Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 29/01/2010 08:11:35
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I was asked on the radio the other day how a dimmer switch works.
Some suggestions as to a neat way to approach this would be gratefully received.
Best
Chris
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"What happens to the excess power?"
There ain't any.
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To elaborate on what Geezer meant (I think):
The electricity is supplied as Alternating Current (50Hz in the UK and 60Hz in the USA for example). To be more correct, when you are not taking power from it, the voltage varies as a sine wave with the "Live" swinging above and below the "Neutral". If you put such a voltage across a light bulb the light bulb filament heats up as current flows backwards and forwards through it. The power it dissipates, as heat and light, is equal to the voltage x current at each instant of time. Also, although not quite true for a light bulb filament, the current that flows is proportional to the voltage.
If you want to reduce the power in the bulb, you therefore have to reduce the average voltage across the filament. You could this simply, by putting a variable resistor in series with the bulb so some voltage is across the resistor and some across the bulb filament, but this would mean that power would be dissipated in that resistor which would be inefficient and give you dangerously hot switches. A better way is to reduce the average voltage by rapidly switching such a resistor between zero resistance and infinite resistance. Zero resistance will not dissipate any power because the voltage across it will be zero and infinite resistance will not dissipate any power because it has no current flowing through it.
Fortunately there are electronic semiconductor devices that can be made to behave this way to a good approximation (see thyristor, triac). With a little simple external control they can be made to turn on and off during each cycle of the voltage sine wave so as to reduce the average voltage and hence the average current (and therefore power) in the bulb filament. Some of these can also work, with a little more complexity, by switching out whole cycles (only switching when the voltage is at a zero crossing in the sine wave) so as to reduce elctromagnetic interference, which can be a problem with simple designs.
It can get more complex when dealing with fluorescent tubes and energy saving bulbs, but the principle in all cases is to "switch" the voltage sine wave between the allowance of conduction or an open circuit state to avoid power dissipation in the controller.
Best of luck.
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I had an unfortunate mishap using a dimmer, I had a small device for plugging into a power socket to measure supply voltage, frequency and power consumption of other devices plugged into it.
for curiosity I plugged it into a socket powered via a dimmer and turned down the power, the device went up in smoke !!!!.
You must wonder why, when I dismantled it I found that the PSU for the internal chip was a half bridge fed via a capacitor apparently when short pulses of power were applied the impedance of this capacitor was reduced letting thru an excess of power.
The same would have happened if it was used on an aircraft 400 Hz supply but it was labeled for use on 50/60 Hz.
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I had an unfortunate mishap using a dimmer, I had a small device for plugging into a power socket to measure supply voltage, frequency and power consumption of other devices plugged into it.
for curiosity I plugged it into a socket powered via a dimmer and turned down the power, the device went up in smoke !!!!.
You must wonder why, when I dismantled it I found that the PSU for the internal chip was a half bridge fed via a capacitor apparently when short pulses of power were applied the impedance of this capacitor was reduced letting thru an excess of power.
The same would have happened if it was used on an aircraft 400 Hz supply but it was labeled for use on 50/60 Hz.
Ah yes! As they say, "If the women don't get you, the harmonics must." Or something like that, I think.
My wife took her hair dryer from the UK to Spain. I was dual voltage, so it worked fine on the 120V(?) in Spain. It worked really well on the 240V supply when we got back to the UK, but only for about five seconds!
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I used to have the same problem with PSU in Apple computers, sometimes when there was a glitch in the mains supply and the voltage dropped low the PSU would switch to the low voltage mode and carry on working fine the trouble was it would stick in this mode and like your hair dryer as soon as the the voltage was restored to normal would blow up!!
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Apparently they didn't have our QA manager working for them. He used to make us do this gawd awful "brownout" test. The general idea was to crank the line voltage up and down in 5 volt steps using a variac, but each step persisted for minutes. If you had a problem with your power supply design, that would find it!