Problem with BTA08400C Triac excessive heat

Discussion in 'The Projects Forum' started by doug08, Feb 25, 2013.

  1. doug08

    Thread Starter Member

    Jan 30, 2011
    For some reason my triac is getting very hot to the touch(Data sheet states 257 degree F max Temp). I have a heat sink attached to the triac. The line voltage input is 124VAC, the load voltage reads 112.5VAC. I have a 10K resistor from MT2 to the gate, and from the gate to neutral is a .1uF 250V. The triac switches on and off perfectly, but too hot. I noticed the load V was 12V lower than the line V, so I tried lowering the resistor value from 10K(which was to allow around 12mA to the gate) to 4.7K which allowed 25 mA to the gate. As a result the load V went up to 116.5V, but the Triac seemed hotter. Gate voltage under a 7 amp resistive load was .94VAC, under the 1.2-1.3V as the data sheet states for all 4 quadrants. Any ideas? Below is the link to the data sheet.
  2. crutschow


    Mar 14, 2008
    The triac will dissipate about 7W average with a 7A load. What is the heatsink?
  3. #12


    Nov 30, 2010
    I suspect that the .1 capacitor is delaying the start of the triac, so that would explain the reduced voltage measurement, as in, "The triac is not dropping 10 volts when it is on".

    If this is true, the only thing left to consider is the heat sink.
  4. DickCappels


    Aug 21, 2008
    Wait...what is your case temperature? These things can reliably run at junction temperatures of 125°C. After taking Crutschow's 7 watts into account, if you are using the TO-220 package, the case can still be over 100 °C -hot enough to boil water. The surface mount packages can be even hotter and still have their junctions under 125°C.

    If you can't get a thermocouple, you can try this test: Unplug the input AC line, put a dab of siliva on the tip of your finger and touch the tab on the triac -if its hot but doesn't "fry spit" then its not too hot.

    By the way, heatsink should be located such that there is sufficient air flow.

    I agree with #12, it looks like the 0.1 uf capacitor is delaying turn-on since dropping the resistor raises the average output voltage. You can also reduce the capacitor value to reduce the delay; perhaps the triac turning on at a lower voltage because of the reduced delay, thereby reducing turn-on losses, but that might not get you as much benefit as using a better heatsink.
  5. crutschow


    Mar 14, 2008
    At power line frequencies, TRIAC switching losses are very low no matter when you turn it on, since it turns on so rapidly. The vast majority of the power dissipation is from the forward conduction loss due to the load current flowing through its ON voltage.