Ultra Low Control Current Optoisolated Power Relay
(September 23, 2017)
designed by David A. Johnson, P.E.
Solid state relays have been used for many decades as a way to control AC power to various loads. A typical relay demands about 20ma to 50ma of DC current to activate the relay. Also, a solid state relay does dissipate some power and typically has to be mounted to heat sink when the AC current exceeds 4 amps. But, there are times, especially with battery powered control devices, when even 20ma is too much current to keep a relay active for a long period of time. The circuit below uses a mechanical latching relay and a custom opto-isolator to perform the AC switching operation. The real advantage of the circuit is that it can switch power on and off up to 10 amps of AC current while drawing only 10ua of current. In fact, I have tested the circuit using just 5ua of control current.