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Author Topic:   Mercury - Burned Rectifier
fester posted 10-03-2000 01:04 AM ET (US)   Profile for fester   Send Email to fester  
I have a 1989 Mercury 80 horsepower outboard on my whaler and I just burned up my second rectifier in the last three months. The symtoms are that within 3 to 5 hours after a new rectifier is installed, the tach on the boat will quit working. This is the only symtom. The motor will continue to run fine. I will then check the rectifier with an ohm meter and find that one of the circuits is burned. This has happened two times. I took the motor to a mechanic and had it looked at. the Mechanic indicated that he thought I had a short some place in the system but was unable to locate it. He indicated that running the motor with a bad rectifier could burn up the stator. I think the stator is fine becuase I replaced it about 9 months ago. Today I installed my third rectifier and also checked all of the electrical connections that I could find. The connections all appeared tight. Any thoughts?
jimh posted 10-03-2000 11:40 PM ET (US)     Profile for jimh  Send Email to jimh     
I have been having a similar run of bad luck with rectifiers, but that's another story.

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...


lhg posted 10-04-2000 01:26 AM ET (US)     Profile for lhg    
Jim, you continue to amaze me! And all the while I though you were only a sailor, a Whaler fan, a website author and a Mills canvas installer!
triblet posted 10-04-2000 10:01 AM ET (US)     Profile for triblet  Send Email to triblet     
You can also blow rectifier diodes by
disconnecting the battery with the engine

Chuck Tribolet

13whaler1983 posted 10-09-2000 04:13 PM ET (US)     Profile for 13whaler1983  Send Email to 13whaler1983     
hey, If I was you, I would remove my merc and replace it with a Johnson. Yamaha, or Honda outboard of comparable HP. I have ran mercurys and almost every other brand of outboard, and I find that merc's and forces will always give you the most problems. I own a Honda and a few Johnsons, all run.
fester posted 10-10-2000 12:12 AM ET (US)     Profile for fester  Send Email to fester     
I want to thank jimh for the thorough analysis of my rectifier problem. This past Saturday I replaced the rectifier and went through all of the electrical connections on the motor. I found a bad ground wire running from one of the batteries to the motor. I replaced the wire which I hope cured the problem. Sunday I took my whaler to Santa Cruz Island which is about 22 miles offshore from where I live in Ventura, California. Probably ended up running the motor for about three hours and the rectifier held up fine. Thanks again.
ofdog posted 01-13-2006 12:13 PM ET (US)     Profile for ofdog  Send Email to ofdog     
I am running a 1995 Mariner 135 and added a '04 Merc 9.9 kicker on one battery. Last season every thing was fine this season, third trip out my crank battery was dead when I got to the water and when I got home I tested it and I have a constant drain (short) on the battery.
I have isolated the short to the 135 Mariner itself. It is not the wiring, it appears to be a short in the charging system itself (regulator perhaps). Is there something I should have installed when I added my kicker motor to keep it from sending power back into the Mariner,or was this just something that just went bad in "it's time? Any past experiance will be appreciated.

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