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Harry Matthew's Weidenhoff magneto charger.
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The following comes from a recent topic on SmokStak, which can
be found on the Internet at: smokstak.cgi. As
ever, various individuals started, commented on and concluded the
following bulletin board thread.

I would like to build a magnet charger like the one John Rex
wrote up in the January 1989 GEM (see Gas Engine Magazine,
January 1989, page 22, ‘Building a Heavy Duty Magneto
Charger’). I’ve wanted to do this for years, but never
could find a source for the wire needed. It takes 40-lbs
(preferably two 20-lb rolls) of 10-gauge, heavy Formvar magnet
wire. Where can the average man find some of this? Also, a good,
heavy, 40-amp knife switch? I like this style charger because, as
Rex states, it will saturate any magnet without removing the magnet
from the rest of the assembly. – Jim

I made a charger like the one you’re building. I got the
wire from a place that rebuilt industrial electric motors. Told
them exactly what I needed and they came up with two spools at a
very good price. As for the switch, poke about in old city electric
buildings or factory electrical rooms. My switch came from the
electric distribution room of a local manufacturing company. They
were ripping all the stuff out and tossing it in a dumpster. Traded
a dozen donuts to the wrecking crew for the switch. – Nick

I recently built a charger to Dave Gingery’s plan. Simple
and easy to build, 115-volt converted to DC with a $5.95 bridge
rectifier, two spools of 6-lb, 18-gauge magnet wire from motor
rewinders. It’s safe, draws 8 amps, weighs approx 50-lbs
finished, and with a bar across the poles when engaged lifts it
right off the bench! It can charge most mags without dismantling.
The total cost was $48, having had the two-inch steel cores in my
scrap pile. – Fenwood

Hi Folks, this is not meant to be rain on anyone’s parade,
but I feel it is necessary to point out a small fact about magnet
chargers. I have been a full-time magneto restorer/repairman for a
number of years, and have owned a number of original magnet
chargers built by some of the leading names in magnetos. These were
built by the manufacturers and intended for use by their own
‘Factory Authorized’ service outlets. Without exception
they have all been LARGE, HEAVY pieces of equipment. The American
Bosch unit that sets on my bench weighs 240 pounds. I am of the
opinion that if F-M, American Bosch and IHC thought a 25-lb charger
would do an adequate job, that is what they would have built.

I am sure that if all you want to do is punch up the charge on
your own mags a little the $50 to $100 units will at least not do
any harm. But if you are truly looking to have a real magnet
charger, take a long, hard look at the one that John Rex designed.
Please don’t just take my word for it. Ask any of the many mag
guys that are in this hobby. – Ted Two things go into design of an
electromagnet; the number of amp-turns (AT) and the magnetic
permeability of the pole pieces. That will determine the maximum
field you can generate to charge magnets. The AT is just the number
of amps flowing through the coil of ‘n’ turns (have to
divide by the area of the pole pieces.) Having a large pole to
charge a wide variety of magnets means you’ll need a
‘Rex’ type charger with many turns carrying high

The permeability is a characteristic number for the pole pieces
used and is the ratio of the amp-turns to magnetic field strength.
Some steels have higher permeability than others. But, almost all
steels max out or saturate at a magnetic field strength of about
1.2 Tesla. If you want to make a better charger, use Alnico

In some cases, bigger is not better! You may think a large
charger is better, but if it is operated at a current above
saturation all it will do is make a nice hand warmer in winter. It
is very important that the magnets you are charging fit snugly
against the poles without an air gap, and that you put a keeper on
the magnet before pulling it off the poles and that you magnetize
in the same direction as the magnet originally was. I have had good
luck with my charger made from two 12-volt truck starter solenoids.
– Paul

I have an Allen heavy-duty magnetizer that will really do the
job. It is very heavy and is 110-volt. I also have made one from
windings from Delco starter solenoids that will charge, but nothing
like the Allen. – Ed

A $5.95 bridge rectifier and two spools of 6-lb, 18-gauge magnet
wire at eight amps is going to work like a toy compared to the big
chargers. You can pretend that you charged the magneto, or you can
charge hell out of it to last forever. Note the size of the Wico EK
compared to the coils (see photo). This one was at least 200-lbs
and cost almost $2 per pound. It works! -Harry

