Solid State Relays

Solid state relays are the best alternative to electromagnetic relays for switching high-voltage AC power lines 220 and 380 Volts without mechanical contacts, based on semiconductors (diodes, transistors, thyristors or triacs).

Since semiconductor relays do not produce sparks when switched and generate significantly less electromagnetic noise, they are widely used in control cabinets for electric motors, where they close the power line to the motor based on signals from the controller or regulator. The control signal is low-voltage.

Principle of operation of solid state relays

Solid state relays can be divided into three types, which operate differently:

• asynchronous, with instant switching
• synchronous, with zero-crossing switching
• with phase control, increasing power proportionate to the signal

Solid state relay with asynchronous switching is the simplest in operation — as soon as the PLC or regulator sends a signal to turn on, the relay instantly closes the load circuit and completely passes the current. It is most often used for electric motors.

Solid state relay with zero-crossing control switches after the control signal at the moment when the AC sine wave crosses the +/- axis. This type of relay is perfect for resistive loads and heating elements.

Solid state relay with analogue control is used for precise power control since they can adjust load current proportionally to the control signal 4-20 mA, 0-5 or 0-10 Volts. There are subtypes, among which triac phase control relays for electric motors are common.