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  #1  
Old 04-20-2020, 04:50 AM
nobubbles63 nobubbles63 is offline
 
Join Date: Sep 2017
Location: Oban, Scotland
Posts: 25
Default Alternator Warning Light Question

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
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  #2  
Old 04-20-2020, 11:34 AM
John Tierney John Tierney is offline
 
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Location: Vonore, TN
Posts: 417
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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
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  #3  
Old 04-20-2020, 11:57 AM
nobubbles63 nobubbles63 is offline
 
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Location: Oban, Scotland
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Default Alernator Warning Light Question

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!
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  #4  
Old 04-20-2020, 12:01 PM
rapid_ascent rapid_ascent is offline
 
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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.
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Old 04-20-2020, 12:11 PM
WingsOnWheels WingsOnWheels is offline
 
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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.
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  #6  
Old 04-20-2020, 01:26 PM
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Bob Martin Bob Martin is offline
 
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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
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  #7  
Old 04-20-2020, 02:31 PM
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AX-O AX-O is offline
 
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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
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Old 04-20-2020, 03:13 PM
wilddog wilddog is online now
 
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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.
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  #9  
Old 04-20-2020, 04:48 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by rapid_ascent View Post
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
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  #10  
Old 04-20-2020, 07:07 PM
BobTurner BobTurner is offline
 
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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.
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