Technical Alternator idiot light - will LED work?

One reason is it stops your battery going flat when the car is switched off, if the regulator pack or diode pack go a bit marginal, prevents "backfeeding" to ground. Generally the resistance of these items is high enough the loss is so little it's not a problem for a car that's run at least once a month or so.

Leaving the battery hooked up you're likely to have something else that's more of a parasitic drain (clock, digital radio, faulty switch, etc etc)

--Phil

 

But you don’t have the total amperage of the vehicle in anything other than the amp meter

 

If your vehicle is wired correctly, that happens only in the brief time at startup that your generator or alternator isn't generating current. At that point in time all of the current is also going through your ignition switch.

Adequate wiring and proper connections should be the norm when using an ammeter. Some folks break those rules.

Others don't know those rules at all.

 

I think that diode is there so the engine will shut down when you turn off the switch.

 

I had always read that you needed a diode in the exciter wire to prevent back feed so I decided to try a LED instead. With current on both sides, light is off. When ALT. not charging, current flows through and LED lights. The next car I tried it on, I couldn't get it to work.

Gary

 

Gary, does the LED light have to be installed in a certain direction? Maybe that's what happened the second time you tried it?

 

When I got my car, somebody had wired in an aftermarket ammeter using 14ga wire, between the switched side of the ignition and the alternator (63 Amp). All it did was register the current being provided by the alternator.

It's oft surprising to find how many people don't understand where it should go in the circuit and what should be on which side of it.
The starter motor solenoid and horns on mine are on the battery side, everything else goes through the ammeter with proper crimped terminals and heavy gauge wire.

Oh, and no, the diode is only to stop backfeed and parasitic drain.

If you hook the alternator up to the ignition coil side of the switch, turning the switch off you suddenly get very uneven voltage because the only current drain will be the coil. Without the battery connected the alternator will self sustain so long as the RPM is high enough to generate enough current to run the ignition.
Generally not a problem, unless you have anything transistorized connected up to the coil side of the switch, which will promptly go pop. The battery acts as a buffer to the electrical output of the alternator and keeps it stable. I learned that one the hard way.

Anyhow, I'm rather getting off point here.

Phil

 

bnm

And protect the LED, which are susceptible to reverse biased voltage levels.

 

Fwiw an LED has no place in or on a traditional Rod or custom in my opinion
The light it emits is just ........wrong !

 

I'll see if I can find a dim yellowish color to match traditional headlights...

 

BTTT

I was just looking into whether an LED can be used as a generator warning light, and as I suspected it's not a good idea. They need to work on opposing voltage, which an LED can't do.

I'm not opposed to the use of LEDs on traditional builds in certain conditions. I have a single cylinder Honda cafe racer that was breaking taillight bulb filaments due to engine vibration, so a switch to LED light cured that. Bulbs are getting expensive where I live. I would consider using LED repop taillights on my roadster, although I struggle with how they look. Most of the aftermarket warning lamps are LED these days, but you can get Lucas bulb type ones from British car restoration parts suppliers.

Interesting article FYI.

"You might realize that function of this lamp would be somewhat different if you installed an LED here, because the LED has much higher resistance and is a one-way electrical valve. An LED will usually light up brightly at much lower voltage and with minuscule current, and will not light up at all with reverse bias (higher voltage on the downstream side). It is definitely not recommended to use an LED for the ignition lamp."

IGNITION WARNING LAMP - What It Does And Doesn't Do (mgaguru.com)

 

When I converted my gen to an alt during an AC installation in my 56 the Powermaster 100 wiring diagram showed how to use all the stock gen/regulator wiring and use the regulator as a junction point. It also showed using one of the original wires relugged for the #1 position on the alt and going to to the i-did-it light which I wanted to keep.

 

There's a misunderstanding on how an idiot light works. One side of the light is connected to a switched 12v source, the other side to the regulator. With the car not running or if the charging system isn't putting out enough voltage, the light comes on, getting its 'ground' from the charging system. Once the charging system is up to speed, the regulator also sends 12V to the light. There is now no 'difference of potential' and the light goes out. A dim light means that the regulator is sending less than 12V, this difference is what lights the lamp.

If you use a LED, it will be polarity-sensitive (you want the current flow to go towards the regulator) and as most LEDs require a minimum voltage to operate, if your system is failing but still sending some voltage the light may not come on. Adding diodes or resistors probably won't help.

I will note that older Ford alternator systems do use a resistor in parallel with the light to bring the light circuit resistance down, you shouldn't need one with an LED.

 

You can use an LED, I use one on my OT race car.

 

Would a 6A 50V diode work in that exciter circut to prevent the back-feed issue? And if its installed right at '1' at the altetnator, will the dash charge light still work?