I put this circuit together to demonstrate latching properties of SCRs but I'm finding that the SCR doesn't behave as I expected it to from the datasheet values.
Specifically, I found that in the circuit as shown it will light when the switch is opened but not latch (i.e. it turns off as soon as the switch is closed again.) If I swap the LED for a small lamp it works fine. After some experimenting I found that it takes about 25 mA to latch.
So the disconnect for me is that according to the datasheet typical latching current is .2 mA and maximum is 5 mA. I know that the test conditions for those numbers are Vak = 12 V, Ig = 20 mA. I think for me Vak is ~ 7V (including LED Vf) so I guess that could explain it, but unfortunately there are no relevant graphs to refer to for more information. (Also I have tried with much higher gate currents with no difference.) Am I reading the datasheet wrong? Any other insights?
I added a 1k resistor between the switch and ground and now the circuit works as expected. So new question: why is the pulldown resistor required to make this function? Does it have something to do with jitter on the switch?