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LED question

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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Fri Feb 18, 2011 3:38 pm Post subject: LED question

Bob: is it OK to wire an LED directly (through a resistor in series) to a 14V system?  Or should a voltage reduction circuit/device be used.  I always though most LEDs were supposed to work on 5V. And 14V would be too much.

 

Noel

 

From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert L. Nuckolls, III

Sent: February 18, 2011 11:25 AM

To: aeroelectric-list(at)matronics.com

Subject: Re: "Starter Engaged" Warning Light Circuit Refresher

 

At 10:34 PM 2/17/2011, you wrote:

--> AeroElectric-List message posted by: Bob Falstad <bobair(at)me.com>



Bob N.,



I'm putting a "starter engaged" warning light in my new IFR panel.  I simply hooked the incandescent bulb from my B&C alternator  "low voltage" warning light between a fused wire that runs from the hot side of the starter contactor and ground.  (I now have the standby B&C alternator and will use its controller's "standby alternator on" light in lieu of the low voltage light on the primary alternator controller.) 



But then I ran across a handwritten sketch that I believe you drew up for this circuit.  It has a 470 Ohm - 1 Watt resistor in the wire between the hot side of the starter contactor and the lamp.  It also shows a 1N4001 protection diode in parallel with the lamp and ground.  It isn't clear from your sketch (and my electrical symbols knowledge is limited) whether the lamp is incandescent or an LED.

This is an incandescent lamp



[img]cid:image001.jpg(at)01CBCFA5.2012AF40[/img]



This is an Light Emitting Diode



[img]cid:image002.jpg(at)01CBCFA5.2012AF40[/img]



  The resistor used with the LED warning circuit as

  sketched serves a dual purpose. It protects the wire

  that runs from starter to lamp by limiting the maximum

  current that could flow in a faulted wire. At the same

  time, it SETS the current intended to illuminate the

  LED.



  For an incandescent warning light, the resistor is

  replaced with a fuse and the incandescent lamp is

  substituted for the LED. No reverse current diode

  is needed.





  Bob . . .

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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Feb 18, 2011 4:21 pm Post subject: LED question

At 06:20 PM 2/18/2011, you wrote:

 	  Quote: 	
		  Bob: is it OK to wire an LED directly (through a resistor in series) 

to a 14V system?  Or should a voltage reduction circuit/device be 

used.  I always though most LEDs were supposed to work on 5V. And 

14V would be too much.


	


    LED's are CURRENT driven devices. They do drop 2-3 volts

    across the device while illuminated (depending on color)

    but they can be BIASED up to run at ANY voltage greater

    than 3 volts by selection of the series resistor . . . which

    is the purpose of the 470 ohm resistor in the sketch. With

    9-12v applied to the starter and 2v dropped across the

    LED leaves 7 to 10 volts drop across the 470 ohm resistor.



    This translates to an LED operating current on the order of

    (7/470) 15 to (10/470) 21 milliampers.

   Bob . . .

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