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IntoductionAn operational amplifier, or op-amp for short is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op-amp produces an output potential that is typically hundreds of thousands of times larger than the potential difference between its input terminals, and is a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. They are used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation.In this video, the basic introduction of the Operational Amplifier (Op-Amp) has been given and different characteristics of ideal and real Op-amp (General Purpose 741 Op-Amp) has been discussed. CatalogIntroductionⅠ Operational Amplifier Basics1.1 Amplification Principle1.2 Balance Resistor1.3 Feedback Resistor in Parallel with a Capacitor 1.4 Pulling Down Resistor and Pulling Up the Capacitor1.5 Parasitical Resistor as an Integrator1.6 Parasitical Resistors and Capacitors1.7 Balance Resistor Failure1.8 Magnification, Input Impedence, Voltage1.9 Open Loop Gain1.10 Virtual ShortⅡ Op Amp ApplicationⅢ Op Amp SamplingⅣ Op Amp Reference VoltageⅤ Importance of Op AmpⅠ Operational Amplifier BasicsWhen using op amp, there will be more problems confusing us, what are they? Listing all of them is impossible, but we can seek the core of these problems, which are the following lists.1.1 Amplification PrincipleThere are many types of op amps with many functions, and their circuits are inconsistent, but the internal block diagrams are basically the same. It consists of three parts: input stage, intermediate stage, and output stage. The input stage consists of a differential amplifier circuit that uses circuit symmetry to improve overall circuit, and the main function of the intermediate voltage amplifier stage is to increase the voltage gain. It can be composed of one or more stages of amplifying circuits; the output stage has a voltage gain of 1, but can provide a certain amount of power, and the circuit consists of two power supplies V+ and V-. The entire amplifier circuit is designed with two inputs P and N, and one output O. The voltages of the three terminals are represented by Vp, Vn, and Vo, respectively. The two ends of P and N are respectively called the non-inverting input terminal and the inverting input terminal, which means that when the P terminal is added with the voltage signal Vp (Vn = 0), it is obtained at the output end. The voltage Vo is in-phase with Vp, when the voltage signal Vn (Vp = 0) is applied to the N terminal, the output voltage Vo obtained at the output is inverted from Vp.The operational amplifier is actually a differential amplifier. Look at its structure, two transistors are connected back to back to share the crossing current source. One of the transistors is the positive input of the op amp and the other is the inverting input. The positive input is amplified and sent to a power amplifier circuit to amplify the output. Thus, if the voltage at the forward input rises, the output naturally becomes larger. If the voltage at the inverting input rises, the reverse current is large, and the forward current is small, because the inverting tertiary tube and the forward tube share a same current source.1.2 Balance ResistorGenerally, there is a balance resistor in the inverting / non-inverting amplifier circuit. What is the role of this balance resistor?(1) Provide a suitable static bias for the transistors inside the chip.The internal circuit of the chip is usually directly coupled, and it can automatically adjust the static operating point, but if an input pin is directly connected to the power supply or the ground, its automatic adjustment function can not work normally. Because the voltage of the ground cannot be raised by the inside transistors, and the voltage of the power supply cannot be reduced, which causes the chip to fail to meet the conditions of virtual short and virtual open.(2) Eliminate the influence of the static base current on the output voltage, and the value should be balanced with the equivalent resistance value of the external DC of the two input terminals.(3) In non-inverting op amp circuit, if it not connecting a balance resistor, the op amp will be burned, because the resistor acts as a voltage divider. 1.3 Feedback Resistor in Parallel with a Capacitor What is the role of the feedback resistor in parallel with a capacitor when using non-inverting op amp?(1) The feedback resistor and capacitor form a high-pass filter, so that local high-frequency amplification is particularly noticeable.(2) Prevent self-excitation. 1.4 Pulling Down Resistor and Pulling Up the CapacitorWhat role does the role of pulling down resistor and pulling up the capacitor at the input of the op amp play?To get positive feedback and negative feedback, depending on the specific circuit connection. For example, if the input voltage signal and the output voltage signal are taken to the input, the partial output signal passes through the balance resistor to obtain a new voltage value, that is, shunting the input voltage to make the input voltage smaller, and this is a negative feedback. Since the signal output from the signal source is always constant, the output signal can be corrected by negative feedback. 