"Eric R Snow" wrote in message
...
On Thu, 16 Jun 2005 22:23:58 GMT, "Pete C."
wrote:

Eric R Snow wrote:

On 16 Jun 2005 13:33:14 -0700, "
wrote:

I don't know what DC voltage a mag chuck needs. So you might need a
transformer in addition to the lamp dimmer and rectifier.
Dan
Oh. The mag chuck needs 100 vdc. Part of the reason for the dimmer is
to limit the voltage to the chuck. But I suppose a transformer would
be OK. I just want to know if a rectifier connected to something that
expects a resistive load will ruin the device.

The lamp dimmers use phase control dimming with triacs. They turn on the
triac part way into the half cycle of the AC, so they do not in any way
limit the peak voltage.

They actually eliminate the first part of the waveform rising (or
falling) from the zero crossing point, generating quite a switching
transient which is what causes filament buzz.

Try a variac (variable autotransformer), they will actually reduce the
voltage supplied to your rectifier.

Pete C.
So the dimmers keep more and more of the waveform on either side of
the peak as the power is increased? And they always reach peak
voltage? I thought that they started conducting at zero and used more
and more of the voltage until peak voltage is reached. But after
reading your post that doesn't make sense to me.
Eric

Nope -- the triac turns on some time after the zero-crossing. That
time is usually simply determined by a simple RC circuit that delays
the trigger (lags). You may not always get the peak -- if it is turned
way down, it may actually turn on sometime after the peak, but it
does not turn off until the next zero crossing. There are different
circuits used for things like heater control that do switch on at the
zero crossing to minimize spikes etc, but they work on a "burst" of
cycles as it were. They may use a 1 hz "cycle time" and vary how
many cycles during that 1 second period are actually turned on but
the typical light dimmer/speed control works with a delayed phase
angle firing of a triac - makes them electrically "noisy". They work
well for lights because you are varying the "average power" under
the portion of the curve (sine wave) that it is turned on - works well
with lights. Not so well for a rectified supply because until you delay
past 90 degrees, you are still getting the peak (and high current pulses
when it turns on).

mikey