# Thyristor Firing circuits assessment

Discussion in 'Homework Help' started by nally85, Jan 7, 2010.

1. ### nally85 Thread Starter New Member

Jan 7, 2010
3
0
please could someone point me in a good direction, i have to look at thyristor (triac) firing circuits using phase control to vary the power to an AC load (lamp). Basicly a dimmer circuit, i have managed to find lots of info on dimmer circuits and constructed a reasonable working circuit.

But i cannot find a good source regarding how to pick/calculate the best value resistor and capacitor in the RC network used to move the phase angle to trigger my diac and then triac at the various degrees required. Also i am struggling to model it in multisim so any help in that department ould be appreciated.

Many thanks and any help would be appreciated

Apr 5, 2008
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3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
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One way to look at the problem is that you only have so much time for the charge on the capacitor to reach the point where the DIAC fires. Taking longer than about 8 ms maximum is not useful.

4. ### nally85 Thread Starter New Member

Jan 7, 2010
3
0
thanks bertus great info, and beenthere for the info.

am i right to understand that using Vc = V(1-e-T/t) to find the time constant for the capacitor, then use it in t = RC to find a capacitor value if i select a reasonable value for R.

I may be off with this as i am sure this is only for DC?

many thanks for your help and kind regards

5. ### Duane P Wetick Active Member

Apr 23, 2009
408
19
I've used lots of these phase controls over the years, and I have found that a C of 0.33 mf and a R of 0-250k will produce a phased waveform that will control lamps, can-vacs & vibrators satisfactorily up to about 1600 VA.

Cheers, DPW [ Spent years making heaters out of op-amps.]