I have an air compressor I'm trying to troubleshoot and I'm trying to determine if the problem is with a small circuit board with a triac. The board takes as input an ac wall outlet (US) and outputs to a motor. The triac is a BTA24 600BW (datasheet: http://www.st.com/web/en/resource/technical/document/datasheet/CD00002264.pdf).

I understand how using a triac as a switch works but this triac is used to control motor speed so it is using phase control which I have zero experience with. How do you measure/test the output of a triac using phase control? I've got two multimeters and one shows output of 0 volts while the other shows 110 volts. The one showing voltage is a more expensive meter so I'm not sure if it is just more sensitive or picking up some slight current leakage that the other isn't or if the phase control stuff is throwing the meters off.

Can you use a multimeter to measure the output of a triac which is being used for phase control? If so, how? And any causal explanations of phase control would be appreciated.

Edit: Originally I thought it was a regular transistor and not a triac.

  • 2
    \$\begingroup\$ If you are trying to troubleshoot the circuit, start by reading up on the difference between transistors and triacs. \$\endgroup\$
    – jippie
    Commented Apr 12, 2014 at 6:54
  • \$\begingroup\$ A triac allows flow in both directions (so AC) and also seems to not require power to the gate once it has been opened. That still seems like a switch to me. Can you explain phase control and how to measure it? \$\endgroup\$
    – Chase
    Commented Apr 12, 2014 at 16:36
  • \$\begingroup\$ en.wikipedia.org/wiki/Phase_control The image on that page shows thyristor phase control, but a TRIAC is pretty much a bipolar thyristor (SCR). \$\endgroup\$
    – jippie
    Commented Apr 12, 2014 at 16:54
  • \$\begingroup\$ So the triac has to be reopened when the sine wave hits zero and a dimmer just delays that? Does it really just cut off the front side of the wave like the wikipedia graph shows or is it some other pattern? \$\endgroup\$
    – Chase
    Commented Apr 12, 2014 at 18:23
  • \$\begingroup\$ Simple as that. Do remember that current and voltage will not be in phase when using a non-resistive load. \$\endgroup\$
    – jippie
    Commented Apr 13, 2014 at 8:34

1 Answer 1


A triac (and snubber circuit, if present) will leak a certain amount of current, so it's not realistic to measure the output without a load.

The best way to test the output is with the motor connected as a load. If the voltage measured across the motor is not more than zero, measured on the AC VOLTS range of any multimeter, it's not going to turn. It's rare that a triac fails in the open state, normally they'll fail "on" and you'll get full RPM.

  • \$\begingroup\$ With the motor disconnected (no load) one very cheap multimeter reads zero voltage while the other reads full voltage. I know the cheap one isn't a true RMS meter but that would just throw off the reading, not zero it correct? Is it possible the cheap meter just has a poor enough input impedance that it is actually functioning as a load? \$\endgroup\$
    – Chase
    Commented Apr 12, 2014 at 18:39
  • \$\begingroup\$ Both on AC volts range? If so, I'd question whether the cheap one was working at all. \$\endgroup\$ Commented Apr 12, 2014 at 19:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.