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IntroductionOperational amplifier (op amp for short) is basically a voltage amplifying device designed to be used with components like capacitors and resistors, between its in/out terminals, or is simply a linear Integrated Circuit (IC) having multiple-terminals. In electronics, the open-loop voltage gain of the actual operational amplifier is very large, which can be seen a differential amplifier with infinite open loop gain, infinite input resistance and zero output resistance. In addition, it has positive and negative inputs which allow circuits that use feedback to achieve a wide range of functions. And meanwhile, it can be further simplified into an ideal op amp model, referred to as an ideal op amp (also called ideal OPAMP).CatalogIntroductionⅠ Ideal Op Amp Characteristics1.1 Infinite Input Resistance1.2 Zero Output Impedance1.3 Infinite Open-loop Gain1.4 Infinite Common-mode Rejection Ratio1.5 Infinite BandwidthⅡ Assumptions of Ideal Op AmpⅢ Working Characteristics of Ideal Operational Amplifiers3.1 Work in Linear Region3.2 Work in Nonlinear RegionⅣ Analysis of Ideal Operational Amplifier CharacteristicsⅤ Balanced Resistance Presets5.1 The Role of Balanced Resistance5.2 Input Balancing Resistor ExplanationⅥ Ideal Op Amp EquationsⅦ Several Common Op Amp CircuitsⅧ Difference Between Ideal Op-amp and Practical Op-ampⅠ Ideal Op Amp CharacteristicsWhen analyzing various application circuits of operational amplifiers, the integrated operational amplifier is often regarded as an ideal operational amplifier. The so-called ideal op amp is to idealize various technical indicators of op amps, and it must have the following characteristics.Characteristics of An Ideal Op-Amp1.1 Infinite Input ResistanceThe input terminal of an ideal operational amplifier does not have any current to flow in. In electronics, op amps are voltage gain devices. They amplify a voltage fed into the op amp and give out the same signal as output with a much larger gain. In order for an op amp to receive the voltage signal as its input, the voltage signal must be dropped across the op amp. If you know the concept of a voltage divider, voltage drops primarily across components with high impedances, proportionally according to ohm’s law by the formula V=IR. So the greater the resistance (or impedance) of a device, the greater the voltage drop across that device is. To make sure that the voltage signal drops fully on the op amp, it must have a very high input impedance, so that the voltage drops fully across it. If it had a low input impedance, the voltage may not drop across it and it would not receive the signal. This is why op amps must have high-input impedances.It’s also easy to make the input impedance lower (put a resistor in parallel) or the source impedance higher (put a resistor in series).Figure 1. Ideal Op Amp Symbol and Transfer Characteristic Curve 1.2 Zero Output ImpedanceThe output of an ideal op amp is a perfect voltage source, no matter how the current flowing to the amplifier load changes, the output voltage of the amplifier is always a certain value, that is, the output impedance is zero. In practice, zero output impedance is actually a distinct property from infinite input impedance, but for a very long time infinite input impedance was approached only with compromises in offset voltage and noise. 1.3 Infinite Open-loop GainIn an open-loop state, the differential signal at the input has an infinite voltage gain. This feature makes the operational amplifier very suitable for practical applications with upper negative feedback configuration. 1.4 Infinite Common-mode Rejection RatioAn ideal operational amplifier can only respond to the difference between the voltages at both ends of V+ and V-. In addition, the same part of the two input signals (ie common mode signal) will be completely ignored. What’s more, a high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI). An example is audio transmission over balanced line in sound reinforcement or recording. 1.5 Infinite BandwidthThe ideal operational amplifier will amplify the input signal of any frequency with the same differential gain, which will not change with the change of signal frequency.Ⅱ Assumptions of Ideal Op AmpThe op amp can be considered a voltage controlled current source, or it is an integrated circuit that can amplify weak electric signals. Based on it, for an ideal OPAMP, what is the relationship between it and these electrical signals?First, assume that the current flowing into the input of the op amp is zero. This assumption is almost completely correct for FET op amps, because the input current for FET op amps is below 1pA. But for dual high-speed op amps, this assumption is not always correct, because the input current of it can sometimes reach tens of microamperes.Second, assume that the gain of the op amp is infinite, so the op amp can swing the output voltage to any value to meet the input requirements. It means that the output voltage of the op amp can reach any value. In fact, when the output voltage is close to the power supply voltage, the op amp will saturate. Maybe this hypothesis does exit, but needs a limit in practical. For example, at higher frequencies, the internal junction capacitors of transistor come into play, thus reducing the output and therefore the gain of amplifier. The capacitor reactance decreases with increase in frequency bypassing the majority of output. The opamp is in saturation state.Figure 2. Op Amp SaturationFor example, as per datasheet of LM741, large signal voltage gain is 200V/mv. It means an open loop gain of 200,000. If you operate an op-amp in open-loop condition(i.e. without negative feedback) ,even microvolts of input voltage (input offset voltage of LM741 is 3mv) will drive the output to saturation.In most of the amplifier circuits op-amp is configured to use negative feedback which greatly reduces the voltage gain (i.e. closed loop gain). In oscillators and schmit triggers, Op-amp is configured to use positive feedback. Comparator circuit is an example of the circuit which utilizes open-loop gain of op-amp. Its output will be always at saturation either positive or negative saturation. In an integrator circuit, the DC gain should be limited by adding a feed back resistor in parallel with capacitor ;else the output will get saturated .Even in amplifier circuits, the amplitude of the input signal and the voltage gain of the circuit should be balanced so that the output voltage does not exceed power supply voltage . For example for a non-inverting amplifier with a voltage gain of 100, the maximum permissible input voltage will be 150 mv if the VCC is 15 Volts. If you apply a signal of 200 mv ,the op-amp output will goto saturation as the required output will be 20 volts which exceeds the VCC of 15 Volts.Third, the assumption of infinite gain also means that the input signal must be zero. The gain of the op amp will drive the output voltage until the voltage (error voltage) between the two input terminals is zero. The voltage between the two input terminals is zero. The zero voltage between two input terminals means that if one input terminal is connected to a hard voltage source like ground, the other input terminal will also be at the same potential. In addition, since the current flowing into the input terminal is zero, the input impedance of the op amp is infinite.Fourth, of course, the output resistance of an ideal op amp is zero. An ideal op amp can drive any load without any voltage drop due to its output impedance. At low currents, the output impedance of most op amps is in the range of a few tenths an ohm, so this assumption is true in most cases. Ⅲ Working Characteristics of Ideal Operational Amplifiers3.1 Work in Linear RegionWhen the ideal op amp works in the linear region, the output and the input voltage show a linear relationship. Where u0 is the output voltage of the integrated op amp; u+ and u- are the voltages at the non-inverting input terminal and the inverting input terminal, respectively. Auo is the open loop differential voltage magnification. According to the characteristics of the ideal op amp, two important characteristics of the ideal op amp in the linear region.1) Zero differential input voltageSince the open-loop differential voltage magnification of an ideal op amp is equal to infinity, and the output voltage is a certain value, the voltage values at the non-inverting input terminal and the inverting input terminal are approximately equal. Just like short circuit between input and output, but it is fake. Because it is an equivalent short circuit, not a real short circuit, so this phenomenon is called "virtual short".2) Zero input currentSince the open-loop input resistance of an ideal op amp is infinite, no current flows into the op amp at either input. At this time, the current at the non-inverting input terminal and the inverting input terminal are both equal to zero. Like an disconnection, but an equivalent disconnection, so this phenomenon is called "virtual break". Virtual short and virtual break are two important concepts for analyzing the ideal op amp working in the linear region.In fact, the ideal operational amplifier has the characteristics of "virtual short" and "virtual break". These two characteristics are very useful for analyzing linear amplifier circuits. The necessary condition for virtual short is negative feedback. When negative feedback is introduced, at this time, if the forward terminal voltage is slightly higher than the reverse terminal voltage, the output terminal will output a high voltage equivalent to the power supply voltage after the amplification of the op amp. In fact, the op amp has a respond time changing from the original output state to the high-level state (the golden rule of analyzing analog circuits: the change of the signal is a continuous change process). Due to the feedback resistance of the reverse end change will inevitably affect its voltage, when the reverse end voltage infinitely close to the forward end voltage, the circuit reaches a balanced state. The output voltage does not change anymore, that is, the voltage at the forward end and the reverse end is always close. (Note: The analysis method is the same when the voltage decreases.) 