Actually two questions. Can a bad alternator cause the battery to drain down? Is that the function of the diodes, to keep juice from flowing backwards?

Please - no lectures on gravity keeping liquid juice from flowing backwards...

Yes, but it's usually the brushes.

Good luck,

Only if the diodes are working right. A popular failure mode of diodes is to short, followed by their reverse voltage to drop.

Alternators need time to replace battery charge. A weaker alternator will need longer than a stronger alternator.

Your driving conditions may not allow enough time, or the alternator is too weak there may not be enough power. A weak alternator will mean that the battery is drained down a little bit at a time.

Pretend the battery is your wallet, money is current and the alternator is your paycheck. If your paycheck isn't big enough, or doesn't come often enough, you'll go into deficit spending and eventually you're broke!

A long time ago, when I was a young'un, : cars used dc generators. With the dawning of the solid-state age, diodes became available and alternating current technology could be used. "Alternating current generator" was a mouthful, so they shortened it to "alternator." Automotive alternators generate more power for their weight than dc generators. All generators have brushes which wear down and need to be replaced at some point. Most alternators do not.

Diodes convert alternating current to direct current by blocking current in one direction. Clever winding and diode connection puts all of these one-way currents together and then out into the electrical system. For single phase, a "bridge" of 4 diodes is used. Most alternators are 3-phase use 6 diodes.

A shorted diode cannot block current, will not rectify ac into dc and will let current from the battery flow into the an alternator winding, draining the battery. In this case the problem appears in a short time

Hope this helps, or even makes sense! Good luck and good hunting.

BCingU, Jim

Check the voltage regulator brushes, behind the alternator. Easy and cheap replacement. If that does not work...hummm...my second guess would be gravity keeping liquid juice from flowing backwards...
Good luck!

__________________
J.H.
'86 300E

Just an adder to Jim H's excellent history lesson.

The way the old two brush generators worked, all the current had to pass through the brushes. In the alternator, only the excitation current passes through the brushes, thus the brushes are smaller and more trouble free. Additionally the alternator provides a good bit of energy at engine idle speeds. The old two brush generators produced virtually no energy at idle speed.

Additionally, I don't know about their application in an automotive alternator, but I can never remember seeing a diode in other circuitry short. If they fail they typically burn open.

Have a great day,

Actually, the diodes are there to produce a pulsating DC voltage. As you all have been wondering why they are called alternators, it's because they produce an alternating current. The diodes rectify this voltage and thus produce a pulsating DC voltage. The size of the stator windings determine the current the alternator will produce. Thus your higher rated alternators have somewhat larger stator winding. If my college physics serve me, the length of the windings, RPM's and strength of the field determines the voltage and the gauge of the windings and the rating of the diodes are the limiters on the current produced. Also, most quality alternators produce a three phase output, which is to say there are three sets of identical windings in the stator, one end of each is grounded and the other end is attached to a rectifying diode. So the end result is a smooth pulsating DC voltage.

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.

If I could draw pictures here, I could get carried away with this. The idea of the three phases is to then rectify it, without the voltage ever going to zero. If you only had one phase, the full wave rectification will allow the voltage to go near zero. With three phases, overlapping as they do, the variation in voltage(ripple) is minimized.

Have a great weekend,

Larry, full wave retification would result in a voltage regulator that would be rather large, as you would have to apply a full wave bridge to each phase winding in the stator. And ground would float between the positive and negative outputs. The is no need for producing a negative voltage in automotive applications. So, instead of having 3 diodes, you would end up with 3X4 (4 diodes in a FWB config)or 12 diodes. Are we there yet???

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.

There are 6 diodes in an alternator, two for each of the phases (only 2 diodes are needed for full-wave rectification when you have a center tap or neutral return).

My alternator failed because the brushes wore out. There is a problem in the design of the dummy light circuit in my car (240D) in that you lose the dummy light when one or both of the brushes lose contact. If you're really observant, you will see that the alternator light doesn't come on when you switch the key on but don't start the car. This should be the clue that you have a problem, but if you're like me, it's easy to miss.

If you have a shorted diode, the battery will be discharged through the alternator when the car is not running. My experience is that the diodes don't fail that often, but you can tell by seeing if the alternator is drawing current from the battery when the car is turned off.

Another tip that I have learned the hard way...never use the cars alternator to charge a completely dead battery, if you have a choice. You are much better off to use a battery charger to bring the battery most of the way back up. Pulling 70 or 80 amps out of any alternator for a period of several hours causes the alternator to get very hot, and heat is the enemy of semiconductor reliability, ie the diodes.

I don't really understand what is meant by a "weaker" alternator. If the voltage regulator, brushes, and coils are in good shape, the alternator will easily put out its rated amps. They really are a very, very good, robust, design.

I hope this wasn't too boring.

Thanks
Joe

Joe, thanks. I did get my numbers wrong. Six is the correct number. I wish I could past a diagram of a full wave rectifier into this post. A picture is worth a thousand words.

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.

How's this:



or this:



[Edited by Webmaster on 12-08-2000 at 10:17 PM]

__________________
Bill Wood - Retired Webmaster
My Personal Website
1998 Mercedes E430
2010 Toyota Sequoia
My Photo Albums

Bill, where is that circuit diagram from??

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.

Two sources:

http://www.ednmag.com/ednmag/reg/1996/031496/06di1.htm

and

http://www.science-ebooks.com/electronics/transistor_circuits.htm

__________________
Bill Wood - Retired Webmaster
My Personal Website
1998 Mercedes E430
2010 Toyota Sequoia
My Photo Albums

I'm certainly not an alternator specialist, but 240D Joe confirmed what I thought it consisted of; 3 phases, 6 diodes, full rectification, very efficient(at least relative to a two brush generator).

Have a great day,