solid state relais (SSR)

Solid state relays (SSR) are the electronic equivalent of mechanical relays. They are electronic circuits made of semiconductor components, of transistors, MOSFETs, thyristors, diacs and triacs, with which a relay function is realized. They are logic circuits for switching voltages, currents and frequencies.

Compared to mechanically operating relays, solid-state relays have no moving parts and are therefore virtually unlimited maintenance and wear-free, offering high system reliability. Since they have no spring contacts, there is therefore no need for additional electronic circuitry to dampen switch bounce. Solid state relays (SSR) are smaller in design than mechanically operated ones, and they can be SMD devices or designed for through-hole technology( THT) and mounted directly on printed circuit boards.

Solid state relay (SSR) for through-hole technology, photo: rs-online.com

In solid state relays, the actual switching element is a switching transistor, triac or MOSFET controlled by a photoresistor or transistor. To ensure the galvanic separation of control voltage and switching voltage, the photosensitive component is controlled by a light-emitting diode. As soon as the light-emitting diode is illuminated by the control voltage, the switching element switches the switching voltage to the load.

Optocoupler in semiconductor relay

The light-emitting diode can be controlled in such a way that AC voltages can be switched in phase, for example in zero crossing. This avoids disturbing switching peaks. A circuit with zero-crossing detector has the further advantage that no voltage drops at the internal resistance, however low it may be, and thus no heat loss occurs. The contact resistance of SSR relays is in the milli-ohm range, but higher than that of electromechanical relays. Their switching frequencies can be 100 kHz and above.