1.5 Parasitical Resistor as an IntegratorWhat is the function of the resistor RF connected to the op amp as an integrator at the two ends of the integrating capacitor?Adjust resistance to prevent the output voltage from running out of control. 1.6 Parasitical Resistors and CapacitorsWhy are resistors and capacitors connected in series at the input of the op amp?Regardless of the type of op amp, it consists of transistors or MOS transistors. In the absence of an external components, the op amp is a comparator actually. When the voltage of the non-inverting terminal is high, it will output a level similar to the positive voltage, and vice versa, but this op amp does not seem to have much use. Only when the external circuit is formed to generate the feedback will make the real op amp function. 1.7 Balance Resistor FailureWhat is the consequence of the balance resistor doesn't work well in non-inverting amplifier circuit?(1) The non-inverting end is unbalanced. For example, there will be an output although the input is 0. When the input signal is output, the output value is always larger (or smaller) than the theoretical output value by a fixed number.(2) The error caused by the input bias current cannot be eliminated. 1.8 Magnification, Input Impedence, VoltageWhat is the amplification factor and input impedence of an ideal integrated operational amplifier? What is the voltage between the non-inverting input and the inverting input?The magnification is infinite, the input impedance is infinitesimal, and the voltage is almost the same (the voltage is not 0V, for example, the non-inverting end is 10V and the inverting end is 9.99V). 1.9 Open Loop GainWhy is the open loop gain of an ideal op amp infinite?1) The actual open loop gain of the op amp is very large, so imagine it as infinity and derive the virtual ground from it.2) Deriving virtual ground is not only an inverting amplifier for the negative feedback connection, because there is no virtual ground for positive feedback.The open-loop gain of the op amp is infinite, when design the circuit, the closed-loop gain can be independent of the open-loop gain, and only depends on the external components. It is to use the large open loop gain in exchange for the stability of the closed loop gain.3) Assuming that the gain is small, the difference between the voltages applied across the op amp is relatively large for an output voltage. If it is connected to a negative feedback state, the voltage across the op amp will be different, causing amplification.We all know that the op amp’s output voltage Vo is equal to the difference Vid between the non-inverting input voltage and the inverting input voltage, multiplied by the op amp’s open-loop gain A, that is, Vo = Vid * A = (VI + - VI-) * A ( 1 ). Since the output voltage of the op amp does not exceed the supply voltage in practice, it is a finite value. In this case, if A is large, (VI+ - VI-) is necessarily small; if (VI+ - VI-) is small enough, then we can actually treat it as 0, at this time,  there will be VI+ = VI-, that is, the voltage at the non-inverting input of the op amp is equal to the voltage at the inverting input. This is what we call “virtual short”. Note that they are not really connected together, and there is resistance between them.In the above discussion, how did we get the result of “virtual short”? Our starting point is the formula (1), which is based on the characteristics of the op amp. Then, we made two important assumptions, one is that the output voltage of the op amp is limited, and it not exceed the power supply voltage; the second is that the open loop gain A of the op amp is large. The A of a normal op amp usually reaches 106 or 107 or even larger, but the actual open loop gain of the op amp is also related to its working state. For example, if the op amp is not working in the linear area, the value A may be small, so second assumption is conditional.Therefore, we know that when the open loop gain A of the op amp is large, the op amp can have a virtual short. But it is one of the possibilities, and it is not suitable for every op amp in any case to say their inputs are virtual short, in other words, virtual short can only be achieved in circuits under certain conditions.The conditions of virtual short:a. The open-loop gain of operational amplifier should be large enough.b. There should be a negative feedback circuit. From the above we know when we need to analyze the virtual short in the circuit. In reality, condition (1) is true for most op amps, and the important point is to look at the work area. If it is a circuit drawing, judge by calculation; if it is an actual circuit, it is reasonable to use the instrument to measure amplifier output voltage.There is also a situation related to virtual short called “virtual ground”, that is, there is a virtual short when the input is grounded. Some books say that virtual short will be exist under deep negative feedback conditions, but in reality, the op amp is more likely to work in the linear region under this situation. But this is not absolute, when the input signal is too large, the op amp with deep negative feedback will still be saturated. Therefore, it should be judged to be the most reliable with the output voltage value. 1.10 Virtual ShortAdd the input signal directly to the non-inverting input, and the inverting input is grounded through the resistor. Why is U-= U+ = Ui≠0? Is it not a virtual short? What are the conditions that the virtual ground exists?