Solid-state relays (SSRs) are a new type of contactless switching device composed entirely of solid-state electronic components. Utilizing the switching characteristics of electronic components (such as switching transistors and triacs), they achieve contactless and spark-free connection and disconnection of circuits, hence the name "contactless switch." Introduced in the 1970s, their contactless operation has led to their increasingly widespread application in electrical control and computer control systems across many fields. I. Principle and Structure of Solid-State Relays SSRs can be broadly classified into AC and DC types based on their application. They function as load switches on AC or DC power supplies respectively and cannot be used interchangeably. The following explanation uses an AC SSR as an example to illustrate its working principle. Figure 1 shows its working principle block diagram. Components ①-④ in Figure 1 constitute the main body of the AC SSR. Overall, the SSR has only two input terminals (A and B) and two output terminals (C and D), making it a four-terminal device. During operation, by applying certain control signals to A and B, the "on" and "off" states between C and D can be controlled, realizing the "switch" function. The coupling circuit provides an input/output channel for the control signals input to A and B, but electrically disconnects the (electrical) connection between the input and output terminals of the SSR to prevent the output from affecting the input. The component used in the coupling circuit is an "optocoupler," which is sensitive, has a high response speed, and a high insulation (withstand voltage) level between the input and output terminals. Since the load at the input terminal is a light-emitting diode, it is easy to match the input signal level of the SSR. In use, it can be directly connected to the computer output interface, i.e., controlled by the logic levels of "1" and "0". The trigger circuit generates a trigger signal that meets the requirements to drive the switching circuit ④ to work. However, since the switching circuit will generate radio frequency interference and pollute the power grid with high-order harmonics or spikes without special control circuitry, a "zero-crossing control circuit" is specially designed for this purpose. The term "zero-crossing" refers to the fact that when a control signal is applied and the AC voltage crosses zero, the SSR is in the on-state; and when the control signal is disconnected, the SSR waits for the boundary point (zero potential) between the positive and negative half-cycles of the AC current before it becomes off-state. This design prevents interference from higher harmonics and pollution of the power grid. The absorption circuit is designed to prevent spikes and surges (voltages) from the power supply from impacting and interfering with the switching device, the bidirectional thyristor (SCR), (even causing malfunctions). It is generally an "RC" series absorption circuit or a nonlinear resistor (varistor). Figure 2 shows the electrical schematic of a typical AC SSR. Compared to AC SSRs, DC SSRs do not have a zero-crossing control circuit and do not require an absorption circuit. The switching device is generally a high-power switching transistor, but the other operating principles are the same. However, when using DC-type SSRs, the following should be noted: ① When the load is inductive, such as a DC solenoid valve or electromagnet, a diode should be connected in parallel across the load terminals, with the polarity shown in Figure 3. The diode current should be equal to the operating current, and the voltage should be greater than four times the operating voltage. ② The SSR should be placed as close to the load as possible during operation, and its output leads should meet the load current requirements. ③ If the power supply is obtained by AC step-down rectification, its filter electrolytic capacitor should be large enough. Figure 4 shows the appearance of several common domestic and foreign SSRs. II. Characteristics of Solid State Relays SSRs successfully realize the control of strong current (output load voltage) by a weak signal (Vsr). Due to the application of optocouplers, the power required for the control signal is extremely low (about ten milliwatts is sufficient for normal operation), and the operating level required for Vsr is compatible with commonly used integrated circuits such as TTL, HTL, and CMOS, allowing for direct connection. This makes SSRs widely used in CNC and automatic control equipment. To a considerable extent, it can replace the traditional "coil-reed contact" relay (abbreviated as "MER"). Because SSRs are composed entirely of solid-state electronic components, unlike MERs, they have no moving mechanical parts and no mechanical action during operation. SSRs achieve their "on" and "off" switching functions through changes in the circuit's operating state, without electrical contacts. Therefore, they possess a series of advantages that MERs lack, namely high reliability, long lifespan (data shows that SSRs can achieve 10⁸-10⁹ switching cycles, hundreds of times higher than the typical 10⁶ of MERs); no operating noise; resistance to vibration and mechanical shock; unrestricted installation location; easy to pot with insulating and waterproof materials to create a fully sealed form, and excellent moisture-proof, mildew-proof, and corrosion-proof properties; and excellent performance in explosion-proof and ozone-pollution prevention. These characteristics make SSRs highly versatile in military applications (such as aircraft, artillery, ships, and vehicle-mounted weapon systems), chemical industries, underground coal mining, and