This circuit is designed
to provide delayed relay switching action at power on. The delay is a
function of the time constant produced by the combination of R1 and C1.
At power on, C1 charges slowly via R1 and the coil of the relay. When
the voltage across C1 exceeds both the base-emitter voltage of Q1 and
the gate trigger voltage of the SCR, gate current flows. This fires the SCR
and switches on the relay. At power off, diode D1 rapidly discharges C1
through the 100O resistor, so ensuring that every time the circuit is
restarted, as in a temporary outage, the delay time is maintained.
Just about any NPN transistor can be used
for Q1, since after SCR1 fires, it is effectively out of the circuit. In
fact, the only part that’s still active after SCR1 turns on is the
relay. You can’t get much simpler than that! This circuit can be used to
delay speaker turn-on, so avoiding the “thump” that occurs in some
stereo systems at power on. A 5-second delay is enough for this
application, requiring approximately 560kO for R1 and 10µF for C1.
Another application might be as a motor protector in a short power
Author: R. Besana – Copyright: Silicon Chip Electronics