Importance of using a lamp, resistor, or diode in alterna...

Bob · Jul 26, 2016

I have often posted the info below about how important it is to use a lamp, resistor, or diode in the circuit to the #1 "excite" terminal on a Delco alternator.

It usually gets pooh-poohed because someone has set theirs up using (for example) a direct connection to the ACCY terminal on the ignition switch and it has worked for years.

Yesterday, I worked on a tractor set up that way. A wire terminal cracked and broke, interrupting the charging circuit between the alternator and battery, and the little diode trio tried to step in and take up the slack by charging the battery through the direct-connected "excite" circuit with the results being shown in the bottom photo.

<img src = "http://www.gondtc.com/~blweltin/Bob/DelcoSIA1A.gif">
<img src = "http://www.gondtc.com/~blweltin/Bob/DelcoSIB1A.gif">

<img src = "http://i.imgur.com/PmUt90E.jpg">

Good trio on left, burnt trio on right.

souNdguy · Jul 26, 2016

Agreed Bob, I've mentioned this before as well, and usually it would get poo-poo'd as you say.

Royse · Jul 26, 2016

Not to poo-poo but it may help to explain to a lay person, how
a lamp or resistor would have avoided this same situation
other than burning out and acting as a fuse.
The diode shouldn't need an explanation but wouldn't hurt.

JMOR · Jul 26, 2016

(quoted from post at 22:11:32 07/26/16) Not to poo-poo but it may help to explain to a lay person, how
a lamp or resistor would have avoided this same situation
other than burning out and acting as a fuse.
The diode shouldn't need an explanation but wouldn't hurt.

ny of the three prevent high alternator current via the diode trio path, the resistance of a resistor or lamp & the one way valve action of the diode.

Bob · Jul 26, 2016

Any of the parts mentioned limit or stop the reverse flow of current from the diode trio to the battery/electrical system.

Hobo,NC · Jul 27, 2016

Bob I value your opinion its folks like you that make the N/tractor world a better place. I have not ran into this issue and a certified ACC excite lubber.

I am also a lubber of the rare 4 wire alt and a hatter of the grubberized all thread monkey bar mounts.

My Deceased charging/stater expert was a believer in hanging a resistor on the back of a 10SI alt, He frowned at a diode.

I take it this may be a rare event that would take out a $2.00 part.

souNdguy · Jul 27, 2016

Rare, in that you need 2 conditions.

1, this happens in systems where the excite line #1 is run to the accy terminal of a key, or other switched power, and where
there is no current limiting in this line like thru a resistor or a light bulb, and where there is no directional current flow
control as with a diode.

2, the charge line from the alt to the battery is interrupted.

when both 1 & 2 happen, then it's goose is cooked.

J20in1st · Jul 28, 2016

If I am using an upgraded electronic distributor shoukd I also use a resistor?

JMOR · Jul 28, 2016

(quoted from post at 03:49:26 07/28/16) If I am using an upgraded electronic distributor shoukd I also use a resistor?

ob is talking alternator, not distributor.

henryv11 · Jul 28, 2016

I have installed alternators on 3 of my tractors. I have used a toggle sw. to excite the
alternator. Could someone explain to me how the lamp,resistor work. Wouldn't the light On
the lamp be on all the time. I know the diode only lets current go in one direction. I
like the toggle sw. because it is a manual operation but if someone else is driving the
tractor they might not know about the sw. so the other method would be better in that case.

R Geiger · Jul 28, 2016

I like my one wire alternator.

JMOR · Jul 28, 2016

(quoted from post at 18:24:48 07/28/16) I have installed alternators on 3 of my tractors. I have used a toggle sw. to excite the
alternator. Could someone explain to me how the lamp,resistor work. Wouldn't the light On
the lamp be on all the time. I know the diode only lets current go in one direction. I
like the toggle sw. because it is a manual operation but if someone else is driving the
tractor they might not know about the sw. so the other method would be better in that case.

o, it would not be on all the time. It would work just like GM had it working on all Chebby, Caddy, BOP cars.

Jim WI · Jul 29, 2016

The lamp is closest to the original design used by GM. One end of the lamp is connected to the key switch and the other is on the excitation terminal of the alternator.

When the alternator is not charging (tractor not running, something wrong with alternator, etc.), the lamp will be lit because the terminal on the alternator is grounded.

When the alternator is charging, the lamp will be lit because the terminal on the alternator is at the alternator output voltage (both sides of lamp at the same voltage so no current flow).

When you turn off the key switch, the alternator continues to provide voltage to the ignition system. The lamp provides enough resistance to starve the ignition system of current. This then stops the tractor and the alternator voltage drops to zero.

Using a resistor accomplishes the same purpose, but without the idiot light function.

Using a diode completely blocks the feed from the alternator to the ignition system.

Importance of using a lamp, resistor, or diode in alterna...

Bob

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souNdguy

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Royse

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JMOR

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Bob

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Hobo,NC

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souNdguy

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J20in1st

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JMOR

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