Electrical dimmers on light bulbs - do they save energy?


Old 02-21-14, 05:00 AM
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Electrical dimmers on light bulbs - do they save energy?

Can anyone comment on this question:

Do dimmers actually cut down the power consumption for a light bulb when you turn down the brightness? or does it simply move where the heat is generated? I know dimmers get warm so I am thinking they get warm rather than the bulb but overall, while the bulb is giving off less light, between the 2 of them, it's still using the same number of watts as the bulb alone?

There are different types of dimmers, right? The older style rotary (resistive?) types vs. newer electronic (smaller package behind the dimmer knob?) is 1 better than another in general or for saving energy?

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Old 02-21-14, 08:53 AM
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Electronic dimmers, just like wall wort power packs are more efficient. I'm not totally certain about the waste factor of old fashioned resistive dimmers. I think, but may be wrong, that dimmers do cut power consumption. Even though old fashioned resistive dimmers heat up I think they choke off more current than they waste in heat. When I dim a 100 watt bulb the dimmer heats up some. Judging by how hot a 40 or 60 watt incandescent light bulb gets I'm pretty certain the dimmer is not generating the same amount of heat so I assume it's not wasting as much energy.

Hopefully someone with an electronics degree will be along to set both of us straight.
Old 02-21-14, 09:46 AM
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Why not set up an experiment with a clamp on on the line side of both style dimmers and see if the load changes.
Just a thought,I would but I don't have a meter available.
Old 02-21-14, 10:08 AM
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I don't have a meter (actually, I do have the kill a watt plug in unit). but these are hard wired.

I turned the lights on bright, went to the power meter and recorded it.

then came in, set to lowest brightness and went back out.

have to watch the videos to count the rotations of the disk on the meter over the couple minutes of each video (rather than take the house wiring apart and get the kill a watt meter in line : )
Old 02-21-14, 10:38 AM
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This is from a manufacturer of dimmers.

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Old 02-21-14, 11:48 AM
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Even a resistance dimmer will lower the power consumed in its subcircuit with the lamp (bulb) but an electronic dimmer set so the lamp is the same brightness will produce yet lower power consumption aka yet greater energy savings.

Measuring power consumption with resistance dimmer in use:

1. Measure resistance of dimmer not connected to circuit.

2. Connect up dimmer and light, turn on and set to medium brightness.

3. Measure voltage across dimmer terminals and also across light fixture terminals.

4. Current equals voltage divided by resistance.

5. Total watts consumed equals supply voltage (approx. 120) times current.

6. Watts consumed by (wasted in) dimmer equals voltage across dimmer terminals times current.

7. Repeat steps 2 - 6 with different dimmer settings to be able to plot (draw graph of) watts consumed by dimmer and total watts consumed versus voltage across dimmer terminals.

Last edited by AllanJ; 02-21-14 at 12:22 PM.
Old 02-22-14, 06:14 AM
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Posted by Furd in another thread:

[Babaganoosh wrote:]
I have a 15+ years old rotary dimmer (resistive?) and in another room we have a 2 year old tiny slide dimmer built right next to the toggle on / off switch. Either is just as good?
There are NO resistive dimmers in any house of average size. The elementary school I attended had a pair of resistive dimmers for auditorium lighting and they were approximately 18 inches in diameter with a lever handle that rotated around the circumference.

ALL residential dimmers used in the last forty years have been variations on a SCR (silicon controlled rectifier) circuit. They operate by only conducting electricity for a portion of the AC sine wave. The DO generate some heat but not anywhere near what a resistor-type dimmer would produce.

Prior to SCR dimmers some high-end residences used variable auto-transformers to vary the voltage level. These used a large wall box to contain the unit. Watch reruns of the old TV show Bachelor Father and you can see one next to the front door, it has a large round dial about three inches in diameter centered in a plate that is probably six inches square. The auto-transformer dimmers were/are very efficient but they cost a comparatively huge sum and take up a lot of space so they are rarely used these days.

Read more: http://www.doityourself.com/forum/el...#ixzz2u3Up2ev5
Old 02-22-14, 09:51 AM
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The elementary school I attended had a pair of resistive dimmers for auditorium lighting
And they had electricity back then??
Old 02-22-14, 11:22 PM
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Yep, we had 'lectricity, and matches too! Granted, it WAS one of the oldest elementary schools in Seattle and today IS the oldest continuously operating elementary school in the Seattle School District. The current building was first opened in 1891 or '92.

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Old 02-23-14, 06:41 AM
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>>> The elementary school I attended had a pair of resistive dimmers for auditorium lighting.

Way back then, stage lighting was done with resistive dimmers, which were heavy and gave off a lot of heat. A common bank of perhaps two dozen dimmers back then occupied a cabinet somewhat shaped like an upright piano. Since most theater shows travel, the number of dimmers in a bank was typically limited by weight. Actually resistive stage lighting dimmers remained in use up into the 1960's in a few places including New York's Broadway because 120 volt direct current was still in use in those places. More modern dimmers do not work on DC.

The school with electricity back in the 1890's had knob and tube wiring or even knob and tube wiring without the knobs and tubes. Its original electrical system may well have maxed out at an average of three amperes per classroom of which practically all was consumed for lights.

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