1 Wire GM Alternator and Generator Light

[Budman1758]

Searched most of the forums here and did not find an exact answer to this one. I have and old vehicle and have installed a newer 1 wire GM alternator in it. These alternators do not have provision for the "idiot" light on the dash. What I am looking for is a simple circuit that will trigger a relay to operate the light. I want it to light at just under 13 volts and go out at just over 13 volts. (or thereabouts)

I had thought I could just use a 13v zener diode but it seems to require a little more than that. I tried to put the zener inline thinking it would conduct "backwards" when the voltage went above 13 volts but it seems they don't work quite like that. There is an outfit that sells something exactly for this purpose but the voltage is set at 11 volts and they want 30 bucks and 10 bucks to ship the darn thing. Diodes are much cheaper. :>)

Anybody have a simple circuit in their back pocket that will do this? Any help would be much appreciated.

 

[duke37]

What does the idiot light do? I think the earlier alternators used separate rectifiers to check if the alternator was producing a voltage.

A LM3914 bargraph display will act as an accurate voltmeter with 10 steps. This can be used to drive an idiot light directly.

A ferrite sleeve over the alternator output can be wound with a secondary to detect the AC ripple when charging. A Hall effect detector could measure the charging DC current.

 

[CDRIVE]

Prior to solid state automobile regulators employed a very basic design that took advantage of the hysteresis that electromechanical relays inherently possess. This simple circuit employs the same technique.

How it works:
EM relays have a Pull-in and Drop-out specification that relates to the minimum current required to energize the relay (pull-in) to open or close the contacts and a maximum current (drop-out) required to hold the contacts in its energized state.

Notes:
(1) This circuit employs a 12V SPDT relay that can be found by following the link shown in the schematic. A SPDT relay was chosen because they're easier to find than a SPST N/C relay. It was also chosen because the spare contact can be utilized to adjust the relay's holding current by connecting a second pot from the N/O contact to the hot side of the coil.

Attached Circuit Theory Of Operation:

Rvar is adjusted so that K1 will energize at ~13V. This will turn off the ALT lamp. If the battery is charging the lamp will remain off. If failure occurs which causes the battery voltage to drop below the hold current of the relay the ALT lamp will turn on.

This is a very simple but robust circuit. It lends itself well to the harsh environment of automobiles. The pot can be replaced with a fixed resistor after the pull-in current has been determined. On that note this circuit is not plug and play. It must be prototyped on a bench using a variable voltage supply.

Chris

 

[Budman1758]

Thanks. That should do just fine....

 

[CDRIVE]

You're welcome but just for the record I like Duke's idea but adding an additional diode to the alternator stator would probably be difficult to say the least. Are you sure that terminal doesn't exist?

Chris

 

[Six_Shooter]

The terminal is there for the charge light, but is used to self excite the alternator.

I would NEVER use a single wire alternator in any vehicle, not even a dedicated race car.

Non-self exciting alternators are so easy to wire up that I don't understand why people insist on buying these still. They also have low RPM charge issues.

 

[CDRIVE]

The terminal is there for the charge light, but is used to self excite the alternator.

I would NEVER use a single wire alternator in any vehicle, not even a dedicated race car.

Non-self exciting alternators are so easy to wire up that I don't understand why people insist on buying these still. They also have low RPM charge issues.

You've tweeked my curiosity. Do you have any links that include photos and schematics showing the two designs?

Chris

 

[Six_Shooter]

No, I don't have any links. Google should be able to help you out there.

I've been around cars for many decades, and have seen the use of single wire alternators rise and fall.

The single wire alternators literally look like it's as simple as connecting the sense and charge light wires together, but I'm sure there's a change in the regulator itself. There are kits to convert the particular style of GM alternator to single wire that includes the "self exciting" regulator.

On the flip side the non-self exciting regulator only needs 3 connections, one of of which IS the charge light connection, that is simple a light bulb, typically a 194, that is in series with a 12 v switched trigger, and then the sense wire that samples the voltage. In an OEM application the sense wire is connected to either the starter battery connection or the battery. Many swaps that I have seen simply loop this wire to the charge terminal on the alternator, though this doesn't usually charge the vehicle correctly and typically experience a slightly low charge condition.

Anyway, there are more in depth descriptions and articles out there, if you're really interested.

 

[Budman1758]

Mad Enterprises has some pretty good info on 1 vs 3 wire setups. Lots if good info there.

I have had the 1 wire setup for several years due to the fact that I got the alternator for free.

I just recently decided to get my idiot light working cause its a much better attention getter that hoping I look at the voltmeter thats under the dash.

Turns out that I can just as easily replace the voltage regulator in the alternator and for pretty cheap too. I see them on EBay for under 10 bucks. Probably gonna go that route.

 

[CDRIVE]

No, I don't have any links. Google should be able to help you out there..

You're correct. In less than the time it took you to type these 5 paragraphs I found all the info I asked for... and then some.

Chris