# Measuring the output of a phase control triac

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.

• If you are trying to troubleshoot the circuit, start by reading up on the difference between transistors and triacs. Commented Apr 12, 2014 at 6:54
• 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? Commented Apr 12, 2014 at 16:36
• 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). Commented Apr 12, 2014 at 16:54
• 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? Commented Apr 12, 2014 at 18:23
• Simple as that. Do remember that current and voltage will not be in phase when using a non-resistive load. Commented Apr 13, 2014 at 8:34