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dimmers and heat


John Dirks Jr
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I notice dimmer switches heat up quite a bit. I assume that this is from the resistor inside of them. I was wondering about the heat.

Does the heat in the dimmer switch mean that there's lost energy?

Point being, you dim the light thinking that your saving energy just to have that energy lost through heat at the resistor in the switch.

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I don't dim lights to save energy, but then, my house doesn't have dimmers.

And didn't we used to dim lights to increase the energy? [:)]

Yes, the rotary dimmer switch is just a big resistive coil with a copper brush that picks up the feed to the light. As you dim the light, you increase the resistance, but the fixture is still drawing whatever, 5 or 6 40 watt bulbs on a chandelier. Better to turn 3 bulbs off.

Or use them to heat the house. That is the direction to head for, capturing the heat from the lights.

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Back in the day - dimmers were rheostats (variable resistors) only.

Modern dimmers are Triac controlled and supply a variable pulse width signal to the bulb. Applied voltage (hence bulb brightness) is proportional to the 'on' time of the triac. Only a small current actually flows through the resistive portion of the circuit. Vastly reducing any heat generated.

http://home.howstuffworks.com/dimmer-switch.htm

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2011123202144_dimmer-switch-diagram-4.gif

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Let's not forget that we aren't all in climates where we need to heat the home more than cool it ...or that losses from inefficient bulbs are not the most efficient way to heat a home. They sure don't help out with cooling loads for those of us with a warmer climate!

Use the most efficient lights available and heat and cool with efficient methods.

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Back in the day - dimmers were rheostats (variable resistors) only.

Modern dimmers are Triac controlled and supply a variable pulse width signal to the bulb. Applied voltage (hence bulb brightness) is proportional to the 'on' time of the triac. Only a small current actually flows through the resistive portion of the circuit. Vastly reducing any heat generated.

I offer that triacs are semiconductor devices that function like a gate. They close when the voltage across them changes direction. This happens 120 times per second for 60 Hz power. The 'gate' re-opens only after a delay whose length is determined by the current signal on the gate lead which is itself controlled by the variable resistor. The resister can be circular (for a knob) or linear (for a slide). More delay means less brilliance.

The heat generated is a result of the less than 100% efficiency of the triac device. When the light is dimmed, the power consumed by both bulb and triac is less than that consumed with the light at full brilliance so there is indeed a savings in energy when dimmed.

I know, you don't need no stinkin' science. [;)]

Marc

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It might also be pointed out, as with any power controlling semi-conductor, the triac is fastened to a heat sink.

The heat sink in this case just happens to be the large aluminum mounting plate wich is found just under the switch plate.

Any heat is immediately apparent by this assembled proximity.

Now all the guys with infrared cameras can post the temperature photos for a dimmer through-out its operating range.

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Another aspect to think about is that many, many times the dimmer switch is not rated for the fixture it is controlling! I can't tell you how many times I have found a dimmer rated for 400w controlling a fixture 10+ 60w chandelier type bulbs in it. This will cause that switch to get hot rather quickly.

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This dimmer was in my junk box. It has a circuit with a capacitor, because when I try to measure resistance with a DMM, I see a capacitor charging up. Can't get a good read.

The metal plate was 11 deg C when I plugged in the 700 Watt load. After 10 minutes or so at about 50% dim, it has heated up to 13 deg C. The heater was up to 37 deg C.

My shop in the garage is a frigid 10 deg C today.

My take is there is minimal energy lost here, relatively modern dimmer design. The dimmer is rated for 500 W max. It seems to be handling 700 Watts, but this is a strictly controlled lab test, eh? [:)]

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tn_2011124205031_dimmer.jpg

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C --- F

10 --- 50

11 --- 52

13 --- 55

37 --- 99

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This got me curious, so I popped a dimmer switch out of the wall and put a clamp meter on it. It's a Lutron dimmer like this one http://www.lutron.com/Products/StandAlo ... rview.aspx that controls three pendants with a 100-watt bulb each. At full brightness, total draw is 2.2 amps, 264 watts. At minimum brightness, total draw is .98 amps, about 118 watts.

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