Building a heavy duty magnet charger

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To protect the coils from damage, I wound several strips of fiberglass cloth and string over the entire coil and painted them with polyester resin. After hardening, the fiberglass and resin were sanded smooth and painted. At this stage, the coils are finished. Assembly and testing are all that remain.

Arc suppressor diodes
The purpose of arc suppressor diodes is to reduce the high voltage arc produced by the coils when the switch is opened. This arc is exactly what is wanted across ignitor points with battery and coil ignition, but in a magneto charger, the arc could lead to switch failure, insulation breakdown or electric shock. Diodes absorb the energy when the switch is opened. A second pair of diodes is necessary to prevent damage to the first set in the event the battery is connected backwards. Alternator diodes will work fine if the ones listed aren't available.

Make sure all diodes are connected exactly as indicated in the wiring diagram (note direction of arrow on diode). Failure to observe this precaution will result in smoked diodes when the charger is powered up!

Knife switch
The choice of a knife switch has one big advantage. You can actually see if it's open or closed. This is important, because there have been cases where an enclosed switch has had the contacts stick closed even though it indicated open. The result was a set of cooked coils. If an ordinary switch is used, always check the ammeter to make sure the current has stopped flowing when the switch is opened.

Ammeter
The use of an ammeter, although optional, is useful to monitor the current through the charger. Once the current has built to a maximum, the switch may be opened because a full charge has been delivered to the magneto magnets. An ordinary automotive ammeter reading 50 amperes is sufficient. The ammeter is connected in series with the switch and mounted to the wood base attached to the hand truck. Fancier ammeters are available from electronic supply houses or surplus electronics stores. The ammeter shown, unlike automotive ammeters, requires the use of an external shunt for operation. No current will be indicated on the meter if the jumper leads have been improperly connected to the battery.

Final assembly
Now things start to get heavy, so it's good to build the charger onto a two wheel hand truck or other cart. Inexpensive ones costing under $15 are available but require reworking so they will stand up to the weight. The one pictured has had the joints welded and additional braces welded on to improve strength. The wheels were replaced with old lawn mower wheels which are stronger. The main base is attached to the hand truck using the two pieces of steel angle cut from bed rails. These may be welded or bolted to the hand truck frame. The coils are now bolted to the base. Arc suppression diodes and connecting wires are attached to the coil terminals using 5/16 inch brass nuts and lock washers.

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