3.2 Work in Nonlinear RegionWhen the op-amp operates in the nonlinear region, the output voltage no longer increases linearly with the input voltage, but saturates. The ideal op amp also has two important characteristics when operating in the nonlinear region.1) When u+ ≠ u-, the output voltage of the ideal op amp reaches the saturation value.When u+ > u-, the op-amp operates works in positive saturation region with a positive output voltage.When u+ < u-, the op-amp operates works in negative saturation region with a negative output voltage.Ideal op amp operates in the nonlinear region, u+ ≠ u-, there is no “virtual short”.2) The input current is equal to zero.Although the input voltage u+ ≠ u- above, the input current is considered to be zero. Ⅳ Analysis of Ideal Operational Amplifier CharacteristicsAs for Op-amp, there's probably a description like this: three-terminal element (circuit structure with double-ended input, single-ended output), ideal transistor, high-gain DC amplifier.(1) High input resistanceUnder this situation, the current flowing into the input terminal is close to 0, almost no signal source current is used, which is close to the voltage control characteristic. And virtual break is derived from this.(2) Lower output resistanceIt has the characteristics of adapting to any load. And the impedance of the subsequent load circuit will not affect the output voltage.(3) Infinite voltage amplification(4) Under a certain supply voltage condition, the amplifier can only work in closed-loop (negative feedback) mode, and the actual amplification is limited. Because op-amps themselves don't have a 0V connection but their design assumes the typical signals will be more towards the center of their positive and negative supplies. Thus, if your input voltage is right at one extreme or forces the output toward one supply, chances are it won't work properly. Working in open-loop mode is the like a comparator, and the output is high level orlow level.In the closed-loop (limited amplification) state, the amplifier is randomly compare the potentials of the two input terminals. The output stage makes immediate adjustments when they are not equal. So the final purpose of amplification is to make the potentials of the two input terminals equal. And virtual short is derived from this. Ⅴ Balanced Resistance Presets5.1 The Role of Balanced Resistance1) A suitable resistance is generally required to ensure that the input impedance is matched.2) In order to reduce the input current imbalance, the in-phase resistor should be equal to the parallel value of the two resistors at the reverse end in theory. In practice, as a result of the closed loop, especially in deep negative feedback conditions, the misalignment is not obvious at the output. And there is no need of in-phase grounding resistor when the misalignment is not the main problem. Because a balanced resistor is the starting point for an ideal op amp. In-phase grounding resistance is useful for bipolar op amps, and has no meanings for MOS-type op amps.3) Ground input termination resistance: it is necessary for impedance matching and high frequency setting.4) Bias current and offset current.For operational amplifiers with bias current greater than offset current, input resistance matching can be reduced, and precision circuits can compensate bias current to a minimum. If the bias current and offset current are similar, the matching resistance will increase the error.5) Set for the bias current at the input, the purpose of which is to equalize the impedance of the invertingand non-inverting inputs, so that two inputs with equal bias currents are assumed to have equal voltage drops, thereby counteraction can be made. 5.2 Input Balancing Resistor ExplanationA op-amp is connected to an inverting amplifier:Set the input resistance for R1, feedback resistance for Rfi,Assume that the non-inverting end is not connected to a balanced resistor, but grounded directly.Set the input bias current for the op-amp IB (same voltage in inverting and non-inverting end).The current flows through R1 and Rf are represented by I1 and If.Inverting voltage is V-, The op-amp gain is A.Use KCL in the inverting end (set the input signal to 0).Where (0-V-)/R1- (A+1)V- /Rf=IBFrom the above equation, it follows that V-=-(IB×R1×Rf/(Rf+(A+1)R1))At this time, the output voltage of the op-amp is Vo=A×(IB×R1×Rf/(Rf+(A+1)R1))The above formula can be approximated as Vo=IB×((A×R1)/Rf)If the in-phase terminal passes through a resistor R2 to ground and R2=R1/Rf, then the voltage at the in-phase terminal is V+=-IB×R2KCL is applied to the inverted terminal, where (0-V-)/R1+(A×(V+-V-)-V-)/Rf=IBAt this time the output voltage of the op-amp is Vo=0. Ⅵ Ideal Op Amp EquationsUnderstanding the basic conditions of an ideal op amp, and combining it with the Kirchhoff's current law (KCL) node voltage method and the superposition theorem of the node, is an effective method to analyze the ideal op amp circuit.As shown below, find the output voltage uoFigure 3. OPAMP Circuit1) Equation based on KCLFrom the concept of virtual break, i+=i-=0, then i1=i2, i3=i4, so (a)Based on virtual break, u+=u-, then (b)2) Node voltage methodList the node voltage equations for node 1 and node 2, and get (c)Note: Because the output current of the op amp is unknown at 1) and 2), it is not possible to list the KCL equation or node voltage equation at the output of the op amp. In addition, the op amp output uo in 2) should be treated as an independent voltage source. 3) Superposition theoremWhen there are multiple signal inputs, choosing the superposition theorem to solve can simplify the analysis and calculation process. The size of the output signal uo can be regarded as the superposition of the output signal obtained by the independent action of u1 and u2. When u1 acts alone, the u2 terminal is grounded, and the op amp output is: (d)Therefore, the final output of the operational amplifier is:   (e) Ⅶ Several Common Op Amp CircuitsNon-inverting Amplifier CircuitA non-inverting amplifier is an op-amp circuit configuration which produces an amplified output signal. It provides a high input impedance along with all the advantages gained from using an operational amplifier. Inverting Amplifier CircuitAn inverting amplifier (also known as an inverting operational amplifier or an inverting op-amp) is a type of operational amplifier circuit which produces an output which is out of phase with respect to its input by 180 degrees out of phase with respect to input signal. In the following figure, two external resistors to create feedback circuit and make a closed loop circuit across the amplifier. Op-amp as AdderAn adder circuit can be made by connecting more inputs to the inverting op amp. The circuit diagram of a summing amplifier is as shown in the following figure. Differential AmplifierDifferential amplifier is an analog circuit with two inputs and and one output in which the output is ideally proportional to the difference between the two voltages. It is a very useful op-amp circuit and by adding more resistors in parallel with the input resistors as shown in the following. Composite AmplifierThe composite amplifier is termed as a combination of multiple operational amplifiers that are cascaded together with a negative-feedback loop around the entire network. The resistance in the circuit is generally selected at the K ohm level, the ratio of the resistance affects the gain and bias, in addition, the supply current, frequency response and capacitive load driving capability of the op amp determine their specific values in circuits. If it is used in a high-frequency circuit, the resistance needs to be reduced to obtain a better high-frequency response, but it will increase the input bias current, thereby increasing the current of the power supply. Ⅷ Difference Between Ideal Op-amp and Practical Op-ampIdeal op amps use no power, have infinite input impedance, unlimited gain-bandwidth and slew rate, no input bias current, and no input offset. They have unlimited voltage compliance.Practical op amps consume some power, have very high input impedance have limited gain-bandwidth and limited slew rate, have some input bias current and input offset voltage. Voltage compliance is limited by the power supply rail, or frequently even less.Still practical op amps are very useful because most of the limitations listed above are way better than what your circuit needs.For an ideal amplifier, it does not draw any current at all from its input. Assuming a two input amplifier the signal current in both input probes is zero. In other words the input impedance must be infinite. The output, should operate as the output of an ideal voltage source. This means that the potential between the output and the ground must be A(v2−v1), no matter how much current would a load connected to the output would draw. In other words the output impedance must be zero.For a real amplifier, the input impedance must be as large as possible while the output impedance must be as low as possible.In fact, An op-amp in real life, however, cannot operate with zero current flow. Frequently Asked Questions about Ideal Op Amp1. What is characteristic of ideal opamp?Ideal op amps will have infinite voltage gain, infinitely high impedance, zero output impedance, its gain is independent of input frequency, it has zero voltage offset, its output can swing positive or negative to the same voltages as the supply rails, and its output swings instantly to the correct value. 2. How does an ideal op amp work?An operational amplifier, or op amp, generally comprises a differential-input stage with high input impedance, an intermediate-gain stage, and a push-pull output stage with a low output impedance (no greater than 100 Ω). ... Open-loop voltage gain runs very high, on the order of 1 million. 3. Why are op amps not ideal?Op-amps with FET inputs have an Ibias that is so small that this method becomes less practical. Instead of measuring the voltage drop across a resistor, one can monitor the change in voltage across a capacitor as it is charged by the bias current. 4. How are real op amps different from ideal op amps?In real op amps, the amplified signal will not fully reach the DC supply rails. They will fall short of it. In an ideal op amp, the output will swing instantly to the amplified voltage value. There will be no time delay between the time the voltage is input into the op amp till the time it is output. 5. What are the four main ideal characteristics of an open-loop op amp?An ideal op amp is usually considered to have the following characteristics:Infinite open-loop gain G = vout / vInfinite input impedance Rin, and so zero input currentZero input offset voltageInfinite output voltage rangeInfinite bandwidth with zero phase shift and infinite slew rateZero output impedance R

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.

