Household mains light dimmers

Say you have a cluster of 3 x 50W mains halogen GU10 lights connected to a standard B&Q dimmer switch. If you turn the dimmer so that the lights are half-brightness, will the lights consume half the power, ie: about 75W in total?


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The simple answer is "overall, no" because there are losses in the dimmer.

I also suspect that a halogen bulb is most efficient when operating at its max rated voltage. Otherwise you could reduce your electric bill by installing twice as many lamps and running them at half brightness (or even four times as many at quarter brightness perhaps )

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says... Quote: However, dimming incandescent lamps reduces their lumen output more than their wattage. This makes incandescent lamps less efficient as they are dimmed. End quote..

This is may also be a factor...

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The accurate answer would be depended on the efficiency of the particular lamp type. Measuring the actual current draw, and calculating the kw/Hrs would be the best way to determine this. Incandescent lamps are not linear in their efficiency. Half the number of lumens will not be an accurate way to say that the lamp is drawing half the power. The heater resistance of an incandescent lamp is not linear. At a lower temperature or average voltage drive, the heater resistance is infact lower, thus it is pulling more current for its amount of lumens output. Therefore, its current pull at different voltages will also not be linear.

Using a true AC sinewave voltage drive, or a true DC source, if you decrease the average drive voltage to a lamp by about 10%, you may increase its life span by more than that amount. At about a 20% reduction of drive voltage, the life span of some lamp types are more than doubled. Some manufactures have published this type of data about their lamps.

As for the low cost thyristor lamp dimmers, these dim by changing the duty cycle by using a phase shifting technique. This infact is not really a lowered voltage as such. It gives the effect of a lower voltage. Because the voltage is phase shifted, it infact is pulse chopped, if you were to look at this on a scope. With many types of lamps, this infact may reduce the lifespan of the lamp, because of this effect.

The best dimmer would be to use a variac transformer. It is also a very expensive way to dim lamps. This would allow the load to have a pure sinewave at an actual reduced voltage. This type of power reduction will not reduce the lamp life as like on the low cost thyristor dimmers, and will allow the filament to work normally, but at a lower voltage.

Many types of compact fluorescents, and quartz lamps cannot be dimmed. Dimming them will damage them, and can even damage the dimmer. Refer to the manufacture's label on the box that the lamp comes in, or contact the manufacture for details.


Jerry Greenberg       GLG Technologies GLG
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Jerry G.

Do you have any references on this, or is it just a theory? The ONLY harmful effect of phase-angle chopping may be the mechanical shock of the sharp wavefront on SOME particularly flimsy filaments. If they can take full mains RMS power, why would they be less capable of handling a fraction?

Incandescent lamp filaments are just big resistors that run white hot. I've never seen or heard of one with so little thermal mass that they could possibly follow the waveform. i.e. the thermal mass averages out the power and "ignores" the waveform.

The perceived reduction of life for halogen lamps is caused by reduced temperature breaking the halogen cycle. This happens regardless of the waveform of the power.

Of course "quartz lamps" can be dimmed. They are incandescent lamps. But you must run them periodically at full power to recycle the tungsten via the halogen cycle. This has been discussed many times here and in related newsgroups.

Now if you mean "quartz lamps" that use step-down transformers, you may have a situation similar to a fluorescent ballast...

Fluorescent lamps can also be dimmed, but the process is made more complex because of the ballast. Special ballasts are made that allow dimming.

"Damage" to halogen lamps is only temporary and easily reversable. OTOH you CAN actually damage dimmer and/ or ballast (but not likely the lamps themselves) if you try to dim conventional fluorescent lamps.

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Richard Crowley

You can use Triac dimmers, professional stage dimmers all use Triacs, with VERY expensive halogen lamps on them (typically a PAR 64 lamp runs about $50 Cnd), they get dimmed all the time, as well as turned on and off rapidly. I guess SCRs chop the wave, and yes, a phase shift occurs. The lamp also starts to make a lot more noise when on a SCR dimmer.

The other problem is the colour temperature is greatly affected. A halogen can have quite a high colour temp, and dimming it brings it quite lower. A 200W halogen can be in the same degree Kelvin as a 500W lamp.

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Davis Redding

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