Solid State Relay (SSR for short) is a non-contact switch composed of microelectronic circuits, discrete Electronic devices, and power electronic power devices. The isolation device is used to realize the isolation between the control terminal and the load terminal. The input end of the solid state relay uses a tiny control signal to directly drive a large current load.

Solid State Relay (SSR for short) is a non-contact switch composed of microelectronic circuits, discrete electronic devices, and power electronic power devices. The isolation device is used to realize the isolation between the control terminal and the load terminal. The input end of the solid state relay uses a tiny control signal to directly drive a large current load.

Analysis of the role and principle of solid state relays in optocoupler circuits


SSR can be divided into two categories: AC type and DC type according to the application. They are respectively used as load switches on the AC or DC power supply and cannot be mixed. The following takes the AC SSR as an example to illustrate its working principle. Figure 1 is a block diagram of its working principle. components ① to ④ in Figure 1 constitute the main body of the AC SSR. On the whole, the SSR has only two input terminals ( A and B) and two outputs (C and D), it is a four-terminal device.

When working, as long as a certain control signal is added to A and B, the “on” and “off” between the two ends of C and D can be controlled to realize the function of “switch”, in which the function of the coupling circuit is for A, The control signal input at terminal B provides a channel between the input/output terminal, but electrically disconnects the (electrical) connection between the input terminal and the output terminal in the SSR to prevent the output terminal from affecting the input terminal. The coupling circuit The component used is an “optical coupler”, which has sensitive action, high response speed, and high level of insulation (withstand voltage) between the input/output terminals; since the load at the input terminal is a light-emitting diode, this makes the input terminal of the SSR very easy to do. Matching with the input signal level, it can be directly connected with the computer output interface when in use, that is, it is controlled by the logic level of “1” and “0”.

The function of the trigger circuit is to generate a trigger signal that meets the requirements and drive the switch circuit ④ to work, but because the switch circuit will generate radio frequency interference and pollute the power grid with high-order harmonics or spikes when no special control circuit is added, it is specially designed for this purpose. “Zero-Crossing Control Circuit”. The so-called “zero-crossing” means that when the control signal is added and the AC voltage crosses zero, the SSR is on-state; and when the control signal is disconnected, the SSR waits for the junction of the positive and negative half-cycles of the AC power (zero potential) When the SSR is in the off state. This design can prevent the interference of higher harmonics and pollution to the power grid.

The absorption circuit is designed to prevent the impact and interference (or even malfunction) of the switching device triac by the spikes and surges (voltage) from the power supply. Linear resistors (varistors).

The role and principle of the solid state relay of the optocoupler circuit

The biggest role of the optocoupler circuit in the electronic design process is to isolate the output and input signals, so as to ensure the normal operation of the circuit.The optocoupler circuit has the characteristics of strong anti-interference ability, stable operation and long service life, and plays an important role in common circuits. This article will introduce the application of optocoupler circuits in solid state relays.

The coupled solid state relay has the advantages of small size, close coupling, small driving power, fast action, and wide operating temperature range. Its left half circuit can be used to convert the input electrical signal VI

It becomes a light-emitting diode in the optocoupler to send a signal; while the right half of the circuit restores the optical signal to an electrical signal through the phototransistor in the optocoupler, so this is a very good electro-optical and optoelectronic combined conversion device. The current transfer ratio of the optocoupler is 20%, the withstand voltage is 150V, and the driving current is between 8~20mA. When the optocoupler circuit acts on the solid state relay, it can be found through actual operation that there is no actual contact in the general relay, so don’t worry. Due to poor contact, the three melons ignited. In addition, there is no need to worry about the misoperation caused by the impact of external force. The hysteresis relay based on the optocoupler circuit has excellent performance in stability and reliability.

Can solid state relays replace optocouplers?

The optocoupler is only an isolation link. If the selection of the electrical components is good enough, the optocoupler can be omitted. But the SSR is not completely isolated, so the optocoupler should not be skipped in applications with strict leakage current requirements (such as medical equipment or something). (But the SSR on your picture doesn’t seem to be able to carry a 380V load).

