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Homebrew Magnetizer-Demagnetizer

Those “nonmagnetic” tweezers remind me of a story and a useful gadget.

Two years ago a lightning strike blasted a football-sized chunk of concrete out of the garage door apron, blew out a bunch of networking gear, magnetized every ferrous object in the house (including the nails in the hardwood floors), yet didn’t do any damage to anything else.

Including us: we were sleeping about 20 feet from the crater. Whew & similar remarks.

Anyhow, all my machine-shop equipment and tooling was magnetized, too. Suddenly, lathe bits attracted swarf like, well, magnets, endmills sported fur coats, scales snapped onto the workpieces they were supposed to measure, and tweezers picked up screws without any pressure. Not a good situation.

Homebrew Magnetizer-Demagnetizer

Homebrew Magnetizer-Demagnetizer

Fortunately, I’d built a demagnetizer loosely modeled on one described in the Sept/Oct 2000 Home Shop Machinist. It got plenty of power-on minutes after that strike, returning my tools to their normal condition.

Those flooring nails will be magnetized forever.

The general idea is pretty simple: recycle the motor from a can opener-class gadget. Strip off all the shading coils and other frippery, saw enough from the pole pieces to position tools in the air gap, plug it straight into the wall outlet, and shake the magnetism right out of your steel.

It has another nice trick: a relatively low DC voltage that magnetizes your tools. The transformer has a 35 VAC center-tapped secondary, a pair of stud diodes yields about 24 V DC, and that honking big cap whacks the bumps off the full-wave rectified DC waveform.

Absolutely nothing is critical, but the original article suggests measuring the AC current into the motor winding, then choosing a DC voltage to force that current (Ohm’s Law: E=IR!) through the coil’s DC resistance. I picked a transformer that was close enough to work; anything in the 10-20 VAC range would probably be fine, too.

The small DPDT toggle switch routes either AC or DC to the winding. If I were doing this again, I’d use a bigger switch, but that’s what I had in the junk box at the time.

Use a momentary pushbutton for the main power switch, as you do not want this thing on for more than a few seconds. The motor windings get warm from the abuse; it was designed to run with the back EMF from the now-missing rotor, making the currents far higher than the design spec. Use fairly husky wire, not doorbell stuff, inside the box.

I used 100% junk-box parts for this project and bolted everything to the outside of a recycled aluminum box because the inside was pretty crowded with that husky wiring.

Demagnetizing: feel the buzz, then pull the tool a goodly distance from the pole pieces before you release the pushbutton.

Magnetizing: stroke the tool over one of the pole pieces, repeat as needed.

That should handle any residual magnetism in those tweezers…

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  1. #1 by andind on 2009-03-16 - 23:41

    i want to know the schematic and make selenoid
    Please help me
    andindcpp at yahoo.com

    • #2 by Ed on 2009-03-17 - 07:05

      I don’t have a schematic, as I made the gadget up on the fly based on the original article in the Home Shop Machininst magazine, but this is actually simpler than you think…
      The AC demagnetizer circuit is just a line cord, a fuse, a big pushbutton switch, and the motor winding!
      The DC magnetizer function requires a suitable transformer, a bridge rectifier to get pulsating DC, and a capacitor to smooth the ripple. If the transformer has a center tap, as mine did, then you can use two diodes in the classic full-wave rectifier layout, but either way gives the same result.
      If you want both functions, then the DPDT switch selects either the AC or DC voltage for the winding. The pushbutton switch turns on the transformer even in AC mode, just because that’s simpler to wire up.
      And, even better, you don’t “make” the solenoid, you salvage it from a dead kitchen appliance. Nearly any small AC motor will work, so picking a discarded blender or even an old clock from the trash is fine.
      Hope that helps…

  2. #3 by peter on 2011-07-01 - 15:08

    I’ve just made a second tool demagnetizer, but slightly differently, using a modified microwave oven transformer, with secondary and magnetic shunts removed. It has a pretty large area so bigger tools (calipers) can be demagnetized properly.

    As I was spying/studying commercial demagnetizers, I noticed that some used a decaying magnetic field so there’s no need to (slowly) withdraw the tool from the demagnetizer. That’s what I wanted too – but had no clue how to design the electronics…. till I recalled how they degauss CRT televisions & monitors, using a PTC in series with the coil. Quickly grabbed a PTC and tried it – works perfectly!

    Thermistor’s cold resistance is 0.2 ohm, hot resistance > 100kohm. Initially about 25A current flows, which decays in .6s to zero.

    Only downside is that to degauss a second tool, you have to wait about a minute for the PTC to cool down.

    Just wanted to let you know about this – I don’t recall reading about using a PTC for a tool-demagnetizer. But it works very well. Have temporarily uploaded some photos should you be interested in it: https://picasaweb.google.com/motorconversion/Tijdelijk

    • #4 by Ed on 2011-07-01 - 15:26

      you have to wait about a minute for the PTC to cool down.

      Sounds like a fair tradeoff for foolproof operation; it also prevents a “fall asleep on the button” burnout.

      I certainly have a PTC resistor or three around here somewhere, although with completely unknown characteristics. Now I know what to do with one when it bubbles to the surface… thanks for the tip!

  3. #5 by david on 2011-07-01 - 16:54

    Those flooring nails will be magnetized forever.

    Not at all! You just need to arrange for an identical lightning strike at the opposite corner of the house! Should be a trivial bit of mad-science for someone like you! :)

    • #6 by Ed on 2011-07-01 - 21:28

      Given the carnage resulting from the first strike, that is just so unattractive… [grin]

  4. #7 by peter on 2011-07-02 - 06:47

    If they can degauss large war vessels, surely a house should be no problem? Think big! :-)

    http://tinyurl.com/5ru9s59

    BTW, this discussion reminded me of highschool after a demonstration of CRTs with an oscilloscope, deflecting the beam using a magnet. When we came back next week in class, we noticed a red/white magnet duct taped to the top of the scope. Teacher grinned sheepishly as he said he had accidently left the magnet on the scope after the demonstration had finished. He had ducttaped it permanently on the scope, reversed polarity. We couldn’t help but wonder how long it would take before he had to reverse polarity again….

    Magnets – guaranteed to provide hours of fun and frustration (magnetized calipers and other tools….)

    • #8 by Ed on 2011-07-02 - 07:12

      Think big!

      Y’know, I’ve always thought this place would look great with a five-element 80-meter Yagi up on the roof. If I could get one of those 136 kHz experimental licenses, that would be even better

      deflecting the beam using a magnet

      IIRC, the degaussing coil in my parents’ TV took quite a while to knock down the residual effects of my cough experimental sessions with an Alnico magnet.

      Those were the days when a child’s toy could contain a drop of mercury, though…

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