What limits the TRIAC gate current here?

Continuation from last question. It is not clear to me what limits the gate current in this dimmer circuit once TRIAC is on. Let see exactly what happens. At first TRIAC is off and current must go through R1 and VR1, once the cap is charged DIAC fires TRIAC gate and it's on. You can see below now there is a low resistance path for gate current only limited by R2, since this is 1kW dimmer we can assume R2 is small resistance. It seems there is nothing here to limit the gate current while TRIAC is on.


Only thing that makes sense is that cap is small and thus limits the gate current according to i = C dv/dt

Update 2:

C2 and R2 and for 1kw output so they are both low resistance. Answer is since current is first forced to flow through VR1 and charge the cap, it continues to flow that way also when TRIAC turns on.

Update 3:

How is it wrong, do you realize this is AC circuit.

Update 4:

qrk why don't you draw the correct gate current path for both half cycles. Let's discuss it.

Update 5:

Well, it is wrong, i already said it above.

Update 6:

Key point is that TRIAC is off at first, if there was just wire in place of TRIAC "gate current" would flow in opposite direction bypassing high resistance of R1 and VR1.

Attachment image

2 Answers

  • 1 month ago
    Favorite Answer

    you have C2, R2 and VR1,  and the diac all in series with the gate, and all will limit the current.

  • qrk
    Lv 7
    1 month ago

    The gate current is limited by R1 + VR1 and to some extent by the diac. Your current path is drawn wrong for the gate current.

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