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Old 29th August 2005, 04:56 AM   #1
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Join Date: May 2003
Location: Colorado
Default 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.

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Old 29th August 2005, 07:04 PM   #2
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See Here: http://www.google.ca/url?sa=t&ct=res...JI78igGAlYCmCg

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.

Do daemons dream of electric sleep()?
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Old 29th August 2005, 07:34 PM   #3
mzzj is offline mzzj  Finland
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using ordinary multimeter A1-A2 should show high or infinite resitance in both ways, G-A1 low ohms, less than 100r
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Old 1st September 2005, 02:25 AM   #4
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Thanks for your help guys -- this was exactly what I needed!
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