# RC Circuit with A.C. applied

#### nally85

Joined Jan 7, 2010
3
I have to build an RC circuit that will provide a break over voltage for a diac to trigger a triac (Phase control circuit), but stuggling to get the RC circuit theory in my head when applying A.C. (circuit Provided below)

All the websites and textbooks disscuss this with DC, not AC, can i use the same equations to find R and C?

I need to find values of R (variable) and C that will provide 27v to the diac across the 0 to 180° of the sinewave?

I understand the practical side of the circuit and how it operates, but cannot design it as my maths is very poor and i struggle to minipulate equations to find and answer.

Is this true, if i have a AC supply at 50hz, so 50 cycles per second. If i devide the 1 sec by 50 i will get the time it takes for one cycle:

1 / 50 = 0.02sec

And as a cycle is over 360°, devide 0.02 sec by 360° to get how many seconds per 1°.

002 * 360° = 0.000055 sec/1°

then mulitiply this by whatever degree i want it to fire:

ie. 50° firing = 0.000055 * 50 = 2.78x10^-3 sec (is this correct)

would this be the time i require the capacitor to charge up to 27v to fire the circuit?

I found the equation Vc = Vs(1-e^-t/RC), can i use this in some form to find R and C, id so how can it be used as i dont have both R or C and would t be the time i found above?

Many thanks and kind regards

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