IntroductionA printed circuit board (PCB) mechanically supports and electrically connects electrical or electronic components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. A PCB allows signals and power to be routed between physical devices.What are PCBs? How Do PCBs Work?CatalogIntroductionⅠ PCB Basics1.1 PCB MaterialsⅡ PCB Product CharacteristicsⅢ Common Sense of PCB ProcessⅠ PCB Basics1.1 PCB Materialsa. Copper Clad Laminate（referred to as CCL, or sheet material）Tg: Glass Transition Temperature, which is the temperature at which glassy substances are transformed between glassy and highly elastic (usually softened). In the PCB industry, this glassy substance is generally referred to as resin or dielectric layer composed of resin and glass fiber cloth. Tg is an important technical index reflecting the heat resistance of printed circuit board substrates. Generally, the higher the Tg value of printed circuit boards, the better the heat resistance. The Tg of general FR-4 copper-clad epoxy glass laminates for SMT printed circuit boards is 130 ～ 140℃, which can meet the requirements when using Sn-Pn solder. For lead-free solders with higher melting points, the Tg of the substrate can not withstand the high temperature of welding if the Tg ≤ 150 ° C. In special cases (high temperature use), the Tg can be greater than 170 ° C. However, excessive Tg will cause the hardness of the substrate to increase and the material to become brittle. Therefore, we cannot simply pursue high Tg. We should comprehensively consider the performance of the board and choose a suitable printed board substrate with a higher Tg, which is one of the requirements for lead-free welding.CTI: Comparative Tracking lndex (or comparative leakage index, tracking index). The highest voltage value that the surface of the material can withstand 50 drops of electrolyte (0.1% ammonium chloride aqueous solution) without the formation of leakage traces. The unit is V.CTE: Coefficient of thermal expansion. Generally, the sheet material performance of a PCB is measured by the linear expansion coefficient, which is defined as the ratio of the increase in length to the original length under a unit temperature change, such as Z-CTE. The lower the CTE value, the better the dimensional stability and vice versa.TD: Thermal decomposition temperature refers to the temperature at which the base material resin loses weight by 5%. It is a sign of delamination and performance degradation caused by the heat of the base material of printed circuit boards.CAF: Conductive Anodic Filament of printed boards is the phenomenon of electrochemical insulation damage on insulating substrates. It refers to the state where dendritic metals are precipitated between wires under the action of an electric field after a voltage is applied to parallel circuits on printed circuit boards. Or conductive anodic filament(CAF) occurs along the glass fiber surface of the substrate, thereby reducing the insulation between the wires.T288: It is a technical index that reflects the welding resistance of the printed circuit board substrate. It refers to the maximum time that the printed circuit board substrate can withstand the high temperature of welding at 288°C without blistering and delamination. The longer this time, the better it is for welding. For traditional Sn-Pb alloys with low welding temperature (220 ～ 230℃), when the thermal decomposition time of printed circuit board substrate is 260 ℃, T260≥30s can meet the requirements of SMT printed circuit board. For lead-free welding, the temperature is generally 250 ~ 260℃, the thermal decomposition temperature of the substrate of the printed circuit board will increase, and only T288≥300s at 288 ℃ can ensure that the substrate does not decompose and the performance is not damaged during welding.DK: dielectric constant.DF: Dissipation factor refers to the ratio of the energy that has been lost in the insulation sheet material of the signal line to the energy that still exists in the line.OZ: oz is the abbreviation of the symbol ounce, which is British measurement unit and also unit of weight. 1 OZ means the thickness of copper with a weight of 1 OZ evenly spread over an area of 1 square foot (FT2). It is the average thickness of copper foil expressed by the weight per unit area. It is expressed by a formula, that is, 1OZ = 28.35g / FT2. b. Copper FoilED Copper Foil: Electrodeposited copper, copper foil commonly used in PCB, cheap.RA Copper Foil: Rolled annealed copper, copper foil commonly used in FPC.Drum Side: smooth side of electrodeposited copper foilMatt Side: rough side of electrodeposited copper foilCopper: Elemental symbol Cu, atomic weight 63.5, density 8.89 g / cm3, and the electrochemical equivalent of Cu2 + is 1.186 g / Ah. c. Prepreg: referred to as PPEpoxy Resin: an organic polymer compound containing two or more epoxy groups in the resin molecule, which is a resin component used in prepregs that are currently commonly used.DICY: Dicyandiamide, a common hardenerR.C: resin contentR.F: resin flowG.T: gel timeV.C: volatile contentHarden: Under certain conditions (high temperature, high pressure or light), the epoxy resin and the hardener undergo cross-linking polymerization to form a polymer with a three-dimensional mesh structure. d. InkViscosity: Viscosity refers to the relative movement between adjacent fluid layers when fluid is flowing, and frictional resistance will be generated between the two fluid layers. Unit: Pascal. Seconds (pa.s).Hardness: The hardness of the ink after pre-baking is 2B, the hardness of the ink after exposure is 2H, and the hardness of the ink after finishing is 6H. Thixotropic: a property that the ink is gelatinous when it is left to stand, but its viscosity changes when it is touched. It is a physical property of a liquid, that is, its viscosity decreases under agitation, and it will return to its original viscosity soon after standing. By stirring, the thixotropic effect lasts for a long time and it is enough to reconstitute its internal structure. To achieve high-quality screen printing, the thixotropic of the ink is very important. In particular, during the squeegee process, the ink is agitated to make it liquid. This action speeds up the speed of ink passing through the mesh, and promotes the original ink with separate wires to be evenly integrated. Once the squeegee stops moving, the ink returns to a stationary state, and its viscosity quickly returns to the original required data. e. Dry Film· Structure of dry film Figure 1. Structure of Dry Film· Dry film consists of three parts and ingredients:   — Supporting film(Polyester)   — Photo-resist dry film   — Covering film(Polyethylene)· Main ingredients① Binder(film-forming resin) ②Monomer ③Photo-initiator ④Plasticiser ⑤Adhesion promoter ⑥Thermal polymerization inhibitor ⑦Dye ⑧Solvent· The types of dry film are divided into three types according to the different methods of developing and removing the dry film: solvent-based dry film, water-soluble dry film, and peel-off dry film; according to the purpose of dry film, it is divided into: dry resist film, masking dry film and solder mask dry film.· Photosensitivity: It refers to the amount of light energy required for the photoresist to react to form a polymer with a certain resistance under the irradiation of ultraviolet light. In the case of constant light source intensity and light distance, the sensing speed is expressed as the length of exposure time. Short exposure time means that the sensing speed is fast.· Resolution: refers to the number of lines (or intervals) that can be formed by the dry film resist within a distance of 1mm. The resolution can also be expressed by the absolute size of the lines (or intervals). f. Net YarnNet density:— T number mesh number: refers to the number of meshes within 1 cm. g. Drill bit· geometry structure name of drill bitFigure 2. Geometry Structure Name of Drill Bit· Point AngleThe point angle is composed of two narrow and long first point angle surfaces and two triangular hook-shaped second point angle surfaces. These four sides meet at the point angle, forming two short edges called chisel edges at the center of the joint. This is the place that the sheet material first touches. This chisel edge is first positioned under pressure and rotation to drill into the stack. A protruding square strip on each of the two outer sides of the first point angle surface is called a margin. This margin tends to spiral upward along with the drill body part, which is the contact part between the drill pin and the hole wall. The right angle at the intersection of the margin and the edge lip is very important to the quality of the hole wall. The point angle has a long edge between the first and second point angle surfaces. The point where the two long edges and the two chisel edges meet in the middle is the point angle. The angle formed by the two long edges is called the point angle. When the drilling paper is made of phenolic resin substrate, the drill point angle is about 90° ~ 110° due to less resistance. When drilling paper is FR4 glass fiber board, the point angle should be slightly blunt at 115° ~ 135°. The most common one is 130°. The angle between the first point angle surface and the horizontal plane of the long edge is about 15°, which is called primary face angle. The second point angle is about 30°, and the angle formed by the chisel edge and the edge lip is called a cheisel edge angle. · Types of drill bitFigure 3. Types of Drill Bit Ⅱ PCB Product Characteristics1) Impedance· The sum of resistance and reactance (capacitance, inductive reactance) on a vector. Common impedance types are characteristic impedance and differential impedance.2) Warpage· Maximum bow (Figure a) and twist (Figure b) of printed circuit boards using surface mount components should be less than or equal to 0.75%3) RoHSRoHS, the abbreviation of restriction of hazardous substances, is the restriction of the use of certain hazardous substances in electrical and electronic equipment. RoHS lists a total of six harmful substances, including: lead(Pb), cadmium(Cd), mercury(Hg), hexavalent chromium( Cr6+), polybrominated diphenyl ether(PBDE), polybrominated biphenyl(PBB).4) BacklightIt is an enlarged visual inspection method to check the integrity of the copper wall of the through hole. The method is to carefully thin the substrate outside the hole wall from a certain direction, and then use the principle of resin translucency to shoot light from the back. If the quality of the chemical copper hole wall is intact and there is no any holes or pinholes, the copper layer must be able to block light and be dark in the microscope. Once there are holes in the copper wall, light spots must appear and be observed , and can be enlarged as photographic evidence. The ground sample is about 4-6mm wide.Figure 5. Backlight Standard Diageam5) Anode Phosphor Copper Ball· Purity requirementsElementContent(%)ElementContent(%)Cu≥99.91Ni≤0.002P0.040-0.06Sb≤0.002Pb≤0.002As≤0.002Fe≤0.0025S≤0.002Sn≤0.002O≤0.002 · Features— A black (or brown-black) film forms on the surface of phosphor copper after power is applied.— Black (or brown-black) film is Cu3P, also called phosphor copper anode film.· Role of phosphor copper anode film— The anode film itself can catalyze and accelerate the (Cu+-E → Cu2+) reaction, thereby reducing the accumulation of CU+.— After the anode film is formed, it can inhibit the continuous generation of Cu+.— The electrical conductivity of the anode film is 1.5X104-1 cm-1, which has metal conductivity.— Phosphor copper is less anodized than pure copper (1A / DM2 P0.04-0.065%, phosphor copper is less anodized than oxygen-free copper, 50MV-80MV) and will not cause anode passivation.— Anode film will greatly reduce the phenomenon of tiny grains falling off the anode.