Umm - but that would also be true when the phototransistor is fried ON at all times, wouldn't it?
-Guess you'll have to provide a continous positive feed to R1, (when the LED will be OFF if the coupler's OK)
and the pushbutton only to pull some current thru the optocoupler's LED (turning the D1 ON).
Having the PB to supply everything will not test for a faulty, continously ON phototransistor.
If there is no voltage at the anode and the cathode, the transistor does not connect the LED to ground and the LED does not work.
The voltage at the anode and cathode opens the transistor and connects to earth, allowing voltage and current to flow to the LED.
A transistor is a switch that is open without activating the base.
(including photo-trannies) they have 2 preferred failure modes:
continously open (where in your test circuit the LED won't turn on when button is pushed), and
continously closed, which state your circuit cannot test for, as you let the button power both the
optocoupler's LED as well as its transistor, simultaneously.
Put another way: Your circuit would (incorrectly) pass a bad optocoupler that that's in this second failure mode.
If you, OTOH, power the phototrannie continously, and merely use the momentary PB to turn the internal LED on/ off,
you'll see both eventual failure modes, (as well as its OFF operation)
I personally use test circuits to check for any possible failure mode, hence my previous suggestions.
But then, though real simple to do if needed, they're merely suggestions.
There are currently 1 users browsing this thread. (0 members and 1 guests)