Date:         Wed, 9 Aug 2006 13:20:12 -0700
Reply-To:     Lucas Thompson <Lucas.Thompson@WATCHGUARD.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Lucas Thompson <Lucas.Thompson@WATCHGUARD.COM>
Subject:      Re: LEDs for the dash
Content-Type: text/plain; charset="us-ascii"

One important thing about white or blue LEDs (at least the cheap chinese
ones I've tested, but I think they all behave this way) is that they
must operate with a constant-current type power supply (I.E. not just a
resistor) or the lifespan is quite short, with a half-life in the
hundreds or low thousands of hours. Dropping the current somewhat by
increasing the resistor value increased the lifespan, but not by a lot.
The same average current supplied with a constant-current supply extends
the lifetime to what you'd expect -- something in the tens of thousands
of hours.
 
Red and Green LEDs don't seem to share this behavior, the lifespan is
long even when operated from just a series resistor to limit current.
 
For diagrams of how to make the circuits, check the "constant current"
section of this site out:
http://girr.org/girr/tips/tips7/white_led_tips.html
It's 3 parts instead of 1 part and a little more complex to build the CC
supply, but the LEDs will last much much longer.
 
Lucas
86 GL 82 Westy
 
 
________________________________

From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Todd P. Last
Sent: Tuesday, August 08, 2006 9:51 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: LEDs for the dash


I think with LEDs you want to be concerned with the current rather than
the voltage when deciding what resistor to use.
And all you need to do is add a resistor to one leg of the LED to drop
the voltage (and current).

Anyway, you can use this link to put in the values for your particular
LED and it will calculate the correct resistor value to use.
http://led.linear1.org/1led.wiz

Todd
'88 Westy


nick greenaway wrote: 

        does anyone know what parts i need to get at radio shack to drop
        12.6v down to 3.7v (i cant remember the amps) for an LED driver?
        im sure someone out there has done it
        thanx
        nick
	
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