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Do rectifiers go bad gradually...or do they just die?

D

djcamera

Contributing Member
I'm trying to diagnose & solve an electrical weird thing on my boat. (75hp Mercury made in 1986). I get power spikes in my 12V accessories. My Garmin fishfinder displays 16.5V (sometimes higher) and when I throttle up, it shows 'Battery Voltage Warning' and shuts down. My stereo head also power cycles sometimes. There is no voltage regulator on this motor, just a rectifier.

I bought a brand-new rectifier, and put my multimeter to the new and existing units side by side. The new rectifier shows continuity of .71 on the two input terminals, and the rectifier off the boat shows .57 continuity. With one lead on ground and the other on the output terminal, the meter shows 1.48 on the new unit, 1.28 on the old.

Are these values a sign that my old rectifier is going bad? In all my years with various outboards, I've found rectifiers are either good or bad, not in-between.
 
Usually the device experiences a voltage overload or a current overload. A voltage overload jumps across the substrate and leaves a carbon path between the different voltages which gets progressively worse and finally generates enough heat to make a short circuit path. Unregulated engine charging systems put out 16.5v at max RPM usually. Usually at least a 50V PIV rated diode is used in 12v circuits so unless you had something in the boat that could get on the 12v line and produce higher voltage spikes, like a power inverter backfeeding....possibly, you could forget that.

Current overloads just melt the circuit and you basically have a piece of wire. Voltage overload could have an intermittent result, current would be continuous (opinion).

The conventional regulator uses a "series pass" transistor acting as a variable resistor. The output is monitored and that information is sent back to the "Base" current of the transistor determining it's resistance and resultant voltage drop for the amount of current being consumed at the time. Anything can and will happen but I'd suspect over voltage (external source) damage to the series pass element in the regulator and in time it will quit regulating completely...become a piece of wire, or burn the path open and the output current to recharge the battery would cease........I retired in 2005 and things get a bit hazy when not used daily but that's a general idea.
 
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