magnetic chuck control

have an Electro-Matic magnetic chuck control that I plan to sell. It is rated at 115 volts AC in and 115 volts DC out at 50 watts 50-60
HZ. It's like new and I'm going to try Craigslist first and if no results, then eBay. Do any of you guys have an idea what my asking price should be?
Engineman
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On Thu, 11 Mar 2010 17:56:34 -0800 (PST), engineman

It sounds like just a bridge rectifier, which can be had for well under $5. Cost to replicate with box, switch, line cord and suitable connector would probably be under $20.
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    Also a capacitor to minimize ripple, and a resistor to limit turn-on surge. A DPDTCO switch does the job of selecting between AC and CD output, as well as turing it off.
    This is exactly what is in the base of the small surface grinder which I have -- and it had an even simpler circuit before I "improved" it. It had not a bridge, but a single Selenium rectifier, and a SPDTCO switch. The original capacitor was marked 8 uF, and actually *measured* something like 0.001 uF or less. I replaced it with about a 200 uF cap, which is total overkill, but is what I had on hand in the proper voltage range.
    Enjoy,         DoN.
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wrote:

Since the spec is 115 VAC in, 115 VDC out, there can't be much of a ripple-reduction capacitor, if any. A cap charges to the peak voltage of 163 volts each half-cycle, decays according to the cap size and load current but the avg DC voltage will always be > 115VDC because (neglecting diode drops) the avg DC voltage is 115 with no cap at all. A substantially inductive load like a magnetic chuck wouldn't have much current ripple anyway. There's also no turn on surge; dI/dt is limited by the inductance of the load and steady state current is determined by the load's resistance.
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wrote:

Just curious, there's also a spring loaded "reverse" switch to de magnetise on these chucks. Are they just switching the + and - leads in there?
Karl
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On Sat, 13 Mar 2010 06:54:03 -0600, "Karl Townsend"

My guess is that it applies AC thru a PTC thermistor to apply a diminishing AC current.
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...

that's the trouble with asking a EE a question. You don't understand the answer <VBG>
Karl
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Karl Townsend wrote:

If I remember right they use a magnetic amplifier type system to regulate the AC voltage going to the magnet. Its been a number of years ago ant to my recollection the transformer was bad on the one I was fixing. I just rewound the transformer and put it back in service.
I learned winding transformers when I was a kid building high power ham transmitters. The old Dumont TV transformers were great for getting 1500 vdc for a pair of 811's.
John
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I took some photos of the device. Can anyone tell from these whether the switch feeds in AC or reverses the polarity to demagnatize? Don, it is a little more elaborate than you speculated, it has a choke coil also. The cap is .001MFD
http://picasaweb.google.com/engineman69/DropBox?authkey=Gv1sRgCJf25e2Q6aiGiQE #
Engineman

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You can't "reverse the polarity to demagnatize" an electromagnet. If current is flowing, it's magnetized. If it happens to be wound for AC, it will be constantly reversing polarity at 60Hz, anyway. If it's wound for DC, reversing the direction of current flow merely reverses the magnetic poles.
OTOH, as suggested, if the chuck were wound for AC, and the power supply were set up to supply a rapidly diminishing AC voltage to the magnet, it would eventually settle down to nearly zero flux.
This is necessary, because suddenly breaking the current can leave the poles magnetized even after current is stopped.
Your power supply apparently has two selenium rectifiers (of old). That would suggest a full-wave-center-tapped DC configuration. But I don't really see that ferro-magnetic thingy as a regular transformer. It looks more like a center-tapped choke or an autoformer. The choke would be to take out the "buzz" of AC, and make the chuck run more quietly. There is a small cap on the control board. It might be there to suppress switch arcing, or it could be used as a fly-back to suppress quick-break magnetization of the chuck, or it could be a little bit of filtering to take out more buzz.
The label sez... 115DC out 115AC in. So there's not much voltage "transforming" going on in the choke/autoformer/transformer.
LLoyd
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Lloyd E. Sponenburgh wrote:

the selenium rectifiers look like they are hooked up as a full wave center tapped rectifier system, but the wires on the secondary also go out to the switch. I would imagine they are just using the ac to feed the magnet while you remove the part from the magnet rather than on other control units that actually demagnetize the part and magnet
John
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    Certainly *not* the latter -- that would just re-magnetize it with the opposite polarity -- still magnetized.

