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Hate to disagree with Mr. Bentley, but I do
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 26 Apr 2006 10:52:49 Share Post by Email
In Reply to: Thanks Ari, but Bentley says D+ should be higher (more, Mike C, Wed, 26 Apr 2006 08:05:40
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The B+ line is connected to the battery. The lowest it will be is battery voltage with the car off, which would be 12.6 or so with a well charged battery. With the engine running, B+ will be at the battery voltage which is the same as the alternator output voltage (since they are all tied together), which would be in the 13 volt range.

The D+ ties to the low side of the light bulb; the high side connects to the battery. In order for the light bulb to light up, it needs voltage across it. 0.7 volts is NOT enough to light a 12 volt incandescent light bulb, which the ALT light is. When the engine is not running and the key is turned to the ON position, the ALT light is supposed to light. That is because B+ will be at battery voltage (and the same voltage as the high side of the light), and the D+ will have to be a few volts lower than B+ for the light to come on. Since the low side is the field coil, and the field coil is essentially a wire to ground, the voltage difference will be pretty big. So D+ will be much less than B+ with the key ON but the engine OFF.

Now, the field coil is a wire to ground, but quite a long one, so it has a resistance. Much lower than a light bulb, but you can pass current through it.

Engine off, key ON, D+ will be a couple of volts or so, assuming the ALT light is working. And the ALT light will be on.
Engine running, D+ will be close to B+. The D+ voltage will vary with engine speed. I would expect it to be close to B+ at idle; less as engine speed increases. Now for the electronics part - since I expect that the VR uses a PNP transistor in an emitter-follower configuration, D+ can get no higher than 0.7 volts less than B+. (Translation - a built in 0.7 volt drop MINIMUM from B+ to D+. But D+ can definately go well below B+, by more than 0.7 volts.

I haven't looked at the Bentley entry - perhaps it has some extra restrictions on when they say the voltage difference is only 0.7 volts.

However, you're missing the bottom line. Your Battery Is Charging. If the alternator is putting out 13 volts with the engine running, and the ALT light is OFF, then the battery is being charged. You should really stop trying to blame the alternator - it's doing its job! If the voltage were in the low 12 volt range with the engine running, then yes, there would be a problem. But that isn't the issue. So don't worry about the B+/D+ voltage drop - it's a nice discussion, but doesn't fix your problem.

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