Triac trigger circuit calculation

Thread Starter


Joined Jul 2, 2005
I would like to design a phase angle control using triac. The controlled
voltage will be applied to a heating element. The triac will be triggered
by breakover of a diac. The diac will get triggered by means of the energy
stored in the capacitor of the RC timing circuit. I am searching for the
formulae that will guide in selecting the appropriate components.



Joined Oct 14, 2005
The easiest way to calculate the timing is first to make sure that the voltage across the capacitor will exceed the DIAC breakdown voltage (~30V) during the time span available for the TRIAC to fire.


If you choose high R and large C so that the voltage across the C would only be above 30V for a short time span right after (due to the phase shift) the main supply has passed its peak voltage then you would only get less than a quarter of sinusoidal period available. If you choose small R and small C so that almost all the voltage is dropped across the C then you would get maximum half sinusoidal period available.

Second, then add the phase shift to the starting time of the span time that has been calculated above to get the total delay from the zero crossing.

θc = tan^-1(-ωRC)

With the above method you can calculate the minimum and maximum trigger delay for various C and R combinations. By increasing the phase shift you can go longer than a quarter of period delay while still able to adjust the R for minimum delay.