Solid-state Relays (SSR) Features and Operation

A solid-state relay (SSR) is a purely solid-state device used for turning ON and OFF AC loads such as motors.

Features of a Solid-state Relay

Let’s consider the figure below:

Physically, the SSR is packaged in a box with four electrical terminals: two input terminals and two output terminals.

The input or control voltage of the SSR is typically 5 Vdc, 24 Vdc, or 120 Vac. The 5 Vdc models are designed to be driven directly from the TTL digital logic circuits. On the output side of the SSR you can notice the load is placed in series with the 120 Vac or 240 Vac power. The output current can be as high as 50 A in some models.

A large number of SSRs incorporate a feature called zero-voltage switching: The line current is switched ON at the precise time that the AC voltage is crossing the 0 V. This eliminates sharp output voltage-rise times and hence minimizes electromagnetic interference noise (EMI).

Operation of a Solid-state Relay (SSR)

Let’s consider the SSR circuit below:

Figure (b) shows a block diagram of the interior of a solid-state relay. The input voltage drives LED, and the light from the LED turns ON the phototransistor, which in turn, turns on the triac. The LED electrically isolates the input and output sections of the solid-state relay. This is significant as it allows the control electronics to have a separate ground from the powerlines and it prevents high-voltage spikes in the power circuit from working their way back upstream to the more delicate control electronics.

The hybrid solid-state relay is similar to SSR but uses a low-voltage, fast acting reed relay instead of an LED on the output triac. Employing the reed relay provides good electrical isolation and may work better than the SSR in some circumstances.

Related: Optoelectronics

Advantages of Solid-state Relays

• They have no moving parts which means that, they are not prone to wear, this makes them practically immune to shock and vibration.
• Because of the inbuilt electronics, they can be driven with a low-voltage source such as the transistor-transistor logic (TTL) signal regardless of the output-current capability.

Disadvantages of Solid-state Relays

• They can be triggered by electrical noise.
• Even when ON, the output resistance is not exactly 0 ohm, so there is some voltage drop resulting in power loss within the relay, and when OFF they may have alarming levels of leakage current.
• Contact arrangements are limited, so they may not work  for all relay applications.
• Even though, they are long-lasting, unlike electromechanical relays (EMR), they do not fail predictably.