My decision to build the Gingery charger was primarily based on
the electrical engineering calculations outlined, i.e., 100 turns
(heavy) at 60 amps, or 600 turns (18-gauge) at 7.5 amps, both give
you 6,000 ampere turns and the same effect at the poles. The
Gingery plan claims, ‘that at 1,600 total turns at a max of 10
amps the charger will provide 12,000 ampere turns of magneto motive
force, at least twice what is required to saturate an ordinary
magneto magnet.’ I felt that was adequate to my needs and it
will accommodate a fully assembled Wico EK quite nicely. –

True, ampere turns do the job, but I still wonder about the
actual mass of the iron. I’m no magnet expert, so I’ll
forgo the fineries other than to be proud of my Weidenhoff. –

I do envy your Weidenhoff, and yes, mass plays a substantial
role. On the Gingery the cores are two-inch diameter by six-inch
mild steel mounted on a four-inch by 10-inch, one-inch thick mild
steel plate. The $5.95 DC rectifier is a state of the art, full
wave bridge rectifier rated to 30 amps. The oldies (chargers) are
scarce as hen’s teeth, so I recommended the Gingery based on
cost, standard, readily available components, step-by-step, clear
instructions and drawings and the electrical calculations that
support the unit’s performance. A great project, and a
confidence builder for the do-it-yourselfer. – Fenwood

I read somewhere that soft iron was best because it doesn’t
stay magnetized well and separating the magnet from the magnet
charger is a little easier. With the mild steel, do you have any
strong residual magnetism after turning off the power? I have
always thought that you can have less turns and more current, or
more turns and less current, in the magnet windings and still do
the same amount of work. That is, as long as the volts times the
amps comes out to the same value. Let’s say 10-volts times
100-amps, versus 100-volts times 10-amps, which in both cases
equals 1,000 watts of power, or energy. The number of turns in the
coils and the wire size, or gauge, has to be proportionate. Any
thoughts? – Russ

I guess I knew what I was thinking, but maybe I didn’t
express it too clearly. I am not a metallurgist, so my knowledge of
these materials is limited. I have read that soft iron, whatever
that is, makes a better electromagnet because when you turn off the
power to it the magnetism decays rapidly. I understand that the
laminated pole pieces in transformers, motor field windings and any
other similar coils used with alternating current have to be soft
iron so the magnetic flux can be reversed at whatever frequency it
is operating at. This may not be as important on a magnet charger,
as the polarity is constant in one direction.

On the other hand, you mentioned using mild steel for the pole
and base pieces. What happens when you remove the power. Is the
decay of magnetism satisfactory? If mild steel is satisfactory for
the pole and base pieces, then I can probably get it here locally
in the smaller quantities. The problem with many of these small
projects is finding the right materials. Some people are fortunate
to be living in places where these supplies are more common than
here in the small city boondocks. – Russ

I am nearly finished with my own design (made from stuff I had
laying around) to charge EK mags. It consists of one 1-inch x
2-inch x 5-1/2-inch mild steel strap with two 1-inch x 2-inch x
2-1/4-inch straps tapped and bolted together to form a wide, flat
U-shaped core. I have taken the four field coils from a Ford auto
starter and I am in the process of mounting them on the short legs
of the ‘U.’ May work, may not. Should I bother finishing it
to see? – Wayne

The ‘REX’ type charger is the way to go and I am
building one now. I talked to a local motor rewind shop about
winding the coils so I don’t have to buy small spools of wire.
The steel for the core is CR 1018, very common. Lots of on-line
companies sell steel by the inch. Rex’s charger is 20,000
ampere turns. – Doug

After following this thread, I just have to build a 200-lb
‘REX’ charger to replace my toy that weighs about 20-lbs. –

SmokStak is an engine conversation bulletin board with over
15,000 messages on file and is part of the Old Engine series of web
sites that started in 1995 as ‘Harry’s Old Engine.’
Harry Matthews is a retired electronic engineer and gas engine
collector from Oswego, N.Y., now residing in Sarasota, Fla.

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