What is a Solid State Relay?

Solid State Relays(SSR) a relay with Isolated input and output whose functions are achieved by means of electronic components without the use of moving parts- NARM(USA) It is a static Switch.

How are they different from the conventional electromagnetic relays?

Their construction is different. No coils , armature and metal contacts. Functionally both do the switching function but are noiseless, arcless switches.

Are they expensive compared to the conventional relays?

Initial cost is high, but in the long run, it works out economical due to long life.

High cost by how much?

May be 3-4 times for the lower currents and 2 times for higher currents (this comparision is with standard makes).

Then why should one use these relays?

SSR's are highly reliable, consumes less power, switching can be controlled electronically,can be operated in harsh environment.

Are they available in 'NC' configuration?

Yes they are avaible in 'NC' configuration.

Can we get multiple contacts like EMR's?

No. Technical limitaions inhibit this configuration.

Can we get series and parallel SSR's?

Series and Parallel connections are possible, but is not recommended by ERI as the characteristics of the semiconductors vary from one device to other.Higher voltage can be achieved by connecting the o/p two SSR's in series.

Can we get o/p from the SSR?

Good Question! Normally this is a misunderstanding.The relay cannor give any o/p on it's own. It is only a switch , which can operate using the external power supply.

What loads can be switched using the relays?

You can switch Resistive, Inductive and Capacitive loads.

What are the examples in each category?

Resistive loads: Lamps, Heaters, resistors etc., Inductive loads: Motors, Solenoid coils, transformers, magnetic clutch etc., Capacitive loads: Capacitors, Power capacitors etc.

What are the "must do" conditions while connecting a SSR?

Make sure to use an external heat sink for load current more than 3 Amps. Apply heat sink compound to he base of the SSR before mounting it on the heatsink.