power consumption with a dimmer

[brendon]

Does anyone know the answer to this question? Does a series of domestic downlights run through a dimmer switch use less electricity when dimmed?

 

[stevez]

There might be two parts to this question. If you look at just the lights the answer is most certainly yes. The efficiency might worse but it will use less electricity (power).

If you were to look at the circuit that includes the lights AND the dimmer than the answer is maybe. If the dimmer is an old fashioned rheostat (variable resistor) then the answer would be that the energy is the same - it's either dissapated in the lights, the rheostat or both. If the dimmer is a more modern electronic (triac, other) dimmer then less energy is likely to be used as the dimmer actually turns the power off for brief moment to achieve the power reduction to the lights.

 

[Dr.EM]

Yes, most modern dimmers will not waste the energy that is not supplied to the bulb. ie, dimming the bulb to half of full brightness may use around half the rated power (not exactly, but similar).

 

[stevez]

Going to fast on some of these. I re-read my answer and realised that even with a rheostat the circuit uses less electricity because the net series resistance of the entire circuit increases therefore the current that flows is less. Sorry about that. What will add some confusion is actually calculating the current. The resistance of a filament changes significantly with temperature so the relationships won't be as simple as a basic resistor.

 

[Someone Electro]

Well resitive limiting burns off most of the energy as heat.While a triac makes next to no heat and all the energy is used by the bulb

 

[Hero999]

But the power factor is very poor when it's dimmed (a harmonic waveform is being drawn) therefore lots of power is wasted in the electricity company's wiring and transformers.

 

[Styx]

But the power factor is very poor when it's dimmed (a harmonic waveform is being drawn) therefore lots of power is wasted in the electricity company's wiring and transformers.

ONLY if it were lots-o-amps which a dimmer aint. The cable impedance will damp out alot of it

plus anyway a dimmer drawns next to nothing!!!
the choppy current will cause next to no voltage distortion since the power levels are so low compared to the power capability on the subnet.

you want to see what a thyristor cct can do, goto a steal mill and check the voltage waveform at the PCC