various industrial and civilian electrical control equipment, giving them a technological advantage over MERs. AC SSRs, employing zero-crossing triggering technology, can be safely used on computer output interfaces without the interference issues that arise from using MERs at these interfaces. In addition, SSRs can withstand surge currents up to ten times their rated current. III. Main Parameters and Selection The characteristic parameters of power solid-state relays include input and output parameters. Taking the GX-10F relay produced by Beijing Ketong Relay Factory as an example, the input and output parameters are listed in Table 1. The operating voltage can be determined based on the input voltage parameter value. When using TTL or CMOS logic level control, it is best to use a low-level drive with sufficient load-carrying capacity and keep the "0" level as low as possible below 0.8 V. In noisy environments, products with small differences between on and off voltage values ​​should not be selected; products with large differences between on and off voltage values ​​must be selected (e.g., products with an on-state voltage of 8 V or 12 V). This will prevent control failure due to noise interference. There are many output parameters; the main parameters are explained below: 1. Rated Input Voltage: This is the maximum permissible effective voltage value of a steady-state resistive load that can be withstood under specified conditions. If the controlled load is non-steady-state or non-resistive, it is necessary to consider whether the selected product can withstand the maximum combined voltage generated by changes in operating conditions or conditions (heating/cooling transition, static/dynamic transition, induced electromotive force, transient peak voltage, change period, etc.). For example, when the load is inductive, the selected rated output voltage must be greater than twice the power supply voltage, and the blocking (breakdown) voltage of the selected product should be higher than twice the peak value of the load power supply voltage. For general low-power non-resistive loads with a power supply voltage of AC 220V, it is recommended to choose an SSR product with a rated voltage of 400V-600V; however, for single-phase or three-phase motor loads with frequent starts, it is recommended to choose an SSR product with a rated voltage of 660V-800V. 2. Rated Output Current and Surge Current The rated output current refers to the maximum effective value of the current that can be withstood under given conditions (ambient temperature, rated voltage, power factor, presence or absence of heat sink, etc.). Manufacturers generally provide thermal derating curves. If the ambient temperature rises, derating should be performed according to the curve. Inrush current refers to the maximum non-repetitive peak current allowed under given conditions (room temperature, rated voltage, rated current, and duration, etc.) without causing permanent damage. The inrush current of an AC relay is 5-10 times its rated current (per cycle), and for DC products, it is 1.5-5 times its rated current (per second). When selecting an SSR, if the load is steady-state resistive, it can be used at full or 10% derated. For electric heaters, contactors, etc., the inrush current at the initial switching moment can reach 3 times the steady-state current; therefore, the SSR should be derated by 20%-30%. For incandescent lamps, the SSR should be used at 50% derated, and appropriate protection circuitry should be added. For transformer loads, the rated current of the selected product must be more than twice the load's operating current. For induction motor loads, the rated current of the selected SSR should be 2-4 times the motor's operating current, and the inrush current should be 10 times the rated current. Solid-state relays are highly sensitive to temperature. Once the operating temperature exceeds the nominal value, cooling or an external heatsink must be added. For example, the JGX-10F product with a rated current of 10A can only operate at 10A without a heatsink. IV. Application Circuit 1. Basic Unit Circuit As shown in Figure 5a, for a stable resistive load, a current-limiting resistor Rx is required to prevent the input voltage from exceeding the rated value. When the load is an unstable or inductive load, a transient suppression circuit should be added to the output circuit, as shown in Figure 5b, to protect the solid-state relay. A common measure is to add an RC snubber circuit (e.g., R=150 Ω, C=0.5 μF or R=39 Ω, C=0.1 μF) to the relay output. This effectively suppresses the transient voltage and voltage rise rate dv/dt applied to the relay. When designing the circuit, it is recommended that users carefully calculate and test the RC circuit values ​​based on the relevant load parameters and environmental conditions. Another common measure is to connect a voltage control device with a specific clamping voltage, such as a bidirectional Zener diode or a varistor (MOV), to the relay output. The varistor current value should be calculated using the following formula: Imov = (Vmax - Vmov) / ZS, where ZS is the sum of the load impedance, power supply impedance, and line impedance, and Vmax and Vmov are the maximum transient voltage and the nominal voltage of the varistor, respectively. For conventional 220V and 380V AC power supplies, the recommended nominal voltage values ​​for the varistor are 440-470V and 760-810V, respectively. Connecting an RC circuit or capacitor in parallel with the AC inductive load can also suppress the transient voltage and voltage rise rate applied to the SSR output. However, experiments show that RC snubber circuits, especially those connected in parallel at the