(1) In the non-inverting amplifier circuit, the output affects by the feedback, so that U(+) automatically tracks U(-), so they will be close to zero. It seems that the two ends are short circuit, so it is called virtual short.(2) Due to the virtual short phenomenon and the high input resistance of the op amp, the current flowing through the two input terminals is small, approaching 0. This phenomenon is called virtual open, which is derived from virtual short.(3) The virtual ground is in the inverting op amp circuit, the (+) terminal is grounded, and the (-) is connected to the input and feedback network. Due to the virtual short, U(-) and U(+) are very close, which is said to be virtual ground.(4) About the conditions: the virtual short is an important feature of the closed-loop (negative feedback) operating state of the non-inverting amplifier circuit; the virtual ground is an important feature of the inverting amplifier circuit in the closed-loop operating state. Ⅱ Op Amp ApplicationWhen a operational amplifier is connected as a non-inverting amplifier, the potentials of the two inputs are the same. If the waveform of the input is measured, it will be the same. This is like a common-mode signal. In fact, there are still small differential mode signal on the two inputs, but the differential mode signal can not be measured by the general instrument. As a result, the virtual short artificially increases the common-mode signal at the two inputs, which poses a challenge to the performance of the operational amplifier. Why is an op amp used like this?(1) The common mode signal of the non-inverting amplifier is much larger than the inverting amplifier, and strict to the common mode rejection ratio.(2) For single-ended input, the equivalent common-mode value is half of the input value, whether non-inverting or inverting input. However, since the input impedance of the non-inverting amplifier is usually larger than the inverting amplification, the anti-interference ability is a little poor.As mentioned above, when the inverting input is performed, the voltage at the inverting terminal is almost zero, so the differential influence on the tube collector voltage that has only one tube change. When the input is in phase, the voltage at the inverting terminal is equal to the non-inverting terminal voltage, so the common mode voltage and the input voltage are equivalent. That is to say, the collector voltage of the differential tube has variable quantity that changes in the same direction when the two tubes have portions that change in different directions at the same time, which is the common mode output voltage. It is added in phase with the voltage of one of the tubes. Therefore, it is easy to cause the tube to become saturated (or cut off), fortunately, the amplification of the common mode voltage is only tens of thousands of parts of the differential mode amplification.However, this does not mean that the common mode rejection suppression ratio of the differential mode input and the common mode input of the amplifier is different. It should be that the non-inverting input is added with a common mode signal equivalent to the input volume, so it should be careful to use non-inverting amplification mode when the input signal is large. Ⅲ Op Amp SamplingWhy is the amplifier circuit composed of operational amplifiers generally sampling the inverting input mode?(1) The significant difference between the inverting input and the non-inverting input mode is:When inverting input, because there is a balanced resistor connected to the ground at the same phase, and there is no current on this resistor (because the input resistance of the op amp is extremely large), this non-inverting terminal is approximately equal to the ground potential, and  the potential at the non-inverting terminal is extremely close to the inverting terminal, so there is a virtual ground at the inverting end. The advantage of having a virtual ground is that there is no common mode input signal, even if the common mode rejection ratio is not high, there is no common mode output. The non-inverting input mode has no virtual ground. When a single-ended input signal is used, a common-mode input signal is generated. Even if an operational amplifier with a high common-mode rejection ratio is used, there is still a common-mode output. Therefore, it is best to use the inverting input method.(2) The positive phase is the oscillator, and the inverting can stabilize the amplifier and access the negative feedback.(3) From the principle point of view, it is possible to connect to the same analog circuit. However, the signal (differential mode signal) that is amplified during the actual application tends to be small, thus it is necessary to pay attention to suppressing noise (usually expressed as a common mode signal). In the same way, the amplification circuit has a poor ability to suppress the common mode signal, and the signal that needs to be amplified is submerged in the noise, which is not conducive to post processing. Therefore, an inverting proportional amplification circuit with better suppression capability is good.