Testing controller outputs MOSFETs & TRIACs

Sometimes, you'll get a channel on your controller that is stuck on. This is often caused by a blown TRIAC (for AC controllers) or MOSFET (for DC controllers).

The most common cause of TRIAC / MOSFET failure is a short circuit in the wiring for that channel. The short circuit causes a current flow much greater than the controller output is designed for.

Items Required


  • A multimeter - an essential piece of test equipment. I recommend an auto ranging meter. This removes the uncertainty and confusion of selecting the correct range. You will still need to select Volts (AC or DC), resistance (Ohms), etc though. One example of an inexpensive auto ranging meter is this one from Jaycar. Do not bother with the cheap nasty $20 types. Buy one with a "CAT III" marking as this indicates that it is suitable for 240Vac use.

DC Controllers


To test for a shorted MOSFET, you need to disconnect the power input as well as the channel you want to test. Select the resistance (Ohms) range on your meter. Touch one probe to the suspect controller output terminal and the other to the negative power supply input terminal. You should get no reading. Most meters will simply display a "1" to the left as indication of an infinite (very high) resistance.

If you get a reading of 0.5 Ohms for example, that indicates that the MOSFET is shorted and must be replaced. The exact reading will depend on the meter, the meter leads and the MOSFET. For this resistance test it is not important which way around the red and black meter probes go.

AC Controllers


When testing an AC controller, it is especially important to disconnect the power (240Vac) and output channel wiring. Otherwise you may be the one who goes "blinky flashy" big time!

To test for a shorted TRIAC, select the resistance (Ohms) range on your meter. Touch one probe to the suspect controller output terminal and the other to the 240Vac Active input terminal. Like the MOSFET test above, you should get no reading. If you do, the TRIAC is defective and requires replacement.

If you do find a TRIAC that needs replacement, make sure you also check the gate resistor that goes between the opto-coupler (6 pin device - MOC3021 or similar number) and the TRIAC. That resistor is sometimes damaged (goes high in resistance or open circuit) as a result of the TRIAC failing. Occasionally the opto-coupler is collateral damage too, so is worth checking (by substitution) if replacing the damaged TRIAC doesn't restore correct channel operation.

Replacing The TRIAC / MOSFET


MOSFETs and TRIACs come in all shapes and sizes. Usually, the faulty part will be the one right next to the terminal of the damaged channel. Trace the copper track back from the terminal to the blown part to make sure you're replacing the correct one!

After Repair


Once you have replaced the damaged part, make sure that you have also removed the cause of the problem. MOSFETs and TRIACs almost always blow for a reason. It is rare to have one just "go faulty". If you do not rectify the root cause of the failure, you will simple blow another part. The TRIAC gate resistor can also fail as a result of poor soldering (cold/dry joints) on the TRIAC.

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