Have a question regarding dimmers and voltages that I just ran into on a new kitchen renovation. I ran all my switch legs down into a crawl space and I am using a Lutron P-PKG1W-WH. Which is a dimmer switch that has a remote capability. I used this for the pendants and high hats on two separate switch legs. When i go up to my lights after installing dimmer i tested for voltage and i got 100V so i adjusted the dimmer to full brightness and i got anywhere from 110-115V. I removed the dimmer and i spliced right through and i got 120V to socket. Can anyone give me some feedback on why i would get that voltage after dimmer installed?
Dimmer Switches
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hbiss
EC, Westchester, New York NEC: 2014
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- Hawthorne, New York NEC: 2014
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- EC
Because a dimmer doesn't vary the voltage like a rheostat. It "chops" the sinewave which in effect will make the light dim but your meter won't respond the distorted waveform. Looks like it did a pretty good job though, probably a true RMS meter?
-Hal
-Hal
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
Welcome to the forum.
Because dimmers introduce a small amount of voltage drop even when all the way up, unless equipped with mechanical bypass contacts.
Because dimmers introduce a small amount of voltage drop even when all the way up, unless equipped with mechanical bypass contacts.
Yes i have an RMS meter. I never ran into this before have you?
Thank you!
Is that across all dimmers or only specific dimmers?
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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All that I'm aware of, due to the internal electronics.
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- Chapel Hill, NC
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- Retired Electrical Contractor
Many dimmers when off will have some phantom voltage at the light source even when the dimmers are off. At least that has been my experience
synchro
Senior Member
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- Chicago, IL
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- EE
I believe there are two effects which each contribute a small voltage drop across the dimmer even when it's turned all the way up.
One is just the voltage drop across the SCR devices inside forward phase dimmers, and the MOSFETs or IGBTs in dimmers capable of reverse phase operation.
Also, to achieve some immunity to noise when creating a timing signal from the zero crossings of an AC waveform, it's typical to have at least some hysteresis which produces a dead zone around the zero crossings. As a result of the dead zone, the dimmer would not conduct over the entire 180 degrees of each half cycle even at the maximum setting. There are ways to reduce this, but it would take extra electronics that would not likely be worthwhile for a dimmer application.
One is just the voltage drop across the SCR devices inside forward phase dimmers, and the MOSFETs or IGBTs in dimmers capable of reverse phase operation.
Also, to achieve some immunity to noise when creating a timing signal from the zero crossings of an AC waveform, it's typical to have at least some hysteresis which produces a dead zone around the zero crossings. As a result of the dead zone, the dimmer would not conduct over the entire 180 degrees of each half cycle even at the maximum setting. There are ways to reduce this, but it would take extra electronics that would not likely be worthwhile for a dimmer application.
Flicker Index
Senior Member
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- Pac NW
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- Lights
The Vrms value at the lamp input is only equivalent to voltage if we're talking about filament lamps. The dimming behavior of LED ballasts don't necessarily follow Vrms value.
mtnelect
HVAC & Electrical Contractor
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- Southern California
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- Contractor, C10 & C20 - Semi Retired
This is a new age for the "Knod & Tube" old age electrician. It's called "Electronics".
Tulsa Electrician
Senior Member
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- Electrician
To help better understand the dimmer types. Read up on forward phase and reverse phase dimming.
There pros and cons for each with effects. One reason they light mfg provides a compatible list of dimmers for there product.
There pros and cons for each with effects. One reason they light mfg provides a compatible list of dimmers for there product.
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