JoeMag

Decided to use an LED as my alternator warning light. When car's running the light is off, but flickering very dimly. ...similar thing on motec engine warning light (also using LED). These are simple LED's from radio shack, but are pretty darn bright. I'm guessing its just induced voltage in the wires running from back to front (would never notice on incandescent light). I'm thinking of just putting relay in because I'm almost positive that will drop out when alternator exciter goes from ground to alternator volts.

...any other ideas?

Left is standard setup, and right is relay setup. (alternator exciter = alternator sense).

ltc

I'm not exactly sure what you want the LED to indicate, but I don't believe your drawing on the left is correct.

If you replace 'Batt' with "GND or Chassis" and verify that the Anode of the LED is connected to the alternator output and the Cathode is connected to GND/Chassis and you select a resistor value to properly limit the current, then it would work/indicate alternator output and not flicker.

If you have specific questions, please feel free to contact me via PM or email.

JoeMag

Lewis -- it's connected so light is ON if alternator fails, so wiring is correct. alternator exciter is grounded when not working and alternator voltage when it is, so batt = alternator volts (theoretically) so LED goes out. It is just barely lit and slightly flickering with car running -- I just think it's sensative enough to the difference in voltages between batt connection and wire runing from alternator. Standard incandescent light would show nothing...

Just had alternator rebuilt so I know it's okay.

ltc

Understood.
There are other circuit implementations possible to have the same functionality but not have the LED 'flicker'.

Premier Motorsp

I am not sure sure what is going on in your case. I recently had to do something similar in my RSR. In checking the factory wiring in the clock I found the following:

It looks like they want about 15 Ohms in the wire to the field coil. They got this by paralleling two 40 Ohm resistors and a 1.2 watt light. The total resistance was around 15 Ohms. The resistors are big so I think they want about 3 watts of power dissipation.

There is NO diode.


I am guessing that your resistor is about 600 ohms to use an LED at 14 Volts. I would switch it to an standard 12V bulb and run the resistor(s) in parallel with the lamp.

Ahsai

I thought this alt warnig light (sometimes called the "idiot light") is built-in in all cars? Why do you have to dulpicate it?

The diagram on the right looks like it will work. Not sure what's the effect of the induced current of the relay when it's being powered on and off...As an alternative, you could put a zener diode (6v?) in series in your left diagram to remove the flickering.

Skip Wolfe

Joe has built this from a bare tub and it is custom wired with a custom dash so there aren't any of the factory warning lights.

Geoffrey

The way the alternator excite wire works is that you have +12v on one side of a light or in this case a diode and the other side goes to the Alternator Excite terminal. When the alternator is not turning, the alternator excite terminal is grounded through the field wires and therefore the light turns on (completed circuit). When the alternator is spinning, it is producing voltage and the excite terminal outputs 12v, so 12v on each side of the light will cause the light to turn off. If you want to use an LED, you need to have the anode wired in the correct direction for it to light, however, the amount of current it takes to light is not enough to turn the alternator circuit on. You will need a 3 watt 250ohm resistor wired in parallel with the LED for it to excite the alternator. Regardless, if you rev the engine past about 2500, the alternator should begin producing voltage on its own and the light should go out.

It sounds like the LED is flickering because the alternator is not spinning fast enough to produce a steady 12v.

Ahsai

Ah, that makes sense now

Yes, I agree he will need that 250ohm resistor. The LED flickers because it's just too "sensitive". Any voltage difference a little over 1.5v will make it flicker. Hence I suggest a 6v zener diode in series with the LED to make the "turn on voltage" at least 7.5v or so, which will be the case if the alternator fails.

JoeMag

Geoffrey -- thx. The alternator is definatly working. I put batt volts on the dash and it's 14V, and engine was running at ~2000 rpm. I do see what you say about reving to excite it... When I was watching this LED flicker, the car stalled, and when I restarted it, it took a few revs to make batt volts jump up to 14V.

Ahsai -- Great idea! Need to stop over at radio shack to what kind of zener's they have.

JoeMag

Geoffrey -- minor detail here, but curious why you recommended a 3 watt 250 ohm resistor? Resistor power being V^2/R. ...at 14V and 250ohm = 0.8W

Bri Bro

If the resistor is used were the ambient air is hot you need to derate the wattage ie use a large wattage resistor. There are charts but the rule of thumb is just double the wattage. Also, the larger 3 watt resistor will run cooler then a one watt resistor, everything else being equal.

JoeMag

Spent a good chunk of today doing some testing on this... I have spare alternator and used a drill (goes to 2.2k rpm, about ~1k engine rpm I think) to do the testing.

Alternator would definately not excite with just LED circuit (have zener in it now too, 5.1V one). Tried with 250 ohm resistor in parallel with LED circuit and could not get it to excite (maybe not spinning fast enough). Put an incandescent light (12V, 10W) in parallel with LED circuit and alternator generated voltage and LED went out, NO dim glow or flicker. Calculated light was equiv to about 22 ohms (was pulling ~0.5,0.6 amps when lit). Got some 50 ohm 10W resistors, put two in parellel (equiv to 25 ohm) and alternator generated voltage, however, LED went out, but as I went up to max speed LED dimmly came on; just like I saw in car. Did some voltage checks and at high rpm's (remember, this is equiv to ~idle), the exciter voltage is about 0.1V higher than the alternator output. This was confirmed by measuring voltage across LED ciruit and watching current change directions through resistor. As soon as the exciter voltage starts to go above the alternator output the LED starts to glow (remember this is with 25 ohm resistor in parallel). This must be back leakage through LED causing it to light (??). In any case, with the 12V, 10W incandescent light in parallel with LED ciruit (as mentioned in 3rd sentense this para) works. I think it's because the resistance is so low in the light (~2 ohms when not lit) the reverse current is shorting through it versus LED circuit. ...I guess I'll have a dim light masked behind my dash.

btw, zener works very nice too. LED is out when incandescent lamp still has faint glow, and without zener LED slowely dims out until just about no voltage is across it.

Thought about just doing the incandecent light, but I have CF dash cut for LED bezels (sp?), and I like the way they look, clear when off and blaring red when on (the LED's actually look pretty much just like the AIM alarm lights).

thx for the help...

ltc

No.

Bri Bro

Did you put the 250 ohm resistor across both the LED resistor and the LED, or just the LED?