is GaAs laser. It is characterized by high efficiency, small size, light weight and simple structure, and is suitable for carrying on airplanes, warships, tanks and infantry. It can be made into range finder and sighting device. However, the output power is small, the directivity is poor, and it is greatly affected by the ambient temperature.2.4 What can Laser Sensor Detect? (1) Laser measurement of lengthPrecise measurement of length is one of the key technologies in precise machinery manufacturing industry and optical processing industry. Modern length measurement is mostly based on the interference phenomenon of light wave, and its accuracy mainly depends on the monochromaticity of light. Laser is the most ideal light source. It is 100 thousand times purer than the best monochromatic light source (krypton-86 lamp). Therefore, the laser measurement range of length is large and the accuracy is high.  According to the optical principle, the relationship between the maximum measurable length L of monochromatic light and wavelength λ and spectral line width δ is L = λ 2 / δ. The maximum measurable length of krypton-86 lamp is 38.5cm. For a long object, it is necessary to measure in sections to reduce the accuracy. If He-Ne gas laser is used, it can measure tens of kilometers at most. Generally, the length within several meters can be measured with an accuracy of 0.1 μ M.Figure2. Laser Measure(2) Laser measurement of distanceIts principle is the same as that of the radio radar. After the laser is aimed at the target, the round-trip time is measured, and then the round-trip distance is obtained by multiplying the speed of light. Because of the advantages of laser, such as high directivity, high monochromaticity and high power, these are very important for the measurement of long distance, the determination of target orientation, the improvement of signal-to-noise ratio of the receiving system, and the guarantee of measurement accuracy, so the laser rangefinder is paid more and more attention.  The lidar developed on the basis of the laser rangefinder can not only measure the distance, but also the azimuth, velocity and acceleration of the target. It has been successfully used in the ranging and tracking of the artificial satellite. For example, the lidar using ruby laser has a distance measuring range of 500-2000 km with an error of only a few meters. At present, ruby laser, neodymium glass laser, carbon dioxide laser and Gas laser are often used as the light source of laser rangefinder.Figure3. Measuring Distance with Laser Sensor (3) Laser measurement of thickness Based on the principle of triangle ranging, a precise laser ranging sensor is divided at the upper and lower part of the C-frame. The modulated laser emitted by the laser hits the surface of the measured object. By sampling the signal of the linear CCD, the distance between the measured object and the C-frame is synchronously obtained by the linear CCD camera under the control of the control circuit. The thickness of the middle measured object is calculated by the data fed back by the sensor. Because the detection is continuous, the continuous dynamic thickness of the measured object can be obtained.Figure4. Thickness Measuring with Laser SensorThickness measurement by single laser displacement sensorPut the measured body on the measuring platform, measure the distance from the sensor to the platform surface, then measure the distance from the sensor to the measured body surface, and measure the thickness after calculation. It is required that there is no air gap between the measured body and the measuring platform, and the measured body is not cocked. These strict requirements can only be achieved offline.Thickness measurement by double laser displacement sensorA laser displacement sensor is installed above and below the measured body respectively, and the thickness of the measured body is d = C - (a + b). Among them, C is the distance between two sensors, a is the distance between the upper sensor and the measured body, and B is the distance between the lower sensor and the measured body. The advantage of this method for on-line thickness measurement is that it can eliminate the influence of the vibration of the measured body on the measurement results.  But at the same time, there are requirements for sensor installation and performance. The conditions to ensure the accuracy of measurement are that two sensor beams must be coaxial and that two sensor scans must be synchronous. Coaxiality is realized by installation, and synchronization depends on the selection of laser sensor with synchronization end.Figure5. Thickness MeasurementIII Laser Displacement Sensor3.1 What is Laser Displacement SensorThe laser displacement sensor is called the eyes of the robot and machine, and has an irreplaceable role in welding, blank manufacturing, mechanical processing, heat treatment, loading and unloading, assembly and other operations. So, what is a laser displacement sensor? The laser displacement sensor is a sensor that uses laser technology for measurement, and is composed of a laser, a laser detector, and a measurement circuit. As a new type of measuring equipment, the laser displacement sensor can accurately measure the position, displacement and other changes of the measured object, and can also measure precise geometric measurements such as displacement, thickness, vibration, distance, and diameter.3.2 How Does Laser Displacement Sensor Work?The laser displacement sensor can accurately and non-contactly measure the position, displacement and other changes of the measured object, and is mainly used