External Voltage Regulator and LED

PaulProe · #1 (**permalink**) 08-12-2008, 05:07 PM

I am using an external Motorcraft regulator in my build. It is the electronic type, not the relay type. For the charging light, the dash light had an LED in it rather than the normal incandescent bulb. Put all the pieces together and everything worked fine.

After starting the motor a half dozen times, now the idiot light stays lit all the time. After a lot of cussing, testing, more cussing, I finally figured out there apparently is enough leakage thru the electronic regulator that the LED remains lit, even though everything is fine. Changed to a incandescent and it works as it should.

I can see if the leakage current on the gate used in the regulator was high enough, the regulator would still be good but the LED would light.

What confuses me is it worked fine for the first few times, then began acting up.

Has anyone else run into this. Really would prefer to use the LED, it looks better and resolves the burned out bulb issues. Should I be looking for something else or do you think I hit on the problem with the gate leaking and the LED subsequently lighting.

? ? ?

Paul

ps: before anyone chimes in about using a 1 wire alternator, I am building a period correct replica so the 1 wire doesn't fit the scheme of things.

Ronbo · #2 (**permalink**) 08-12-2008, 08:24 PM

You'd have to put a resistor in parallel to simulate the current draw of the lamp...

LED's are typically around 10~20ma at 2v, (you'll have a dropping resistor in the assembly about 1k ohms). To bump the current up, solder a 330 ohm 1/2watt resistor across the LED lamp assembly (including it's dropping resistor if it's external). This will put you closer to 50mA like the lamp would draw.

It's still a lot of leakage though... Even bipolar semiconductors typically only produce a few microamps of leakage.

trularin · #3 (**permalink**) 08-12-2008, 08:35 PM

Well, the "bulb" is generally the excite resistance for the alternator. Hence there is a difference between the alternator and the battery. This ranges between 40 and 80 Ohms and will allow anywhere between 5 and 70 milliAmps of current to flow between the two points.

The high point is the battery. If you feel you must have an LED, you should use an excite resistor of about 52 Ohms at 20W. Then at the point the alternator/regulator connect to the excite resistor, you can place a 750 Ohm 1W resistor followed by the LED of your choice.

I highly recommend a 1K ( 1000 ) Ohm resistor in parallel with the LED to remove "ANTIGLOW" and a .1 uF/50V capacitor in parallel with the LED as well.

The LED will glow when the alternator side drops indicating a charging problem.

You can PM me if you need more help. I can provide a schematic if necessary.

Hope this helps.

trularin · #4 (**permalink**) 08-12-2008, 08:39 PM

BTW, what you may be seeing is caused by difference current we in the cluster trade call "glow". It is a very small current that is getting through the junction causing the LED to glow even when it is not supposed to. Very common.

PaulProe · #5 (**permalink**) 08-12-2008, 09:20 PM

I forgot to list the parallel resistor.

In addition to the LED, I have a 1/4watt dropping resistor for the LED - 560 ohm. Then parallel to them, I have a 510 ohm, 3w exciter resistor.

Trularin, I've sent you a PM

thanks

Paul

Ronbo · #6 (**permalink**) 08-12-2008, 09:53 PM

Yow! It needs that much current? 52 ohms is around 1/4A... Also this is around 3W dissipation so a 20W resistor is rather large...

PaulProe · #7 (**permalink**) 08-13-2008, 07:07 PM

Trularin,
Received your PM and image of the schematic. Had a little trouble with the size til I went into your gallery and downloaded that image. A little fanagling in Paintshop and I got it large enough to read. Thanks so much for your help.\

In the notes, you refer to R3 as a "sandbox" I am not familiar with that term. Is that lingo for a ballast resistor?-the 52 ohms sounds familiar.

Again, thanks. Will wire up this weekend and report back.

Paul