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Old 02-22-2009, 04:59 PM   #1
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dimmed lights dim more with a power draw


I've always wondered why when you have a light turned down on a dimmer and something like the fridge or an AC compressor kicks on they dim a little more.

I assume this is normal since I've noticed it for years in multiple homes.

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Old 02-22-2009, 06:11 PM   #2
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dimmed lights dim more with a power draw


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Originally Posted by handifoot View Post
I've always wondered why when you have a light turned down on a dimmer and something like the fridge or an AC compressor kicks on they dim a little more.

I assume this is normal since I've noticed it for years in multiple homes.
When a large motor is in operation, an uncanny kind of "voltage drop" occurs where the voltage and current get out of phase.

You would almost need a Ph.D. in electrical engineering to understand this phase relationship. It is not related to the difference between single phase and 3 phase power.

A process called "power factor correction" puts the voltage and current back in phase. This is typically accomplished by putting a big capacitor between hot and ground. The size of the capacitor should depend on the average load of motors and other inductive devices, or more or fewer capacitors switched in as needed.

For household use this phase difference is usually not much but some dimmers are sensitive to it and produce greater or less dimming when the phase changes.

Also the normal voltage drop that occurs when the overall load is increased might translate into a more noticeable difference in the brightness of the lamp at that particular level of dimming.

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Last edited by AllanJ; 02-22-2009 at 06:19 PM.
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