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Re: An electrical problem

To: bullwinkle <yd3@nvc.net>, mghirsch <mghirsch@netzero.net>
Subject: Re: An electrical problem
From: Barney Gaylord <barneymg@ntsource.com>
Date: Mon, 12 Nov 2001 10:42:44 -0600
At 07:35 PM 11/11/01 -0600, bullwinkle wrote:
>Maynard:
>
>I am sorry but I don't buy it.  There's got to be some other
>explanation.
>
><<<snip>>>
>To be concise.  The dynamo light is triggered by amperage, the
>alternator by
>voltage.
><<<snip>>>

Shucks bullwinkle, I understood it alright.

When the generator output drops too low its output voltage drops below 
battery voltage, and currrent flows from the battery through the warning 
lamp to the generator, making the light glow.  The brightness of the lamp 
is related to the amount of current flowing through it.  And if the voltage 
regulator should go whacky and the generator start overcharging at maybe 18 
volts or higher, then current will flow from the generator through the 
warning lamp to the battery, and the lamp will also glow with the 
brightness related to the amount of the current.  As such, "the dynamo 
light is triggered by amperage."

The warning lamp for the alternator is hooked up to a relay contact, or 
otherwise to a solid state regulator circuit involving a diode (or three 
diodes in the alternator).  The signal that powers the warning lamp is a 
voltage that is generally not proportional to the output of the alternator, 
but switches suddenly at a prescribed threshhold voltage.  As such, the 
alternator light is triggered by voltage.  What causes the glow of the 
warning lamp with a partially failing alternator is current leakage through 
one failed diode in the rectifier circuit inside of the alternator (at 
least I think that's a legitimate way to state it).

With one diode failed you can still get partial output from the alternator, 
often sufficient current to keep the battery charged without any appearant 
problem, and sometimes continuing for years.  When a second diode fails the 
rectifier fails to work completely, and the output current from the 
alternator stops, and the warning lamp lights brightly.

Barney Gaylord
1958 MGA with an attitude (and occasional dim bulb)
     http://www.ntsource.com/~barneymg

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