SSR output, can easily cause oscillations if improperly combined with inductive loads. This increases the peak transient voltage at the relay output when the load power is on or the relay switches, increasing the likelihood of the SSR misfiring. Therefore, specific application circuits should be tested first to select appropriate RC parameters; sometimes, omitting the RC snubber circuit altogether is even more advantageous. Inrush currents caused by capacitive loads can be suppressed using inductive components, such as magnetic interference filters or chokes, to limit rapidly rising peak currents. Additionally, if the rate of change of the output current (di/dt) is large, an inductor with a high permeability and a soft core can be connected in series at the output to limit it. SSRs are typically designed to be "normally open," meaning the output is open when there is no control signal input. However, "normally closed" SSRs are often required in automated control equipment. In this case, a simple external circuit can be connected at the input, as shown in Figure 5c, which constitutes a normally closed SSR. 2. Multifunctional control circuit. Figure 6a shows a multi-output circuit. When the input is "0", transistor BG is cut off, and there is no input voltage at the input terminals of SSR1, SSR2, and SSR3, so their respective output terminals are disconnected. When the input is "1", transistor BG is turned on, and there is input voltage at the input terminals of SSR1, SSR2, and SSR3, so their respective output terminals are connected. Thus, the purpose of controlling multiple output terminals to "on" and "off" with one input port is achieved. Figure 6b shows a single-pole double-throw control circuit. When the input is "0", transistor BG is cut off, there is no input voltage at the input terminal of SSR1, and the output terminal is disconnected. At this time, the voltage at point A is applied to the input terminal of SSR2 (UA-UDW should reliably connect the output terminal of SSR2), and the output terminal of SSR2 is connected. When the input is "1", transistor BG is turned on, there is an input voltage at the input terminal of SSR1, and the output terminal is connected. At this time, although there is voltage at point A, the voltage value of UA-UDW is no longer sufficient to connect the output terminal of SSR2, so it is in the open state. Thus, the function of "single-pole double-throw control circuit" is achieved. (Note: When selecting the voltage regulation value of Zener diode DW, it should be ensured that the voltage at the "+" terminal of the conducting SSR1 will not turn on SSR2, while also taking into account that the voltage at the "+" terminal during the cutoff period of SSR1 can turn on SSR2.) 3. Computer-controlled motor forward and reverse rotation interface and drive circuit. Figure 7 shows the computer-controlled single-phase AC motor forward and reverse rotation interface and drive circuit. During commutation control, the pause time between forward and reverse rotation should be greater than 1.5 cycles of the AC power supply (using a "falling edge delay" circuit) to prevent short circuits between lines caused by too fast commutation. The relays in the circuit should be AC ​​solid-state relays with a blocking voltage higher than 600 V and a rated voltage of 380 V or higher. To limit the discharge current of the capacitor during motor commutation, a current-limiting resistor Rx should be added to each circuit. Its resistance and power can be calculated using the following formula: Rx = 0.2 × VP/IR (Ω), P = Im²Rx. Where: VP—peak voltage of the power supply (V); IR—rated current of the solid-state relay (A); Im—motor operating current (A); P—power of the current-limiting resistor (W). Figure 8 shows the computer-controlled three-phase AC motor forward and reverse rotation interface and drive circuit. The figure uses four NAND gates, with two signal channels to control the motor's start, stop, forward rotation, and reverse rotation respectively. When changing the direction of motor rotation, the sequence of command signals should be "stop—reverse—start" or "stop—forward—start". The minimum delay of the delay circuit should not be less than 1.5 AC power supply cycles. RD1, RD2, and RD3 are fuses. When the motor allows, current-limiting resistors can be connected at positions R1-R4 to prevent the half-cycle short-circuit current from exceeding the surge current that the relays can withstand if any two relays between the two lines are mistakenly connected, thus avoiding relay burnout and ensuring safety; however, the side effect is that a voltage drop and power consumption will occur across the resistors during normal operation. It is recommended to use an SSR product with a rated voltage of 660 V or higher for this circuit. V. Conclusion As can be seen from the foregoing, SSRs have many advantages over electromagnetic relays, especially in terms of ease of computer programming control, making control implementation more convenient and flexible. However, it also has some weaknesses, such as: on-resistance (several Ω to tens of Ω), on-state voltage drop (less than 2 V), and off-state leakage current (5-10mA), which can easily cause overheating and damage; leakage resistance exists at cutoff, preventing the circuit from being completely disconnected; it is susceptible to temperature and radiation, resulting in poor stability; its high sensitivity makes it prone to malfunctions; and in control circuits requiring interlocking, the addition of protection circuits increases cost and size. Therefore, the unique performance of SSRs must be correctly understood and used cautiously to fully utilize their capabilities and ensure their trouble-free operation.