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Re: Singing light bulb on dimmer switch

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

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!


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