Alternator Warning Light Question

nobubbles63 · #1 04-20-2020, 04:50 AM

I am looking, belatedly, at fitting an alternator failure warning light from the L post on the standard alternator. I have read on other posts that an LED light will need a resistor fitted into the circuit to stop the LED glowing dimly all of the time.
Can anyone advise what size of resistor is needed to fulfil this task.
Thanks
Derek

John Tierney · #2 04-20-2020, 11:34 AM

Not sure what you mean by standard alternator, but the plane power 60 amp alternator allows an optional light on the white wire with no resistor. I tried to use an LED but didn't figure out the current flow and ended up with a standard incandescent light from ACS (can't seem to find the one I used).

http://planepower.aero/pdf/AL12-EI60_C.pdf

nobubbles63 · #3 04-20-2020, 11:57 AM

Thanks John. That's the alternator I have and am trying to wire up. I had just found an earlier thread that said the incandescent was good, but if you go LED then a resistor was needed otherwise it glowed all of the time. But the original posters did not say what the value of that resistor was and I'm no electrician!

rapid_ascent · #4 04-20-2020, 12:01 PM

Just a general comment. A resistor is always needed when using an LED to limit the drive current. The only time a resistor would not be needed is when you are driving it with a constant current driver.

I'm not familiar with your application so I don't know if that unit has an internal resistor.

WingsOnWheels · #5 04-20-2020, 12:11 PM

For a 14v system (assuming that output is system voltage), generally a resistor in the 400ohm range is good, but it depends on the specifics of your chosen indicator.

R= V - Vled / I

R= 14-3 / 0.03

R= 11 / 0.03

R= 366.6 ohm (use the next higher value available), so 470ohm is probably your best bet.

Bob Martin · #6 04-20-2020, 01:26 PM

I just went through this with B&C.
470 Ohm Resistors, 1/2 W,
But they said to install one on each side of the bulb.
Haven't tested it yet.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

AX-O · #7 04-20-2020, 02:31 PM

I have a plane power alternator and that light has never worked at all. I even replaced the light bulb with the ridiculously priced one they say is compatible.

https://www.aircraftspruce.com/catal...?clickkey=3929

wilddog · #8 04-20-2020, 03:13 PM

I?m using B&C alternator and regulator with a LED low v indicator. Light is dim most of the time but starts flashing at 12.6v. No resistors yet but plan to add them to get rid of the dim glow. Glow is not visible in sunlight, flashing is.

lr172 · #9 04-20-2020, 04:48 PM

Quote:

Originally Posted by rapid_ascent

Just a general comment. A resistor is always needed when using an LED to limit the drive current. The only time a resistor would not be needed is when you are driving it with a constant current driver.

I'm not familiar with your application so I don't know if that unit has an internal resistor.

That is not universally true. The LED indicator lights that I use on my planes are set up for 12V and have built in resistors. I can even dim them with a standard Pot. Many 12V rated LEDs has the resistor built in.

Lary

BobTurner · #10 04-20-2020, 07:07 PM

There's a bit of arguing here over terminology. Strictly speaking, an LED is a diode like device that emits light. In nearly all applications something must be used to limit the current. While some manufacturers pre-package the LED with a resistor, it is not "just" an LED. It's an LED with built in resistor. If you install "just" an LED without a current limiter of some type it will self-destruct almost as soon as you apply power.