# SCR (thyristor) - Practical Dimmer Circuit

Could someone please explain the main function of the capacitor C3 connected between the gate of the thyristor (through the diac) and the cathode of the thyristor, in this Dimmer circuit schematic

• What is the circuit used in or where did it come from i.e. please add some context. – Andy aka Sep 12 '18 at 9:28

That circuit uses a traic (not a thyristor) triggered via a diac.

C3 charges up via R3 until it reaches about 20 V or so, the diac breakdown voltage. C3 then provides a reservoir of charge to ensure that the triac receives enough of a pulse to turn the device on properly.

Some rough caluclations with R2 at 100k:

• $\tau = R_2C_2 = 100k \times 0.1 \mu = 10 \ \text {ms}$.

• $\tau = R_3C_3 = 15k \times 0.1 \mu = 1.5 \ \text {ms}$.

So C3 is following C2 with a 1.5 ms delay roughly.

The main purpose of the diac is to give a reasonably precise trigger voltage rather than rely on the triac itself which may differ on positive and negative half-cycles.