zero crossing triac drive explained?

Discussion in 'General Electronics Chat' started by ssutton, Sep 16, 2011.

  1. ssutton

    Thread Starter New Member

    Sep 12, 2011
    I need to construct a circuit that will drive a resistive load (< 8 amps) with a triac operating at 120VAC 60 HZ. I am planning to use a MOC3031M to drive the triac along with a PIC. I have never used a triac in a circuit nor a zero crossing detector. My question is; What is the control scheme, in other words will the pic turn on the triac at the beginning of the zero crossing and then turn it off during some point in the cycle to limit total power? What about RFI issues with this setup?

    Thank you,
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Triacs and thyristors can´t open where you want them to, they open when current is 0.
    The zero crossing controllers baffled me too for a while, until I realized they are only suitable for regulating in multiples of the half-period, not for small parts of the half-period.
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Rather than turning the triac on at the zero crossing one would normally delay the turn-on time by using phase control to trigger the triac. This is normally how AC load power is controlled with a triac. Yes you will get some RFI developed. Alternatively it is possible to use integral cycle control which has potentially fewer issues with RFI.
  4. eblc1388

    Senior Member

    Nov 28, 2008
    Zero-crossing Optical triac simplifies the control scheme for the user.

    You don't need to worry when exactly in the AC cycle to turn on the optical triac, just enable the signal, the optical triac will wait until it is the right moment to turn ON the power TRIAC. Therefore it is possible you will have max. half cycle delay in turning on the load.

    You can also turn off the optical triac signal at any time. The main power TRIAC will keep on conducting until its current crosses zero.

    The whole point is to guarantee that complete half period of the AC waveform is being switched, to avoid unnecessary RFI issues.