Different types of solid state relays (SSR) and their working

Classification of Solid State Relays (SSR)

SSR Definition: A solid state relay may be defined as an electrical switch that changes its state when an external voltage is applied to it. Solid state relay has a capability to control a larger load current or voltage.

There are mainly three types of Solid State Relays:

  1. Reed-Relay-Coupled SSR
  2. Transformer-Coupled SSR
  3. Photo-coupled SSR

Let us discuss these solid state relays one by one.

1)      Reed-Relay-Coupled SSR’s

Reed Relay Coupled SSR
Reed Relay Coupled SSR

Above circuit shows the circuit diagram for Reed Relay coupled SSR. In this type of SSR control signal is applied directly or using preamplifier (if control signal is weak) to the coil of a reed relay. This relay causes the operation of the trigger circuit which is used to trigger the power transistor or TRIAC.

2)      Transformer Coupled SSR’s

Transformer Coupled SSR
Transformer Coupled SSR

Second type of solid state relay is transformer coupled SSR. It uses a transformer for sending the control signal from primary windings to the secondary winding. Secondary winding is connected to the trigger circuit which is used for the triggering of thyristor switch. It also uses DC-AC converter if the input control signal is DC because transformer can not work with DC.

3)      Photo coupled SSR:

Photo Coupled SSR
Photo Coupled SSR

Another type of solid state relay is photo coupled SSR. Above figure shows the circuit diagram of the photo coupled SSR.In this type of SSR control signal is applied to the infrared source or light source like LED. When it emits light, a photo sensitive element like photosensitive diode, photo-sensitive transistor or a photo-sensitive thyristor will sense that light and operates the load current through trigger circuit. It is obvious that the input and output circuit are completely isolated from each other since only light is used between them. Therefore sometimes this SSR is also called as “optically coupled” or “photo-isolated” SSR.

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