— Anode film prevents the copper anode from dissolving too quickly.6) Method for Estimating Surface Area of Electroplated Copper Anode· Method for estimating surface area of round titanium basket copper anode: pDLF/2p = 3.14 D = diameter of titanium basket L = length of titanium basket F = factor· Method for estimating surface area of square titanium basket copper anode: 1.33LWFL = length of titanium basket W = width of titanium basket F = factor· F is related to the diameter of the copper ball:Diameter = 12 mm    F = 2.2Diameter = 15 mm    F = 2.0 Diameter = 25 mm    F = 1.7Diameter = 28 mm    F = 1.6 Diameter = 38 mm    F = 1.27) ICD IssuesInternal Connection Defects Ⅲ Common Sense of PCB Processa. Etching FactorThe index used to consider the amount of etching lateral erosion, because the amount of lateral erosion will be different for different copper thickness, so the etching factor is different from the total copper thickness.Calculation method:b. Lateral ErosionThe etching of the side wall of the wire under the resist pattern is called lateral erosion, and the degree of lateral erosion is expressed by the width of the side etching:· lateral erosion is related to the type, composition and etching process and equipment of the etching solution. c. Pool EffectDuring the etching process, the circuit board passes through the etching machine horizontally. Due to the gravity acting on it, the fresh medicine is blocked by the old one and cannot effectively react with the copper surface. d. Different Stages of ResinsA-stage resin: Some thermosetting resins are liquid in the early stages of manufacture or liquid when heated, and can still dissolve in some liquids at this time.B-stage resin: Some thermosetting resins can soften when heated in the middle stage of the reaction, but they cannot be completely dissolved or melted. At this time, they can swell or partially dissolve when contacted with some solvents.C-stage resin: The later stage of the reaction of some thermosetting resins, when it is practically insoluble or infusible. e. Font Color of SubstrateRed font: flame retardant grade, other fonts: non flame retardant grade. f. OthersMSDS: Material Safety Data Sheet provides a variety of information on safety, health and environmental protection for chemical substances and products, and can provide information on basic knowledge, protective measures and emergency actions of chemicals. In some countries, MSDS is also called SDS, and SDS terminology is used in ISO 11014.SGS: Societe Generale de Surveillance S.A. Founded in 1887, it is currently the world's largest and oldest non-governmental third party engaged in product quality control and technical certification of multinational companies. Headquartered in Geneva, it has 251 branches around the world, 256 professional laboratories and 27,000 professional and technical personnel, and carries out product quality inspection, monitoring and assurance activities in 142 countries.UL: UNDER WRITERS LABORATORIES INCIPC: The Institute for International and Packaging Electronic CircuitsISO: International Standards OrganizationMIL: Military StandardJPC: Japan Printed Circuit AssociationCOV: Coefficient of variationFR4: Abbreviation for Flame Retardant Type 4. It is the name of a flame-resistant printed circuit board material composed of a composite material of glass fiber and epoxy resin. It is the most widely used printed circuit board. g. pH ValueAlso known as the hydrogen ion concentration index and pH value, it is a scale of the hydrogen ion activity in a solution, which is a measure of the acidity and alkalinity of a solution in the usual sense. The concept was proposed by Danish biochemist Søren Peter Lauritz Sørensen in 1909. P stands for German Potenz, which means strength or concentration, and H stands for hydrogen ion (H +). Sometimes pH is also written in Latin as pondus hydrogenii.Under normal conditions (25°C, about 298K), when pH <7, the solution is acidic, when pH> 7, the solution is alkaline, and when pH = 7, the solution is neutral. h. Hull Cell TestHull designed Hull cell in 1939. The Hull cell test only requires a small amount of plating solution. After a short time test, the plating effect of the plating solution can be obtained in a wide range of current density. Because this test is sensitive to the composition and operating conditions of the plating solution, it is commonly used to determine the concentration and pH value of each component of the plating solution, and to determine the current density range for obtaining a good coating. The Hull cell has become an indispensable tool for electroplating research and electroplating process control.Hull cells are usually made of insulating materials such as plexiglass or rigid polyvinyl chloride. The bottom surface is trapezoidal, and the cathode and anode are placed on two sides that are not parallel. There are five types of capacity: 250ml, 267ml, 320ml, 534ml, and 1000ml. Generally, 250ml plating solution is often added to a 267ml test cell, which is convenient for converting the additives into how many grams per liter.Figure 8. Dimensions of Hull Celli. Current Density A / dm2A/dm2 — how many amperes per square decimeter area, 1A / dm2 — This is the current density of electroplating, which means that the current passes through the plating area of the workpiece per square decimeter is 1A and dm means decimeter. Generally, we use a 267ml Hull cell, place the test piece at the cathode and immerse it in the test solution. The area of the test piece is approximately 1dm2. j. TP: THROUGH POWERCalculation method:k. Replacement ReactionA replacement reaction is a chemical reaction in which a simple substance reacts with a compound to form another simple substance and a compound. Any replacement reaction is a metathesis reaction, including a reaction of metal and metal salt, the reaction of metal and acid, etc.; the replacement reaction must be a redox reaction, and the redox reaction is not necessarily a replacement reaction; the replacement reaction occurs according to the active list of metals. l. EDS：Energy Dispersive X-ray SpectroscopyThe surface of the sample being tested is irradiated with a condensed electron beam. Due to the interaction between the electron beam and the sample, various electrons or X-rays, photons, and other information are generated. Then, this information is collected and processed in different ways to display various characteristics of the sample (morphology, microstructure, composition, crystal plane, etc.) m. SEM：Scanning Electron MicroscopeAn electron beam with a diameter of 20 mm to 30 mm emitted from the cathode of the electron gun is accelerated by the voltage between the cathode and the anode, and is directed toward the lens barrel, and is condensed by the condenser lens and the objective lens, and is reduced into an electron probe with a diameter of about several nanometers. Under the action of the scanning coil on the upper part of the objective lens, the electron probe scans the surface of the sample in a raster pattern and excites a variety of electronic signals. These electronic signals are detected by corresponding detectors, amplified, converted into voltage signals, and finally sent to the grid of the picture tube and modulate the brightness of the picture tube. The electron beam in the picture tube is also raster scanned on the phosphor screen, and this scanning movement is strictly synchronized with the scanning movement of the electron beam on the sample surface, so that a scanning electron image corresponding to the contrast and the intensity of the received signal is obtained. The image reflects the topographical features of the sample surface. n. BONDINGBonding is a wiring method in the chip production process. It is generally used to connect the chip's internal circuit with gold or aluminum wires to the package pins or gold-plated copper foil on the circuit board before packaging. Ultrasonic waves from an ultrasonic generator (generally 40-140KHz), which generates high-frequency vibration through the transducer, are transmitted to the splitter through the horn. When the splitter is in contact with the lead wire and the welded part, under the action of pressure and vibration, the surfaces of the metal to be welded rub against each other, the oxide film is destroyed, and plastic deformation occurs, causing the two pure metal surfaces to closely contact each other to achieve the combination of atomic distance, and finally form a strong mechanical connection. After bonding (ie, after the circuit is connected to the pins), the chip is packaged with black glue. o. The Giovanni EffectIt means that due to the potential difference between two metals, an electric current is generated through the medium, and then an electrochemical reaction occurs, and the anode with a high potential is oxidized. p. Vacuum Degree; Degree of VacuumThe degree of thinness of the gas under vacuum is usually expressed by "high vacuum" and "low vacuum", high vacuum indicates "good" vacuum, and low vacuum indicates "poor" vacuum. If the pressure in the device under test is lower than atmospheric pressure, a vacuum gauge is required for pressure measurement. The value read from the vacuum gauge is called the degree of vacuum. The degree of vacuum is a value indicating that the actual value of the system pressure is lower than the atmospheric pressure, that is: the degree of vacuum = atmospheric pressure-absolute pressureThere are usually two ways to identify the degree of vacuum:   — First, it is marked with "absolute pressure" and "absolute vacuum degree" (that is, how much pressure is higher than "theoretical vacuum"); in actual cases, the absolute pressure value of the vacuum pump is between 0 and 101.325KPa. The absolute pressure value needs to be measured with an absolute pressure meter. The initial value of the gauge (absolute vacuum gauge) for measuring the vacuum degree at 20°C and the place where the altitude = 0 is 101.325KPa (ie, a standard atmospheric pressure).   — The second is to use "relative pressure" and "relative vacuum degree" (that is, how much pressure is lower than "atmospheric pressure") to identify. "Relative vacuum degree" refers to the difference between the pressure of the measured object and the atmospheric pressure at the measurement site and is measured with an ordinary vacuum gauge. In the absence of vacuum (that is, at atmospheric pressure), the initial value of the gauge is 0. When measuring vacuum, its value is between 0 and -101.325KPa (usually expressed as a negative number).Commonly used vacuum units are Pa, Kpa, Mpa, atmospheric pressure, kilogram (Kgf / cm2), mmHg, mbar, bar, PSI, etc. The approximate conversion relationship is as follows:   — 1MPa = 1000KPa   — 1KPa = 1000Pa   — 1 atmospheric pressure = 100KPa = 0.1MPa   — 1 atmospheric pressure = 1 kg (Kgf / cm2) = 760mmHg   — 1 atmospheric pressure = 14.5 PSI   — 1KPa = 10mbar   — 1bar = 1000mbar Frequently Asked Questions about PCB Basic1. What is PCB and types of PCB?A printed circuit board (PCB) is a thin board made from fiberglass, composite epoxy, or other laminate materials. PCBs are found in various electrical and electronic components such as beepers, radios, radars, computer systems, etc. Different types of PCBs are used based on the applications. 2. What is the basis of PCB?PCB is the acronym of Printed Circuit Board, a mechanical base that contains tracks and footprints reflecting the schematic of the design. Modern PCBs are typically made of a non-conductive substrate that is overlayed by copper layers. 3. What are the basic steps of PCB design?Here's the full list of PCB layout and design steps:Create the Schematic.Create a Blank PCB Layout.Schematic Capture: Linking to Your PCB.Designing Your PCB Stackup.Defining Design Rules and DFM Requirements.Place Components.Insert Drill Holes.Route Traces. 4. What are the common types of PCB?Common Types of Printed Circuit BoardsSingle Layer PCB. Single layer printed circuit boards are among some of the simplest to design and manufacture. ...Double Layer PCB.Multi-Layer PCB.High Density Interconnect (HDI) PCB.High Frequency PCB. 5. Which material is used in PCB?copper circuitryPrinted circuit boards (PCBs) are usually a flat laminated composite made from non-conductive substrate materials with layers of copper circuitry buried internally or on the external surfaces. They can be as simple as one or two layers of copper, or in high density applications they can have fifty layers or more.

