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How to test a dimmer switch with Ohm tester #188131 07/22/09 05:24 PM
Joined: Sep 2003
Posts: 114
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bwise121 Offline OP
Member
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

Work Gear for Electricians and the Trades
Re: How to test a dimmer switch with Ohm tester [Re: bwise121] #188132 07/22/09 05:37 PM
Joined: Jun 2004
Posts: 56
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Paulusgnome Offline
Member
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.


Mark aka Paulus
Re: How to test a dimmer switch with Ohm tester [Re: Paulusgnome] #188133 07/22/09 06:42 PM
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Posts: 823
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JoeTestingEngr Offline
Member
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

Re: How to test a dimmer switch with Ohm tester [Re: Paulusgnome] #188134 07/22/09 06:48 PM
Joined: Mar 2005
Posts: 1,803
Alan Belson Offline
Member
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!
Re: How to test a dimmer switch with Ohm tester [Re: Alan Belson] #188142 07/22/09 10:07 PM
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gfretwell Offline
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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
Re: How to test a dimmer switch with Ohm tester [Re: gfretwell] #188146 07/23/09 01:44 AM
Joined: Jul 2007
Posts: 1,335
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sparkyinak Offline
Member
Ocilliscope but you are just better off pig tailing a light and a cord and just bench test it.


"Live Awesome!" - Kevin Carosa
Re: How to test a dimmer switch with Ohm tester [Re: sparkyinak] #188150 07/23/09 07:04 AM
Joined: Jul 2005
Posts: 223
A
aussie240 Offline
Member
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.

Re: How to test a dimmer switch with Ohm tester [Re: aussie240] #188155 07/23/09 01:07 PM
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Posts: 613
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mikesh Offline
Member
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.

Re: How to test a dimmer switch with Ohm tester [Re: mikesh] #188156 07/23/09 01:15 PM
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gfretwell Offline
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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

[Linked Image from gfretwell.com]


Greg Fretwell
Re: How to test a dimmer switch with Ohm tester [Re: gfretwell] #188166 07/23/09 07:42 PM
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Posts: 763
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KJay Offline
Member
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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