to measure the displacement, thickness, vibration, distance, diameter and other geometric quantities of the object. According to the measurement principle, the principle of laser displacement sensor is divided into laser triangulation method and laser echo analysis method. Laser triangulation method is generally suitable for high-precision and short-distance measurement, while laser echo analysis method is used for long-distance measurement. The following is the introduction to two measurement methods of laser displacement sensor principle.TriangulationFigure6. Laser Displacement SensorThe laser emitter shoots the visible red laser to the object surface through the lens, and the laser reflected by the object passes through the receiver lens, which is accepted by the internal CCD linear camera. According to different distances, the CCD linear camera can "see" this light point at different angles. According to the distance between the laser and the camera known from this angle, the digital signal processor can calculate the distance between the sensor and the measured object. At the same time, the position of the beam in the receiving element is processed by analog and digital circuits, and the corresponding output value is calculated by microprocessor analysis, and the standard data signal is output in proportion in the analog quantity window set by the user. If switching value output is used, it will be conducted in the settings window and cut off outside the window. In addition, an independent detection window can be set for analog quantity and switch quantity output.Echo analysisThe laser displacement sensor can achieve a certain degree of accuracy by using the echo analysis principle to measure the distance. The sensor is composed of processor unit, echo processing unit, laser transmitter and laser receiver. The laser displacement sensor emits one million pulses per second through the laser transmitter to the detector and returns to the receiver. The processor calculates the time required for the laser pulse to meet the detector and return to the receiver, so as to calculate the distance value.  The output value is the average output of thousands of measurement results. It is the so-called pulse time method. The laser echo analysis method is suitable for long-distance detection, but the measurement accuracy is lower than the laser triangulation method, and the longest detection distance can reach 250m.3.3 Application of Laser Displacement Sensor(1) Dimension measurement: position identification of small parts; monitoring of whether there are parts on the conveyor belt; detection of material overlapping and covering; control of manipulator position (tool center position); device state detection; detection of device position (through the small hole); monitoring of liquid level; thickness measurement; vibration analysis; collision test measurement; automobile-related test, etc. (2) Thickness measurement of sheet metal: laser sensor measures the thickness of sheet metal. Thickness change detection can help to detect wrinkles, small holes or overlaps to avoid machine failure. (3) Cylinder measurement: angle, length, eccentricity of inner and outer diameter, conicity, concentricity and surface profile.Figure7. Application of Laser Displacement Sensor(4) Length measurement: place the measured component on the conveyor belt at the designated position, the laser sensor detects the component and simultaneously measures it with the triggered laser scanner, and finally obtains the length of the component. (5) Uniformity check: place several laser sensors in a row in the tilt direction of the workpiece movement to be measured, and directly output the measurement value through one sensor. In addition, the software can be used to calculate the measurement value and read out the result according to the signal or data. (6) Inspection of electronic components: two laser scanners are used to place the tested components between them. Finally, the data is read out by the sensor, so as to detect the accuracy and integrity of the component size. (7) Inspection of filling level in production line: laser sensor is integrated into the production and manufacturing of filling products. When the filling products pass through the sensor, it can detect whether the filling is full. The sensor can accurately identify whether the filling product is qualified and the quantity of the product by using the extended program of laser beam reflecting surface. 3.4 What are the Parameters to Know When Choosing a Laser Displacement Sensor?Some parameters that must be understood when selecting a laser displacement sensor are very important.(1) Resolution: generally refers to the minimum range of the sensor, that is, the maximum recognition rate of the sensor. If the parameter is marked as 1mm, then the resolution is equal to 1mm. (2) Repeatability: We must know that even if the measured object is at rest, the measured value will fluctuate slightly. The error margin of repeated measurement of the measured object at the same position in the static state is the repeat accuracy. For example, if the parameter is marked as 1μm, the repeat accuracy of the sensor is 1μm. (3) Full range (effective range): the rated effective range of the sensor. When selecting a sensor, we must select the sensor that contains the effective range according to the required detection distance. (4) Linear accuracy: the error between the measured value and the actual displacement. Linear accuracy is expressed as a percentage, but since the range is a range and the measurement accuracy is more difficult to reach the apex of