Testing a SCR
"This simple SCR tester can be made with only a couple components. It will
provide a visual 'go' or 'no-go' indication--it is simply a voltage source, a
lamp, and a resistor through which gate current is supplied."
This handy tester will provide a visual "on" or "off" switching and latching
indication. When finished, you can test all those possible 'duds' in your
junkbox and dump some of those in the garbage. If the scr is latching and can
hold the latch it is most likely okay.
Look at the circuit diagram, it shows a 3-amp, 50-volt SCR (under test) and a
test circuit. Points "Gate" and "Kathode" are temporary connections, so that
they can easily be opened. I used toggle switches for each, but use whatever
you feel comfortable with, a simple jumper wire would do the trick. I use this
gadget in my shop and so have it mounted in a small case. This circuit can
even be bread-boarded for your purpose.
When "Kathode" is closed, the lamp doesn't light. When "Gate" is also
closed, the lamp lights to its full intensity. The lamp remains lit even if
"Gate" is opened again. But when "Kathode" is opened, even momentarily, the
lamp does not close again when "Kathode" is closed. That illustrates the "ON"
and "OFF" operation of the SCR.