Solid State Relay Primer

Introduction

Solid state relays (SSRs) are devices that can be used to control the power being supplied to other devices. They are turned on by applying a DC signal to the input contacts.

Functionally, they are the same as Mechanical Relays, but have the following advantages:

• SSRs produce less electromagnetic interference during operation, as opposed to mechanical relays whose internal contacts tend to spark when switching.
• While the switch contacts of a mechanical relay will eventually wear down from sparking, an SSR will have a longer life because its internals are purely digital.
• SSRs exhibit faster response times than mechanical relays (≈1ms compared to ≈10ms).
• SSRs are less susceptible to electric shock and physical vibrations than mechanical relays.

Of course, since SSRs are more complex, they are more expensive to produce and will dissipate more energy (1-2% of the energy intended to power the load).

Principle of Operation

The two inputs are connected internally to an optocoupler. An optocoupler is a circuit component that uses an LED and a light sensitive element to link two parts of a circuit without directly connecting them electrically. The light sensor is connected to the transistors which open and close the path that supplies the relay's load with power.

Some SSRs are designed to switch AC current, some switch DC current, and others can switch both. Make sure you check your SSR's data sheet to ensure that it can switch the type of current your load is using.

Noise Protection

AC relays will typically have built-in noise protection in the form of a "snubber". A snubber is a simple RC circuit that will absorb small transients and will protect against unintentional turn-on.

For large transients and highly inductive loads, additional protection is necessary. A simple way to do this is to add a Metal Oxide Varistor (MOV) across the load terminals.