    Hmm ... perhaps the ringing capacitor discharge which I postulated in the other article I just posted.
    Enjoy,         DoN.
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On Sat, 13 Mar 2010 18:40:02 -0800 (PST), engineman

I'm wondering if the thing that looks like a choke might be a transformer used to apply low-voltage AC for degaussing. There are only three terminals in view on the choke/xfmr thingy, but all three of them are more than halfway from center of coil to outside of coil. That makes me wonder if there isn't another winding whose terminals aren't visible in the photos.
That's about all I can deduce from the photos.
There could still be a PTC thermistor in there. They've been around about as long as color TV has. They were used with the degaussing coils that demagnetized the picture tube upon turnon.
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On Sat, 13 Mar 2010 23:19:06 -0600, Don Foreman

Difficult to sort out from the photos.
The two rectifiers are clearly AC in at the centre and DC out at the ends. Since the the ends of the two rectifier stacks are strapped together they are configured as a full wave bridge rectifier with output to the magnetising coil.
The outer wires of the iron cored component connnect to bridge rectifier AC input so this must be a transformer - it cannot be a choke.
It looks as if there are only three leadouts. The AC input is connected to the centre tag and RH tag(connection at rectifier centre tag) so this makes it an autotransformer - possibly 2:1 stepup for a 230V magnetising coil.
This sounds pretty unlikely unless it is of European origin. However the nameplate lists both 60 and 50 HZ operation so clearly European supplies are a possibility. 230V operation would simply need moving the yellow wire from centre to LH tag.
Jim
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I suspect it is an isolation transformer first off and then has various taps to adapt to a full wave.
I can't see it well - so if the switch is DC to grip and AC on the windings - it might demag the unit while 'off'.
Big time guess - have no idea what the PCB is doing...
Martin
Don Foreman wrote:

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On Sat, 13 Mar 2010 23:19:06 -0600, Don Foreman

Difficult to sort out from the photos.
The two rectifiers are clearly AC in at the centre and DC out at the ends. Since the the ends of the two rectifier stacks are strapped together they are configured as a full wave bridge rectifier with output to the magnetising coil.
The outer wires of the iron cored component connnect to bridge rectifier AC input so this must be a transformer - it cannot be a choke.
It looks as if there are only three leadouts. The AC input is connected to the centre tag and RH tag(connection at rectifier centre tag) so this makes it an autotransformer - possibly 2:1 stepup for a 230V magnetising coil.
This sounds pretty unlikely unless it is of European origin. However the nameplate lists both 60 and 50 HZ operation so clearly European supplies are a possibility. 230V operation would simply need moving the yellow wire from centre to LH tag.
Jim
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On Sat, 13 Mar 2010 14:52:07 -0600, "Karl Townsend"

Ask easier questions! <G>
Wanna beer? Hot enough for ya? Cold enough for ya? Ja git chur deer? How 'bout them Vikings, eh? Been catchin' enny? Waddya usin fer bait?
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On Sat, 13 Mar 2010 14:52:07 -0600, "Karl Townsend"

    Well -- to give a partial answer at least:
1)    PTC    Stands for "Positive Temperature Coefficient" --         when cold the resistance is much lower than when hot.
2)    If the cold resistance is low enough and the hot resistance     is high enough, it can start out with high current when first     switched on, and over a fairly short time decrease to a much     lower current as the resistance increases.
    This sort of thing was used to degauss color TV tubes when the set was first switched on without continuing to apply an AC magnetic field strong enough to cause wiggling of the image as you watch. (If you switch the set off and back on fairly quickly, you would probably see just that as the PTC resistor heats up again. The presumes that the circuit is designed to drop all power to the PTC and degaussing coil once the current drops below a certain value.
    Enjoy,         DoN.
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    [ ... mag chuck info snipped ... ]

    Usually -- they just switch from DC to AC into the coil, which weakens the attraction, and as you pull the workpiece away, the AC field slowly decreases removing the permanent magnetism otherwise potentially left in the workpiece. In the case of mine, it has no spring return, so you can lift the workpiece away at leisure.
    Ideally -- you would have a circuit which caused a steadily decreasing AC voltage to demagnetize everything and then just go away. You might be able to do it with just a good quality charged capacitor switched onto the coil, which would ring for a few cycles and this may be what you have with the momentary switch.
    Enjoy,         DoN.
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Glad you got the Selenium rectifier out of there - when it fails, a poisonous gas is released! Rather nasty in the small shop. The 8uf cap was an electrolytic type and dryed out - becoming an insulator almost.
Martin
DoN. Nichols wrote:

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