CatalogⅠ What is Biosensor?Ⅱ Principle of Biosensors Ⅲ Characteristics of Biosensors Ⅳ Types of Biosensors  4.1 Acoustic Biosensor  4.2 Optical Biosensor  4.3 Magnetic Biosensor  4.4 Electrochemical Biosensor  4.5 Optical Fiber Nano BiosensorⅤ FAQⅠ What is Biosensor?Biosensor is an instrument that is sensitive to biological substances and converts its concentration into an electrical signal for detection. Biosensor has the function of receiver and converter. Because enzyme membrane, mitochondrial electron transport system particle membrane, microbial membrane, antigen membrane and antibody membrane have the selective recognition function to the molecular structure of biomaterials and only have the catalytic activation function to specific reactions, so biosensors have very high selectivity. The disadvantage is that the biofilm is not stable.Biosensors are mainly used in clinical diagnosis, treatment monitoring, fermentation industry, food industry, environment and robotics. Biosensor is an interdisciplinary subject combining bioactive materials(Enzyme, protein, DNA, antibody, antigen, biofilm, etc)with physical and chemical transducers. It is an advanced detection method and monitoring method necessary for the development of biotechnology, and it is also a rapid and microanalysis method at the molecular level. In the 21st century, in the development of the knowledge economy, biosensor technology will be a new growth point between information and biotechnology. It will have a wide application prospect in clinical diagnosis, industrial control, food and drug analysis(including biopharmaceutical research and development), environmental protection, biotechnology, biochip and other research in the national economy. All kinds of biosensors have the following common structures: including one or several related bioactive materials(Biofilm) and physical or chemical transducers(sensors) that can convert the signals expressed by bioactivity into electrical signals. The two are combined to reprocess the biological signals with modern microelectronics and automatic instrument technology to form a variety of usable biosensor analysis devices, instruments and systems.Ⅱ Principle of Biosensors The substance to be measured enters into the bioactive material through diffusion, and after molecular recognition, biological reaction occurs. The information generated is then transformed into a quantitative and treatable electrical signal by the corresponding physical or chemical transducer, and then amplified and output by the secondary instrument, the concentration of the substance to be measured can be known.Ⅲ Characteristics of Biosensors     （1）The biosensor uses the immobilized bioactive substance as the catalyst, and the expensive reagent can be reused many times, which overcomes the shortcomings of the high cost of enzyme analysis reagent and complicated chemical analysis in the past.    （2）Strong specificity only reacts to a specific substrate, and not affected by color and turbidity.     （3）The analysis speed is fast, and the results can be obtained in one minute.    （4）High accuracy; general relative error can reach 1%.    （5）The operating system is simple and easy to realize automatic analysis.    （6）Low cost; only a few cents per measurement in continuous use.     （7）Some biosensors can reliably indicate the oxygen supply and by-products in the microbial culture system. In the process of production control, much complex information can be obtained only by the comprehensive action of physical and chemical sensors. At the same time, they also pointed out the direction of increasing the yield of products.Ⅳ Types of BiosensorsAccording to the classification of life substances used in biosensors, biosensors can be divided into microbial sensors, immune sensors, tissue sensors, cell sensors, enzyme sensors, DNA sensors, etc. According to the principle of sensor detection, it can be divided into a thermosensitive biosensor, FET biosensor, piezoelectric biosensor, optical biosensor, acoustic channel biosensor, enzyme electrode biosensor, mediator biosensor, etc.According to the type of interaction between sensitive substances, it can be divided into two types: affinity type and metabolism type. 4.1 Acoustic BiosensorAcoustic biosensor is a kind of sensor to detect the change of acoustic frequency caused by the substance to be detected. Among them, quartz crystal microbalance（QCM） biosensor has been studied most. In the piezoelectric crystal of quartz crystal microbalance biosensor, AT mode is often used to form two parallel metal((Au，Ag，Pt，Ni，Pd etc.)) membrane electrodes on both sides of the crystal by ion beam deposition. (At cutting means that the cutting surface is 25.15 ° to the main optical axis of quartz crystal.  At this moment, the temperature coefficient of crystal resonance is close to zero at room temperature)The recognition molecules are fixed on the surface of the membrane electrode. Because of their specificity, the recognition molecules combine with the molecules to be detected, causing the quality change of the electrode surface, thus changing the oscillation frequency of the quartz crystal. If the nanoparticles are modified on the molecules to be detected, the quality of the molecules to be detected will be significantly improved, and the detection signal will also be enhanced. Ward et al. Labeled the antibody with nano colloidal particles, and combined it to the surface of quartz crystal by antibody antigen immunoassay.  Because the modified colloidal particles(The diameter of sol particles is 5 "100nm) improved the quality of the labeled molecules, according to Sauerbrey equation, the oscillation frequency of quartz crystal was correspondingly increased, so the detection signal was amplified, the detection sensitivity was improved, and the detection lower limit was also reduced.4.2 Optical BiosensorNano metal particles can be used for optical resonance detection. Bauer et al. Fixed nano-metal particles on the surface of conductive materials by antigen-antibody or protein receptor binding methods. Due to the interaction of reflection dipoles of nanoparticles, the resonance of reflected light is enhanced. The materials to be detected can be detected by detecting resonance signals. Nanoparticles can also be used to locate tumors. Fluorescein labeled recognition factors bind to tumor receptors, and then the size and location of tumors can be displayed in vitro. Nano metal particles can also be used as a general fluorescent annihilation group. Maxwell and other scientists labeled gold nanoparticles and fluorescence excitation groups at both ends of oligonucleotide probe molecules respectively. The probe formed a "hairpin" structure due to complementary bases, and the proximity of fluorescence excitation group and gold nanoparticles resulted in excitation fluorescence annihilation. When the probe combined with specific target DNA, its conformation changed, and the gold nanoparticles and fluorescence excitation groups were separated, so as to excite Fluorescence. The principle can be used for real-time fluorescence detection of nucleic acids and single base mutation polymorphism detection.4.3 Magnetic BiosensorMagnetic nanoparticles have important application value in biological detection and drug analysis. By using magnetic materials to label biomolecules and molecular recognition technology, complex operations such as sample mixing, separation and detection can be realized. Scientists label molecules with magnetic materials, and realize the separation and detection of samples under the magnetic field gradient. Richardson et al. Used magnetic counter to detect magnetic labeled molecules by magnetic immunoassay.  In addition, the distribution and position of magnetic particles in vivo can be measured in vitro after the identification factor is labeled with nanomagnetic particles and combined with the target recognition device on the tumor surface, so as to locate the tumor. Chemla and other scientists used paramagnetic nanoparticles and a microscope based on high-temperature transient DC superconducting quantum interface device (SQUID) to propose a novel rapid detection technology for biological samples. Firstly, the magnetic particles of the fixed antibody are suspended in the solution, and then the magnetized nanoparticles are generated under the instantaneous magnetic field pulse.  When the magnetic field disappears, the particles tend to be free distribution, because the particles without the antibody are Brownian motion, so there is no detection signal; while the nanoparticles with the target molecule move in the way of Neel relaxation, resulting in a slowly attenuated magnetic signal, The substance to be detected can be analyzed by the signal collected by the squid. This technology can directly detect the labeled molecules without separating the nanoparticles which are not combined with the molecules to be detected, which shortens the detection time and improves the detection efficiency.4.4 Electrochemical BiosensorColloidal gold is the most common metal nanoparticles, which can be used to mark biomolecules, thus realizing signal detection and amplification; in addition, it can also be widely used in TEM, SEM characterization and paper strip color. Many literatures also reported the signal amplification of colloidal gold in various biosensors. Gonzalez Garcia and other scientists used colloidal gold labeling and electrochemical methods to study the interaction between biotin and avidin.  By modifying biotinylated albumin on the electrode surface and then reacting with avidin labeled by colloidal gold with a diameter of 10 nm, scientists found that the current response caused by colloidal gold was linearly related to the concentration of avidin(2.5×10-9mol/L "2.5×10–5mol/L). Nanoparticles have an excellent specific surface area, which can be used to immobilize biomolecules, increase the number of fixed molecules, and achieve signal amplification. Singh and other scientists used the sol-gel method to synthesize silicon nanoparticles with a diameter of 20 nm or 200 nm. Acetylcholinesterase immobilized on the surface of nanoparticles can be used to make organophosphorus pesticide biosensor.  Because of its high specific surface activity, combined with the detection of ion-sensitive field effect tube, the metal nanoparticles with rapid response can be used as the carrier of catalyst, which can greatly improve the performance of catalyst. Enzyme colloidal gold is fixed on the surface of the electrode and can be used for the electrochemical detection of H2O2, glucose, xanthine and hypoxanthine. Xu et al. Modified the surface of the screen-printed carbon electrode with colloidal gold, combined with immunity and horseradish peroxidase (HRP) to make H2O2 biosensor. The results showed that the electrocatalytic performance and current response of HRP were significantly improved, the linear range of signal was greatly improved(0.8μM"1.0mM), and the detection limit was also reduced to 0.4 μ M.4.5 Optical Fiber Nano BiosensorCompared with other types of biosensors, fiber-optic nano biosensors are not only small in size and high insensitivity, but also free from electromagnetic interference and do not need reference devices. It can enter the interior of cells and measure the changes of structure and cytoplasm in vivo. （1）Optical fiber nano fluorescence biosensorKopelman was the first to use a fluorescent fiber-optic nanosensor to detect the pH value in the microenvironment. Its working principle is to fix the fluorescent agent at the head of the optical fiber. When the fluorescent agent reacts reversibly with the proton, the optical property of the liquid changes.  