Most light bulbs hum loudly when dimmed via a dimmer switch. A few are ok, because they contain extra filament supports at critical positions.
My hanging (swag) kitchen tiffany-style light takes a G40-150w bulb. It hums badly when dimmed.
(1) Is there a brand of G40-150 bulb that does not hum when dimmed?
(2) Alternatively, is there a small, in-line filter available, or a filtered, table-top dimmer switch, that would create a smoother short-duty-cycle output than the intermittent, alternating square-wave created by a typical dimmer switch? Would that be safe for a household lamp application? Would that eliminate the audible humming?
From my experience, some lamp types whine more than others. Typically the longer the filament stanchions are, the more vulnerable to whine the lamp is. I've also had the best success preventing whine with Lutron dimmers
I don't think there's much you can do in a practical sense to "filter out" the the non-sinusoidal waveform without engineering a special purpose high chopping frequency dimmer, which would prolly waste a lot much power in the form of heat.
BTW, it's not an alternating square wave, it's a sinusoidal waveform which has a portion "removed" from every half cycle when you turn the dimmer down from full brightnes. The lower you set it the less of the original waveform is left, but the parts that are left follow the line voltage waveform.
Before solid state stuff was a twinkle in Shockley's eye they used to make wall mounted variac light dimmers for use in rich folks houses, and I'd bet that they didn't make the light bulbs hum.
I only have three dimmers for lights, but I've never had a lightbulb hum because of one. Until a couple weeks ago, I was using a 150 watt bulb in a desk lamp with a built-in dimmer right next to me. Unfortunately, I knocked the lamp off the table and broke the bulb so I don't have it, but I think it was the standard GE 150 watt bulb they sell in supermarkets.
In the dining room I also have a 150 with a dimmer, and I'll check the brand if you want me too. I only have to take off the cover. It's probably not G-40 since I've never heard of that, so let me know if you want me to check.
Since the output of the dimmer is still AC, that would dim it further. by cutting the current in half, unless you used something like a bridge rectifier (4 diodes arranged in a square) that is full wave.
(Sorry, maybe htat is what you mean to begin with.)
Then you'd be running yhour lightbulb on DC, and I would be very intersted in how well that would work.
Filament singing is a common problem with phase control dimmers. I design phase control dimmers for theatre and television use. The amount of sing can be reduced by limiting the rise time when the switching device switches on. With triacs and SCRs, this is normally done with a choke in series with the device. Wall mount and other consumer dimmers have relatively small chokes due to the amount of space available and cost considerations. The choke is often a a "stick" of ferrite with wire wrapped around it. We use toroid cores that are considerably larger.
Some professional dimmers are now using IGBTs as the switching device. These can be turned on more slowly, increasing risetime and limiting filament sing. The slow turn-on increases power dissipation in the device, but there are also losses in chokes used in thyristor dimmers, so the total losses may be equivalent. These dimmers often vary the rise time with heat sink temperature, speeding up the rise time as the unit heats up, limiting total temperature rise.
Some IGBT dimmers also do "reverse phase control" where the lamp is dimmed by an "early turn off" instead of a "late turn on." Fall time control is used here to limit singing.
IGBTs are considerably more complex to drive when compared with SCRs and triacs, so IGBT based dimmers are generally more expensive.
There are also some "true sine" dimmers that high frequency chop the incoming AC, then filter out the high frequency. Due to complexity of the high frequency drive of the IGBTs and the requirement for a high power high frequency filter on each channel, these dimmers are also more expensive than phase control.
As Jeff points out, Variac light dimmers (variable autotransformer) output a variable voltage sine wave which keeps lamps from singing. My high school auditorium had a dimmer system based on these. It had really big handles you run up and down to control the lights.
Way way back, theatres used salt water for dimming. Electrodes would be dipped in and out of salt water to bring the lamps up and down. I don't suggest this at home!
If we are worrying about lights bulbs 'singing/humming' it makes one wonder what other RFI (radio frequency interference) these somewhat unecessary devices are causing? Why not use cheap bulbs and turn off a lamp or two?
And since many (most?) dimmers are manufactured 'to a price' is it correct to assume that compromises are made?
A capacitor "attempts" to maintain a constant voltage (I=C dv/dt), causing a current to keep the voltage the same. An inductor "attempts" to maintain a constant current (V=L di/dt), causing a voltage to keep the current the same. When the thyristor turns on, there's 0A going through the inductor, so the full line voltage ends up across the inductor. The current then ramps up at a rate inversely proportional to the inductance (lamp resistance makes this an RL circuit, so it ends up being an exponential ramp with time constant of L/R, but when the thyristor first fires, the ramp starts out linear). This is exactly what we want. The instantaneous current rise is slowed down to several hundred microseconds of rise time. I can't think of how you'd be able to limit the current slew rate with a capacitor. Sorry!
So the bulb itself shows. And it's 5 inches, so it might have a much bigger filament than the minimum size 150 incandescent.
You might be better off removing the dimmer and buying the size you most want to use.
I wanted the 150 for the dining area, because I only have one lamp, a floor lamp, in my living area. It's always sort of awkward to get to it, and sometimes near impossible, and it doesn't light up the whole area. So I want a bright dining room bulb. If it weren't for that, I think I'd use a 100W.
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.
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.