Ⅳ Op Amp Reference VoltageSome op amps will have an output even if no voltage is input after power-on, and the output is not small, so VCC/2 is often used as the reference voltage.The output is output signal without any input, this is called the input offset voltage Vos, which is caused by the asymmetry of the design structure of the op amp. It is a very important performance indicator of the op amp. The op amp commonly used VCC/2 as the reference voltage is because the op amp is in a single power supply state. At this time, the real reference of the op amp is VCC/2, so a DC offset of VCC/2 is often provided at the positive terminal of the op amp. When having positive and negative dual power supply, it is often referenced to the ground.The selection of op amps requires attention to many things. Under less stringent conditions, it is often necessary to consider the operating voltage, output current, power consumption, gain bandwidth product, and price of the op amp. Of course, when using it under special conditions, different factors must be considered in practice. Ⅴ Importance of Op Amp(1) If the voltage on both inputs of the op amp is 0V, the output voltage should also be equal to 0V. But in fact, there is always some voltage at the output, that is, the offset voltage Vos. If the offset voltage at the output is divided by the noise gain of the circuit, the calculated result is called the input offset voltage or the input reference offset voltage. The Vos is considered to be a voltage source in series with the inverting input of the op amp. A differential voltage must be applied to both inputs of the amplifier to produce a 0V output.(2) The input impedance of an ideal op amp is infinite, so no current flows into the input. However, a real op amp using a bipolar junction transistor (BJT) in the input stage requires some operating current, which is called bias current (IB). There are usually two bias currents: IB+ and IB-, which flow into the two inputs, respectively. The range of IB values is large, with bias currents of lower at 60fA for special op amps and up to tens of mA for some high-speed op amps.(3) The power supply voltage range required for the first single-chip op amp to operate normally is ±15V. Today, op amps are moving toward low voltages due to increased circuit speeds and power supplies from low-power sources such as batteries. Although the op amp’s voltage specifications are usually specified as symmetrical two-pole voltages ±15V, these voltages do not necessarily require a symmetrical voltage or a two-pole voltage. For an op amp, as long as the input is biased in the active region (within the common-mode voltage range), the ±15V supply is equivalent to a +30V/0V supply, or a +20V/-10V supply. The op amp does not have a ground pin unless the negative voltage rail is grounded in a single-supply application.The input voltage swing of high speed circuits is smaller than that of low speed devices. The higher the speed of the device, the smaller its geometry, which means the lower the breakdown voltage. Due to the low breakdown voltage, the device must operate at a lower supply voltage. Today, op amps typically have a breakdown voltage of around ±7V, so high-speed op amps can work at a supply voltage of ±5V, and they can also operate at a single supply voltage of +5V.For general-purpose op amps, the supply voltage can be as low as +1~8V. These op amps are powered by a single power supply, but this does not mean that a low supply voltage must be used. Because the terms single supply voltage and low voltage are two related and independent concepts. Frequently Asked Questions about Operational Amplifiers Problems1. How can you tell if an op amp is blown?Re: how to tell whether an op amp is burned out? measure the DC voltage at the +input. then measure the DC voltage at the output. if the results are significantly different, the opamp is most likely shot. 2. How do I know if my op amp is broken?measure the DC voltage at the +input. then measure the DC voltage at the output. if the results are significantly different, the opamp is most likely shot. if they are the same, the opamp is most likely ok and the problem is something else. 3. What errors you have to consider with real operation amplifiers?These errors include input bias current, input offset current, input offset voltage, CMRR, PSRR, and finite input impedance. In reality, all these errors will occur at the same time. 4. How do op amps fail?The common failures I have seen including with comparators involve either the output being shorted or open to one supply or the input differential pair or input protection circuits being damaged causing excessive input bias current and/or input offset voltage which usually ends up pinning the undamaged output. 5. Why do op amps fail?The common failures I have seen including with comparators involve either the output being shorted or open to one supply or the input differential pair or input protection circuits being damaged causing excessive input bias current and/or input offset voltage which usually ends up pinning the undamaged output.

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.

