Replacing Old LEDs

[Chuck Guzis]

On 10/22/2005 at 10:56 AM woodelf wrote:

>Well the current does add up, if you are looking at a front panel. Just 
>remember what you are driving
>the led with. A regular TTL gate has only 16  ma of sink current. If you 
>are replacing a old led, I would
>make sure the size of the led is right, and  avoid the high brightness 
>ones. Think what they had in the
>70's  compared to today.

It really doesn't matter.  Consider a typical 5v OC driver, driving an LED
through a current limiting resistor to limit current to, oh, no more than
15 ma. across the 1.8v of drop across the LED.  So,assuming that we have
the full 5 volt supply to drop across our load (not really, but close
enough for worst-case calculations), the voltage drop across the resistor
would be about 3.2v, making the resistor

    (3.2/.015) = 213, call it 220 ohms, the closest "standard" value.

Now, suppose we replace the LED with a dead short (i.e. an infinite current
sink).  The current through the 220 ohm resistor would rise to only

     (5.0/220) = 22 ma.

Which, while causing the LSTTL gate to break a sweat, probably wouldn't
damage it under normal circumstances (I've used 74LS06's to drive 220/330
terminated loads for years with no problem).  In other words, as long as
the LED is large enough to handle the current and the current limiting
resistor is intact, the size doesn't matter much (when was the last time
you heard that?).  Most plain old red LEDs were rated for about 20 ma.,
maximum current, so that's the rating I'd shoot for.

Cheers,
Chuck