Electronic – Optoisolator with EL wire won’t turn off, TRIAC works fine. Why


I'm pretty new to electronics, and I'm making a circuit to control EL wire with a DC logic control signal. Specifically, I would like to blink the EL wire with the output of a 555 timer.

My first pass at this was with an optoisolator. MOC3031 data sheet

The circuit looked like this:


(C1 is a stand-in for the EL wire, which I'm told is a capacitive element, and the 555 outputs 5V)

The good news is that if I turn the EL wire inverter on when the 555 outputs 0V, the EL wire will be off until the 555 jumps back to 5V. The bad news is that the EL wire never shuts back off, even when the 555 outputs 0V again (I've confirmed that the 555 circuit works w/ a multimeter).

I ripped out the optoisolator and replaced it with a triac, so now it looks like:

enter image description here

And everything works perfectly. I'm happy in the sense that I have a working circuit, but I'd like to know what changed. Can anyone explain why?


I want to switch some of the EL elements on and off while the inverter remains switched on and drives other EL elements continuously. The frequency is likely higher than 50-60 Hz and the voltage is probably not a clean sine wave.

Best Answer

Something is being missed here. The optotriac is quite capable of switching that load current, however the triac in the optoisolator is handling the inverter's high frequency AC differently from your 'triac'. It's designed to control mains frequency AC - 50 or 60Hz, and is too slow to turn off at the very brief zero crossings when it is fed with ~1kHz AC current

Edit: there is a good app note from Vishay here which makes it a bit more clear. You cannot just use the static dv/dt spec - that is what prevents the opto from turning on when it is off, rather you need the (lower) commutating dv/dt, which may not be given.

In fact, perhaps it's not actually a 'triac' but an alternistor that you're using, which would tend to perform better because the semiconductor structure is different.

The answer in that case is to use a more appropriate switching element, such as a OptoMOS SSR.

enter image description here