What is a solid state relay and what is it for?

  • Dec 26, 2019

When you create a variety of electrical devices and systems often require controlled switching. Usually used for this electromechanical relays have a number of disadvantages (the main ones large size, the need to supply high power signals in the control circuit, short term service).

Effectively overcome these shortcomings can transition to new element base. One of such developments, which successfully replaces the traditional technique, elements of steel, known as solid-state relays.

Design

The basis of the design of solid-state relay based on two parts: an electronic key and a control circuit which, under the influence of standardized input signals turn the key in one of two possible states of: open and Closed.

Electronic components relays designed in the form of assembly, the body of which is provided with the necessary conclusions, including ground and ready for installation in the circuit. Housing some of the models has the appropriate cutouts for direct mounting on DIN-rail. Figure 1 shows a typical example of the design of the relay case.

Picture 1. Constructive design of solid state relay SSR-25 Series
Picture 1. Constructive design of solid state relay SSR-25 Series

The relay is used as a key element:

  • conventional or FETs of different patterns for the DC relay;
  • thyristors or semistors AC relay.

Typical solution to ensure reliable electrical isolation of the power and control circuits - various opto circuit with penetrative resistance to several thousand volts. An example of one of them is shown in Figure 2.

Because of the relatively low power output circuits photocoupler its complementary amplifier. An alternative way of solving this problem is considered to be the realization of the output relay stage Darlington.

Stability of key functioning in conditions typical of the focal field of application of voltage instability is reached by introducing an internal voltage regulator.

Figure 2. The block diagram of a solid-state relay and its interaction with the control circuits and the load
Figure 2. The block diagram of a solid-state relay and its interaction with the control circuits and the load

Semiconductor components are mounted on the circuit board, which is installed in a sealed housing. On the outer surface of the housing are provided corresponding contacts for power and control circuits.

Typical relay switching circuit shown in Figure 2. No major differences from electromechanical counterparts, she has not.

Figure 3. Circuit of the solid state relay SSR-25 Series
Figure 3. Circuit of the solid state relay SSR-25 Series

Advantages and disadvantages

Compared to the classical solid-state analog switch it has a number of advantages.

Among them:

  • high performance due to the lack of massive moving parts;
  • the possibility of use in potentially explosive environments due to the absence of an electric arc occurring at break electrical contacts
  • rigid control characteristic without hysteresis and bounce;
  • good weight and size parameters.

According to the current carrying capacity (up to 150 A) older model of solid-state relays are not inferior to their electromechanical counterparts. At the same time, these elements are successfully competing with traditional relays and contactors on functionality. For example, they are able to provide direct control of even large motors. In addition, some elements of this species nominally allows operation in circuits not only permanent, but also AC.

The main disadvantages are:

  • poor resistance to overloads and high probability of failure at short circuit;
  • relatively high resistance in the open state, resulting in some cases require additional forced cooling;
  • typical semiconductor switches for high nonlinearity of the output current-voltage characteristic.

conclusion

Solid state relay is efficient replacement of traditional electromechanical devices and can be recommended for use when creating a broad class of electrical devices.