Replacing Old LEDs

Tony Duell ard at
Fri Oct 21 17:42:38 CDT 2005

> I believe that all these LEDs are on ISA or EISA cards, circa 1987.
> Let me be more specific now that I have gone to radio shack.
> In 5mm red LEDs they have
> 12volt, 2.6v, 1.8v

Ignore the 12V one (and a 5V one if they listed that). Those have 
built-in resistors or current source ICs and are not what you want at 
all. Rememebr an LED is a semiconductor diode and this had a V-I 
characteristing with a very steep slope (almost no change in voltage over 
a wide range of currents), so you can't connect a bare LED to a voltage 
supply. You would normally put a resistor in series with it.

Msot common red LEDs have a Vf (forward voltage) of 1.8V. Higher voltage 
tend to be the modern low-current ones that you don't want. I'd get the 
1.8V one and see what happens. 

Except for very speicalised aplications I've never really worried about 
the characteristics of the LEDs I am using. Just get a generic red LED 
and pick a reisstor to pass between 10 and 20 mA through it.


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