the range, most sensors will mark the linear accuracy of the apex of the range to intuitively reflect the performance of the sensor. (5) Sampling frequency/sampling period: frequency refers to the number of measurements per second. The higher the frequency, the shorter the time it takes to make a measurement. The shorter the measurement time, the more suitable it is for the detection of high-speed moving objects. (6) Average sampling times: even in the static state, there will be slight measurement fluctuations. At this time, multiple measurements are required to calculate the average number to make the measured value stable and accurate.Figure8. Laser SensorIV Laser Distance SensorLaser ranging is one of the earliest applications of the laser. This is because the laser has many advantages such as strong directivity, high brightness, and good monochromaticity. Before 1965, the Soviet Union used a laser to measure the distance between the earth and the moon (384401km) with an error of only 250m. In 1969, the Americans landed on the moon with a retro-reflector on the lunar surface. They also used a laser to measure the distance between the earth and the moon, with an error of only 15cm. The basic principle of using laser transmission time to measure the distance is to determine the target distance by measuring the time required for the laser to travel to and from the target.Related recommendation: Proximity SensorFigure9. Laser Distance Sensor4.1 Classification of Laser Distance Sensors Laser distance sensor technology is divided into absolute distance measurement method and micro displacement measurement method according to the measurement range. Subdivided according to the measuring method, the absolute distance ranging method mainly includes pulse laser ranging and phase laser ranging, and the micro displacement measuring method mainly includes triangulation laser ranging and interferometric laser ranging.4.2 Measuring Principle of Different Laser Distance Sensors(1) Pulse Laser Distance SensorA pulse laser with a very short duration is emitted by a pulsed laser, and after reaching the target to be measured after the distance to be measured, part of the energy will be reflected back. The reflected pulsed laser is called an echo. The echo returns to the rangefinder and is received by a photoelectric detector. According to the interval between the main wave signal and the echo signal, that is, when the laser pulse travels from the laser to the target to be measured, the distance of the target to be measured can be calculated.  (2) Phase laser Distance SensorThe emitted laser light is emphasized, and the phase change of the modulated signal is used when the laser is propagated in space. According to the wavelength of the modulated wave, the distance represented by the phase delay is calculated. That is, the indirect method of phase delay measurement is used instead of directly measuring the time required for the round trip of the laser to achieve distance measurement. The accuracy of this method can reach the millimeter level.Figure10. Working Principle of Laser Distance Measuring Device (3) Triangulation Laser Distance SensorAs mentioned above, this measurement principle is that the light emitted by the laser is focused on the surface of the measured object after being focused by the condensing lens, and the receiving lens receives the scattered light from the incident light spot and images it on the photoelectric  position detector On the sensitive side. When the object moves, the relative distance of the object movement is calculated by the displacement of the light spot on the imaging surface. The resolution of triangulation laser ranging is very high, which can reach the order of microns.   Figure11. Triangulation Principle (4) Interferometric Laser Distance SensorBy moving the measured target and measuring the coherence, the distance increment measurement is completed by counting, so the sensitivity of the interferometric measurement is very high, which can reach the nanometer level.4.3 Application of Laser Distance SensorThe laser distance sensor is mainly used for: monitoring the position of moving objects; measuring the railway contact network, measuring the boundary of buildings; measuring unsuitable objects; industrial automation and intelligent production management; vehicle speed and flow statistics; industrial monitoring signal trigger control; tower crane XY positioning of crane; automatic target distance control; monitoring of ship's safe docking position; positioning of container; measurement of vehicle's safe distance; measurement of overhead cable and height limitation; measurement of width of boxes on conveyor belt.V Laser Sensor Application Case(1) Over-limit detection of vehicle width and heightThe laser sensor is used for rapid measurement, the network core of the PC industrial control computer and the visual programming software VB are used for real-time data transmission and processing, and the friendly interface control software is also designed. Field test data shows that the system has good real-time performance and high measurement accuracy, and has certain practical value. (2) Expressway toll stationUsed in highway toll stations to count and protect vehicles. Malaysian Teras has applied hundreds of BEA laser sensors to its manual and automatic toll station systems. The laser sensor uses the time-of-flight (TOF) measurement principle, which can form 4 planes in the detection area to detect the vehicle. At the same time, the product also has functions such as anti-collision and vehicle safety protection. Compared with the