posted 10-03-2000 11:40 PM ET (US)            

The general configuration of mid-range outboards is to have a coil ("charging" coil or "lighting" coil or "stator") which produces Alternating Current. The coil is located so that permanent magnents (on the flywheel) pass over it as the engine shaft rotates.

Each pass of a magnet over the coil produces a pulse of current. The function of the "rectifier" is to convert these to pulsating direct current, which then charges the battery.

The "rectifier" is usually an encapsulated hunk of epoxy that contains four diodes arranged in a FULL WAVE BRIDGE circuit.

The AC from the coil ("stator") connects directly to the "rectifier", the positive lead from the rectifier connect to the battery, the negative lead to ground.

There are two things which will damage the rectifier: excessive voltage or excessive current.

The voltage that the stator can generate is somewhat fixed by the number of turns of wire that are wound on it. The engine maker should have chosen diodes for the "rectifier" that can withstand the maximum voltage that the engine's stator can produce, plus additional margin for reliability.

The current that flows through the rectifier is limited only by the ability of the stator to generate it, and by the load connected. If you have a very low resistance load, and the engine is being run at high speeds (where the higher number of pulses of current increase the supply of current available) it is possible that the stator could generate enough current to destroy one or more of the diodes in the "rectifier".

Of course, the solid-state nature of the device means that it only takes one overload to destroy it.

But if the designer of the circuit chose the diode with enough margin, it will be able to handle all the current that the stator can throw at it.

Of course, the old problem of cost-vs-profit come in, and there is a tendency to save a nickel here and there. So the diode may not have a lot of excess current capacity.

Another instantaneous way to blow the diodes in the rectifier is to connect the battery up with the polarity wrong.

If you do this, the current from the battery will flow through the rectifier and dissipate as heat in the stator. If the rectifier does not blow soon, the stator will overheat and melt. Or, maybe a fuse (if there is one) will blow first.

So to understand why you are blowing the rectifier so often, look at your load.

Is there excessive load that draws too much current when it is available at high RPM?

Also, in DC circuits involving batteries, the extreme low impedance of the battery when it is fully charged is shunted across the circuit. This tends to limit the voltage that can be developed across the circuit. The voltage cannot do lower than the battery's voltage, and it cannot go much higher because the battery will represent a very stiff load for any voltage that tries to exceed the terminal voltage of the battery.

Thus, a full-charge battery acts like a regulator in the circuit.

If the battery is old, weak, and discharged, the voltages can raise to higher levels; the battery's shunt impedance is much higher.

Inductive loads, like motors, can also generate very strong voltage kicks when they are turned off. The collapsing fields of magnetic energy make the windings of the motor become generators, and it is possible to produce a voltage several times higher than the operating voltage when the circuit is broken. Voltage spikes like this could also cook the rectifier in the engine.

So if you are using a trolling motor, perhaps it is causing the problem.

Some ideas...