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Charles Hansen 29th August 2005 04:56 AM

How to measure a Triac with a multimeter?
Hello -

I have a bad light dimmer and suspect that the problem is with the triac. I have never used a triac before. Is there a way to measure them with a multimeter to determine if they are bad? I also have a curve tracer if that will help.

The triac in question is the BTB04-600SL from ST. The data sheet can be found at:

Any insights will be appreciated.


wes-ninja250 29th August 2005 07:04 PM

See Here:

Is this a PWM dimmer? If so, it should be easy to test with a 'scope if you have access to one. If it's not, I can't imagine what it's using a triac for. :)

If it's a PWM dimmer, I imagine that you will find that there is a varying-width pulse being applied to G, which allows AC to conduct through A1 and A2.

Have you done any in-circuit testing? Have you tried triggering the gate yourself, with some method of feeding it around 20mA of current? Chances are pretty good that if the gate will trigger at low frequency, then it will also trigger at high frequency. That's not a common failure mode.

Oh, you can probably sub in whatever beefy SCR or TRIAC you have lying around (PC SMPSes are a good place to look) for testing purposes. Just test with a 40W or less light bulb. With an SCR, the bulb will be dimmer, since you'll only be feeding it half the AC cycle.

And finally -- I hope you are using incandescent bulbs with your dimmer. Most compact fluorescents will not work with a PWM dimmer.


mzzj 29th August 2005 07:34 PM

using ordinary multimeter A1-A2 should show high or infinite resitance in both ways, G-A1 low ohms, less than 100r

Charles Hansen 1st September 2005 02:25 AM

Thanks for your help guys -- this was exactly what I needed!

Karl vd Berg 19th April 2014 04:30 AM

Hi there!

Is there a way to test the TRIAC with the DMM in diode mode? If yes, what values for it?

My DMM is autorange, and in resistance mode it changes values all the time!

Thank you!

P.S.: should the TRIAC be measured off the board?

Max Headroom 19th April 2014 05:36 AM


Originally Posted by Charles Hansen (Post 712535)
Hello -I have a bad light dimmer and suspect that the problem is with the triac. I have never used a triac before. Is there a way to measure them with a multimeter to determine if they are bad?

Hi Charles.
For the price of a BTB04-600SL, I would just plain replace the triac....or the whole dimmer assembly.
If the former, while you are at it, clean the board, replace any electros and be suspect of solid caps too, blanket resolder, clean again, conformal coating of some sort....the control pots go noisy also.

Triacs can fail fully open, fully closed, conducting one half cycle only, and become erratically noisy.
What is the nature of this fault of your dimmer ?.


Semiconductor relays TRIAC failure modes
In some cases the triac in your circuit can be damaged typically because of overcurrent, overvoltage or too much heating. Triac failure modes can be either "blown open" or "blown short".

If the Triac blows open, all connections are opened up as the semiconductor material has failed catastrophically and the explosive force inside the device clears disperses the internal wiring.
If the Triac blows short, the shorts can be to any combination of the terminals. MT1 to MT2 (gate open), MT1 to gate (MT2 open), MT2 to gate (MT1 open) or MT1 and MT2 to gate.
There are normally no half-way houses in the failure modes. They go either fully short or fully open. The most common failure mode seems to be a short circuit.

Unsolved TRIAC failures Re: TRIAC failures
I have seen this failure many times it happens when a overvoltage situation has occurred (usually lightning). It fails on one side or the other because the strike is polar one side of the TRIAC conducts without damage however the other is destroyed in the process. You can fix this by adding a bi-directional transorb from the high voltage side (not the load) to the gate. Choose your tranzorb so that it only turns on when higher than normal voltage is present. When the transorb turns on it will activate the gate and protect the TRIAC. The load in this case your christmas lights will get hit with the lightning. They may or may not survive but your controller will be ok. Incandescent lights will probally survive but leds will surely die.

davidsrsb 19th April 2014 10:26 AM

There should be a diode between Gate and MT1
MT2 should be open circuit to the other two terminals

Usually triacs fail short

Mooly 19th April 2014 11:38 AM

Triacs are strange devices to measure... they shouldn't be but...... :D

Typically, an SCR reads "a bit lower" to "a lot lower" than a normal diode. If a diode reads ".600" on your DVM then expect somthing nearer .400 or lower, even down to 0.050

I have a new BTA16 in front of me.

Gate to Anode 1 should conduct with the leads either way around.
Gate to Anode 2 should not conduct with leads eiher way around.
Anode 1 to Anode 2 should not conduct with leads either way around.

And yes, unless you know the circuitry around it and can use judgement then always test out of circuit.

I have some old T066 triacs that were used in Grundig TV's in a commutating line output stage. They read as low as 0.160

Karl vd Berg 19th April 2014 12:59 PM

Hi, Mooly,

It's a BTB16-700BW... and I have .041 on diode mode.

Gate to Anode 2 is open (not conducting) and same to Anode 1 to Anode 2 (also not conducting)...

.041 the is an acceptable value and the TRIAC seems OK, right?

Karl vd Berg 19th April 2014 01:19 PM

Just did some Resistance readings as well:

MT1 to G is 72 ohms
MT1 to MT2 is 13 Mohms
MT2 to G is 11.5 Mohms

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