I'm Kinda Dense: Alternators Again

Please explain to me again why you fellows put a diode in the exciter circuit when installing an internally regulated alternator?

Is this for gas engines only if you tie the exciter to the same ignition switch terminal? I guess I'm just not understanding the purpose for the diode.

I wire the darned things hot and it works just fine without any drain to speak of on the battery.

Allan

So I just threw away all my diodes, idiot lights, etc. Snice most Farmalls have an extra wire in the harness, I just wired up a push button starter type switch to excite my alternators. I just give that button a push after starting the tractor and if amp gauge jumps you know she is a chraging. With the push button I know I will get no back feed when tractor is not running. I have three done this way, one has been going since 1992. The one going since 1992 is a 6 volt alternator and the battery put in new in 1992 is still doing the job. I started it about a month ago, hadn't been going since before Christmas, fired right up on about 3rd turn of starter.

I tried one, a 12 volt with all the diodes, idiot lights, etc. Worked alright, but quite frankly I don't have the patience for running to auto parts store every year for lights, diodes. etc. It's close to 0 degrees F here this morning and I know I could go to unheated shed, start all three tractors and be back in time to imediately respond to any reply you might make.

Problem is my good better half, tells me I have other assignments as we have company coming for dinner. Take the garbage out, go to store, etc, etc. Funny that stuff just wouldn't matter if it were just going to be the two of us. It's our daughter and husband coming, she just doesn't want them to think it may be time to ship us off to old folks home.

Don't sweat the retirement home. Your daughter knows you're too darned ornery for that! :>)

Yep, juice isn't my strong suit either, although I had to work on these darned alternators for many years.

Back in 1972, kind of by accident and in a rush, I hooked up that darned exciter circuit direct to the battery voltage to get an alternator to work. Heck, it seemed to work just fine with no ill effects, so I've wired a lot of the silly things this way.

I've never had one drain the battery, so if they are a bleedin', they aren’t bleeding very darned much! :>)

Later

I always figure if daughter sees me fooling with these old trators when it's this cold, will quell any thoughts on the old folks home. Then again most young people may think we've lost it completely. I've really never had many fears. My good better half on the other hand has arthritis quite badly and she finds this big house a bit much, so I got to cover for her.

I always figure if I get half those years from a battery it's done well. Oh well this makes up for the ones that calved a day after the warrenty ran out. I do have better than average luck with batteries though, always been quite fussy about the charging system. That is why I always say no shame to put an alternator on these oldies. Why it is even our responcibility to make sure these run better than new. Afterall, I doubt the engineers, designers, builders, etc. never dreamed they were building something that would be running 50 years down the road. I just wonder sometimes how many guys are still living who bought a letter series Farmall new.

I can't lay claim to that. 300 was sort of my first input although I wanted dad to buy a Super 77 Oliver diesel, didn't get my way. Farmall 130 in 58, yes, dad wanted a 230, I talked him out of it, already had H and 300, we needd something compact. Had a fair bit of input on 560 diesel, dad was leaning to another breed. His niece and husband were selling David Brown and New Holland. I loved that New Holland equipment, but couldn't hack DB.

The diode prevents this backfeed of current. It is NOT needed to prevent draining of the battery with the tractor shut down. Using an "idiot lamp" does not PREVENT this backfeed of current, however, it limits the current to a much lower "safe" level.

I always use either a diode or a #194 lamp, as this is how the alternator was designed to be wired.

With a direct connection from the switch to the alternator #1 terminal, it will work just fine, however, if there is ever a poor connection in the high-current circuit from the alternator's large output post to the battery, the small wire, the voltage regulator, and the diode trio inside the alternator will provide an alternate current path for the alternator's output, and the smoke will be let out of the system, requiring alternator repairs.

I repair alternators for a living, and have seen this a few times. People who don't fully understand what goes on in the regulator cicuit will tell you the "idiot light" or diode is not necessary, however, there are times having one, as the designers of the alternator intended, will save on $$ repairs.

As originally intended by the alternator designers, a small lamp is used instead of a diode. The lamp serves 2 purposes: 1) It limits the backflow current to a level below that required to fire the coil, and 2) It works as an alternator "idiot light" - it will light if the ignition is on but the alternator is not generating current.

Make sense now?

Thanks a lot and that makes perfect sense. I thought it had to be something like that.

