10-SI excite voltage?

PJH

Well-known Member
Re-wiring an old tractor. I got in a hurry and tapped the wrong side of the ignition resistor for the excite feed to the alternator. I don't know what the resistor drops the voltage to, and I wonder if my alternator will care. I can change it easily enough, but it will require a splice and another 2 " of wire. If it will work as is, I'd rather not have the splice. I'm overhauling the old tractor, and I don't have it ready to start yet. Just thinking ahead (which I should have done sooner).

Thanks for all advice.
 
It probably will not work unless you have a diode in the wire. In this case the Alt will put 12v in the wire to the points when running. An idiot light, or resistor will add a tiny bit of voltage to the coil supply, but again probably no issue. If it will not excite when started, I would wire it to the supply side. Jim
 
Wiring the alt excite wire to the coil side of the ballast resistor will add at least 2 or more amps load to the ballast resistor, This will pull the coil voltage well below where it should be for a strong spark. If no diode in the excite wire, once running, the alternator will supply full 12V from the alternator to the 6V coil which will overheat the coil and burn the points.
 
These other guys didn't answer your exact question, so yes, you need to move it to the other side of the resistor... for the exact reasons listed above. It won't work well/long where it is.
 
10SI can start charging with 5 volts at 1 amp of field current. Maybe even less but I have ha 10SIs, 12SIs, and 17SIs cut in with excite power at that level.

Spend a buck on a 5 amp diode and you are all set. If you have an automotive ignition switch - no diode or resistor needed. Just hook the exciter wire to ACC (with nothing else).
 
How does a 10 ohm resistor or idiot light allow 2 amps to pass back to the alt to excite it? I thought it only needed a modest 12v ish signal that the key was on. I can stand correction. Jim
 
(quoted from post at 11:43:32 01/17/18) How does a 10 ohm resistor or idiot light allow 2 amps to pass back to the alt to excite it? I thought it only needed a modest 12v ish signal that the key was on. I can stand correction. Jim
have measured more than one 10SI and the number is 0.3 ampere.
 
Less than 1/4 amp through a charge light supplied to the excite terminal will excite the alternator, problem is if a charge light or resistor is not used, the alternator will full "field" where the rotor will draw 3+ amps from the 12V source. If using a diode inline which drops voltage a bit, the field current will still be above 2 amps. Only with a charge light or other resistor will the excite current be whatever the light or resistor flows. (usually about 1/4 amp)

I see it every time I put a repaired SI series alternator on my test stand which has a field current meter. An alternator being supplied 12V to the #1 excite terminal will draw 2-3 amps through the field rotor and regulator. Only when the alternator starts charging so the field current can be supplied by the diode trio, will the excite current draw on the #1 regulator terminal go to 0.
 
Thanks for all of the comments. I'm making this harder than it oughta be, ha.

To kinda explain it - I kicked myself for connecting to the wrong terminal. I intended to hook it to the upstream side of the ignition resister, and got on the downstream terminal instead. The wiring is all new, and taped up into a real nice neat harness, and then I noticed that I screwed up. So I'll splice on another piece of wire and do it right. I'm trying to make this tractor leak free and neat looking - it will be a lifetime first for me. No used lamp cord dangling from under the sheet metal. No wire nuts anywhere. No loose tape tails flipping in the wind. No flickering lights.

It's not the only mistake I made - I completely forgot to include the wiring for the hour meter.

I also should have mentioned that I have a diode in the excite circuit so it won't backfeed the ignition. I have a question for Jon on that subject.

Jon - would you clarify your comment please? Are you saying that a diode is not the best choice in the excite circuit? I have already installed a diode in this one, but on other projects I've always used a resister to limit the current, with no problems.

Thank you
 
Yes, I prefer to use a charge light on any alternator conversion that excites from the ignition circuit.
Not only does the glowing charge light serve as a reminder that the ignition switch is on, but it also tells you that the alternators excite circuit is intact and indicates the alternator is /is not charging when the tractor is running.
Using an inline diode is compact and reliable, but it adds a couple amps more current draw to an ignition circuit that was designed only to power the coil, so may cause voltage drop to the coil and weaker spark.
Using a resistor or charge light which is just a glowing hot resistor, limits the additional current load to the ignition circuit to about 1/4 amp, which will not add a significant load / voltage drop to the ignition circuit that in addition to warning you if you leave the ignition on and is an easier to spot charge indicator that an ammeter needle.
I think the confusion in this thread is that while 1/4 amp of current flow through a light or resistor will excite a Delco SI alternator, connecting one without a resistor or light, either directly or through a diode will "full field" the alternator which draws 2-3 amps and can overload the ignition circuit while starting.
 
Not Jon, but a diode has three options: it can work as intended allowing its rated amps to flow in the forward bias direction and preventing all but a very tiny minuscule flow in the reverse bias, it can go open and not conduct at all in either direction, or it can short, conducting in both directions. In this application it is really not a problem if it fails in the open condition, the alt will just not excite till the RPM get high enough to self excite. With today's rebuilds it may do this at a very low RPM anyway. If it fails shorted the tractor will not shut off, which is not a catastrophe. If the alternator regulator fails and the when it did the unlikely happened and the #1 terminal would go to ground, it could allow its rated current to be drawn from the ignition circuit. This would burn out a 4 amp rated diode and thus act as a fuse in a circuit that can probably handle 8 amps. If the alternator failed open nothing happens except there would be no charging. If it were attached to the coil side of the ballast resistor, and failed shorted, it could take the ballast resistor out as well, but I think that is all. I have seen no alternators fail with ground at the #1 terminal. I have seen no diodes fail in this application. That is my opinion. Jim
 
Thank you Jim and Jon - I always try to check on what you guys think about the different subjects that come up on YT.

You've both given me something to think about, and I'm leaning toward replacing the diode with a resister. I have a little drawer full of hefty diodes, and they were just too tempting and too handy, but I also have a box of heavy resisters, and I'll get the color chart out and start looking. I had an idiot light on this tractor for 30 years - it was an interior dash light out of some long forgotten vehicle, and it hung from it's wires, which were wrapped around the left headlight bracket, complete with wire nuts and loose ended tape that flapped in the fan breeze. But it worked. I don't have a good place to properly mount an idiot light on this tractor, so resister it is.

Thanks to both of you for taking the time to explain this issue.
 

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