Weidenhoff Model 818 Magnet Charger

Author Photo
By David Cave | Jul 7, 2020


I have long had an interest in old equipment used around our gas engines. In pursuit of that interest, nearly 20 years ago, I found and bought — sight unseen — a Weidenhoff Model 818 magnet charger. Sitting on a bluff overlooking the Mississippi River, even unrestored, it was a beautiful sight to see. But why the fork lift? The advertisement made no mention of its weight. Back to Arizona we headed with the rear of our 4-wheel-drive SUV sitting low. Loading it with a forklift was easy, unloading it with a cherry picker was another story. Getting the cherry picker boom in under the SUV roof line and above the Weidenhoff and chains to the corners of the pallet was a challenge. Once it was unloaded, I estimated its weight to be between 500 and 600 pounds by measuring how many pounds it took to pull the picker pump handle and multiplying that by piston diameters, arm lengths, etc.

Since then, it has sat in my shop on a Harbor Freight dolly regularly recharging magnets. There have been a few cases where I have tested a low-tension magneto whose magnet had just been recharged on a home brew charger and one case after being recharged on a super charged John Rex style running on 24-volt. After a second recharge on the Weidenhoff the magneto output current of those magnets improved as much as 30 percent which translates to a 69 percent hotter spark.

Shown from the top, the Weidenhoff 818 features a John Deere magneto with pole piece storage area to left.

More recently, after I had originally written this article, I received a very weak welded John Deere magnet from Mitch Malcolm of Lightning Magneto. Knowing the condition of the magnet, Mitch felt I would be unable to revive it, and if I did it wouldn’t last more than a few days laying around without a keeper. The Weidenhoff brought it back to a very good level, not the best but never the less very good.

I pulled the magnet off the magneto base and test stand and laid it on my wood workbench. The following week it was put back on the set up and tested again. Indeed, it lost 12 percent of its output but that still left it in the range of pretty good magnets I have tested. Five Mondays in a row I have put it back on the test stand, it has lost no strength since the first week.

A huge bottom tie bar, 14 x 8 x 3 inches thick, and two 4-pole pieces poured as an integral part. 

These examples would indicate that the Weidenhoff is a pretty good machine, but over the years I have been unable to find any documentation. After a search again this year for documentation with no results, I decided it was time to reverse engineer it and create my own documentation.

First, an external look. Its physical dimensions are 27 x 20.5 x 14 inches (LWH). I now have an accurate digital scale that I can use to lift the charger under my gantry crane. Wow, 470 pounds. When the pole pieces, shown in are added in, the total weight is exactly 500 pounds. I felt pretty good that my very crude estimate was 500 to 600 pounds when the actual weight is 500 pounds.

Continuing the external look, when turned on, the charger draws 15 amps from the 120-volt line or 1,800 watts. For a home brew machine connected to a 12-volt car battery, the equivalent would be 150 amps.

The rear panel has the tag with model and serial number 17264. The front panel has two switches. The ON/OFF switch is some form of contactor. As the switch handle is moved smoothly and goes over center a set of springs slams the switch points open or closed with a noticeable bang. Why the contactor? Opening the switch here is identical to the igniter popping open on a battery and coil ignition. But rather than a small coil we have two giant coils and rather than a small spark we get a big arc. There was no snubber technology available at the time this machine was built so the switch takes a big hit every time it’s opened. The second switch selects 2-pole or 4-pole operation. There is a large, stiff wire attached to the ON/OFF switch that forces the 2/4-pole switch to be in one position or the other. I’m not sure of a need for that other than to make the user think whether he needs to be in 2-pole or 4-pole mode.


Top view of the north pole with pole piece removed.

The top view shows the two main poles. The view from the top is north and the view from the side is south. The two smaller four pole operation poles are between them. The other end has storage, with a green velvet lining, for the pole pieces.

Side view of the north pole.

The core of each magnet coil is 10 inches tall with a 5-inch diameter. The wire bundle is 2 inches thick giving each coil the apparent diameter of 9 inches. The center to center separation is 10.5 inches. As best I can measure, on each coil, there are 21 layers of insulated 14-gauge wire with 77 turns end-to-end for a total of about 1600 turns. I estimate each coils wire length to be about 2,500 feet. Each coil has 7.5? resistance and 700mH inductance.  The chemistry of the big tie bar and the coil cores is unknown at this time.

Weidenhoff Model 818 schematic drawn by David Cave using reverse engineering.

All of that was interesting but not very useful as it didn’t tell me the amp turns nor the magnetic field strength. For those, I needed the schematic. After several hours, the illustration above is the result. Very interesting! The 120-volt line comes into a large transformer with 16 taps. The output of the transformer feeds two complete and separate circuits for driving each coil individually. Each circuit uses two Tungar bulbs to rectify the transformed AC to a 120 pulses-per-second DC voltage. Tungar bulbs are Argon gas filled with a very hot filament (2.8 volt at 23 amps) cathode and a cold anode located above it. When the anode is positive — with respect to the cathode — electrons can leave (boil off) the hot cathode and travel to the cold anode but when the cathode goes positive they cannot leave the cold anode. The result is a pulsing, one-way DC current. In operation the filament becomes very bright and the Argon gas glows. You may have noticed a 5th dark Tungar bulb, it’s a spare.

The Tungar bulbs under power with an additional, unlit, spare bulb. A large, stiff wire attaches the ON/OFF switch (right) to the 2/4-pole switch.

For 4-pole operation the big south pole coil is reversed by the 2/4-pole switch, becoming a second north pole. When connected in this manner, the huge underlying tie bar becomes a south pole for both big coils causing both smaller shoes to become south poles at the top. In so doing, going around a circle at the surface, we see north, south, north, south.

Now that we have a schematic and understand how these big Weidenhoff machines operate, maintenance, trouble shooting and repairing in the future should be much easier. For example, replacing the Tungar bulbs with modern semiconductor diodes would be straightforward and eliminate the wasted power in the filaments and the voltage drop across the bulbs.

The cold anode looks like a hat, the heated cathode looks like a tiny spring.

If any readers happen to have or come upon more information on the Weidenhoff Model 818 or similar big 4-pole chargers, I would appreciate it if you would pass it along.

David Cave is a retired electrical engineer with a doctorate in Electrical and Computer Engineering. He spent his whole career as a computer chip designer at Motorola Semiconductor. He had a gas engine in grade school that never got running. About 20 years ago, after retiring, the bug bit him again. David now focuses on collecting John Deere engines. His collection is made up of one of every model and every horsepower (of every model) John Deere made plus pulleys and other peripheral things. But, with his electronics background, he actually spends more time collecting and fixing magnetos. 

Contact David Cave at jdengines@cox.net

Applying Voltage to Each Coil

This shows the pulsing DC voltage applied to each coil after passing thru the Tungar rectifiers, it’s about 120-volt peak. This is a distorted, pulsing wave form and will likely read differently on all DC meters. On mine, it reads about 65 volt average and a current of 9 amps average which is estimated to be about 16-amp peak. For those of you who like to turn your charger on and off while charging or like to bang the magnet with a hammer, the Weidenhoff does that 120 times a second. On average, the machine is about 28,800-amp turns but 51,200-amp turns at the peak of each 120-cycle pulse. At 0.070-inch separation the average magnetic field measures 1,746 milliTesla, that would put the peak field at over 2 Tesla at the top of each pulse.

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