Building a heavy duty magnet charger
(Page 5 of 6)
January/February 1989
By John D. Rex
Other components, such as the knife switch and terminal board, are mounted to a wood base on the hand truck. If an ammeter is used, it should be placed away from the coils so the magnetic field won't affect meter accuracy. A set of automotive jumper cables attached to the terminal block connects the charger to any 12 volt automotive battery or power source.
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Testing the charger
Once the unit is wired according to the wiring diagram, it's time to test it. Connect the leads to a 12-volt automotive battery (observing polarity) and close the switch. Don't leave the charger on for more than a few seconds at a time, because the coils and diodes will overheat. The current should build up to a maximum (about 40 amps) in about 3 seconds. If not, check the wiring, paying special attention to proper battery polarity and diode polarity.
If the wiring and winding have been done as indicated the north and south poles should be as shown in the figures. Mark the poles with the letters N and S to indicate their polarity. The easiest way to see if the poles are as marked is to test with an ordinary compass.
As everyone knows, opposite magnetic poles attract while like poles repel. A compass needle points north, because the end of the needle is charged south. The needle is really a 'north seeking needle' and seeks the north pole of the earth or magnet. The north pole of the magneto charger can be verified by bringing the compass to about a foot from the charger pole while the current is switched on. The north seeking compass needle should point to the north charger pole as the compass is circled around it. Mark the north pole 'N'. The other pole should attract the south seeking needle and should be marked 'S'.
Warning! Don't bring the compass too close to the charger (or to magnets) because the needle may be demagnetized or remagnetized in the opposite direction. It's always a good idea to periodically check the compass to make sure the north seeking needle still points north (my compass gets messed up a couple of times a year and has to be remagnetized so it points properly).
Charging magnetos
Magnets have the unique property of magnetizing opposite to the polarity of the charging source. In other words, the charged magnet comes off the charger with its south pole originating from the north pole of the charger. Because of this fact, when charging magnetos, remember the pole which you desire to be north should be placed at the south pole of the charger (first test the magnet with a compass to determine the north pole). The pole pieces of the charger should be adjusted to concentrate the magnetic field across the magnets while on the magneto. Use the auxiliary pole pieces if necessary. The figures show optimal pole piece placement for charging most popular magnetos.
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