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EDIT: DIAC part numnber is DB3TG

I'm trying to understand how a dimming circuit works, specifically TRIAC control. The TRIAC (D1) in this circuit is a BT136S-6. Where does the Gate Trigger Current come into play? I understand that the combination of R1/R3/C2 control the time that C2 is charged up and therefore hits the threshold of the DIAC (D2). Is the TRIAC just consuming full power at trigger point? Is that bad news? Should the gate current be limited by a resistor?

enter image description here

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  • \$\begingroup\$ R1 & R3 are resistors ... they'll limit current ... \$\endgroup\$
    – brhans
    Dec 15 '20 at 20:24
  • \$\begingroup\$ Phase- fired controller name of method. Wiki has it. \$\endgroup\$
    – user263983
    Dec 15 '20 at 23:33
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    \$\begingroup\$ Your triac symbol is upside-down. The MT1 and MT2 designations are correct for the circuit. \$\endgroup\$ Dec 16 '20 at 3:19
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The Diac is a highly non-linear device. Below the diac breakover voltage, minimal current flows. When the breakover is reached, the diac conducts strongly and discharges C2 through the Triac gate.

The Triac gate current should easily exceed the minimum trigger current. How high the current will be is not as easy to calculate. Some diac specifications will give a maximum current under certain conditions. If you tell us your diac part number, we may be able calculate the maximum current.

A power triac can handle a fairly large gate current momentarily. A series resistor to limit the current is usually not needed.

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  • \$\begingroup\$ This is what I suspected, thanks for your answer. I added the DIAC part number to my question. The peak output is 2A which is more than enough to trigger the TRIAC as I understand it. \$\endgroup\$
    – Craigfoo
    Dec 18 '20 at 16:52
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In circuits, like this, using usually diac with 6V breakout voltage. R1, R3 charging C2. When voltage on C2 exceeds diac breakout voltage, C2 discharge to triac gate, triac opened. Varying R3, changing delay from zero crossing to triac opening moment, changing effective voltage.

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