Thanks again,

Allan

Like we were talking in the other thread, I take a 2” piece of 16 gauge wire and just loop the thing from the exciter lug to the battery post on the alternator (I wouldn’t do this on someone else’s rig, but have done many, many setups of my own). The regulator stays “hot” all the time.

In essence, this makes it truly a “one wire” alternator, as far as the electrical system goes, but the darned thing is always “On” and is never isolated from battery voltage.

You mentioned that the field circuit would drain the battery. On an externally regulated alternator, this is certainly true.

But, on an internally regulated alternator (I’m talking GM), the exciter circuit does not provide field current in any way whatsoever and the battery does not provide any field current to the alternator in any form.

If this were the case, the darned alternator would stop charging if you removed the battery.

It doesn't, take the battery out of the picture and the alternator just keeps right on a doin' it's job of maintaining system voltage and providing field current as needed.

The exciter current only turns on the regulator gate and takes the alternator out of isolation. Yes, by hot-wiring the exciter, the regulator stays on all the time, but so what? There is always power at the light switch too.

Since the regulator uses the alternator's own induced voltage as a field voltage source, without rotor rotation, there is absolutely no draw thru the field circuitry anyway.

I would almost bet that there is more of a drain thru the dirt/dust between the two battery posts than the miniscule current that is holding that regulator gate open.

Just my view, even tho it might be somewhat up-side-down. :>)

Allan

Not quite! The exciter circuit uses battery current to provide the initial “flash” current to the field at startup. It draws this current until the alternator develops an output voltage higher than the battery voltage.

>If this were the case, the darned alternator would stop charging if you removed the battery. It doesn't, take the battery out of the picture and the alternator just keeps right on a doin' it's job of maintaining system voltage and providing field current as needed.<

Not recommended! The battery serves as a stabilizing ballast and helps the regulator system voltage under control. With the battery removed the system is liable to go unstable and either exhibit wild voltage swings (25 – 30 volts or more), or the alternator may simply self-destruct. Don’t ask how I might know this!

>The exciter current only turns on the regulator gate and takes the alternator out of isolation. Yes, by hot-wiring the exciter, the regulator stays on all the time, but so what? There is always power at the light switch too. Since the regulator uses the alternator's own induced voltage as a field voltage source, without rotor rotation, there is absolutely no draw thru the field circuitry anyway.<

Again, not quite. If the rotor stops, the regulator draws a couple hundred milliamps from the #1 terminal as it attempts to reflash the field. It will continue to draw this current until the alternator resumes generating current.

If you run the alternator daily it won’t be a problem. But if left idle for several days the constant small drain will kill the battery. Thus you must have a switch to cut off voltage to the #1 terminal when shut down.

Hope this makes sense! Bob M

Here's a generic schmatic of the regulator/alternator.

Allan

Boy! You ain't the only one; I can't even remember back that far, seems like just a fog now after all these years. :>)

I'm not disputing the initial charge flow in the field circuit; yes, of course it happens. This is what Bob describes as the "flash".

But once that happens and the two legs of the field are at the same potential, current flow then becomes nil 'thru' the field and (I think) the main offending drop, or our controversial "bleed" if you will, is then seen as the remaining voltage drop across that gate.

Does this make any sense or am I still looking at it upsidedown? (I have a really bad habit of viewing from this perspective) :>(

Allan

The flash that Bob refers to is the initial exciter current that starts the alternator charging. The charging voltage is higher than the initial voltage from your battery, so the current from your ignition switch will be overridden by the higher voltage from the alternator voltage at this point in the initial cycle. Look at the schematic - do you see that line that goes from one of the charging phases (after the diode) to the tie point on the right side of the regulator? This is where that voltage will be introduced. From this point on, the regulator circuit will determine how much current is allowed to flow through the exciter - thus regulating the output voltage. If your alternator does not start producing its own voltage, the exciter will continue to draw current from your battery. After all, if an idiot light were in that circuit, it would be lit while the alternator is not charging. This lit lamp means that current is being drawn. We are talking about at least .1 amps (100 ma). If you have a 40 AH battery, it means that your battery will in theory be dead in 400 hours. Remember, without a lamp in there - a short circuit - you will draw even more current.

I still maintain that you have a one-wire alternator, through a mistake or some other reason. A three-wire would run your battery dead if you left the exciter circuit energized.

I was not going to dispute what the young man said.

:)

As I recall, we've had this discussion before, haven't we? :>)

Allan