Solid State or Electronic Relays

 Solid State or Electronic Relays

Reed Relay Coupled SSR

Solid State uses solid state components to perform the switching operation with one or more semiconductor switching devices like a power transistor, thyristor and TRIAC without moving any parts. 

Since the control energy required is much lower, compared to the output power to be controlled by this relay, which may result the power gain higher

compared to the electromagnetic relays. 

These relays are of different types

Reed Relay Coupled SSR

Reed Relays consist of a pair of magnetic strips (also called as reed) that is sealed within a glass tube. 

This reed acts as both an armature and a contact blade. 

The magnetic field applied to the coil is

wrapped around this tube that makes

these reeds move so that switching

operation is performed.

Based on the dimensions, relays are differentiated as micro, miniature, subminiature and miniature relays. 

Also, based on the construction, these relays are classified as hermetic, sealed and open-type relays. 

Furthermore, depending on the load operating range, relays are of micro, low, intermediate and high-power types.

Transformer Coupled SSRs

The below figure shows a photo-coupled SSR where the control signal is applied by LED and it is detected by a photo-sensitive semiconductor device. 

The output form this photo detector is used to trigger the gate of TRIAC or SCR that switches the load. 

The electric isolation between the input and output is high due to optical coupling between them as compared to the magnetic one.

Direct Control AC SSR Relays

In this relay, operating load circuit AC power is also given to the control circuit to trigger the TRIAC. 

This is a simple design and less expensive, but it doesn’t provide any isolation between the control and power circuit.

Direct Control DC SSR Relays

As similar to the above one, in this type of relay also the control circuit is energized with the same DC power used in the load circuit. 

This DC power is utilized to bias the transistor so that it is conducted to pass the current to the load. 

This is also less expensive and simplest circuit than the more sophisticated designs, but without isolation between the load and the control circuit is the biggest problem or disadvantages.

Photo-Coupled SSRs

The below figure shows a photo-coupled SSR where the control signal is applied by LED and it is detected by a photo-sensitive semiconductor device. 

The output form this photo detector is used to trigger the gate of TRIAC or SCR that switches the load. 

The electric isolation between the input and output is high due to optical coupling between them as compared to the magnetic one.