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So I am putting in a dimmer switch, replacing a simple single pole switch. It is a simple two way, single pole dimmer, should be pretty easy. The existing switch works fine. AFter installation, the light is always on, turning the dimmer off, and moving the slider up and down, the light is always on. Using a voltmeter, I tested the supply and made sure it was hot. after the switch, the line is always 120v, moving the dimmer and silding it the voltage remains at 120v.

This is where is gets even weirder. I removed the dimmer, and tested the conductivity. regardless of the position of the dimmer (off or all the way up), there is not conductivity between the wires (using the beeping mode on the meter). Using the resistive setting on the meter, it also confirms that there is no conductivity regardless of the slider settings.

So both of these (light always on and no conductivity) would tell me the dimmer is bad, but they are telling me opposite things. The light always on tell me that the dimmer is shorted and always closed, but the meter measurements tell me that the dimmer is always open. Weird. How is this possible?

Regardless, this seems like a bad dimmer and I went back to the old switch until I can pick up a new one.

For reference, this is the dimmer: http://www.lutron.com/Products/StandAloneControls/Dimmers-Switches/GlyderDimmer/Pages/Overview.aspx

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3 Answers

up vote 3 down vote accepted

The dimmer switch has some solid-state electronics in the path between the electrical mains and the light that it controls: definitely some form of transistor, and most likely one or more MOSFETs. MOSFETs have very high resistance when they're off, well into the gigaohm range (or higher), and very low resistance when on, in the range of 1 ohm (or lower). Other transistors types have a less extreme swing between on- and off-resistance.

The rest of the circuitry in the dimmer is designed to operate on AC mains voltages, so the small DC voltage that your multimeter drives through the dimmer isn't enough to turn on any of the transistors in the conduction path, so you're reading it as being open-circuit.

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ah, makes sense. Basically, the dimmer needs power itself in order to function. –  mohlsen Oct 8 '10 at 14:32
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The reason you might not see any conductivity using a volt meter is because it probably uses Triac thyristors, which are designed for AC, instead of variable resistors which work with both. This still doesn't explain why you're having a problem, unless there's something strange about your power source, or perhaps something strange with the lights.

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The direct answer to your question is "yes", you have a defective dimmer. You cannot test conductivity on a solid state dimmer with a VOM. Get to the hardware store and your problem will be solved. Good luck.

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