Testing 3-way dimmer with a digital voltmeter

Hi,
We are suspecting that our 3-way dimmer by Leviton is defective. We are trying to run connectivity tests with a digital voltmeter, and very high resistance (1.6MOhms). Perhaps the mechanism in these devices is such that it cannot be measured with a cheapo digital voltmeter?
Thanks!
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Jim O'Brien wrote:

I think it likely is solid-state switch/dimmer, not mechanical.
--
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Jim O'Brien wrote:

Hi, Pardon me first, do you know what you are trying to do with the DVM? Today's dimmer is not a rheostat. It is S.S. device.
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By measuring resistance, I'm hoping the detect connectivity b/w the common and one of the travelers. When I flip the switch, I want to see connectivity b/w the common and the other traveler. I'm not seeing that. Defective dimmer?
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Something's wrong and I can't post so I'm emailing.
On Sat, 13 Jun 2009 22:48:30 -0700 (PDT), "Jim O'Brien"

One can have no idea from the kind of testing you're doing. There is a whole little IC chip inside that thing and it won't switch to ON if it's not connected to 110volts. If you do connect it to 110 volts, you can't measure resistance without burning out your meter.
The way to do this is to leave the switch fully connected, to all three wires. You probably have two three way switches. There are a variety of ways to wire these things, so I'm can't tell you that one wire will be 110 all the time, or when the light is on. It's more complicated than that.
But you might get lucky, so I'd measure the voltage btween each wire and ground. If done correctly the metal mounting flange of the switch is grounded, or the metal box it's mounted in, if the box isn't plastic. If any of these voltages are a full 110 volts AC, then one of the other two should be also. Then when you flip the switch, the common wire should stay at the same voltage and the other two should interchange which is 110 and which isn't. If one wire is 110 and neither position of the switch makes either other wire 110 the switch is broken. Also if one wire is 110 and an another wire is 110 but when you flip the switch, only one wire is 110, the switch is broken.
If you don't find 110 at any wire in the switch, do the following:
What I would do then depends on whether you know which wire in that switch is the common wire.
If you do, I woudl measure the voltage between the common wire and each of the other wires when the switch is in one position. Then flip the switch and make the same measurements.
In one switch position, there should be less than a volt or two AC between the common wire and remaining wire A, and there should be 110 volts between the common wire and the other wire B.
When you flip the switch, those readings should reverse.
If you find over five but under 90 volts somewhere, I'd calibrate your meter by measuring the voltage at a good receptacle. If it's 110 or 120 there, but under 90 and over 5 at that other location, it's probalby a phantom voltage that is picked up by your very sensitive voltmeter, but one which has no ooomph, no power behind it. In a simple situation like this, you can ignore such voltages in your logic. Assume they are zero.
If you don't know which is common, I'd make 3 measurements for each switch position, write them all down and figure out which one is the commnon and then continue as above. each pair of wires
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On Sat, 13 Jun 2009 22:48:30 -0700 (PDT), Jim O'Brien

Defective wetware.
The dimmer operates at 120vac and puts out a pwm (chopped) signal to a bulb. Why, pray tell, do you think measuring its resistance at the DMM's battery voltage will tell you anything useful? You're measuring the resistance of solid state devices that aren't switched on.
The only hope you have with a DMM is to connect the dimmer to a load and measure it then; however, at that stage, if the load is a lightbulb you could simply look at the bulb's brightness to get the same information.
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On Mon, 15 Jun 2009 11:39:14 -0500, AZ Nomad

It's a 3-way switch, so he won't know which one of them is broken?
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wrote:

If either of them are. If he didn't disconnect the hot wire and forget about it when he was working on something else, for example.
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Put one output leg to the load. test. Try the other output leg to the load. test.
Hit switch to flip output legs. retest.
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Too technical, forget the meter. First, if you have a Leviton dimmer, it's a pos, so just get a Lutron of your choice and replace it. When you have the existing dimmer out of the wall for "testing", remove it from the three wires, noting the common wire. Touch the common wire to each of the travelers. One of the two connections should turn on the light. Flip the other switch in the circuit, then touch the common to the travelers again, and this time the other connection should make the lights come on. If things don't happen as I describe, you have a wiring problem

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*I agree with RBM. Throw away the dimmer and stop using a VOM for any house wiring. Change the dimmer to a three-way switch to see if your problem (You didn't specifically state what your problem is) is corrected. You must identify the common wire in the switch box. Don't go by color code.
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In general, you can't get any useful information by trying to test for connectivity/resistance over a solid state device. Among other things you normally won't get any output unless the device is powered, and you can't test connectivity/resistance while it's powered. The output in this case is probably also a fixed voltage over a varying wave pattern, Who knows what that will look like to a solid-state volt meter...
You would need at least an oscilloscope and an ohmmeter to test it conclusively. If you had a working one to compare it to, you could probably get a good idea with a non-solid-state voltmeter and a known good incandescent bulb.
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Agree; forget the meter; with solid state devices any high impedance meter (including cheap or expensive digital ones) can give misleading readings. Just use basic fault finding; touch or bridge the connections over (past the dimmer) to simulate what the dimmer/three-way switch is supposed to do. If that doesn't work then there is another problem somewhere.
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