Eric R Snow wrote:

On Thu, 16 Jun 2005 23:53:58 GMT, "Pete C."
wrote:

Jeff Wisnia wrote:

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.

I don't see any inherent reason why a full wave bridge rectifier after
the dimmer won't work. Try it.

I didn't say it wouldn't work, just that it did not reduce the voltage.
If you want to try this approach, don't use a light dimmer, use a fan
speed control. Only a couple $ more and they are at least designed to
deal with an inductive load.


You could also use just a full wave bridge rectifier with an appropriate
sized light bulb in series with the chuck coil to reduce the applied
voltage.

A filter capacitor across the chuck coil might be needed if it acts a
little buzzy running on just the rectified AC waveform.


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

Too expensive, unless you've already got a spare one. A 120 to 18 or 24
volt transformer wired up as an autotransformer to subtract its
secondary voltage from the 120 line will reduce the AC voltage to around
100 volts pretty cheaply.

I've got an entire milk crate full of 1 kw variacs. A few 2 kw kicking
around as well.

Pete C.
Pete,
How do the fan speed controls work? And, you wanna get rid of any of
those variacs for cheap?
Eric

I've not dug into the fan versions much. I believe they work in much the
same way as the lamp dimmers, but probably use higher rated triacs and
larger snubbers to deal with the inductive load. The may also use
reverse phase control with a switching device other than a triac. In
reverse phase control instead of turning on after a delay into the half
cycle, the device turns off after the delay which lets the voltage
gradually come up (or down) from zero crossing before being cut off.

A little more info:

http://www.lutron.com/technical_info...mingBasics.pdf

Seems to indicate that dimmers rated for magnetic low voltage lighting
might work as well.

Still more info:

http://home.howstuffworks.com/framed...ghtdimmer.html

As for the variacs, I have to track down where they are. I think they
are still at my other facility which I will not be at for a while (1,700
miles away). If I can get my hands on them I'd be happy to send you one
since I'll never use all of them.

Pete C.