Warm hints: The word in this article is about 2500 words and reading time is about 12 minutes.   This paper mainly introduces that how to use a solid state relay to drive a thermostat.    As we all known,relay is an electrical control device, an electrical appliance that makes the predetermined step change in the electrical output circuit when the input (excitation) changes reach the required requirements. Catalog   I. Solid State Relay Basics 1.1    What is solid state relay 1.2    Solid state relay working principle 1.3    Solid state relay appliances II. Thermostat Basics 2.1    What is thermostat 2.2     Types of thermostat 2.3    Features of thermostat 2.4    Applications of thermostat III. Drive Thermostat by Using Solid State Relay 3.1    Power thermostat 3.2    Case of driving thermostat FAQ   I. Solid State Relay Basics   1.1 What is Solid State Relay   A solid-state relay(SSR) is a contactless switch consisting of microelectronic circuits, discrete electronic devices and power electronic power devices. The isolation between the control end and the load side is realized by isolation devices. The input of the solid-state relay is controlled by a tiny control signal to directly drive the large current load.   SSR takes advantage of the switching characteristics of electronic components, such as switch triode, bidirectional thyristor and other semiconductor devices, to achieve the purpose of connecting and disconnecting the circuits without contact and sparkless, and therefore is also called "contactless switch".    A solid-state relay is a four-terminal active device, of which two terminals are input control terminal, and the other ends are output controlled ends. It has both amplification and isolation function. It is suitable for driving high power switching actuator, which is more reliable than electromagnetic relay and has no contact, long life, fast speed and interference to the outside. Because of its small size, it has been widely used.   1.2  Solid State Relay Working Principle   SSR can be divided into two types: AC type and DC type according to the use occasions.    They can switch loads on AC or DC power supply, and they can not be mixed. The following is an example of the AC type SSR as an example of its working principle. The following diagram is a block diagram of its working principle. The components in the diagram constitute the main body of the AC SSR. From the whole, SSR has only two inputs (A and B) and two output terminals (C and D), and is a four-terminal device.   Working principle block diagram of solid state relay   When a certain control signal is added to the A and B, the "switch" and "break" between the two ends of C and D can be controlled and the function of "switch" can be realized. The function of the coupling circuit is to provide a channel between the input/output terminal of the control signal input from the A and B ends, but it disconnects the input and output terminals of the SSR in the electrical circuit.    In order to prevent the effect of the output end on the input end, the coupling circuit used the "optical coupler", which is sensitive, responsive, and high in the input/output insulation (voltage resistance) level; because the input terminal load is a light-emitting diode, this makes the input end of the SSR easily matched with the input signal level. When used, it can be directly connected to the output interface of the computer, that is, the logical level control of "1" and "0".    The function of the trigger circuit is to generate the required trigger signal, drive the switch circuit 4, but because the switch circuit does not add the special control circuit, it will produce the radio frequency interference and pollute the power grid such as the high order harmonic or the peak, so the zero-crossing control circuit is set up. The "zero crossings" means that when the control signal is added and the AC voltage is over zero, the SSR is a passing state, and when the control signal is broken, the SSR is to wait for the junction point (zero potential) of the positive half of the alternating current and the negative half of the half-week (zero potential), and the SSR is broken.    This design can prevent high-order harmonic interference and pollution to the power grid. The absorption circuit is designed to prevent the shock and interference (or even misoperation) of the peak, surge (voltage) transmitted from the power supply to the bidirectional thyristor in the switch device, usually using an "R-C" series absorption circuit or a nonlinear resistor (varistor).   1.3  Solid State Relay Appliances   The special solid-state relay can have the function of short circuit protection, overload protection and overheating protection. With the combined logic curing package, the intelligent module can be realized by the user. It is directly used in the control system.   Solid-state relay has been widely used in computer peripheral interface equipment, thermostat system, temperature regulating, electric furnace heating control, motor control, CNC machine, remote control system, industrial automation device, signal light, light adjustment, scintillator, lighting stage lighting control system, instruments, medical instruments, duplicator, automatic laundry. Machine, automatic fire protection, security system, and power capacitor switching switch as power factor compensation for the power grid, and so on, in addition to the chemical, coal mine, explosion-proof, anti-corrosion, corrosion prevention and so on. The logical curing encapsulation can realize the intelligent modules that users need and is directly used in the control system.     