# How does a dimmer switch work?

Originally Posted By: James D Mosier
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On another forum (home improvement topics on an automotive site) someone suggested using a dimmer switch to help lower electricity costs. It got me thinking about how the heck these work.

I'm guessing that they just add resistance in series to the circuit and that they don't reduce electrical consumption, but again that's jsut a guess.

I know this isn't really inspection related but figure this is the most likely place to get the facts.

Thanks.

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Jim Mosier

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Jim there are a lot of ways to build a dimmer and a lot depends on the type of lighting you want to dim.

The most common type of dimmer used with incandescent lamps uses an electronic switch to chop the the peaks off the AC sine wave.

In other words it basically pulses the AC, they do save energy.

It can be done with a rheostat (variable resistor) and that used to a common way to do it. But they trade voltage for heat which is major waste of power.

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Electrical Construction & Maintenance
Moderator at ECN

Originally Posted By: mboyett
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Here’s a visual: http://www.silentdimming.com/flash_utility.asp

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A normal AC sine wave has a smooth ramp from the peak voltage below the 0 volt line to the peak voltage above the 0 volt line.

Here is an AC sine wave that shows what happens when dimming with a rheostat.

Here is an AC sine wave controlled by a dimmer that stays 'off' until the peak voltage than lets it ramp down.

Notice that type above come 'on' fast and fades to 'off'

That works for incandescent lamps but if that dimmer was used on a transformer for low voltage lighting it would cause high inrush current over heating the transformer.

In that case you have to buy the correct dimmer for a low volt transformer and the dimmer works the opposite way. It comes 'on' slow but drops out fast at the peak.

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Electrical Construction & Maintenance
Moderator at ECN

Originally Posted By: mboyett
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 bbadger wrote: In that case you have to buy the correct dimmer for a low volt transformer and the dimmer works the opposite way.
Like the "Reverse Phase" in my example.

 bbadger wrote: It comes 'on' slow but drops out fast at the peak.
Or anywhere in the cycle, thereby providing less overall power.

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Mike Boyett
Capital City Inspections
Austin, Tx
www.capcityinspections.com

This post was automatically imported from our archived forum.

 mboyett wrote: Like the "Reverse Phase" in my example.

I was typing while you where posting, but yes definitely. ![icon_cool.gif](upload://oPnLkqdJc33Dyf2uA3TQwRkfhwd.gif)

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Electrical Construction & Maintenance
Moderator at ECN

Originally Posted By: dvalley
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David Valley

MAB Member

Massachusetts Certified Home Inspections
http://www.masscertified.com

"Some cause happiness wherever they go; others, whenever they go."

Originally Posted By: pabernathy
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I get this asked alot in residential wiring that we do…the home owner says they want 22 recess lights…well then they say because they are burning alot of wattage and power they say…we will just put them in a rheostat dimmer…this way they turn it down and saves them money…lol

I end up laughing and saying....look the resistor coil in the rheostat does not care if you turn the light down...it is still going to pull the same amount of power...you are only ruducing the output....not the demand.

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Paul W. Abernathy- NACHI Certified
Electrical Service Specialists
Electrical Contractor
President of NACHI Central Virginia Chapter
NEC Instructor
Moderator @ Doityourself.com