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Old 05-03-2010, 12:43 PM   #1
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Dimmer Question


I bought some pretty fancy Pass and Seymour brand dimmers. Have installed them but the lights are not up yet. Anyhow, when putting a volt meter on them, I get a reading of 120 volts no matter where the dimmer is set at?

The instructions say a minimum "load" of 50 watts if required. Does this mean the dimmer will not affect the output voltage unless the lights are hooked up as the "load"?

Thanks.

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Old 05-03-2010, 12:57 PM   #2
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Dimmers work in many ways, not just by adjusting the voltage.

As a simplistic example (without getting into the details of sinusoidal wave forms), a dimmer can work by basically turing the power off part of the time.

To make for a simplistic explination, if we were dealing with DC power, your light bulb would get 120 volts all the time without a dimmer. But if you install a dimmer and set it to half power, the dimmer would cut the power half the time. Basically, it would give the light 120 volts for a millisecond, then 0 volts for a millisecond. Your bulb would effectively be flashing, but it's happening so fast, that your eye can not detect the flashing, and the bulb's filliement never goes completely out. The net effect would be a light bulb that averages glowing at a lower light level than what it glows at when getting 120 volts constant.

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Old 05-03-2010, 01:01 PM   #3
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Okay...but...does the "50 watt load required" mean the dimmer will be non-functional (no change in voltage output or frequency of any kind) unless hooked up to a light source?
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Old 05-03-2010, 01:07 PM   #4
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Originally Posted by MoreBeer View Post
Okay...but...does the "50 watt load required" mean the dimmer will be non-functional (no change in voltage output or frequency of any kind) unless hooked up to a light source?
Ideally your true RMS voltmeter should read the correct value of the chopped-up-sine-wave reduced voltage.
http://www.st.com/stonline/books/pdf/docs/3566.pdf
see fig. 2

If you load the dimmer and now you can read the correct values, there must be some reason it only works with a load.
Without seeing a schematic I can't say what the reason would be. It may have to do with the physics of the particular Triac that they are using, e.g., possibly no Triac can be equally usable at 600w and at 40w or at 700 microwatts [the power your meter uses].
This also implies that the performance you get at 600w will not be quite the same as what you get at 50w.

Last edited by Yoyizit; 05-03-2010 at 01:14 PM.
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Old 05-03-2010, 01:21 PM   #5
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I guess that it must need the load. This brand dimmer is kind of jerky, in fact had problems with one in the past. This model can be hooked up as either single pole or 3-way so there's a load of circuitry inside.

Of the 3 I bought, the indicator light doesn't work on one and needs to be returned. I think the next time I'll pass on Pass and Seymour products.
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Old 05-03-2010, 02:14 PM   #6
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So, to properly check dimmers I'd need a minimum 60w incand. lamp in my toolkit. The 25w bulb I use for general testing will give me a 'false negative' [says the dimmer is bad when it isn't].
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Old 05-03-2010, 04:17 PM   #7
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It is a good practice not to install dimmers until the fixtures are installed, lamped and tested.
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Old 05-03-2010, 05:28 PM   #8
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It is a good practice not to install dimmers until the fixtures are installed, lamped and tested.
And if you want to save dimmers and switches from having 400A go through them for the few dozen milliseconds until the breaker trips, check your work with an ohmmeter.
I had a cust that burned out two 3-way dimmers by not checking.

600w worth of incand. lamps will read about 2 ohms. A short can read up to about 0.3 ohms.
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Old 05-03-2010, 05:34 PM   #9
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Wire the cans, put lamps in the cans, turn on the power to the switch location, attach the hot wire to the switch leg and see if the cans light. If all ok, turn the power off and install the dimmer.
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Old 05-03-2010, 08:07 PM   #10
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Yes, I'm going to set up a temporary light on line from the dimmer and test it. Good idea.

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