traditional light curtain, the laser sensor has the advantages of high sensitivity, high accuracy, easy installation, high cost performance and strong stability. (3) Google's second-generation unmanned vehicleIn addition to the laser sensor on the top, Google’s second-generation driverless car prototype is still quite obvious, and the other sensors are set very concealed. The front, rear and sides of the vehicle are clearly marked with the Google unmanned vehicle logo. The driving principle of Google's unmanned vehicle is to continuously collect various accurate data of the vehicle itself and the surroundings through many sensors installed around the car, analyze and calculate it through the processor in the car, and then control the driving of the car according to the calculation results . Unmanned vehicles will use GPS equipment and sensors to accurately locate the vehicle's position and speed, and judge pedestrians, vehicles, bicycles, signal lights and many other objects around it.Figure12. Google's Self-driving CarThe roof of this Lexus is equipped with a 360° rotating laser holographic sensor, which can sense the front, side and rear conditions of the car almost simultaneously. The data collected by the sensor will be input to the processor located on the right rear side of the vehicle through the green data line. This laser sensor can also allow unmanned vehicles to be accurately positioned globally. The original L-shaped Lexus logo on the front of the car was also removed and replaced with a radar sensor; it was used to measure the distance ahead and the speed of the vehicle in order to determine the condition of the vehicle ahead and control the safe acceleration and deceleration of the vehicle. The wheel hub of the tire is also equipped with a position sensor, which is used to detect wheel rotation and help the vehicle to locate. The heart of Google's unmanned vehicles-the processor is located on the right rear side of the vehicle, the data information from each sensor will be transmitted here through the data wire, and analyzed and processed through the software in order to accurately sense and judge the difference between the unmanned vehicles object. In addition to analyzing and judging the current position of objects around the unmanned vehicle, the unmanned vehicle also needs to be calculated by software to accurately predict the possible next position of each object. Finally, the unmanned car will make safe driving decisions based on all the collected data, including controlling the speed of the car and the surrounding distance. VI FAQ1. How does laser sensor work?The basic principle is optical triangulation using a CMOS linear imager. A diffuse triangulating laser distance sensor transmits a laser through a lens and to the target, which reflects the light back to the sensor. A lens focuses this reflected light into a small spot onto the CMOS linear imager. 2. What is the use of laser sensors?The definition of a laser sensor is, it is an electrical device used to sense minute objects and precise positions. This sensor uses a laser to produce light within a straight line. Its visible ray mark of the laser makes the arrangement very simple. Laser light includes light waves with similar wavelengths. 3. What are the types of laser sensors?Laser distance sensors.Displacement sensors.Laser projectors.Laser light curtains.Laser photoelectric sensors.Positioning lasers.Laser edge detection sensors. 4. Are laser sensors dangerous?Improperly used laser devices are potentially dangerous. Effects can range from mild skin burns to irreversible injury to the skin and eye. The biological damage caused by lasers is produced through thermal, acoustical and photochemical processes. 5. Is a laser a sensor?A laser sensor uses a 'laser' to emit light in a straight line. Its visible beam spot makes alignment and positioning very easy. Since the light beam is focused, the sensor can be installed without worries about stray light. The major types of laser sensors include reflective, thru beam, and retro-reflective. 6. What is the range of the laser sensor?Laser distance sensors are designed for non-contact distance measurements: laser gauges for measuring ranges up to 10m, laser distance sensors for up to 3,000m. 7. What is CMOS Laser Sensor?A CMOS image sensor combines with a step-less laser power adjustment algorithm to produce stable detection of all types of workpieces from black rubber with low reflectivity to stainless steel and other highly glossy materials. 8. Which laser sensor is used for measuring very long distances?LDM301 laser distance sensor series – fast measurement of long distances. The laser distance sensors of the LDM301 series use a measured time-of-flight principle to measure distances of 300 m for natural surfaces and 3,000 m for reflective surfaces. 9. How does a laser sensor measure distance?The distance measurement is based on the triangulation principle. The laser beam strikes the object as a small point. The receiver of the sensor (photodiode line) detects the position of this point. The angle of incidence changes according to the distance, and thereby the position of the laser point on the receiver. 10. How accurate are laser distance sensors?Compared to other types of laser sensors, OM70 sensors feature one of the thinnest beam shapes, helping to ensure a more precise measuring focus. For example, most point-type lasers typically only go down to 0.2mm x 0.75mm whereas the OM70 goes down to 0.05mm x 0.05mm. 

