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#188131 - 07/22/09 03:24 PM How to test a dimmer switch with Ohm tester
bwise121 Offline
Member

Registered: 09/11/03
Posts: 113
Loc: Sacramento, CA USA
If I have a customer bring in a dimmer switch he thinks is not working, how can I use a multi-meter to test it without hooking it to a 120 volt source? This is a normal incandescent 600 watt dimmer.


Thanks,
Byron

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#188132 - 07/22/09 03:37 PM Re: How to test a dimmer switch with Ohm tester [Re: bwise121]
Paulusgnome Offline
Member

Registered: 06/15/04
Posts: 56
Loc: Christchurch, New Zealand
I don't think that there are many meaningful tests that you can perform on a dimmer with a multimeter.
I would hook it up to a lamp and supply on the workbench and see how it behaved.
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Mark aka Paulus

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#188133 - 07/22/09 04:42 PM Re: How to test a dimmer switch with Ohm tester [Re: Paulusgnome]
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
You can test if it is shorted out with a DMM but that's about it. You can test some individual components if you tear it apart but hooking it up to a lamp would be the sane way to learn if it works.
Joe

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#188134 - 07/22/09 04:48 PM Re: How to test a dimmer switch with Ohm tester [Re: Paulusgnome]
Alan Belson Offline
Member

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
You can't test a triac circuit dimmer with an ohmeter. It's not a rheostat, it works by chopping the a.c waveform.

I don't think a digital multimeter will give any valid results when the dimmer is powered up either, you need a scope to check the waveforms under load. Bench test it as Paul says. If it's u/s, get the customer to buy a new one.
If it works ok on the bench, he's got wiring problems.
_________________________
Wood work but can't!

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#188142 - 07/22/09 08:07 PM Re: How to test a dimmer switch with Ohm tester [Re: Alan Belson]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
The best way to test a dimmer is to swap it out with another dimmer. If that one works, the other one is bad.
_________________________
Greg Fretwell

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#188146 - 07/22/09 11:44 PM Re: How to test a dimmer switch with Ohm tester [Re: gfretwell]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1296
Loc: Alaska
Ocilliscope but you are just better off pig tailing a light and a cord and just bench test it.
_________________________
"Live Awesome!" - Kevin Carosa

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#188150 - 07/23/09 05:04 AM Re: How to test a dimmer switch with Ohm tester [Re: sparkyinak]
aussie240 Offline
Member

Registered: 07/06/05
Posts: 222
Loc: Blue Mountains, NSW, Australia
The battery in an ordinary ohmmeter has insufficient voltage to overcome the breakdown voltage of the Diac (about 30V) so it won't show anything. Unless of course the Triac is shorted (usual fault), in which case the light cannot be dimmed. As others have said, a real world test jig is the best way.

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#188155 - 07/23/09 11:07 AM Re: How to test a dimmer switch with Ohm tester [Re: aussie240]
mikesh Offline
Member

Registered: 06/07/06
Posts: 614
Loc: Victoria, BC, Canada
Bench test using a big light bulb and an rms voltmeter. You should see a smooth increase or decrease in the voltage across the bulb. If you need a more detailed measurement you need a scope.

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#188156 - 07/23/09 11:15 AM Re: How to test a dimmer switch with Ohm tester [Re: mikesh]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
I think you are overthinking this. If the bulb lights and dims, it works.

I have scoped a dimmer but it was for no practical purpose, just "gee whiz" info

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Greg Fretwell

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#188166 - 07/23/09 05:42 PM Re: How to test a dimmer switch with Ohm tester [Re: gfretwell]
KJay Offline
Member

Registered: 11/27/07
Posts: 763
Loc: MA, USA
The most common problem I see with dimmer switches is that they are blown and won't pass voltage.
I'll do basic "continuity" checks of suspected blown single-pole and 3-way electronic dimmer switches, like a Lutron Ariadne, between screw terminals, with a Fluke 87 meter, which has 40-MegOhm impedance.
This doesn't really tell you much of anything other than that the switch function of the dimmer is working, or if the dimmer is not allowing voltage through.

If the dimmer switch is in the OFF position or blown, the display will always read "OL".
If it's in the ON position the display will bounce unsteadily at around 23 Ohms between terminals.
I'm only guessing that the display bounce has something to do with the Triac firing due to the DC voltage, but I don't do this test any longer than necessary.

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