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