IntroductionThyristor is a four semiconductor layers or three PN junctions devicea solid-state semiconductor device with four layers of alternating P- and N-type materials. It is also known as “SCR” (Silicon Control Rectifier). The term “Thyristor” is dervid from the words of thyratron (a gas fluid tube which work as SCR) and Transistor. And It acts exclusively as a bistable switch in electronic circuit.What is a Thyristor?CatalogIntroductionⅠ Types of ThyristorsⅡ Thyristor Selection2.1 Specific Requirements of Applying Circuit2.2 Main Parameters of the ThyristorⅢ Replacement of ThyristorⅣ Detection of Thyristor4.1 Detection of Unidirectional Thyristors4.2 Detection of TRIACⅠ Types of ThyristorsCommonly used thyristors include unidirectional thyristors, TRIAC, and turn-off thyristors, etc., which should be selected reasonably according to the needs of the circuit.— Unidirectional ThyristorThe unidirectional thyristor is characterized in that the current can only flow from the anode A to the cathode k, and is mainly used in the control of DC power supply or pulsating direct current, AC power rectification, and DC power inverter.Unidirectional thyristors can be divided into ordinary thyristors and high-frequency thyristors (the working frequency is above 110kHz). Commonly used unidirectional thyristors are 3CT series, 3DT series, KP series and KK series (high frequency thyristors), and imported MCR series, SF series, BST series etc.— TRIACTRIAC was developed on the basis of the unidirectional thyristor and is an AC power control device. TRIAC can not only replace two unidirectional thyristors in anti-parallel, but also requires only one trigger circuit, which is more convenient to use.The characteristic of TRIAC is that alternating current can pass through it, which is mainly used in the control of AC power supply and the adjustment of AC voltage. Commonly used TRIAC include 3CTS series and KS series, as well as imported MAC series, SM series, BCR series, etc.— Gate Turn-off ThyristorThe characteristic of the gate turn-off thyristor is that it can be switched off by the control electrode. It is mainly used in gate turn-off contactless switches, DC inverters, dimmer, speed regulation and other occasions.Gate turn-off thyristors are power-type control devices developed on the basis of ordinary thyristors. After the ordinary thyristor is triggered to be turned on, its control electrode does not work. To turn off the thyristor, the power must be cut off, or the forward current flowing through the thyristor must be less than the holding current. Gate turn-off thyristor overcomes the above drawbacks. When the control electrode G is added with a positive pulse voltage, the thyristor is turned on, and when the control electrode G is added with a negative pulse voltage, the thyristor is turned off.Gate turn-off thyristors are ideal high-voltage, high-current switching devices. For example, the DG series high-power gate turn-off thyristors can reach a maximum voltage of 4500V and a maximum current of 3000A. Ⅱ Thyristor Selection2.1 Specific Requirements of Applying CircuitThere are many types of thyristors, which should be selected reasonably according to the specific requirements of the application circuit.For AC/DC voltage control, controllable rectification, AC voltage regulation, power inverter, switching power supply protection circuit, etc., ordinary thyristors can be selected.For AC switch, AC voltage regulation, AC motor linear speed regulation, lamp linear dimming, solid state relay, solid state contactor, etc., a TRIAC should be selected.For AC motor variable frequency speed regulation, chopper, power inverter and various electronic switch circuits, you can choose gate turn-off thyristor.For sawtooth wave generator, long time delay, over voltage protector and trigger circuit with power transistor, etc., BTG thyristor can be selected.In electromagnetic cookers, electronic ballasts, ultrasonic circuits, superconducting magnetic energy storage systems, switching power supplies and other circuits, reverse conducting thyristors can be selected.In the photocoupler, light detector, light alarm, light counter, photoelectric logic circuit and operation monitoring circuit of automatic production line, the light-control thyristor can be selected.2.2 Main Parameters of the ThyristorThe main parameters of the thyristor should be determined according to the specific requirements of the application circuit.The selected thyristor should have a certain power margin, and its rated peak voltage and rated current (on-state average current) should be higher than the maximum operating voltage and maximum working current of the controlled circuit by 1.5 to 2 times.The parameters of the thyristor's forward voltage drop, gate trigger current, and trigger voltage should meet the requirements of the application circuit (this refers to the control circuit of the gate), and should not be high or low, otherwise it will affect the normal operation of the thyristor. Ⅲ Replacement of ThyristorAfter the thyristor is damaged, if no thyristor of the same type is replaced, another type of thyristor with similar performance parameters can be used instead.When designing an application circuit, a large margin is generally left. When replacing the thyristor, just pay attention to its rated peak voltage (repeated peak voltage), rated current (on-state average current), gate trigger voltage and gate trigger current, especially the two indicators of rated peak voltage and rated current.The switching speed of the thyristor used for replacement should be consistent with the switching speed of the damaged thyristor. For example: After the high-speed thyristor used in the pulse circuit and high-speed inverter circuit is damaged, only