According to the change of the fluorescence intensity, the pH value can be determined. The optical fiber processing method is as follows: the optical fiber is drawn into a fiber probe with a head diameter of 100nm "1000nm by a fiber drawing instrument, and aluminum is plated on the surface of the optical fiber by a vacuum evaporator to prevent light from leaking during transmission. Then, the exposed optical fiber head is silanized, and the surface is modified into an active surface containing hydroxyl or amino group, and the antigen or antibody of the molecule to be detected is fixed and identified.  Finally, the light The fiber head is combined with a pH selective fluorescent dye polymer. The response time of the nano sensor is 250ms, and it can detect the ion concentration of μ M. These characteristics are suitable for the detection of single cell and subcellular structure, such as the detection of pH value of mouse embryonic cell fluid. （2）Optical fiber nano immune biosensorOptical fiber nano immunosensor is a kind of sensor which applies optics and photonics technology to immunoassay. It can convert the amount of antigen or antibody to optical signal by using the characteristic that antigen and antibody can combine specifically. This kind of sensor combines the advantages of traditional immunoassay, optics and biosensor technology, and has high specificity, sensitivity and stability.  At the same time, the fiber-optic nano immune sensor only uses nano products on sensitive components, so it not only retains many advantages of the original but also makes it suitable for the measurement of single cells. Dinh et al. Have successfully developed an optical fiber nano immunosensor for the detection of BPT (benzopyrene tetrol, a biomarker of DNA damage related to exposure to carcinogenic benzo [α] pyrene). They first made quartz fiber with a diameter of 10nm "100nm with a fiber drawing instrument, then silanized the fiber head, modified the fiber head with BPT antibody, and then plated the whole length of the fiber (except the modified fiber head) with silver to prevent light from leaking out.  Finally, cell puncture and detection experiments were carried out on a single cell operated micromanipulator/microinjector, they used photomultiplier PMT to record the fluorescence produced by the binding of BPT and antibody, and detected the content of BPT in cells by measuring the change of fluorescence intensity. The minimum detection limit of the sensor can reach 10 – 21mol. Ⅴ FAQ1. What is the principle of piezoelectric biosensors?Piezoelectric Biosensors are also known as Acoustic Biosensors as they are based on the principle of sound vibrations i.e. acoustics. When a mechanical force is applied to a piezoelectric biosensor, they produce an electrical signal. The biological elements are attached to the surface of the piezoelectric biosensor. 2. What are the different types of biosensors?• Electrochemical Biosensors.• Magnetic Biosensors.• Thermometric Biosensors.• Acoustic Biosensors.• Optical Biosensors. 3. How does a basic biosensor work?The term ‘biosensor’ is short for ‘biological sensor.’ The device is made up of a transducer and a biological element that may be an enzyme, an antibody or a nucleic acid. The bio element interacts with the analyte being tested and the biological response is converted into an electrical signal by the transducer. 4. What are the main components of biosensors?A biosensor typically consists of a bio-receptor (enzyme/antibody/cell/nucleic acid/aptamer), transducer component (semi-conducting material/nanomaterial), and electronic system which includes a signal amplifier, processor & display. Transducers and electronics can be combined, e.g., in CMOS-based microsensor systems. 5. How do you classify biosensors?Biosensors can be classified according to the transduction methods they utilize (Fig. 4). Most forms of transduction can be categorized in one of five main classes: electrochemical, electrical, optical, piezoelectric (mass detection methods) and thermal detection. 6. What are wearable biosensors?Wearable systems are devices that allow physicians to overcome the limitations of technology and provide a response to the need for monitoring individuals over weeks or months. Wearable Biosensors typically rely on wireless sensors enclosed in bandages or patches or in items that can be worn. 7. What is an amperometric biosensor?Amperometric biosensors are self-contained integrated devices based on the measurement of the current resulting from the oxidation or reduction of an electroactive biological element providing specific quantitative analytical information. 8. What is voltammetric biosensor?Cyclic Voltammetry (CV) Voltammetry belongs to a category of electro-analytical methods, through which information about an analyte is obtained by varying potential and then measuring the resulting current. It is, therefore, an amperometric technique. 9. What is the electrochemical biosensor?An electrochemical biosensor is a self-contained integrated device, which is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor) which is retained in direct spatial contact with an electrochemical transduction element. 10. What are Piezoelectric Biosensors?Piezoelectric biosensors are a group of analytical devices working on the principle of affinity interaction recording. A piezoelectric platform or piezoelectric crystal is a sensor part working on the principle of oscillations change due to a mass bound on the piezoelectric crystal surface. 

Introduction Diodes are widely used in electronics, such as rectification in power supply, as detection and mixing, etc. in communications, and are often used in voltage regulation and protection in various circuits (such as freewheeling diodes, TVS and so on). Due to the wide variety and versatility, the following is an analysis of the simple application of Schottky diodes in digital circuits.In this video, the Schottky diode has been explained.CatalogIntroductionⅠ Schottky Diodes CharacteristicsⅡ Schottky Diode Applications2.1 As Dual Power Supply2.2 As AND Gate2.3 As OR GateⅢ Schottky Diode ParametersⅣ Example AnalysisⅠ Schottky Diodes CharacteristicsThe Schottky diode is structurally different from the PN junction diode. It is made of an anode metal (a barrier layer made of a material such as molybdenum or aluminum), SiO2 (electric field eliminating material), and N- epitaxial layer (arsenic material), the N-type silicon substrate, N+ cathode layer, and the cathode metal, which are as shown in the following figure. A Schottky barrier is formed between the N-type substrate and the anode metal. When a forward bias is applied to both ends of the Schottky barrier (anode metal is connected to the positive electrode of the power supply, and the N-type substrate is connected to the negative electrode), the Schottky barrier layer is narrowed, and the internal resistance becomes small. On the contrary, when a reverse bias is applied across the Schottky barrier, it becomes wider and its internal resistance becomes larger. Figure 1. Schottky Diode Structure Ⅱ Schottky Diode ApplicationsThe problem with Schottky diodes is that the withstand voltage is relatively low and the reverse leakage current is large. At present, the general condition of the Schottky diode used in the power conversion circuit is that the withstand voltage is below 150V, the average current is below 100A, and the reverse recovery time is between 10 and 40ns. Therefore, Schottky diodes are ideal device for use in high frequency and low voltage circuits.2.1 As Dual Power SupplyAt present, in the electronic design with the main controller, the real-time clock (RTC) is basically used, and the RTC needs an additional button battery to support, to avoid information lost after the system is powered off. And meanwhile, after the system is started, in order to extend the battery life, the main system is often supplied with power. Therefore, RTC often requires dual power supply, and the diode can be used for power isolation due to its single-conductivity. Taking the small-signal Schottky diode BAT54C as an example, the forward voltage drop is only 0.24v (the forward current is 0.1mA), and the RTC current consumption is uA-level, after adding the Schottky diode to isolate power supply to save info security.2.2 As AND GateAs shown in the figure below, n Schottky diodes form the AND gate of the n input. As long as there is a signal output logic 0 in A1~An, the Output is logic 0, only all signals in A1~An output logic 1, Output can output logic 1. That is, the phase sum of the signals A1~An is realized. Since the chip signal input stage is basically high-resistance in the digital circuit, the overall current of the AND gate circuit composed of the Schottky diode is uA-level, and the Schottky diode voltage drop is extremely small. In the case of it, the flat can still meet the design requirements.  Figure 2. Schottky Diode as AND Gate2.3 As OR GateAs shown in the figure below, n Schottky diodes form an n-input OR gate. As long as there is a signal output logic 1 in A1~An, Output outputs a logic 1. Only all signals in A1~An output logic 0, and Output can output logic 0. That is, the phase sum of the signals A1~An is realized.  Figutre 3. Schottky Diode as OR Gate Ⅲ Schottky Diode ParametersNote: Because Schottky diodes are used differently in different electronic circuits, we also need to consider the following parameters when using them.1）Forward voltage drop VFVF is the forward voltage drop when the diode is forward conducting. The greater the current through the diode, the larger the VF, in addition, the higher the diode temperature, the smaller the VF.2）Reverse saturation drain current IRIR refers to the current flowing through the diode when a reverse voltage is applied to the diode. The Schottky diode has a large reverse leakage current, therefore, selecting a Schottky diode with a smaller IR.3）Rated current IFIt refers to the average current value calculated from the allowable temperature rise when the diode is in a long time operation.4）Maximum surge current IFSMExcessive forward current that is allowed to flow. It is not a normal current, but an instantaneous current, which is quite large.5）Maximum peak inverse voltage VRMEven if there is no reverse current, as long as the reverse voltage is continuously increased, the diode will be damaged sooner or later. This reverse voltage is not the instantaneous voltage, but the reversed voltage that is added repeatedly. Since the rectifier is supplied with an alternating voltage, its maximum value is a specified important factor. And the maximum reverse peak voltage VRM refers to the maximum reverse voltage that can be applied to avoid breakdown. At present, Schottky's highest VRM value is 150V.6）Maximum DC reverse voltage VRVR is the value when the DC voltage is continuously applied. For DC circuits, the maximum DC reverse voltage is important to determine the allowable and upper limits.7）Maximum operating frequency FMDue to the junction capacitance of the PN junction, when the operating frequency exceeds a certain value, its unidirectional conductivity will deteriorate. And Schottky diodes have high FM values up to 100 GHz.8）Reverse recovery time TrrWhen the operating voltage changes from a forward voltage to a reverse voltage, the ideal operation of the diode is that the current can be instantaneously turned off. In fact, it usually takes a little delay. The amount that determines the current cut-off delay is the reverse recovery time. Although it directly affects the switching speed of the diode, it does not mean that this value is small. That