Replacing Old LEDs

Tony Duell ard at p850ug1.demon.co.uk
Sat Oct 22 15:55:22 CDT 2005


> Well the current does add up, if you are looking at a front panel. Just 

True, but the PSU was designed to handle it :-)

> remember what you are driving
> the led with. A regular TTL gate has only 16  ma of sink current. If you 

That's 15mA _while maintaining the specifed output voltage_. You can 
actually sink a little more current without damaging the chip if you're 
happy for the output voltage to rise a bit.

> 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.

Exactly. In the 1970s they didn't have the high-brightness LEDs, they 
didn't have the low-current ones. Typical red LEDs had a Vf of 1.8V, and 
an If of 10-20mA

Now, think of the circuit. Typically, you have the LED and a resistor 
(220 ohm to 330 Ohm) in series between the output of a TTL gate and +5V. 
That reistor _will_ limit the current. If the LED was a dead short, and 
the resistor 220 Ohms, then the current would be 5/220 = 22.7mA. OK, 
that's out of spec for a TTL gate, but I doubt it would do any damage. 
And no real LED has a zero forward voltage anyway.

Suppose you fitted a high-brightness LED. Whatever the forward voltage 
is, it's likely to he higher than the original 1.8V. Which means the 
actually current will be _less_ (less voltage dropped across the 
resistor). The LED might end up looking somewhat dim, but it won't do any 
damage.

The real problem would be using a low-current LED. The forward voltage of 
those is a little higher than the 'stnadard' type, so the current would 
be a bit less than the original, but still probahly high enough to damage 
the new LED (but no damage would aoccur to anything else).

-tony



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