the same type of fast thyristor can be used instead of the ordinary thyristor.When selecting a thyristor to be used for replacement, it is not necessary to leave too much margin for any parameter, and the parameter of it should be as close as possible to the parameter of the replaced thyristor, because an excessively large margin is not only a waste, but also sometimes has side effects, such as non-triggering or insensitive triggering.In addition, the appearance of the two thyristors should be the same, otherwise it will cause inconvenience to the installation. Ⅳ Detection of ThyristorThyristors are usually represented by the letters "SCR" in circuit schematic diagrams. For example, SCR2 refers to the thyristor numbered 2. The symbol of the thyristor in the schematic diagram is shown in figure 1. Figure 1. Symbols of Thyristor4.1 Detection of Unidirectional Thyristors(1) Discrimination of each electrode: According to the structure of an ordinary thyristor, it can be seen that there is a PN junction between the gate G and the cathode K, which has unidirectional conductive characteristics, while there are two PN junctions of opposite polarities connected in series between the anode A and the gate. Therefore, by measuring the resistance between the pins of an ordinary thyristor with the R × 100 or R × 1 k Q level of the multimeter, three electrodes can be determined.The specific method is: use the black probe of the multimeter to connect one electrode of the thyristor, and use the red probe to touch the other two electrodes in turn. If the measurement result has a resistance value of several thousand ohms (kΩ) and another resistance value of several hundred ohms(Ω), it can be determined that the black probe is connected to gate G. In the measurement with a resistance value of several hundred ohms, the red probe was connected to the cathode K, and in the measurement with a resistance value of several thousand ohms, the red probe was connected to the anode A. If the measured resistance values are both very large, it means that the black probe is not connected to gate G. Apply the same method to test other electrodes until three electrodes are found.You can also measure the forward and reverse resistance between any two pins. If the forward and reverse resistance are close to infinity, the two electrodes are anode A and cathode K, and the other pin is gate G.Each electrode of the ordinary thyristors can also be judged according to its packaging form.For example, the bolt end of the bolt-type ordinary thyristor is anode A, the thinner lead end is gate G, and the thicker lead end is cathode K.The lead end of the flat thyristor is gate G, the flat end is anode A, and the other end is cathode K.A thyristor of metal package (T0-3) is a common thyristor and its shell is anode A.The middle pin of the plastic thyristor (T0-220) is anode A, and it is mostly connected with its own heat sink. Figure 2. Pin Arrangement of Several Common Thyristors(2) Judging whether it is good or bad: Use the R×1 kΩ level of a multimeter to measure the forward and reverse resistance values between anode A and cathode K of ordinary thyristor, which should normally be infinite (∞) ; If the forward and reverse resistance values are zero or the resistance values are both small, it indicates that a breakdown short circuit or leakage occurs inside the thyristor.Measure the forward and reverse resistance values between gate G and cathode K. Normally, there should be forward and reverse resistance values similar to diodes (the actual measurement results are smaller than those of ordinary diodes), that is, the forward resistance value is small (less than 2 kΩ) and the reverse resistance value is large (greater than 80 kΩ). If the resistance values of the two measurements are both large or small, it means that the thyristor is open or short-circuited between electrode G and K. If the forward and reverse resistance values are equal or close, it indicates that the thyristor has failed, and the PN junction between its electrodes G and K has lost its unidirectional conduction effect.Measure the forward and reverse resistance value between anode A and gate G. In normal conditions, both resistances should be several hundred kiloohms (kΩ) or infinite. If the forward and reverse resistance values are not the same (there is unidirectional conduction like a diode). One of the two PN junctions connected in reverse series between gate G and electrode A has been short-circuited.(3) Detection of triggering capability: For ordinary thyristors with low power (working current is below 5A), it can be measured with R×1 level of the multimeter . During the measurement, the black probe is connected to anode A and the red probe is connected to cathode K. At this time, the watch hand does not move, and the resistance value is displayed as infinite (∞). Use tweezers or wires to make anode A and gate G of the thyristor be short-circuited(see figure 3), which is equivalent to applying a forward trigger voltage to gate G. At this time, if the resistance value is several ohms to tens of ohms (the specific resistance value will vary according to the part number of the thyristor), it indicates that the thyristor is conducting due to the forward trigger. Then disconnect electrode A and gate G(the probes on electrode A and K do not move, only the trigger voltage of gate G is cut off). If the value indicated by the watch hand is still in the position of several ohms to tens of ohms, it indicates that the triggering performance of the thyristor is good. Figure 3. Detection of Triggering CapabilityFor medium and high power