Official Luthiers Forum!

I don't do a great deal of custom pickup work, but for things like old lap
steel pickup magnets or the occasional evil experiment, I'm getting tired
of the crude recharging of pickups with permanent magnets. I know it can
be done well, and I've a handful of magnet holders I've made to get an
even field across the length of a pickup, but I'd still like a bit more
convenience and control. So I've been meaning for some time to make a
coil system for discharging and recharging, and was hoping someone
may have some suggestions on a few details.

My main question is, does anyone have a ballpark wattage I should be
shooting at for adequate power? Here's my general design idea if it helps.

Two coil system, one stationary on bottom, and a moveable one on top.
Perhaps laid out similar to an arbor press, or simply guide rods past the
ends of the coil and a simple clamping system. The idea is simply to
clamp the magnet(s)/pickup to be charged between the two coils, in as
simple a means as possible. My designs for projects like this often tend
to form around the shape of materials near the top of my scrap bins, so
finer mechanical aspects are still up in the air. Dimensionally, thinking
1"x4" by 1" height soft iron cores, in efforts to keep the charging field
(relatively) straight and even across the entire pickup.

Where I start with making the coils depends on a sense of what an
appropriate wattage would be to start with . If I was looking at the
100-200 watt range, then a lawn tractor battery would be fine and
economical. If I needed to be closer to the 800-1000 watt range, I may
not want to heat the coils with the 75 amps, and would perhaps start
looking for a higher voltage DC power supply. Unfortunately those get
expensive once you want more than a few amps. I could make some sort
of a rectifier I suppose, but the simplicity of a car battery appeals to me
more. I won't be trying to charge any neodymium or samarium cobalt
here, but more along the lines of things like alnico 2 or 5.

Of course decisions on wire gauge, turns, etc., all depend on those
variables, so I'm just fishing for suggestions on wattage range here. Of
course this would all be different with AC, but I'm just concerned with DC
at the momet. Should I ever want to use it as a demagnetizer I have a pile
of Variacs to bring voltage to whatever I find necessary. Those and a few
good heavy rheostats for dropping the DC voltage from a battery for
adjustment as well.

I know the actual workings should be more appropriately discussed in
joules, oersteds, gauss, ampere-turns, etc. If you think the idea is too
simple, too small, or can't be solved in volts, amps, watts, and trial and
error, let me know.

Any thoughts are welcome.

_________________
Eschew obfuscation, espouse elucidation.

Here ya go, David.   From what I've seen of your work and read of your posts, this sucker should be right up your alley!

http://www.lindsaybks.com/dgjp/djgbk/magneto/index.html

You could even do some fabulous aesthetics on it.

I built one many years ago with some parts I got from Dowling Magnetics, and it was very amusing when we got one of the capacitors hooked up backwards... I built it into a nice mahogany base with the guts inside and a brass lever switch poking out.   I painted the steel parts dull black and the coils fire engine red.   Very 19th century looking thing.

Basically, you're using a full wave rectifier to charge up some big capacitors, and then you're discharging the caps into the coils.   Blam, major magnetic pulse, plenty enough to recharge Anico magnets or charge unmagnetized ceramics.    I used to cut my own ceramic V magnet stock on a diamond saw for the early Alembic pickups.   

Don't wear a watch when you fire one of these babies off.

Perfect! Simple power supply and at least some general guidelines for coil
size/wire/etc. I hadn't thought of looking to automotive sources, but it
looks like a good starting point. Thanks, and I'll make sure to get a sign to
warn customers in the shop with pacemakers...

_________________
Eschew obfuscation, espouse elucidation.

You can scale this thing up, too. You might just have to mess around to get it right, but that's what we do, innit?

David-
We had this sort of device around the high-school physics lab for years for re-magnetizing lab magnets. As I recall, it had a momentary-type pushbutton switch- it certainly went 'thump' when you fired it, and it did work.
I notice that there's an article in:
The Physics Teacher -- March 2003 -- Volume 41, Issue 3, p. 185 titled; Remagnetizer/degausser
If you know a teacher who has access to this, or can get it through your public library, you'll save the $16 cost of buying the article online.
If you get in touch with the author, he'd likely help you out:
Frank Butcher
Deer Park High School, Deer Park, TX 77536

Warning: this may be the first step on a path that leads to the 10 foot high Tesla coil in your backyard, right next to the railgun!
Cheers
John

Thanks John,

I'm sure I can get the article, either through a friend who is a librarian at U of
M (always a very valuable resource) or an old physics teacher of mine who I
keep occasional contact with when I hit a brick wall.

The giant Tesla coil was in my plans for a home security system, but I ran in
to problems with city codes. They're so strict around here.....



Just try and steal my lawnmower. I dare ya.

_________________
Eschew obfuscation, espouse elucidation.

[QUOTE=David Collins]

Just try and steal my lawnmower. I dare ya.[/QUOTE]

Love it!
A couple of weeks ago there was a TV (Discovery channel??) item about a fellow proving that Faraday was right (Faraday shield) by putting tinfoil over his head and getting a 'lightning bolt' to jump from his Tesla coil to the tinfoil...while standing in his swimming pool. His wife was watching, but it was hard to figure out how she wanted the experiment to work out!

John