II. Thermostat Basics   2.1  What is Thermostat   The thermostat is a device that directly or indirectly controls one or more hot and cold Yuanlai to maintain the desired temperature. In order to achieve this function, a thermostat must have a sensitive element and a converter. The sensitive element can measure the change of temperature and produce the function required for the converter. The converter converts the function from the sensing element to the proper control of the device that changes the temperature. Thermostat 2.2 Types of Thermostat The types of the thermostat are generally the following: （1）Insert thermostat is installed on the pipe and sensitive element is inserted into the pipeline. （2）Immerse sensitive elements immersed in liquid in pipes or containers to control liquids. （3）Surface sensitive elements installed on the surface of pipes or similar surfaces.   2.3  Features of Thermostat This thermostat pressure gauge setting range (5~35 C) This thermostat measurement accuracy: plus or minus 1 DEG C The thermostat. Size: 86 x 86 (mm) Power supply: AC220V thermostat. This thermostat using ultra-thin design, electrical interface It has a large LCD screen with an LCD thermostat (backlight green, Lan Beiguang) You can display the thermostat in international language (Chinese + English) The thermostat has the function of automatic and manual. This thermostat for refrigeration heating and ventilation three working modes This thermostat high low-speed automatic selection Thermostat timing shutdown function. This thermostat control fan coil end of the fan, water valve, air valve You can also set the password on the thermostat setting temperature and wind speed according to the requirements of users.   Features of thermostat 2.4  Applications of Thermostat   The most common use of the thermostat is to control the room temperature.   Typical uses include: control the gas valve; control the fuel furnace regulator; control the electric heating regulator; control the refrigeration compressor; control the gate regulator.   A room temperature regulator can be used to provide a variety of control functions, such as heating control, heating - cooling control, day and night control (at night at lower temperatures), multistage control, primary or multistage heating, primary or multistage cooling, or multistage heating and cooling control.       III. Drive Thermostat by Using Solid State Relay   3.1  Power Thermostat   There are two kinds of power supply for the thermostat: battery and 24VAC power.    The thermostat needs battery power to run without interruption. It is very important that these batteries consume as low energy as possible, but even if you minimize the power consumption, the users are still inconvenient because the battery needs to be replaced from time to time. In order to reduce the replacement frequency, you can use a 24 VAC power supply. When the C line in the system is not available, the bridge rectifier shown in Figure 1 can convert the AC (AC) voltage to a DC (DC) voltage by the load. Single thermostat signal relay connection with HVAC load   3.2  Case of Driving Thermostat When the HVAC load (compressor, fan, gas valve, etc.) is turned off, the contact of the signal relay is broken. When the contacts are open, the terminals of the rectifier bridge see the voltage of the HVAC transformer is 24VAC, and convert the AC power to DC power, as mentioned earlier. The resulting DC voltage is used to drive the thermostat or subcircuit.   During the HVAC load conduction, the contacts of the signal relay are closed. When the contact is closed, the voltage across the bridge terminal is reduced to zero. This eliminates the need to use 24VAC as a power supply, so the thermostat battery power must be controlled. The range of current required for operating electromechanical relays ranges from tens to hundreds of Ma, which can have a significant impact on battery life.   If there is a way to drive a relay without using a thermostat battery, what will happen? Battery life will increase and replacement frequency will be further reduced. One way is to turn on the relay and charge the control system briefly during the HVAC load conduction (signal relay contact closure).  Compared with the turn off time of the power relay, the time required during charging is very short, which can stimulate the power relay and its corresponding load. Unfortunately, electromechanical (signal) relays are not likely to achieve this goal due to their switching speed limits. The time taken by the contact to the desired location is in milliseconds and will interrupt the HVAC load.   Fortunately, a device can achieve the appropriate switching speed: solid-state relay (SSR). SSR is a semiconductor repeater based on a thyristor or power transistor to perform on / off control.   This recharge method requires a dual MOSFET SSR because it can turn off MOSFET based SSR when necessary. Besides, body diodes of each MOSFET can assist in 24VAC rectification. A full-wave rectifier bridge is built with two diode MOSFET diodes, as shown below.   