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


Work Gear for Electricians and the Trades

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


Mark aka Paulus

#188133 - 07/22/09 06:42 PM Re: How to test a dimmer switch with Ohm tester [Re: Paulusgnome]  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 791
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


#188134 - 07/22/09 06:48 PM Re: How to test a dimmer switch with Ohm tester [Re: Paulusgnome]  
Alan Belson  Offline
Member
Joined: Mar 2005
Posts: 1,803
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!

#188142 - 07/22/09 10:07 PM Re: How to test a dimmer switch with Ohm tester [Re: Alan Belson]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,142
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

#188146 - 07/23/09 01:44 AM Re: How to test a dimmer switch with Ohm tester [Re: gfretwell]  
sparkyinak  Offline
Member
Joined: Jul 2007
Posts: 1,328
Alaska
Ocilliscope but you are just better off pig tailing a light and a cord and just bench test it.


"Live Awesome!" - Kevin Carosa

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


#188155 - 07/23/09 01:07 PM Re: How to test a dimmer switch with Ohm tester [Re: aussie240]  
mikesh  Offline
Member
Joined: Jun 2006
Posts: 613
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.


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


Member
Joined: Jul 2004
Posts: 9,142
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

[Linked Image]


Greg Fretwell

#188166 - 07/23/09 07:42 PM Re: How to test a dimmer switch with Ohm tester [Re: gfretwell]  
KJay  Offline
Member
Joined: Nov 2007
Posts: 763
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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