Automotive alternator windings & rectifier

has photos of an alternator with 4 connections for windings (each of 3 phases

• common), and the 8 diode rectifier. But wiring diagrams show only 3 connections and 6 diodes.

Where does the common winding terminal connect (there is a connection on the rectifier plate)? Where do the other 2 diodes connect?

I handled one of these disassembled units but didn't have time to ohm out the connections.

Thanks.

Not one of the pages shows four connections brought out from the windings.

I don't see any 8 diode rectifier.

Yup.

Common is created by the way the diodes are connected. >THAT< common is connected to the auto frame.

Hmm.

Wo uld you ask your question again and maybe clarify the situation?

The common point is sometimes used for fault detection.

When I was at Motorola SPD in the '60's I developed a circuit that could detect just about any fault, shorted or open diodes and shorted or open windings, by viewing that node. That node, BTW, is often labeled "S". Old GM alternators brought that out as a post on the back of the alternator (before regulators were internal... that's what some of my patents are all about... integrating the regulator).

That was before CAD ;-) If I can find my hand drawing, I'll post it. ...Jim Thompson

The common in the Wye connection (called "Neutral Junction" in the photo on page 18) goes to the 4th pair of diodes (and typically the regulator). The other three pairs of winding ends ("Stator Lead Ends") go to the other three pairs of diodes.

Not the alternators I've taken apart. The star point is typically a crimp, and serves no other useful purpose.

The field, and regulator power, are normally taken from an additional set of diodes, usually 3, ie one per phase, such that the regulator is effectively isolated from the battery when the engine is not turning.

dumb question.

what EXACTLY does the charge lamp indicate?

what triggers it to go on and off?

can it simply be removed and ignored?

One cause that can light the lamp is a broken belt. You would like to know as soon as that happens.

It's called an "IDIOT" light for a reason... as presently implemented it mostly tells you that the alternator is turning and has current in the field winding.

As I designed, the circuit compared the average value of "S" with 1/2 of the "A" terminal. Deviations were correlated with fault type.

Some GM versions enable the alternator regulator to function, so you can't remove it. (Saves GM the cost of a wire ;-) ...Jim Thompson

Photo on p. 18.

Photo on p. 25. Note callouts for 6 diodes but you can see 8 round circles -- those are the diodes. Eight. (And yes, I've seen the rectifier and each of the 8 have leads soldered to each bus (or something...). I can confirm that firsthand.)

My conundrum remains:

By "common" are you referring to the (-) terminal? We agree that that terminal goes to chassis (presuming neg. ground chassis).

I'm talking about the common point among all 3 windings -- the center terminal of the "Y" configuration.

There ya' go...

Thanks.

afaik the common of the 'Y' does not connect anywhere else. To do so would make it a "Wye" setup.

You're right, I'm wrong. I see the four a little hidden by the picture angle.

I'll take that as so, but I don't see it.

The + and - from the six diodes is connected to a series pair of diodes. The junction of that pair goes to the neutral junction of the stator coils and the regulator. Like this:

• --->|---+--->|--- - | | Neutral Junction-+->to regulator

See

Ed

Thanks Ed. I see that there are 2 additional figures:

ALTERNATOR WITH NEUTRAL SAMPLING CIRCUIT

which shows use of 8 diodes, and

ALTERNATOR WITH STATOR COIL SAMPLING CIRCUIT

which shows use of 9 diodes.

Appreciate your pointing out this web page. Thanks.

That pictured alternator is one used on Hondas. ISTM it's a Nippon-Densi or such.

I have too much experience with same. I went through 3-4 boneyard ones before I bought a rebuilt one from Retarded Auto Parts. That brand/model alternator is nice because the diode array unscrews easily. No unsoldering needed.

That particular alternator has 4 pairs of power diodes; the center point of the wye has a pair from there as well.... and I have no idea why... Jim??

That said, all recent alternators also have a diode trio of tiny diodes. Their function is to rectify enough power to excite the rotor. It takes a few amps to drive it. Until it's up to speed, that excitation comes from the battery, through the red idiot light. When the alternator is up to speed, there is 12V on the battery side of the lamp, and 12v on the load side....and it goes out.

Those were alternators with only six power rectifiers, right?

The power rectifiers connected to the Wye common are used to extract

3rd harmonic power- yielding up to 10% additional current capacity- when operating at high speed.

Best regards, Spehro Pefhany

One problem with this arrangement... if the red idiot light burns out or a wiring fault opens the circuit, the alternator won't "bootstrap" the next time you start the car, and you won't (of course) get any red-light warning. Dead battery, here we come.

Ask me how I know :-(

But THAT'S why I'm asking here: this Denso alternator (used in MANY Hondas, Toyotas, and myriad other makes) does have a common terminal connection to the rectifier. The rectifier has EIGHT diodes.

Yes I agree that common configuration is 6 diodes. That's why I'm confused. The system is quite populous and apparently functions. I just don't understand how.

That's why you LOOK at the Idiot Light when you turn the ignition switch on, just before you start the engine. A lit ALT bulb means it is present and functioning.