A few months ago I built a small demagnetizer, as I was getting tired of all my tools ending up magnetic and having to take them to a friend to have them demagnetized. Figured I might as well do a little write-up about the project, it may perhaps interest someone here.
The first, small demagnetizer was made using a coil out of an old 3-phase energy-meter, the kind with a rotating aluminium disc. There were 3 coils in it, I used one. The trick is to have a transformer or coil without a fully-closed magnetic circuit, so the magnetic flux will travel through the air (or rather, through your workpiece) from one pole to the next. It's hard to see in the pictures, but the 'working-end' of the coil has an airgap; just like the modified microwave transformer below.
Forgot the exact inductance of the coil, but the reactance was about 6000 ohm, so safe to connect directly across the mains without too much current flowing. In fact, it's safe to have it permanently left on, unlike the second, larger model. I installed a push-button but for this small demagnetizer it isn't really needed.
Modus operandi: plug demagnetizer in a wall outlet, hold tool to be demagnetized on the pad, push button, slowly withdraw tool. When tool is about 20-30 cm away, the demagnetizer can be turned off again. And the tool should be demagnetized.
Works like a charm, but only on smaller tools (screwdrivers etc.), so I built a larger one soon after, using a modified microwave oven transformer. Carefully sawed the top half off, removed the high-voltage windings and removed the magnetic shunts. That's about it. However, unlike the smaller version, this one can't be connected directly to the mains for any length of time (at least, not without the circuitbreakers tripping....) as it is a virtual short. Initially I was designing fancy electronics to provide the decaying AC current, till I thought of PTCs; grabbed an old one, taken from a colour TV set, put in series with the transformer (now: coil), plugged it in, and it worked. Initially about 25A flowed, but it decayed to practically zero in a few milli-seconds as the PTC heated up.
Despite what the schematic shows, I haven't bothered with the fuse or indicator lamp. Just the transformer/coil, a PTC and a momentary switch. I haven't got around to encasing the large version yet, but will do that someday too.
The only downside of the PTC is that, after you've demagnetized a tool, you've got to wait a minute or two for it to cool down again to do another tool. That's a downside I can live with though, and considering how simple the 'electronics' is, it seems like a reasonable trade-off to me.
I have seen small demagnetizers as projects on the web, but never saw one using a PTC to provide the decaying AC current, though no doubt others have thought of this.
Anyway, a few hours spent on a rainy sunday yielded two handy gadgets for me, using recuperated parts that cost me nothing (the cabinet of the small demagnetizer had a few holes, most of which were re-used).
Perhaps this idea is also of use to someone else.
Peter.