A power supply for SSR in a HVAC system The following figure shows the rectified waveform corresponding to the color coded diode in the above figure. The voltage ripple of the final waveform can be eliminated by connecting a suitable capacitor to the output of the rectifier bridge. Then, you can reduce the DC voltage of the control system to the desired voltage. Full wave rectifying waveform The use of SSR enables the HVAC system to fully supply the thermostat and reduce the power utilization rate of the battery. When SSR closes, the HV1 and HV2 pipelines will see the full 24VAC voltage and provide a constant 33VDC voltage at the output of the rectifier bridge. When SSR is connected, it may still be circulated through a short-time on/off state to recharge the power supply capacitor. This design can greatly reduce the energy requirements of the thermostat battery and reduce the battery replacement frequency.   FAQ   1. What is solid state relay and how it works? A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. It serves the same function as an electromechanical relay, but has no moving parts and therefore results in a longer operational lifetime.   2. What is the difference between a relay and a solid state relay? The main difference between solid state relays and general relays is that there is no movable contacts in solid state relay (SSR). In general, solid state relays are quite similar to the mechanical relays that have movable contacts. ... SSR provide high-speed, high-frequency switching operations.   3. How fast is a solid state relay? The SSR output is activated immediately after applying control voltage. Consequently, this relay can turn on anywhere along the AC sinusoidal voltage curve. Response times can typically be as low as 1 ms. The SSR is particularly suitable in application where a fast response time is desired, such as solenoids or coils.   4. Do solid state relays get hot? All solid state relays develop heat as a result of a forward voltage drop through the junction of the output device. Beyond a point, heat will cause a lowering (or derating) of the load current that can be handled by the SSR. ... Loads greater than 4 Amps will require heat sinks.   5. What causes solid state relay failure? What are the main causes and solutions of the Solid-state Relays (SSR)'s failures? If an inrush current exceeds the rated making current of the SSR due to the high inrush current of loads such as motors and lamps, SSR output elements are damaged. Consider using an SSR with a higher capacity.   6. Can a solid state relay switch DC? Solid state relays can be designed to switch both AC or DC currents by using an SCR, TRIAC, or switching transistor output instead of the usual mechanical normally-open (NO) contacts.   7. How do you test a solid state relay with a multimeter? Using Multimeter:  1. Set the multimeter in continuity test mode. 2. Place the probes of the multimeter on the coil terminals. 3. If the multimeter beeps (or show any sign of continuity), the coil is electrically closed (good). 4. If the multimeter does not beep, the coil is open & damaged. The relay needs to be replaced.   8. How reliable are solid state relays? Solid-state relays are the preferred choice for system reliability because they have no moving parts or contacts. Over time, the plating on the contacts inside EMRs can erode. This erosion can cause the contacts to weld shut; therefore they no longer open/close properly, and the relay has to be replaced.   9. Is a solid state relay a transistor? Solid-State Relay: A sort of hybrid between a conventional relay and a transistor, these relays switch a load using an LED activated by the control circuitry. The LED activates a light-activated MOSFET that controls the load.   10. How do I know if my solid state relay is bad? Solid-state relays should be checked with an ohmmeter across the normally open (N.O.) terminals when control power is off. The relays should be open, switched to OL, and closed (0.2 , the internal resistance of the ohmmeter) when control power is applied.   11. How do I choose a solid state relay? When selecting a Solid State Relay, consider: Current rating, as a general rule consider using the relay at no more than 70% of its rated current. Electrical environment,. i(In harsh electrical environments, consider a relay with an line voltage rating above the application line voltage.)   12. Do solid state relays need a diode? 2 Answers. The control side of solid state relays is usually just a LED, sometimes two LEDs back to back, and sometimes with integrated resistor. ... If the relay is on the same board as whatever is driving it, then no inductive kickback diode is needed. It's no different than driving any other on-board LED.   13. Do solid state relays leak voltage? Solid State relays have leakage. If you want to repeatedly switch something on / off, use them. But when you want the SSR to be fully off, say after pressing an off switch, a mechanical relay should be across the load to take it off the SSR. ... The SSR control is attached to the atmega328 through a 200ohm resistor.   Relevant information about "How  to Drive Thermostat by Using Solid State Relay " About the article "How  to Drive Thermostat by Using Solid State Relay", If you have better ideas, don't hesitate to write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles.   Making a Arduino Variable Timer Relay Comprehensive Introduction of the Time Delay Relays