Sparkfun's EL Escudo Dos breakout board uses this circuit for an EL wire driver channel, with R2 = R3 = 330.
I was wondering what the function of R2 and R3 is? Are they required? What would happen if they were simply replaced with a short?

EDIT: additional info
The HV lines are 110VAC and the channel will be driving EL tape (I think resistive and capacitive load?)

enter image description here

  • 2
    \$\begingroup\$ You're way too fast with accepting. Questions with an accepted answer get less new answers, which otherwise might be interesting too. I suggest to wait for a day or so, so that the question has time to go around the world. (I'm awake, but the rest of Europe isn't.) Or at least until there are a couple of other answers. \$\endgroup\$
    – stevenvh
    Commented Aug 13, 2012 at 0:40
  • \$\begingroup\$ @stevenvh Hmm, good point. Will keep in mind. \$\endgroup\$
    – Shubham
    Commented Jan 9, 2013 at 23:02

2 Answers 2


Current limiting. The MOC3043 has a photodiode, and just like any diode can fry if not current limited.

  • \$\begingroup\$ I understand that, but I was asking about R2 and R3. \$\endgroup\$
    – Shubham
    Commented Aug 12, 2012 at 23:19
  • \$\begingroup\$ So was I. R1 is for the LED, R2 and R3 are for the photodiode. \$\endgroup\$
    – mordac
    Commented Aug 12, 2012 at 23:22

It's there to limit the peak current through the driver triac.
In the datasheet for the MOC3043 this is given as 1A. So for a 115V RMS line with a pk-pk of ~170V, a resistor of around 180 ohms is suitable. The 330 ohms given is more suited to a 240V line, but if it works okay for your application then fine - the resistor value also sets the turnon angle, and too high a value may prevent the power triac getting enough gate current to turn on fully.

This App Note has some good info on (optocoupled) triac driving.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.