Solid State Relay Basics: Working, Features and Structure

Catalog

Ⅰ Introduction

The solid state relay is a new type of non-contact switching device composed of solid-state electronic components. It uses the switching characteristics of electronic components (such as switching transistors, triacs, and other semiconductor devices), which can achieve the purpose of making and breaking the circuit without contact and spark, so it is also called "non-contact switch". A solid state relay is a four-terminal active device, two of which are input control terminals, and the other two terminals are output controlled terminals. It has both amplifying and driving functions and isolation functions, which are very suitable for driving high-power switching actuators. Compared with electromagnetic relays, it has higher reliability, no contact, long life, fast speed, and less interference to the outside world. It has been widely used.

Ⅱ Working Principle 

SSR can be divided into AC type and DC type according to the occasion of use. They are used as load switches on the AC or DC power supply, and cannot be mixed. The following takes an AC SSR as an example to illustrate its working principle. Figure 1 is a block diagram of its working principle. The components ①～④ 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 output terminals (C and D), it is a four-terminal device.

Figure 1. block diagram of working principle of SSR

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, and the function of "switch" can be realized. The function of the coupling circuit is to provide a channel between the input/output terminals for the input control signal from the A and B terminals, 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 component used in the coupling circuit is an "optical coupler", which is sensitive, has a high response speed, and has high insulation (withstand voltage) level between the input and output terminals. Since the load on the input terminal is a light-emitting diode, this makes the input terminal of the SSR easy to match the input signal level and can be directly connected to the computer output interface when in use, which 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 to drive the switch circuit ④ to work, but because the switch circuit does not add a special control circuit, it will generate radio frequency interference and pollute the power grid with high harmonics or spikes. Therefore, the "Zero Crossing Control Circuit" was designed. The so-called "zero crossing" means that when the control signal is added 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 intersection of the positive half cycle and the negative half cycle of the alternating current, SSR is off state. This design can prevent high-order harmonic interference 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 from the spikes and surges (voltage) from the power supply. Generally, the "RC" series absorption circuit or non- Linear resistance (varistor) are used.

Ⅲ Features

The solid state relay is a non-contact electronic switch with isolation function. There are no mechanical contact parts during 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 positions, good moisture, mildew, and corrosion resistance, and excellent performance in explosion protection and prevention of ozone pollution. It has 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 adopts a fully-sealed perfusion method. Therefore, the SSR has the advantages of vibration resistance, corrosion resistance, long life, and high reliability, and its switching life is up to 10.1 million times;

(2) Low noise: AC SSR adopts zero-crossing trigger technology, so the voltage rise rate dv/dt and current rise rate di/dt value are effectively reduced on the line, so that the SSR has minimal interference to the mains during long-term operation ;

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

(4) Photoelectric isolation is used 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 and COMS circuits;

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

(7) Strong load capacity.

1 Advantage 

(1) Long life and high reliability: The solid state relay has no mechanical parts, and the contact function is completed by solid devices. Since there are no moving parts, it can work under high-impact and vibration environments. Because of the components that make up the solid state relay, the inherent characteristics of solid state relays determine the long life and high reliability of solid state relays.

Figure 2. solid state relay

(2) High sensitivity, low control power, and good electromagnetic compatibility: The solid state relay has a wide input voltage range and low drive power, and is compatible with most logic integrated circuits without the need for buffers or drivers.

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

(4) Small electromagnetic interference: The solid state relay has no input "coil", no arc ignition and rebound, thus reducing electromagnetic interference. Most AC output solid state relays are a zero-voltage switch, which is turned on at zero voltage and turned off at zero current, reducing the sudden interruption of the current waveform, thereby reducing the switching transient effect.

2 Disadvantage

(1) The voltage drop of the tube after it is turned on is large, the forward voltage drop of the thyristor or triac can reach 1~2V, and the saturation voltage drop of the high-power transistor is also between 1~2V, and the general power field effect tube’s on-resistance is also greater than the contact resistance of mechanical contacts.

(2) The semiconductor device can still have a leakage current of several microamperes to several milliamperes after it is turned off, so ideal electrical isolation cannot be achieved.

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

(4) The temperature characteristics of electronic components and electronic circuits have poor anti-interference ability and radiation resistance is also poor. If effective measures are not taken, the work reliability is low.

Figure 3. solid state relay 2

(5) Solid state relays are more sensitive to overload and must be protected against overload with a fast fuse or RC damping circuit. The load of the solid state relay is obviously related to the ambient temperature. As the temperature rises, the load capacity will drop rapidly.

(6) The main shortcomings are the on-state voltage drop (corresponding heat dissipation measures are required), off-state leakage current, AC and DC cannot be used universally, the number of contact groups is small.

Ⅳ Structure 

The solid state relay is composed of three parts: input circuit, isolation (coupling), and output circuit.

1 Input circuit

According to 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 inversion functions, and can be easily connected with TTL and MOS logic circuits.

For a control signal 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 reliable operation of the current greater than 5mA within the entire voltage variation range.

2 Isolation coupling

The input and output circuits of solid state relays can be isolated and coupled in two ways: photoelectric coupling and transformer coupling: photoelectric coupling usually uses photodiode-phototransistor, photodiode-bidirectional light-controlled thyristor, photovoltaic cell to realize control side and load side Isolation control; high-frequency transformer coupling uses the self-excited high-frequency signal generated by the input control signal to be coupled to the secondary, detected and rectified, and processed by the logic circuit to form a drive signal.

3 Output circuit

The power switch of the SSR is directly connected to the power supply and the load side to realize the on-off switch of the load power supply. Mainly use high-power transistors, one-way thyristor (Thyristor or SCR), bidirectional 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/DC output circuit. According to the load type, it can be divided into DC solid state relay and AC solid state relay. Bipolar devices or power FETs can be used for DC output, and two thyristors or one bidirectional thyristor are usually used for AC output. The AC solid-state relay can be divided into single-phase AC solid-state relay and three-phase AC solid-state relay. AC solid-state relays can be divided into random AC solid-state relays and zero-crossing AC solid-state relays according to the timing of turn-on and turn-off.