# Voltage divider capacitive dimmers

Discussion in 'General Electronics Discussion' started by Iuval Clejan, Sep 28, 2017.

1. ### Iuval Clejan

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Sep 28, 2017
Are there any such on the market? If not, why not? I am calculating that to dim 100W incandescent bulb with 120V AC from 0 to 0.9 the voltage, a variable capacitor on the order of 0-230 uF is needed. Are there no reasonable size variable capacitors available in that range? If we only needed to dim from 0 to 0.5 the voltage it would be 0-66 uF. And if we need to dim a 10W bulb it would be 0-6.6 uF. I know there might be issues with non-purely resistive loads such as CFLs or LEDs? Same question for inductive voltage dividers...

2. ### Audioguru

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Sep 24, 2016
A high value variable capacitor was used 60 years ago in an AM crystal radio. Its maximum value was 365pF which is 0.000365uF. It was pretty big and expensive because it was mechanical. You want one that is 630 thousand times larger and you want it non-polarized.

For many years a triac fed different pulse widths of AC electricity to a light bulb to dim it.
100W incandescent bulbs are not made anymore because they waste a lot of electrical power making heat instead of light. Also they do not last long.
LEDs are efficiently dimmed with Pulse Width Modulation.

3. ### Iuval Clejan

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Sep 28, 2017
I was off by a factor of 2 Pi, so divide all my capacitance estimates by that. But OK, maybe they are too big. The ones I saw online looked pretty small, but they were not variable...

4. ### Iuval Clejan

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Sep 28, 2017
The size can be made small with a strong dielectric, but I suppose it would be hard to vary the capacitance when you have a liquid between the plates. I bet it could be done though. Maybe not economically though. I suppose PWM works well enough. Oh, I forgot about the polarized part. I suppose all the small ones use electrolytes?

5. ### duke37

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Jan 9, 2011
If you wish to use capacitors, then the only reasonable sort are those with solid dielectric. These are big and expensive and you will need one for each output level. See motor capacitors, 3μF would be typical for motors in the UK.