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28v switches in a 12v system?

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CJohnston(at)popsound.com
Guest

Posted: Wed Feb 21, 2007 6:36 pm Post subject: 28v switches in a 12v system?

Alright smart guys...  here's my dumb question of the day...



Can I use LED illuminated switches designed for a 28 volt system in a 12

volt system?  Why or why not?  What would theoretically happen?  I

imagine that I'm fully displaying my lack of understanding in this area,

but try to be gentle.



Thanks!

cj

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nuckollsr(at)cox.net
Guest

Posted: Wed Feb 21, 2007 7:23 pm Post subject: 28v switches in a 12v system?

At 06:32 PM 2/21/2007 -0800, you wrote:



 	  Quote: 	
		  

<CJohnston(at)popsound.com>



Alright smart guys...  here's my dumb question of the day...



Can I use LED illuminated switches designed for a 28 volt system in a 12

volt system?  Why or why not?  What would theoretically happen?  I

imagine that I'm fully displaying my lack of understanding in this area,

but try to be gentle.


	


   Sure, they'll just be dim compared to the 14v illumination

   level. Better get one, hook it to a battery and see if

   the illumination level is still satisfactory to your needs.





        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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mprather(at)spro.net
Guest

Posted: Wed Feb 21, 2007 8:05 pm Post subject: 28v switches in a 12v system?

It won't hurt anything, but they'll be dimmmer.  LED's like most diodes

have very non-linear current vs. voltage curves.  Below the knee voltage

the current is nearly zero.  Once the voltage goes above gets above the

knee, the current increases rapidly for small increases in voltage.  A

typicl LED knee voltage is about 1.5V.  Changing from 28V to 12V will

still keep the voltage applied to the LED above the knee voltage.



For the purposes of your question, let's assume that for both 12V and 28V

the voltage across the LED is 1.7V - a safe assumption, though not

perfectly accurate.



Let's also assume that the current through the LED at 28V was 20mA.  That

says the switch has a dropping resistor with a value given by R = V/I =

(28V - 1.7V) /20mA = 1315ohm.



Next, let's calculate the current through the circuit when running with a

12V supply.  I = V/R = (12V - 1.8V) / 1315ohm = 7mA



When considering light output, the key is about power.  Let's compare.



P(28V) = 20mA * 1.7V = 34mW



P(12V) = 7mA * 1.7V = 12mW



So, I would expect that when running the 28V switches at 12V, the emitted

light would be about one third.  Maybe less, actually.  At the lower

current, the voltage drop across the diode is going to be a little less,

so the power is going to be a little less too.  At 1.5V the power is

10.5mW.



Hope that helps..

Regards,



Matt-



 	  Quote: 	
		   

 <CJohnston(at)popsound.com>



 Alright smart guys...  here's my dumb question of the day...



 Can I use LED illuminated switches designed for a 28 volt system in a 12

 volt system?  Why or why not?  What would theoretically happen?  I

 imagine that I'm fully displaying my lack of understanding in this area,

 but try to be gentle.



 Thanks!

 cj

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Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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