However, there is generally an LED light on the photocouple to indicate the status. In order to facilitate the debugging of the equipment, it is generally still used. This light is much more intuitive than the small light on the PLC, and it can also be used for labeling.

Even for a certain brand of pneumatic components, there is no light on the wire package of the electric control valve. To have this function, you need to use this brand of light-emitting gasket. Come on, it costs 50 yuan per piece. So I just gave up this extortion light-emitting gasket and added a set of optocouplers directly to the electrical. Oh, I used a product of a very good quality Japanese brand, and it was only 8 yuan per channel on average, which is convenient and affordable. Even the debugging master said that this is more intuitive than before.

Features of Solid State Relays

Solid state relay is a non-contact electronic switch with isolation function, no mechanical contact parts in the switching process,

Therefore, in addition to the same functions as electromagnetic relays, solid state relays also have logic circuit compatibility, vibration resistance and mechanical shock resistance, unlimited installation position, good moisture-proof, mildew-proof and corrosion-proof performance, and excellent performance in explosion-proof and ozone pollution prevention. It has the characteristics of low input power, high sensitivity, low control power, good electromagnetic compatibility, low noise and high operating frequency.

(1) There are no mechanical parts inside the SSR, and the structure is fully sealed by perfusion. Therefore, the SSR has the advantages of vibration resistance, corrosion resistance, long life and high reliability, and its switching life is as high as 10.1 million times;

(2) Low noise: The AC-type SSR adopts the zero-crossing trigger technology, so the voltage rise rate dv/dt and current rise rate di/dt are effectively reduced on the line, so that the SSR has minimal interference to the mains when it works for a long time. ;

(3) The switching time is short, about 10ms, which can be used in high frequency occasions;

(4) Photoelectric isolation is adopted between the input circuit and the output circuit, and the insulation voltage is above 2500V;

(5) The input power consumption is very low, compatible with TTL, CMOS circuits;

(6) There is a protection circuit at the output end;

(7) Strong load capacity.


(1) High life and high reliability: solid state relays have no mechanical parts, and solid state devices complete the contact function. Because there are no moving parts, they can work in high shock and vibration environments. Because of the components that make up solid state relays The inherent characteristics of the solid state relay determine the long life and high reliability of the solid state relay.

solid state relay

solid state relay

(2) High sensitivity, low control power and good electromagnetic compatibility: solid state relays have a wide input voltage range and low drive power, and are compatible with most logic integrated circuits without buffers or drivers.

(3) Fast switching: Because solid-state relays use solid-state devices, the switching speed can be from a few milliseconds to a few microseconds.

(4) Small electromagnetic interference: The solid-state relay has no input “coil”, no arcing and rebound, thus reducing electromagnetic interference. Most AC output solid state relays are a zero-voltage switch that conducts at zero voltage and turns off at zero current, reducing sudden interruptions in the current waveform and thus reducing switching transient effects.


(1) The voltage drop of the tube after being turned on is large, the forward voltage drop of the thyristor or bidirectional thyristor can reach 1~2V, and the saturation voltage drop of the high-power transistor is also between 1~2V. The on-resistance is also higher than the contact resistance of mechanical contacts.

(2) After the semiconductor device is turned off, there may still be a leakage current of several microamps to several milliamps, so ideal electrical isolation cannot be achieved.

(3) Due to the large voltage drop of the tube, the power consumption and heat generation after conduction are also large. The volume of high-power solid state relays is much larger than that of electromagnetic relays of the same capacity, and the cost is also high.

(4) The temperature characteristics of electronic components and the anti-interference ability of electronic circuits are poor, and the radiation resistance ability is poor.

Solid-state relays are also poor, and if no effective measures are taken, the reliability of the work will be low.