is, when the diode is suddenly reversed by conduction, the reverse current is greatly attenuated to a time required to approach IR. This indicator is important when the high-power switch is operating in the high-frequency state.9）Maximum dissipation power PWhen a current flows through the diode, it absorbs heat and raises its temperature. In reality, the external heat dissipation condition also has a great influence on P. Specifically, the voltage applied across the diode is multiplied by the current flowing through and the reverse recovery loss. Schottky Diode Symbols Ⅳ Example AnalysisIn digital circuit design, it is often necessary to make simple phase, or phase inversion of some signals. If the logic chip such as the 74 series is directly used, not only the layout area is greatly increased, but also the wiring is not flexible. The use of small-signal Schottky diodes and OR gates is more flexible and easy to use. The following figure shows a simple two-way reset circuit. JTAG generating a reset signal needs to reset the master, and the external reset button also needs to reset the master when pressed. If the JTAG reset and button reset directly to the reset pin of the master, it may cause damage to the JTAG emulator. For example, when the reset button is pressed, the JTAG output reset pin will be directly lowered. The phase and circuit are formed by the Schottky diode BAT54A , and the signal outputs do not affect each other. The following figure allows the master to reset as long as the JTAG output logic 0 or pressing the button reset output logic 0.Figure 4. Schottky Diode BAT54A ApplicationIf it is to be used as a non-gate, a triode can be used. Of course, the triode is widely used in electronics, such as a switching device in a digital circuit, as a current drive, level shifter, and the like. Frequently Asked Questions about Small Signal Schottky Diodes1. What is the Schottky diode and how it works?A typical diode combines p-type and n-type semiconductors to form a p-n junction. In a Schottky diode metal replaces the p-type semiconductor. This metal can range from platinum to tungsten, molybdenum, gold, etc. When metal is combined with an n-type semiconductor an m-s junction is formed. 2. What does small signal mean?A small signal is an AC signal (more technically, a signal having zero average value) superimposed on a bias signal (or superimposed on a DC constant signal). This resolution of a signal into two components allows the technique of superposition to be used to simplify further analysis. 3. Which statement is correct for Schottky diode?Explanation: The majority charge carriers in a Schottky diode are electrons not holes. Explanation: Due to the metal-silicon junction there are no stored charges hence, no reverse recovery time, due to which the switching is faster. 4. What are the two important features of a Schottky diode?We have seen here that the Schottky Diode also known as a Schottky Barrier Diode is a solid-state semiconductor diode in which a metal electrode and an n-type semiconductor form the diodes ms-junction giving it two major advantages over traditional pn-junction diodes, a faster switching speed, and a low forward bias.

  Light emitting diode (LED) is a light source that meets the requirements of green lighting. LEDs are safe, efficient, environmentally friendly, long-lived, responsive, small in size and robust in construction with many features that are unmatched by ordinary light-emitting devices. Moreover, it is one of the first semiconductor devices and is widely used. Currently, LEDs are widely used as indicators for various electronic products and as light sources for fiber optic communication.     How dose a diode work?     Catalog I What is a diode? II How dose a diode work? III What is diode characteristics? IV What are diode parameters? V What are the types of diodes? FAQ   I What is a diode?   Diode is an electronic device made of semiconductor materials (silicon, selenium, germanium, etc.) . It has a unidirectional conductivity, that is, the diode anode and cathode to add a forward voltage, the diode conducts. When the reverse voltage is added to the anode and cathode, the diode cuts off. Therefore, the on and off of the diode is equivalent to the on and off of the switch.   In almost all electronic circuits, semiconductor diodes are used. The use of semiconductor diodes in the circuit can play a role in protecting the circuit, extending the life of the circuit. The development of semiconductor diodes has made integrated circuits more optimized and has played an active role in various fields. Diodes have many roles in integrated circuits and maintain the proper functioning of the integrated circuits.     Diodes were one of the first semiconductor devices to be created, and their applications are very widespread. Especially in a variety of electronic circuits, the use of diodes and resistors, capacitors, inductors and other components to make a reasonable connection to form a circuit of different functions, you can achieve a variety of functions such as rectification of alternating current, detection of modulated signals, limiting and clamping, and voltage regulation of the supply voltage. Whether in common radio circuits or in other household appliances or industrial control circuits, diodes can be found.   A diode is made of a PN junction with corresponding electrode leads and a tube housing package. The diode has two electrodes, the electrode leading from the P area is the positive electrode, also known as the anode; the electrode leading from the N area is the negative electrode, also known as the cathode.       Diode structure         There are many kinds of diodes:  - According to the semiconductor materials used, they can be divided into germanium diodes and silicon diodes.   - According to their different uses, they can be divided into detector diodes, rectifier diodes, zener diodes, switching diodes, etc.   - According to the structure of the tube core, they can be divided into point-contact diodes, surface-contact diodes and planar diodes.     --  The point contact diodes are pressed on the smooth surface of the semiconductor wafer with a thin metal wire. With pulse current, one end of the contact wire is sintered firmly with the wafer to form a "PN junction". Due to point contact, only small currents (no more than a few tens of mA) are allowed, which is suitable for high frequency low current circuits, such as radio detection, etc. The area of "PN junction" of surface contact diode is large, which allows large currents passing through and is mainly used in "rectifying" circuits that convert AC to DC.    --  Planar diode is a kind of special silicon diode. It not only can pass through large current, but also has stable and reliable performance. It is widely used in switching, pulse and high frequency circuits.   II How does a diode work? The crystal diode is a p-n junction formed by p-type semiconductor and n-type semiconductor. It forms a space charge layer on both sides of the interface and has a self-built electric field. When there is no applied voltage, the diffusion current caused by the carrier concentration difference on both sides of the p-n junction is equal to the drift current caused by the self-built electric field, so it is in an electric equilibrium state. When the external positive voltage is biased, the mutual suppression of the external electric field and the self-built electric field results in the increase of the carrier diffusion current, which is shown in the conduction region below. When the external reverse voltage is biased, the external electric field and the self-built electric field are further strengthened to form a reverse saturation current I0 independent of the reverse bias voltage value within a certain reverse voltage range, which is shown in the cut-off region below. When the applied reverse voltage is high enough to a certain extent, the electric field intensity in the space charge layer of p-n junction reaches the critical value to produce the multiplying process of the carriers, resulting in a large number of electron hole pairs and a numerical reverse breakdown current is generated, known as the diode breakdown, which is shown in the breakdown region below. III What is diode characteristics? The most important characteristic of the diode is the unidirectional conductivity. In the circuit, the current can only flow from the positive electrode of the diode, and flows out from the negative electrode . The forward and reverse characteristics of the diode are illustrated by simple experiments.   3.1 Forward characteristics In electronic circuits, if the diode is connected to the high potential terminal and the negative electrode to the low potential terminal, the diode will be switched on. This connection is called forward bias. It must be noted that when the forward voltage applied to both ends of the diode is very small, the diode still cannot be switched on, and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (about 0.6 V of the silicon tube) can the diode be truly switched on. The voltage at both ends of the diode after conduction is called the forward voltage drop of the diode.   3.2 Reverse characteristic In the electronic circuit, the positive end of the diode is connected to the low potential end, the negative electrode is connected to the high potential terminal, and the diode is in the cutoff state. This mode of connection is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, called leakage current. When the reverse voltage of the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose the single direction conduction characteristic. This state is called the breakdown of diode.   IV What are diode parameters? The technical specifications used to test the performance of diodes are called diode parameters. Here are some of the main parameters in diode testing:   4.1 Rated forward working current (IF) Refers to the maximum forward current that is allowed to pass through the diode when it is in continuous operation over a long period of time. When a larger current passes through the diode, the dice is heated and the temperature rises, and when the temperature exceeds the allowable limit, the dice is overheated and damaged. Therefore, it should not exceed the diode rated forward operating current value when the diode is in use. Eg. The rated forward working current of DFM is 1A   4.2 Forward Voltage(VF) Refers to the voltage at both ends of the diode when the rated forward working current IF is passed through the diode. Eg. The voltage at both ends of the diode is about 0.9V when the forward working current of DFM is 1A.   4.3 Maximum reverse operating voltage (VR) When the reverse voltage at both ends of the diode is raised to a certain value, the diode will be broken down and the unidirectional conductivity will be lost. In order to ensure the safety of operation, the maximum reverse operating voltage is specified. Eg. The maximum reverse operating voltage of DF10M is 1100V and the breakdown voltage is about 1400V   4.4 Reverse current IR Refers to the reverse current that flows through the diode when the maximum reverse operating voltage VR is applied to both ends of the diode. The smaller the reverse current, the better the unidirectional conductivity of the diode will be. Eg. When the reverse voltage of DF10M is 1100V, the VR is about 0.2uA.   4.5 Reverse critical current (IZ) Refers to the reverse current of the diode increases sharply to close to the breakdown phenomenon.   