I have build a circuit with the purpose of driving a ~1000VA AC motor. The motor controller fires a triac only at zero crossings. The circuit is as follows:


simulate this circuit – Schematic created using CircuitLab

Both C2 and C1 are ceramic capacitors. R4 is a 5W resistor, the rest is regular 1/8W. The BTA208S-800E has a proper heatsink mounted directly to it.

The problem is this: after a few seconds of operation the circuit gets really hot and a little later starts to malfunction (random firing, loss of speed control).

The motor is from a old vacuum cleaner and therefore I have no datasheet for it (Google didn't yield any results either).

The circuit is one found on the internet, since I do not have the required knowlegde to calculate RC snubbers.

  • 1
    \$\begingroup\$ What's the question? \$\endgroup\$
    – Phil Frost
    Jul 3, 2014 at 10:56
  • \$\begingroup\$ does the problem occur with a lower load? \$\endgroup\$
    – miceuz
    Jul 3, 2014 at 11:14

2 Answers 2


The BT208S is a smallish SMT triac- you will be dissipating 5-6W (assuming European mains) with a 1000VA motor. That's a lot of power.. Probably your heat sink is woefully inadequate for the power dissipation and the triac is overheating.

If just mounted on FR4 board the Rth-ja is 75K/W so 5W would heat it to about 400C -- so maybe your heatsink is better than that, but nowhere good enough.

Edit: For that amount of power, to keep the junction temperature sensible, you either need a BIG heatsink coupled thermally to the MT2 of the triac (might require an aluminum board) or a fairly large heatsink with a lot of air flowing over it.

If you just want to turn the motor on and off, forget about the triac and use a power relay.

  • \$\begingroup\$ because of the desired footprint size of the PCB, I would like to keep SMT components. Also, the TRIAC is rated up to 8 amps, so there must be a way to use this TRIAC at 4-5 amps right? If a heatsink isn't the solution, what is? \$\endgroup\$
    – Roel
    Jul 3, 2014 at 11:21
  • \$\begingroup\$ @Roel See my edit above. \$\endgroup\$ Jul 3, 2014 at 11:29

R3 = 2.7k, R2 = 220R, C1 = 100n and connects to NEUTRAL, NOT gate. Works fine for me.

  • \$\begingroup\$ This sounds like it might be a worthwhile answer, but can you expand on it a bit to explain your reasoning? \$\endgroup\$
    – Dave Tweed
    Sep 13, 2014 at 21:14

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