(5) Solid state relays are highly sensitive to overload and must be protected from overload by fast fuses or RC damping circuits. The load of the solid state relay is obviously related to the ambient temperature. When the temperature increases, the load capacity will drop rapidly.

(6) The main disadvantage is that there is an on-state voltage drop (corresponding heat dissipation measures are required), there is an off-state leakage current, AC and DC cannot be used universally, the number of contact groups is small, and overcurrent, overvoltage and voltage rise rate, current rise rate, etc. Poor indicators.


A solid state relay consists of three parts: input circuit, isolation (coupling) and output circuit.

input circuit

According to the different types of input voltage, the input circuit can be divided into three types: DC input circuit, AC input circuit and AC/DC input circuit. Some input control circuits are also compatible with TTL/CMOS, positive and negative logic control and inverse equality functions, which can be easily connected with TTL and MOS logic circuits.

For control signals with a fixed control voltage, a resistive input circuit is used. The control current is guaranteed to be greater than 5mA. For the control signal with a large variation range (such as 3~32V), a constant current circuit is used to ensure that the current can work reliably at more than 5mA in the entire voltage variation range.

Isolated coupling

The isolation and coupling methods of the input and output circuits of solid-state relays include photoelectric coupling and transformer coupling: photoelectric coupling usually uses photodiode-phototransistor, photodiode-bidirectional photo-controlled silicon, photovoltaic cell, and realizes control side and load side. Isolation control; high-frequency transformer coupling is that the self-excited high-frequency signal generated by the input control signal is coupled to the secondary, detected and rectified, and processed by the logic circuit to form a drive signal.

output circuit

The power switch of the SSR is directly connected to the power supply and the load terminal to realize the on-off switching of the load power supply. Mainly used are high-power transistors (switch-Transistor), one-way thyristor (Thyristor or SCR), two-way thyristor (Triac), power field effect transistor (MOSFET), insulated gate bipolar transistor (IGBT) . The output circuit of solid state relay can also be divided into DC output circuit, AC output circuit and AC and DC output circuit. According to the load type, it can be divided into DC solid state relays and AC solid state relays.

Bipolar devices or power FETs can be used for DC output, and two thyristors or one triac are usually used for AC output. The AC solid state relays can be divided into single-phase AC solid state relays and three-phase AC solid state relays. AC solid state relays can be divided into random type AC solid state relays and zero-crossing type AC solid state relays according to the timing of turn-on and turn-off.

How to use the proper radiator

Except for the solid-state relay with a rated current of 1-5A directly installed on the printed circuit board, the rest should be equipped with appropriate heat sinks, and the SSR base plate and the heat sink should be coated with thermal grease, the two are in close contact, and tightened with screws. .

Below are some recommended radiators for SSR with specifications for reference. With the different usage conditions, the user can make appropriate adjustments.

How to Secure SSR

A. Overcurrent protection. SSR is a semiconductor power device, which is extremely sensitive to temperature changes. Overcurrent will damage the SSR, and a fast fuse is usually used. However, it is necessary to understand its protection characteristics, know the relationship between its fuse current and time, and correctly select the fast fuse that is compatible with the SSR’s nominal current.

B. Add RC absorption circuit. Adding RC loop not only has the effect of preventing overvoltage, but also is good for improving dv/dt. It is recommended that R is 20-100Ω, power is 2-5W, C is 0.1-0.47uf, and withstand voltage is 250-630v. SSR nominal current is small, R takes the upper limit of 100Ω, C takes the lower limit of 0.1uf, otherwise, R takes the small value, C takes a large value.

C, overheating protection

The SSR is overheated and its characteristics are degraded. If it is out of control, it will cause permanent damage. It is recommended to install a temperature control switch near the bottom of the SSR. The temperature control point is 75 to 80 °C.

D. Inductance L is connected in series in the inductive load. In inductive loads, the SSR is usually damaged due to the high rate of change of current di/dt. The amount of L inductance depends on the size and cost.

The Links:   SKD145-18 SKB72/16