ordinary thyristors with a working current above 5 A, the on-state voltage drop VT, holding current IH and the gate trigger voltage Vo are relatively large. The current provided by the R × 1 kΩ level of the multimeter is low, and the thyristor cannot be completely turned on, so a 200Ω adjustable resistor and one to three 1.5 V dry batteries can be connected in series at the end of the black probe (depending on the capacity of the thyristor being tested, if its working current is greater than 100 A, three 1.5 V dry batteries are applied), as shown in figure 4. Figure 4. Detection of Trigger VoltageYou can also use the test circuit in figure 5 to test the triggering capability of an ordinary thyristor. In the circuit, vT is the thyristor under test, HL is a 6.3 V indicator (small electric beads in a flashlight), GB is a 6 V power supply (four 1.5 V dry batteries or 6 V regulated power supply can be used), and S is the button, R is the current limiting resistor. Figure 5. Test Circuit to Test the Triggering CapabilityWhen the button S is not connected, the thyristor VT is in a blocking state, and the indicator light HL is not on (if HL is on at this time, there may be breakdown of vT or leakage damage). After pressing the button S once (turn S on for a moment to provide the trigger voltage for gate G of the thyristor VT), if the indicator HL is always on, it means that the thyristor has a good triggering capability. If the brightness of the indicator is low, it indicates that the thyristor has poor performance and a large conduction voltage drop (the conduction voltage drop should be about 1 V under normal conditions). If button S is on, the indicator light is on, and when button S is off, the indicator light is off, indicating that the thyristor is damaged and the triggering performance is poor.4.2 Detection of TRIAC(1) Discrimination of each electrode: Use the R×1 or R×10 level of the multimeter  to measure the forward and reverse resistance values between three pins of the TRIAC. If it is measured that one pin is not connected with the other two pins, then this pin is the main electrode T2.After finding the electrode T2, the remaining two pins are the main electrode T1 and the gate G3. Measuring the forward and reverse resistance values between these two pins will gain two smaller resistance values. In a measurement with a small resistance value (about tens of ohms), the black probe is connected to the main electrode T1, and the red probe is connected to gate G.One end of the bolt of the bolt-shaped TRIAC is the main electrode T2, the thinner lead end is gate G, and the thicker lead end is the main electrode T1.    The shell of the metal-encapsulated (TO-3) TRIAC is the main electrode T2.The middle pin of the plastic-encapsulated (TO-220) TRIAC is the main electrode T2, which is usually connected to its own small heat sink. Figure 6. Pin Arrangement of Several TRIAC(2) Judging whether it is good or bad: Use the R×1 or R×10 level of a multimeter to measure the forward and reverse resistance values between the main electrode T1 and the main electrode T2 and between the main electrode T2 and gate G of the TRIAC. Normally it should be close to infinity. If the measured resistance values are all very small, it means that the electrodes of the TRIAC have been broken down or are short-circuited.Measure the forward and reverse resistance of the main electrode T1 and gate G. Normally, it should be between tens of ohms (Ω) and one hundred ohms (Ω) (when the black probe is connected to electrode T1 and the red probe is connected to gate G, the measured forward resistance value is slightly smaller than the reverse resistance value). If the forward and reverse resistance values between electrode T1 and gate G are measured to be infinite, it indicates that the thyristor has been damaged by an open circuit.(3) Detection of triggering capability: For small power TRIAC with working current below 8A, it can be measured directly with R×1 level of the multimeter. When measuring, first connect the black probe to the main electrode T2 and the red probe to the main electrode T1, then use tweezers to make electrode T2 and gate G be short-circuited, and add a positive polarity trigger signal to gate G. If the resistance value measured at this time changes from infinity to more than ten ohms (Ω), it means that the thyristor has been triggered to conduct, and the conduction direction is T2 → T1.Then connect the black probe to the main electrode T1, and the red probe to the main electrode T2. Use tweezers to make electrode T2 and gate G be short-circuited, and add a negative polarity trigger signal to gate G. If the resistance value measured at this time changes from infinity to more than ten ohms (Ω), it means that the thyristor has been triggered to conduct, and the conduction direction is T1 → T2.If gate G is disconnected after the thyristor is triggered to be turned on, the low-resistance conduction state cannot be maintained between electrode T2 and T1 and the resistance value becomes infinite, it indicates that the TRIAC has poor performance or is damaged. If a positive (or negative) polarity trigger signal is added to gate G, the thyristor still does not conduct (the forward and reverse resistance values between T1 and T2 are still infinite), then the thyristor is damaged and has no trigger continuity.For medium and high power TRIAC with a working current of 8A or more, when measuring their triggering capability, one to three 1.5V dry batteries can be connected in series to a probe of a multimeter, and then measure by using R×1 level as described above.For a TRIAC with a