Solid State Relays: A Basic Overview

Here’s the Solid State Relay (SSR) blog I promised in my last one on mechanical relays. SSRs perform the same basic functions as mechanical relays, but here I will cover how they function internally, why you would choose them over a mechanical relay, and go over some terminology.

SSRs are made up of three main pieces: a sensor, a switching device, and a coupling mechanism. Typically, the coupling is done optically to provide electrical isolation between the control and signal circuits. The input will turn on an internal LED that switches on a photosensitive diode. The diode will turn on an SCR, thyristor, or MOSFET, allowing flow to the output pins.

SSRs are typically faster than mechanical relays because there aren’t any moving parts so release time is greatly reduced, contact bounce is not a factor, they last longer, and there is no acoustical noise. Some downsides to SSRs are that they have a higher contact resistance than a mechanical relay and they are more vulnerable to damage from surge currents. If the internal switching device is damaged, the relay will be rendered useless.

One piece that usually puzzles people when looking at SSRs are the output types. They are able to switch DC, AC, or a combination of the two. When switching AC there are multiple options such as: Zero Cross, Proportional Control, or Asynchronous. Let’s look at the differences between these types.

Zero Cross (‘Synchronous’): After the control voltage is applied, the relay will not switch on until the load voltage crosses zero volts. The image below shows the DC control voltage being sent to the input, but the AC load voltage does not conduct through the output until line 1, which is the first time the sine wave crosses zero volts. The load does not turn off again until the sine crosses zero for the first time after the control voltage is turned off (Line 1 marks the turn on point and line 2 marks the turn off point).

Figure 1: Zero Cross Relay (Image Source: DigiKey)

Proportional Control: In this case, the power provided to the load is directly proportional to the analog control signal provided to the input. The control signal can take several different forms such as 0 - 5 V_DC, 4 - 20 mA, and 0 - 10 V_DC. These varying signal outputs are typically found in lighting or heating applications.

Asynchronous (‘Instantaneous’ or ‘Random Turn-On’): The output of these relays turns on as soon as voltage is applied to the input, and turns off as soon as voltage is removed and the sine wave reaches zero.

For more information on electromechanical relays or relays in general, please take a look at this blog, Mechanical Relays: A Basic Overview, or watch the video below.