Solid State Relay Requires Only 50uA Drive Current
Most solid state relays require at least 5ma and often more
input control current, to fully turn on the device. But such current levels often force battery
powered circuits to use excessively large batteries. The relay hobby circuit shown below demands only
50uA of input current. This about 100
times lower than that needed by a typical optically isolated solid state
relays.
The circuit uses a combination
of a high current triac and a very sensitive low current SCR, to control up to
600 watts of power to a load, while providing full isolation and transient
protection. 
At the heart of the circuit is a Darlington type
opto-isolator A1 from NEC. This device
needs only 50uA of current through the LED section to activate the Darlington
side. A bridge rectifier and a couple
capacitors, strips off a bit of current from the 120vac line, through the
load. A zener diode limits the generated
DC voltage to 8v. When the opto-isolator
is turned on, current is routed to the gate of a sensitive SCR. When turned on, the SCR routes current pulses
to the main control triac, through a bridge rectifier. A 15v zener delays the trigger point of the
triac slightly, so a minimum 30 volts peak to peak is always available to
maintain current to the SCR circuit.