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What did I do wrong?

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jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Wed Mar 21, 2007 9:24 pm Post subject: What did I do wrong?

I am installing a flexible USB 5v, 50ma LED  used on computers for cockpit/map light. To figure out which contacts light the  LED, I soldered a 5.1v , 1 watt,  Zener diode on to the positive lead of my  power supply and went probing the contacts. The two outside contacts were the  ones powering the LED. But the LED promptly fried.

  

 Obviously there's more to this than I thought. What  do I need to do to here to keep my LED lights from burning up?

  

 Thanks,

 John

   [quote][b]

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nuckollsr(at)cox.net
Guest

Posted: Thu Mar 22, 2007 5:33 am Post subject: What did I do wrong?

At 10:23 PM 3/21/2007 -0800, you wrote:



 	  Quote: 	
		  I am installing a flexible USB 5v, 50ma LED used on computers for 

cockpit/map light. To figure out which contacts light the LED, I soldered 

a 5.1v , 1 watt,  Zener diode on to the positive lead of my power supply 

and went probing the contacts. The two outside contacts were the ones 

powering the LED. But the LED promptly fried.



Obviously there's more to this than I thought. What do I need to do to 

here to keep my LED lights from burning up?



Thanks,

John


	


  adding a 5v zener in series with your powersupply

  only DROPS the supply voltage by 5v. Assuming a 12v

  supply, this leaves you 7v. Try hooking your lamp up

  with a series RESISTOR instead. Put a mulitmeter in

  series with your supply and adjust the resistor to get

  30 milliamps of flow when pwered up. This is the operating

  point for most LEDs. White LEDs need about 4 volts to work

  so the 1v drop at .03 amps suggests a 33 ohm resistor

  inside the fixture. Applying 7 volts would have tripled

  the current trough the LED and probably induced the failure.



  If you don't have a tightly regulated 5v supply to run

  the light, then the series resistor is better. Try

  200-270 ohms. A 1/2 watt resistor will do.



  Bob . . .

        ----------------------------------------

        ( IF one aspires to be "world class",  )

        ( what ever you do must be exercised   )

        ( EVERY day . . .                      )

        (                  R. L. Nuckolls III  )

        ----------------------------------------

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu Mar 22, 2007 6:00 am Post subject: Re: What did I do wrong? (LEDs)

 	  Quote: 	
		  I am installing a flexible USB 5v, 50ma LED used on computers for cockpit/map light. To figure out which contacts light the LED, I soldered a 5.1v , 1 watt, Zener diode on to the positive lead of my power supply and went probing the contacts. The two outside contacts were the ones powering the LED. But the LED promptly fried. 



Obviously there's more to this than I thought. What do I need to do to here to keep my LED lights from burning up? 	
  

 

John, 



A USB connection is nominally 5VDC current limited initially to 100 ma. So the LED assembly probably has inside it a current limiting resistor of



   R= (5V-Vf)/0.050    (You could assume it is about  47 ohms 1/4W)



Diagnosing how you used the zener would complicate the issue. I suspect your assumption of how the zener works out needs some rethinking. But if you want to put this into your airplane on 14V add a series 470 ohm 1.5W resistor and be happy.



Ring the bells that still can ring 

Forget your perfect offering 

There is a crack in everything 

That's how the light gets in. 

---Leonard Cohen

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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

mprather(at)spro.net
Guest

Posted: Thu Mar 22, 2007 7:41 am Post subject: What did I do wrong?

Hey John,



If I understand what you described, the zener would have only been capable

of reducing the voltage seen by 5V.  In other words, if the supply were

set at 20V, the LED would see 15V...  If the supply makes 12V, the LED

sees 7V, etc.



A way to make the LED see 5V is to connect the zener to ground, and then

an appropriate resistor, and then to the positive lead of the power

supply.  Probe the LED from the node between the zener and the resistor.

Matt-

 	  Quote: 	
		   I am installing a flexible USB 5v, 50ma LED used on computers for

 cockpit/map light. To figure out which contacts light the LED, I soldered

 a 5.1v , 1 watt,  Zener diode on to the positive lead of my power supply

 and went probing the contacts. The two outside contacts were the ones

 powering the LED. But the LED promptly fried.



 Obviously there's more to this than I thought. What do I need to do to

 here to keep my LED lights from burning up?



 Thanks,

 John

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jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Fri Mar 23, 2007 8:04 am Post subject: What did I do wrong?

Bob, Eric & Matt,

  

 A thousand thanks to you all for my education on  Zener diodes and how to get my lamps to work. Eric, you're right about my  concept of Zeners...totally backward. I thought they "spilled" voltage over the  nominal amount instead of absorbing the nominal amount. Now I think I understand  what happened.

  

 John

  

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   [quote][b]

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