Magnetic Chuck Control Circuit Wanted

Jim: those caps are pretty small and wouldn't have a large impact on the waveform. Actually as Pete mentioned, it might be better to have a voltage
suppressor (MOV) right across the chuck terminals- you could dispense with the 4 diode caps most likely. They won't protect the switch anyway.
Pete was also concerned about AC residue in the DC output causing heating- my feeling is the chuck is going to heat up anyway even if you fed it pure DC, so
I thought the small amount of AC harmonics wouldn't make much difference. I've never used an electromagnetic chuck so I'm kind of making educated guesses here
about the proper way to power it. Main thing would be to make sure the rectifier and switch don't get clobbered by the chuck voltage spikes.
Pete: I hear you about the SCR situation, that's a whole 'nuther level of complexity. Rick (I think) wants the minimum parts count that's reliable.
Rick: There should be a suppressor on the chuck terminals. Metal Oxide Varistor. I have a couple, I could send you one. They are about the size of a quarter with two wires coming out. It will limit the voltage spike to about 180 volts. Part number V130L20. Amazon may have them too.
Mark S.
 
MozamPete said:
I have an old 8" x 24", 110Vdc magnetic chuck...I want to build a proper power supply/controller with a de-mag function (with the setup last night the work piece stayed stuck for about 20 second after the power was turned off)
Click to expand...

I like the motor-control idea (also consider transformer-capable dimmers,
which also use a triac with inductive-switching capability) for current control.

The demagnetization is best accomplished with an AC excitation that diminishes
with time; a PTC thermistor, such as is used for motor starting in refrigerators,
is an elegant way to accomplish this. These are easily available as repair parts,
with several seconds time delay when given ~ 1A .

<http://www.ebay.com/itm/3PIN-Terminals-Refrigerator-PTC-Starter-Relay-12-Ohm-Resistance/172347087382>

The old (Klixon) data sheets indicate about 1:5 cold/hot resistance ratio for these items
<https://web-beta.archive.org/web/20070221224341/http://www.sensata.com:80/files/8ea.pdf>
 
Good idea Whit- I'm going to have to edit the schematic for sure now. Not sure if a variac would even be needed if you did that instead... just throw the switch and let it auto-demag.
You following this Rick? I would hold off buying any parts just yet, we are still pushing ideas around.
Mark S.
 
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Actually I was going to insert the PTC in one of the ac lines from the bridge to the switch. So, switch up: pulsating 120 Hz DC out. Switch down: AC diminishing (demag) out. Yes you would need a pretty beefy DPDT switch. PTC elements have a finite lifespan too.
Variac optional if one wanted a means of adjusting the mag force. Can't see a need for it tho. Seems like either full on or off would be fine.
Mark S.
 
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Maybe a multitap transformer can be gleaned from an old stereo; my antique Pioneer has +/-44VDC derived from a power transformer, with low-voltage windings that could be connected in series.
Line isolation is good practice, but a GFI module could be the other safe option.

I'm thinking the high price of electronics for these chucks is a figment of the
market, not a realistic cost for the modern bill of materials.
 
I built a power supply for my mag chuck a few years ago now. I kept it simple. Full on for the mag, not variable and the AC was on a momentary switch. Have not needed an adjustable force of magnetism. And the little shot of straight AC is all the chuck needs to let go. I was concerned at first about my design and application. So I was feeling the chuck for heating and put a scope on the switch looking for spikes. For my set-up everything is happy and works great…Dave.

PS: my power supply has an isolation transformer.
 
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Hi Chips (Dave): So you haven't had any problems blowing up your bridge rectifier?
Mark
 
Sorry, I do not have any pics of the power supply I made. And it’s been too long to recall exactly what components I used. But here is a pic of the switches and face plate that I made. The holes were already in the machine base so I had to work around them…Dave.
I do remember at first having a current limiting resistor in the circuit. But that did not give me ALL the magnetism that I could achieve. So I took it out. I just made sure I have a carefully selected circuit breaker.
magchuck.JPG
 
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