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Re: Singing light bulb on dimmer switch

mm wrote:

> On Wed, 24 May 2006 00:08:24 GMT, "D&SW" <dsweatherwax@xxxxxxxxxxx>
> wrote:
>>Years ago I tried to use a bridge and two 40mF 450V caps to banish some
>>nasty hum coming from two chandeliers, each with five 50W bulbs. Way too
>>much load for the caps. The hum remained and the caps were seconds away from
>>exploding due to their instant heat buildup. It might have worked with a
>>smaller load.
> I don't get it.  I thought that if the capacity of a cap was exceeded,
> it just filled up on one side, and after that the rest of the current
> behaved as if there were no cap.    In fact it occurs to me that in a
> DC power supply circuit for a radio or tv, the only reason the cap
> ever gets below full charge is that the load is *high* enough to draw
> more than is currently, during low parts of the cycle, being provided
> through the diodes, so it drains the cap.

In this case, the cap is way too small for the 250W load, and it is well
drained before the next 'fill up'.

> During the high points, the peaks of the 120 cycles per second power
> (after rectification) there is more than enough power and that's when
> the the caps are refilled.
> Lowering the load would mean the cap would fill up on one side, and
> then just stay filled all the time.
> Two 40mF sounds like a lot, but if it wasn't enough, it seems to me
> there would have been no current in or out of the caps after the first
> charge.

Since the cap is pretty fully drained, there is a very high current at
each 'fill up' followed by high current to the load that drains the cap.

> If 80mF was enough to filter, maybe the internal leads couldn't handle
> the current in and out without getting hot, even though current in and
> out is what caps do.  Maybe that level of heat was within range.

In addition to the leads, the capacitor has an internal resistance which
you might find specified as ESR (equivalent series resistance). Trying
to filter for light bulbs causes a relatively high current charging the
caps and then discharging to the lights. That will cause heating when
flowing through the capacitor's internal resistance. A reasonably sized
(larger) filter cap would have a far smaller ESR. But the dimmer may not
like the high peak currents to charge a large cap.

> And I would also think that nothing 110 volts could do, even
> full-rectified to make it higher than 110, could damage a 450V cap.

The AC voltage is a sinewave whose value is constantly changing. 110
volts is the RMS value - a form of average. The peak voltage is 1.4
times that - about 155 volts, which the cap would charge to with no
load. Far lower than 450V as you said.

> I would also wonder if caps are necessary, since an incandescent bulb
> with pulsing DC current would remain hot and giving light, despite the
> pulsing.  Don't electronic dimmers work by completely turning off the
> current parts of the time? And yet all we see is a constant but dimmer
> light.  They don't use caps at all except maybe little ones to make
> them oscillate.

Dimmers work by turning on late in each 'hump' in the sine wave. The
dimmer stays on until the next zero crossing. At full brightness the
dimmer turns on at the start of the 'hump'. At low brightness the dimmer
turns on late in the 'hump' and only the end of each 'hump' is there.
Rectifying just makes all the 'humps' positive instead of half being
positive and half being negative. The same basic waveform, which causes
the singing, is still there. Capacitor filtering would change the
waveform to DC. Series inductors, which some (all?) dimmers have also
changes the waveform, and if there is enough inductance will eliminate


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