Singing light bulb on dimmer switch

[D&SW]

[David D.]

Thanks, everyone, for your replies, suggestions, and electronics clarifications.

I haven't quite decided what solution I am going to explore long term, but, for the moment, I am using the light undimmed.

- David

[mm]

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.

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.

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.

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.

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.

Posted and mailed because it's been almost 3 days and ahr is so busy, I'm not sure anyone is reading this thread anymore. So I wanted the poster to know I had replied.

[Bud--]

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

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.

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.

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.

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 singing.

bud--

[Peter Pan]

Always thought the singing to be related to the harmonics due from phase control (the sound isn't 60 Hz), so the capacitors might not like seeing all that harmonic current.

Also putt> mm wrote:

[Bud--]

Going further back in the thread, the discussion was on filtering rectified AC.

bud--