Re: How much current does an LED take?

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Yes, parallel ports are relatively easy to damage by shorting them
out, etc. I've done this a few times. :-/ Serial ports are usually
more goof-resistant, but of course they have fewer pins...

Re your other post, the LED will still light if you feed it less than
20 mA; it'll just be dimmer. Even 1 mA should still produce an easily
visible glow. What you need to do is insert a resistor in series
with each LED to limit the current to the amount that the parallel
port can supply.

LEDs (and diodes in general) have an exponential current/voltage relationship.
To a first approximation, this means that above a certain voltage,
they'll pass all the current you can throw at them (possibly overheating
and burning up in the process); below that voltage, they'll pass very
little current. (Including for negative voltages.) Another way of looking
at this is that, if more than a little current is flowing, the voltage across
the diode will be almost constant for that diode. This is the diode's
"forward voltage drop", Vf.

So let's say you have an LED and a resistor connected to your parallel
port. You want to size the resistor so that (for example) 1 mA is flowing.
The parallel port is supplying 5 volts. The forward voltage drop of
the LED is in the neighborhood of 1.5-2v. That leaves 3-3.5 volts across
the resistor. You know the voltage across the resistor, and you know the
current you want; using Ohm's law you can divide in order to find what
the resistance must be (in this case, about 3000 to 3500 ohms).


Re: How much current does an LED take?

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depends how shitty the LED is. I've just had an unfortunate experience
with some 0603 orange LEDs, that at 20mA were extremely dim, and no
detectable light at 1mA. cf some of the high-efficiency LEDs I use that
are really bright (calibrated to a traceable standard eh wot) at 3mA.


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