Magnetic Chuck Control Circuit Wanted

The PTC (positive temperature coefficient) device is the same little gizmo that they used to control the degaussing coils around your color tv picture tube. It tapers the current down as it heats up. John H. and I (and others) were thinking it would give a better demag effect than just a shot of AC because if your shot happened to stop right on the peak of the AC sinewave- well you get the idea. Of course if you have a variac in the circuit you could just flip the switch to demag and turn the voltage down. Same effect.
The MOV (metal oxide varistor) is a voltage clamp like a double zener diode. Protection for the bridge rectifier and toggle sw. from inductive kick from the chuck coil. Small 0.1uf cap also. Omit them if you dare LOL Someone else (Pete?) zapped a bridge already. But his setup was somewhat unusual.
Extra filtering caps- optional as far as I'm concerned. Another part to dry out and fail someday. If you can get good results without it...Large high voltage electrolytics are always the weak link- and expensive. Use the smallest uf value you can.
Mark
 
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Be careful out there everyone- don't do the 60 cycle shuffle unless you have a partner.
MS
 
I was studying this while you were writing, Mark. OK, I understand using the variac and dialing it down to demag, bypassing the rectifier. I like that idea, and have the stuff to do it.

I thought that a fairly large cap was needed to smooth the DC ripple created by the rectifier. It sounds like I may be missing something here... My electronic knowledge leaves a lot to be desired.
 
YES, I'm still here and learning, think I have located a variac, will know the details by Saturday.
 
Bob Korves said:
I thought that a fairly large cap was needed to smooth the DC ripple created by the rectifier.
Click to expand...
A smoothing of the DC ripple is also accomplished by use of an inductor, like... the
hulking electromagnet. It is so effective at smoothing current, in fact, that
the switch and Variac windings are likely to get arc discharge damage, if the MOV is
omitted from the circuit. A large capacitor would smooth low-frequency ripple,
but a small one prevents RF switching hash (so the Variac adjustment doesn't
put static onto every AM radio in the vicinity). An early schematic has small
capacitors across every diode in the bridge rectifier (a sure sign that someone
is a ham, IMHO).
 
:D No, never keyed up, actually. A little shortwave listening only. One can only have so many hobbies LOL
I think whitmore and I are on the same frequency. I didn't know if a big cap would be needed on the dc output so I didn't show one.
If you do use a big filter cap be sure to hang it across the bridge rectifier output not the chuck terminals. You don't want to put AC on it. AC is ok for the little cap and the MOV.
MS
 
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markba633csi said:
:D I didn't know if a big cap would be needed on the dc output so I didn't show one.
If you do use a big filter cap...
Click to expand...

The magnet, as inductor, takes the rectified AC and makes its RMS value
(about 120V) average across the load. A big filter cap, takes the rectified AC
and potentially makes the peak (170V) appear, on average, across the load. That
exceeds the expected applied DC voltage... by too much. The filter capacitor
is not particularly useful, unless the AC ripple on the inductor causes
hum (in some way that disturbs the cut). If that were to be an issue,
a stepdown transformer, or variac with a stop, would fix it, at some cost.

The switching of a 170VDC power source is hard to accomplish: most
switches cannot handle DC voltages that high. The DigiKey offerings top out
at 125 VDC for toggle switches (but 277VAC if switching alternating current).
 
whitmore said:
The magnet, as inductor, takes the rectified AC and makes its RMS value
(about 120V) average across the load. A big filter cap, takes the rectified AC
and potentially makes the peak (170V) appear, on average, across the load. That
exceeds the expected applied DC voltage... by too much.
Click to expand...

The inductance of the magnetic chuck will tend to smooth out and average the current flowing, not the voltage. Without a capacitor the voltage waveform will still be the same raw rectified AC voltage and you will still be exposing the coils to the 170V peak, just only for a short period 120 times a second (instead of continually if you had smoothing capacitor).

This has been my main problem with trying to develop a transformerless design - and exasperated by the fact I'm starting with a 230V ac supply over here so have a potential 325V peak.
 
MozamPete said:
The inductance of the magnetic chuck will tend to smooth out and average the current flowing, not the voltage. Without a capacitor the voltage waveform will still be the same raw rectified AC voltage and you will still be exposing the coils to the 170V peak, just only for a short period 120 times a second (instead of continually if you had smoothing capacitor).
Click to expand...

Yes, the current (which determines resistive heating) is limited by the 120V average
if there's no big filter capacitor. Resistive heating, and overcurrent causing
saturation effects, are things to avoid. Overvoltage, on the other hand... that's
no problem for any reasonable wiring insulation: all the wiring in motors and
such gets kilovolts applied for safety testing. The peak DC voltage doesn't
threaten the electromagnet. The average voltage, though, does.

A variant on a light dimmer (the kind of light dimmer that runs transformer-operated
lights) can effectively lower average voltage, by switching the AC input. Another way
to feed less voltage from a too-high-voltage input, is to use a ballast (like,
the old-fashioned magnetic fluorescent ballasts) before the rectifier.
One might build a decent 100V/1A current source by connecting AC through
old-style magnetic fluorescent ballasts (one or more in parallel).

Dimmers are, alas, likely to fail in full-brightness mode, so fuses are not
optional if the dimmer is the only voltage limit.
 
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