What are opto-coupler relays?

Optocoupler relays are relays in which the changeover of contacts is effected by the switching on or off of a light source which is linked to a SCR or a TRIAC. The SCR or the TRIAC is switched on or off when the LED is switched unlike conventional relays, where it is done electromagnetically.

These relays are faster than electromagnetic relays. More importantly, the provide isolation between the control and the power circuits.

These relays do not have any moving parts which can deteriorate due to arcing or operational wear. However, they are expensive over conventional relays and hence still find limited application.

Another advantage of the Solid state relay is that it can open an AC circuit when the sinewave crosses zero. This ensures that the back-emf is minimum and this ensures that there are no voltage kicks in the opposite direction when the circuit is open due to inductance. This is because the Triac or the SCR used in the solid state relays conduct current till the waveform reaches the zero point even after the optocoupler LED has been switched off. This prevents premature failure of the contacts.

The Advantages of Optocoupler relays include
  • Smaller Size
  • Faster Response time
  • Noiseless operations as there is no mechanical movement of the contacts.
  • Optocoupler relays can withstand high vibration compared to conventional relays
  • They do not generate Electromagnetic radiation as there is no coils to be energized.

However, they also have some disadvantages.
  • They are more expensive
  • They generate heat and require special heat-sinking fixtures.
  • They cannot switch on very low currents
  • When Solid state relays fail, they fail in the "closed position". In this situation, the machine which is connected will continue to be in the operating condition and there will be difficulty in isolating it. Electromagnetic relays usually fail in the "open" position.