withstand voltage of 400V or more, its trigger capability and performance can also be tested by using 220V AC voltage.Figure 7 is a test circuit of a TRIAC. In the circuit, FL is a 60W /220V incandescent bulb, VT is the TRIAC under test, R is a 100Ω current limiting resistor, and S is a button. Figure 7. TRIAC CircuitAfter the power plug is connected to the working frequency AC, the TRIAC is in the off-state and the light bulb is off. (If the bulb is glowing normally at this time, it means that electrode T1 and T2 of the thyristor under test have been broken down and short-circuited; if the light bulb is slightly light, it means that the thyristor under test is damaged by leakage). Press the button S once to provide the trigger voltage signal for gate G of the thyristor. In normal conditions, the thyristor should be immediately triggered to turn on, and the light bulb will glow normally. If the bulb fails to emit light, the internal circuit of the tested thyristor is damaged. If the light bulb is turned on when the button S is pressed, and the light bulb is turned off when the button is released, it indicates that the triggering performance of the tested thyristor is poor.When using a multimeter to detect low-power light-controlled thyristors, put the multimeter in R × 1 level, connect one to three 1.5V dry batteries in series to a black probe, and measure the forward and reverse resistance values between the two pins. Normally it should be infinite. Then use a small flashlight or laser pen to illuminate the light receiving window of the light controlled thyristor. At this time, a small forward resistance value can be measured, but the reverse resistance value is still infinite. In a measurement with a small resistance value, the black probe is connected to the anode A, and the red probe is connected to the cathode K.The following method can also be used to measure light-controlled thyristors. Turn on the power switch S and illuminate the light receiving window of the thyristor VT with a flashlight. After adding a trigger light source (high-power light-controlled thyristor has its own light source, as long as the light-emitting diode or semiconductor laser in its optical cable is added with the working voltage, no external light source is required), the indicator EL should be on. After the light source is evacuated, the indicator light EL should remain illuminated. There is only one PN junction. Therefore, you just need to measure electrode A and G with a multimeter.Put the multimeter in the R × 1 kΩ level, and the two probes can be connected to one of the two pins of the thyristor under test (measure their forward and reverse resistance values). If a pair of pins is measured with a low resistance value, the black probe is connected to the anode A, while the red probe is connected to gate G, and the other pin is the cathode K. (2) Judging whether it is good or bad: Use the R×1 level of a multimeter to measure the forward and reverse resistance values between the electrodes of the BTG thyristor. Under normal conditions, the forward and reverse resistances between the anode A and the cathode K are infinite; the forward resistance between the anode A and gate G (when the black probe is connected to electrode A) is several hundred ohms to several thousand ohms and the reverse resistance value is infinite. If the forward and reverse resistance values between two electrodes are measured to be very small, it indicates that the thyristor has been short-circuited and damaged.(3) Detection of triggering capability: Put the multimeter in the R × 1 Ω level, connect the black probe to anode A, and the red probe to cathode K. The measured resistance should be infinite. Then touch gate G with your finger and add a human body induction signal to it. If the resistance between electrodes A and K changes from infinity to low resistance (a few ohms) at this time, it indicates that the thyristor has a good triggering ability. Otherwise, the performance of the thyristor is poor. Frequently Asked Questions about Thyristors1. What is thyristor and its types?A thyristor is a four-layer device with alternating P-type and N-type semiconductors (P-N-P-N). In its most basic form, a thyristor has three terminals: anode (positive terminal), cathode (negative terminal), and gate (control terminal). The gate controls the flow of current between the anode and cathode. 2. What is thyristor diagram?In general, Thyristors are also switching devices similar to the transistors. ... SCR or Thyristor is a four-layered, three-junction semiconductor switching device. It has three terminals anode, cathode, and gate. Thyristor is also a unidirectional device like a diode, which means it flows current only in one direction. 3. Where is thyristor used?Thyristors may be used in power-switching circuits, relay-replacement circuits, inverter circuits, oscillator circuits, level-detector circuits, chopper circuits, light-dimming circuits, low-cost timer circuits, logic circuits, speed-control circuits, phase-control circuits, etc. 4. Why SCR is called Thyristor?Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor device made of silicon. This device is the solid state equivalent of thyratron and hence it is also referred to as thyristor or thyroid transistor. 5. What is thyristor diagram?In general, Thyristors are also switching devices similar to the transistors. ... SCR or Thyristor is a four-layered, three-junction semiconductor switching device. It has three terminals anode, cathode, and gate. Thyristor is also a unidirectional device like a diode, which means it flows current only in one direction.