Eg. Set the IZ of DF10M to 0.1 Ma (Ma) 4.6 Reverse critical voltage (VZ) Refers to the reverse voltage of the diode when the reverse current is IZ. If the reverse voltage is greater than this value, the reverse current increases dramatically and the unidirectional conductivity of the diode is destroyed, thus causing reverse breakdown. Eg. The VZ is about 1300V when IZ of DF10M is 0.1mA.   4.7 Reverse recovery time (Trr) When diodes are in low frequency applications, it generally do not need to consider its conduction to the cut-off, or cut-off to the transition time. But if the diode works in a high-speed switching circuit environment, when diode suddenly turns to reverse bias from the forward biased conduction state, it will take a certain time to become a cut-off state, which is called reverse recovery time. But if the diode works in a high-speed switching circuit environment, when diode suddenly turns to reverse bias from the forward biased conduction state, it will take a certain time to become a cut-off state, which is called reverse recovery time. Eg. The maximum Trr of EDF1DM is 50nS.   V What are the types of diodes?   5.1 Light emitting diode Light emitting diode, also called LED, is a semiconductor diode that converts electrical energy into luminous energy. Like ordinary diodes, LEDs are made up of a PN junction and have unidirectional conductivity. When a forward voltage is applied to a light-emitting diode, Holes injected from P region to N region and electrons injected from N region to P region are recombined with N region electrons and P region holes in several microns near PN junction to produce spontaneous emission fluorescence.    The energy states of electrons and holes in different semiconductor materials are different. When electrons and holes are combined, the energy released is different. The more energy is released, the shorter the wavelength of light is. The commonly used diodes are red, green or yellow light.The reverse breakdown voltage of a light-emitting diode is greater than 5 volts. Its forward volt-ampere characteristic curve is so steep that it must be used in series to control the current passing through the diode. The current limiting resistance R can be calculated by the following formula: R=（E－UF）/IF . In this formula, E is the power supply voltage, UF is the forward voltage of LED, IF is the running current of LED.   5.2 Zener diode Zener diode, is also called voltage stabilizing diode. By using the reverse breakdown state of pn junction, the current can be changed in a wide range and the voltage is basically unchanged, thus form a diode which has voltage stabilizing function. This diode is a semiconductor device with high resistance until it reaches the critical reverse breakdown voltage.  The following picture is a typical Zener diode application circuit diagram: At this critical breakdown point, the reverse resistance is reduced to a very small value, where the current increases and the voltage remains constant in this low resistance region, and the Zener diode is divided according to the breakdown voltage, because of this characteristic, The regulator is mainly used as a voltage regulator or voltage reference element. Zener diodes can be connected in series for use at higher voltages, and higher stable voltages can be obtained by serializing them.   5.3 Switching diode  Working principle: The semiconductor diode is equivalent to switch-on when it is turned on (the circuit is turned on), and is equivalent to switch-off when it is turn-off (the circuit is cut off), so the diode can be used as a switch. The common used model is 1N4148. Due to the unidirectional conductivity of semiconductor diodes, the PN junction is on at positive bias, and the resistance is very small at the on-state, which ranges from tens to hundreds of ohs. At reverse bias, it is in a cut-off state, and its resistance is very large. Generally, silicon diodes are above 10 μ Ω and germanium diodes have tens to hundreds of kilos. By using this property, the diode will play the role of controlling the current on or off in the circuit and become an ideal electronic switch.   At high frequency, the barrier capacitance of the diode exhibits extremely low impedance and is parallel to the diode. When the capacitance of the barrier itself reaches a certain level, the switching performance of the diode will be seriously affected. In extreme conditions, the diode will be short-circuited, and the high-frequency current will no longer pass through the diode, but will pass directly through the barrier capacitance, and the diode will fail to work. The barrier capacitance of the switching diode is generally small, which is equivalent to blocking the barrier capacitance path and achieving the effect of maintaining good unidirectional conductivity at high frequency.  Classification: General switching diode, high speed switching diodes, ultra-high speed switching diodes, low-power switching diodes, high reverse voltage switching diodes, silicon voltage switching diodes and so on.   5.4 Variable capacitance diode( Varactor Diodes ) Variable capacitance diode, also known as varactor Diodes, are semiconductors that change the junction capacitance according to the voltage supplied. That is, as variable capacitors, they can be used in resonant circuits such as FM tuners and TV tuners and FM modulation circuits. Working principle: Varactor Diodes is a kind of special diode. When applied forward bias voltage, the depletion region of PN (positive and negative electrode) junction is narrowed and the capacitance becomes larger, which results in diffusive capacitance effect. However, the leakage current will be generated when the forward bias is added, so the reverse bias is supplied in application. In fact, we can think of it as a PN junction. If a reverse voltage V is added to the PN junction (the varactor diode is used in reverse direction), the electrons in the N-type semiconductor are directed to the positive electrode and holes in P-type semiconductor will be led to the negative electrode. Then forms a depletion layer that has neither electrons nor holes, and the width of the depletion layer is set to d, which changes with the reverse voltage V. In this way, when the reverse voltage V increases, the depletion layer d becomes wider and the diode capacitance C decreases (according to C=kS/d), and the reverse voltage decreases, the depletion layer width d becomes narrower and the diode capacity becomes larger. The change of reverse voltage V leads to the change of depletion layer, which changes the junction capacity of the variable capacitance diode. - Application: the varactor diode is a semiconductor device based on the principle of variable capacitance between PN junctions. It is used as a variable capacitor in high frequency tuning and communication circuits. As shown in the following figure, the reverse voltage of the diode is changed by changing the different R2. This will result in a change in the capacitance of the diode, thus changing the resonant frequency in which the varactor diode can pull out the full range of the required capacitance in the parallel resonant band-pass filter.   FAQ   1. What is diode and its symbol? Diode, an electrical component that allows the flow of current in only one direction. In circuit diagrams, a diode is represented by a triangle with a line across one vertex.   2. What is special about a diode? Some semiconductor junctions, composed of special chemical combinations, emit radiant energy within the spectrum of visible light as the electrons change energy levels. Simply put, these junctions glow when forward biased. A diode intentionally designed to glow like a lamp is called a light-emitting diode, or LED.   3. Are diodes AC or DC? It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction. Diodes are also known as rectifiers because they change alternating current (ac) into pulsating direct current (dc). Diodes are rated according to their type, voltage, and current capacity.   4. Why do we use zener diode? Zener diodes are used for voltage regulation, as reference elements, surge suppressors, and in switching applications and clipper circuits. The load voltage equals breakdown voltage VZ of the diode. The series resistor limits the current through the diode and drops the excess voltage when the diode is conducting.   5. What is unit of diode? A diode is not a measurable quantity. Hence,it does not have a unit. Generally,for a diode,we measure characteristics like forward voltage drop,reverse voltage drop and reverse breakdown voltage which are usually measured in Volts.   6. Do diodes have resistance? Just like a resistor or any other load in a circuit, a diode offers resistance in a circuit. Unlike resistors, though, diodes are not linear devices. This means that the resistance of diodes does not vary directly and proportional to the amount of voltage and current applied to them.   7. Does diode reduce current? Ideally, diodes will block any and all current flowing the reverse direction, or just act like a short-circuit if current flow is forward. Unfortunately, actual diode behavior isn't quite ideal. Diodes do consume some amount of power when conducting forward current, and they won't block out all reverse current.   8. How are diodes classified? Diodes are classified according to their characteristics and are offered in a number of different types, including rectifiers, switching diodes, Schottky barrier diodes, Zener (constant voltage) diodes, and diodes designed for high-frequency applications.   9. What is the most common diode? The most commonly used signal diode is the 1N4148. This diode has a close brother called 1N914 that can be used in its place if you can't find a 1N4148. This diode has a forward-voltage drop of 0.7 and a peak inverse voltage of 100 V, and can carry a maximum of 200 mA of current.   10. What is the difference between a Zener diode and a Schottky diode? As their switching speed is very high, Schottky diodes recover very fast when the current reverses, resulting in only a very small reverse current overshoot. ... A special type of diode, called the Zener diode, blocks the current through it up to a certain voltage when reverse biased.   11. What is difference between Schottky diode and normal diode? In the normal rectifier grade PN junction diode, the junction is formed between P type semiconductor to N type semiconductor. Whereas in Schottky diode the junction is in between N type semiconductor to Metal plate. The schottky barrier diode has electrons as majority carriers on both sides of the junction.   12. Why it is called diode? A diode is called a diode because it has two distinct electrodes (i.e. terminals), called the anode and the cathode. A diode is electrically asymmetric because current can flow freely from the anode to the cathode, but not in the other direction. In this way, it functions as a one-way valve for current.   13. Is a diode the same as a resistor? Key Difference: A diode is a type of electrical device that allows the current to move through it in only one direction. ... A resistor is an electric component that is used to provide resistance to current in the circuit. They are mostly used to produce heat or light.   14. How much voltage can a diode take? Silicon diodes have a forward voltage of approximately 0.7 volts. Germanium diodes have a forward voltage of approximately 0.3 volts. The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.   15. Can a resistor replace a diode? Diodes only conduct in one direction whereas resistors conduct in both directions. Without analyzing the actual circuit the results would be unpredictable but, generally speaking, being that diodes & resistors are designed to do different things, substituting one for the other is something you wouldn't want to do.