# zero voltage crossing and triac holding current in phase control

Thread Starter

#### denison

Joined Oct 13, 2018
307
I am using a H11L1 zero crossing detector for phase control of 250vac mains. This is very successful. I am using a random phase triac optocoupler.
What happens at zero voltage crossing is that no signal is applied to the emitter side of the opto. Then you apply your delaying signals to the opto for phase control. The triac is held off at zero crossing.
At zero crossing the triacs holding current falls so that it doesn't conduct. So my question is why you would need a zero crossing detector at all? You would think that the non conducting triac at zero crossing would be a suitable zero voltage crossing detector.
Why wouldn't that work?

#### ericgibbs

Joined Jan 29, 2010
16,367
hi den,
If I follow your description correctly, using the Triac to cut off at zero crossing, if you did not sense the crossing by some other circuitry ie: the H11L1, how would you when to apply the phase control signal to the Triac.?
E

#### crutschow

Joined Mar 14, 2008
30,785
A zero crossing circuit is only needed where the TRIAC is controlling multiple AC cycles on and off (such as an on/off circuit or a heater controller with long duty-cycle).
The zero crossing is to turn the TRIAC on at the zero crossing to minimize EMI.
The TRIAC, of course, automatically turns off at the zero crossing.

For single-cycle phase-control, as you are doing, a zero crossing circuit is not needed (or useful).

#### ericgibbs

Joined Jan 29, 2010
16,367
hi Carl,
He does say he is using Phase control, not Burst control I am using a H11L1 zero crossing detector for phase control
If he controlling the triac trigger from say 0 deg thru 180 degree, he needs to know the zero crossing point,so that it can trigger at the correct instant in the half cycle.
E
https://www.ppi-uk.com/news/thyristor-controller-firing-methods/

Thread Starter

#### denison

Joined Oct 13, 2018
307
hi den,
If I follow your description correctly, using the Triac to cut off at zero crossing, if you did not sense the crossing by some other circuitry ie: the H11L1, how would you when to apply the phase control signal to the Triac.?
E
I always assumed that a zero crossing detector would be needed. If the triac turns off at zero crossing there is your starting point for your phase control signal. I was wrong about the circuit working. The attached globe is flickering when trying phase control and only works for full on or full off. The power to the globe has no relation to the signal being applied.
Crutschow says a zero crossing circuit is not needed for single cycle phase control. Anyway this is easily tested. I will let you know. I have found Crutschow to really know his stuff.

#### ericgibbs

Joined Jan 29, 2010
16,367
hi den,
Ref your comment regarding the results you have observed.
ie: globe is OK on 'burst' control and flickers when using 'phase' angle triggering shows that some method of zero phase crossing detection is required, in order to sync the phase angle timing of the triac Gate trigger.

Opto coupler with inbuilt ZCD are available.
E

BTW: you can build your own ZCD circuit.
https://www.google.com/search?client=firefox-b-d&q=mains+zero+crossing+detector+circuit

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