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Glowing Warning Lights

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MikeEasley(at)aol.com
Guest

Posted: Sun Sep 10, 2006 4:59 am Post subject: Glowing Warning Lights

I have 2 of the B&C LR3C-14 regulators hooked up to warning lights on  my panel.  The warning lights are LED.  They work fine, flashing when  the voltage is low.  I finally got my plane out at night an noticed that  both warning lights have a dim glow.  My other warning lights are totally  dark.

  

 Any ideas?

  

 Mike Easley

 Lancair ES

 Colorado Springs

  [quote][b]

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mprather(at)spro.net
Guest

Posted: Sun Sep 10, 2006 6:49 am Post subject: Glowing Warning Lights

Mike,



I seem to remember that the regulator was designed to drive incandescent 

lamps instead of LED's.  I can't remember why the regulator produces a 

bias on the warning circuit..  At any rate, lamps won't produce any 

light at low voltage/current bias, but LED's will.  I think the fix was 

to put a high value resistor (1kohm?) across the leads of the LED.  It's 

in the archive as I recall.

Regards,



Matt-

MikeEasley(at)aol.com wrote:



 	  Quote: 	
		   I have 2 of the B&C LR3C-14 regulators hooked up to warning lights on 

 my panel.  The warning lights are LED.  They work fine, flashing when 

 the voltage is low.  I finally got my plane out at night an noticed 

 that both warning lights have a dim glow.  My other warning lights are 

 totally dark.

  

 Any ideas?

  

 Mike Easley

 Lancair ES

 Colorado Springs



*

*

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nuckollsr(at)cox.net
Guest

Posted: Sun Sep 10, 2006 9:37 am Post subject: Glowing Warning Lights

At 08:48 AM 9/10/2006 -0600, you wrote:



 	  Quote: 	
		  



Mike,



I seem to remember that the regulator was designed to drive incandescent 

lamps instead of LED's.  I can't remember why the regulator produces a 

bias on the warning circuit..  At any rate, lamps won't produce any light 

at low voltage/current bias, but LED's will.  I think the fix was to put a 

high value resistor (1kohm?) across the leads of the LED.  It's in the 

archive as I recall.

Regards,



Matt-


	


   Out of my archives:



 	  Quote: 	
		  I believe I have seen that two 220 ohm resistors can be used in

conjunction with an LED to act as a OV light on the B&C LR3s.  Can

someone help me with how 220 ohm was determined and why two of them?

Normally, I use a 600 ohm resistor in series with an LED to operate at

13.8V. (drop 12V at 20ma). What am I missing here?


	


    To replace an incandescent lamp on the LR-3 Alternator Controller

    with and LED, one has to account for a built-in leakage current

    for the LR-3's lamp driver picked so that the lamp will illuminate

    even when ALL power is removed from the LR-3 as long as the warning

    lamp has + power.



    This allows the LR-3 to be used with incandescent annunciator

    panels that get power from a third source and still annunciate

    regulator failure if all power is removed from the regulator.



    This leakage is too small to cause an incandescent lamp to glow

    but it will cause an LED to glow even when it's supposed to be

    dark. Hence, the resistor around the lamp to drive up it's minimum

    illumination current to something on the order of 7-8 mA.



    So taking the 220 ohm resistors as recommended, let's figure

    2v across the illuminated lamp which means we have about 9 mA

    used up in the parallel resistor. Figure 12.5v for the bus voltage

    while flashing which leaves 10.5 volts across the series resistor

    and 47 mA total through it.  With 8 mA being sucked off by

    the parallel resistor, this leaves 39 mA or so for the LED.

    A bit more than its "rated" current but by no means overly

    stressful.



    The 10.5 volt drop on series resistor at 47 mA suggests

    that a 490 milliwatt resistor is called for . . . except

    that this is used in a flashing light system with about 50%

    duty cycle which cuts dissipation in half. So the 1/2

    watt callouts on the drawing at



http://www.aeroelectric.com/Pictures/Schematics/LV_Led.jpg



    are plenty conservative.



    Why 220 ohm resistors? That was a size stocked by

    Radio Shack (stock #271-1111) in packages of

    5 for about a dollar. Other resistors could have

    worked just as well for the parallel value . . . but

    then one would have to buy two 5-paks and throw

    away 8 resistors. The design offered gets the job

    done with one purchase.



    Why the "leakage" in the LR-3?  We had a design goal

    for the LV warning light to illuminate BOTH for

    real Low Volts warning -AND- when the LR-3 had lost

    power entirely. To achieve the second goal, the LR-3

    had to appear "very leaky" in the OFF state, hence the

    resistor optimized for incandescent lamps but something

    of a nuisance for LEDs when attempting to directly

    substitute the LED for incandescent.



    Bob . . .

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MikeEasley(at)aol.com
Guest

Posted: Wed Sep 13, 2006 4:18 am Post subject: Glowing Warning Lights

After studying the schematic (I'm a little slow with this stuff), it  appears that the resistors can be installed at the regulator, one across  terminals 3 and 5, and the other in series from the wire running from terminal 5  to the LED.  I'm assuming some heat will be generated by the resistors and  installing them at the regular would work better for me with cooling.

  

 One other question, why not use a relay?  I understand the added  complexity issue, but that would give you a dark LED without sacrificing any  brightness when it's illuminated.  Right?  Any part number  recommendations would be appreciated.

  

 Mike Easley

 Colorado Springs

 Lancair ES

  [quote][b]

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nuckollsr(at)cox.net
Guest

Posted: Wed Sep 13, 2006 3:09 pm Post subject: Glowing Warning Lights

At 08:16 AM 9/13/2006 -0400, you wrote:



 	  Quote: 	
		  After studying the schematic (I'm a little slow with this stuff), it 

appears that the resistors can be installed at the regulator, one across 

terminals 3 and 5, and the other in series from the wire running from 

terminal 5 to the LED.


	


   Look at it again. The two resistors would go in series between 3 and 5

   and the LED fed from the protected end of the fuse and the TAP BETWEEN

   the two resistors.



 	  Quote: 	
		     I'm assuming some heat will be generated by the resistors and 

 installing them at the regular would work better for me with cooling.


	


   The RECOMMENDED resistors are at least 1/2 watt and 1 watt is preferred.

   Like the selection of 1N540X series diodes over 1N400X series is their

   mechanical robustness. The actual power dissipated in the 220 watt series

   resistor is about 11(squared)/220 = 550 milliwatts which is divided in

   two again due to the approx 50% duty cycle of the flashing lamp.



   So heating is an insignificant concern.



 	  Quote: 	
		  

One other question, why not use a relay?  I understand the added 

complexity issue, but that would give you a dark LED without sacrificing 

any brightness when it's illuminated.  Right?  Any part number 

recommendations would be appreciated.


	


   LED's are current operated devices, the resistors don't sacrifice

   brightness, they prevent darkness by keeping the LED from being

   destroyed by using it to directly replace an incandescent lamp

   sans resistors. A relay would suffer from the same kinds of issues

   as a barefoot LED. Relays can be held closed by a tiny fraction of

   the current that it takes to energize it. Further, you would still

   need at least one of the two resistors to set the LED's operating

   current. Finally, relays are rated amongst the least reliable of

   components. It behoves us to limit their use where ever practical.



   Bob . . .

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