# How much current does an LED take?

Discussion in 'General Electronics' started by Sea Squid, Mar 17, 2005.

1. ### Sea SquidGuest

I want to experiment the parallel port with eight LEDs tied to
a cut parallel port cable, then send instructions with Visual Basic
to create some patterns. Is there any danger to my laptop?

Thanks.

2. ### Sea SquidGuest

I found PP is unable to drive such LEDs, which needs 20mA, but what is the
converter chip I shall order?

Thanks

3. ### Wim LewisGuest

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).

4. ### Robert MonsenGuest

Look here:

http://www.logix4u.net/parallelport1.htm

There is a schematic for doing exactly what you want to do.

(comp.arch.fpga?)

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

5. ### Terry GivenGuest

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.

Cheers
Terry

6. ### Rich GriseGuest

ULN2803 - Eight darlingtons in a DIP
http://www.st.com/stonline/books/ascii/docs/1536.htm

Of, course, you'll need a separate supply - there is no reliable +5V. Vcc
at the LPT port.

Good Luck!
Rich

Use a series resistor of at least 3.3K Ohm to keep the current under 1
milliAmp. Most LEDs will give out enough light at this current to be
visible.

Glenn.

8. ### Gregory ToomeyGuest

Off the top of my head I would say a 500-1K Ohm resistor in series, and the
schematic here is using 1K.

If you try to connect directly without a resistor it will work but you could

gtoomey

9. ### dmmGuest

Have a look at
http://www.boondog.com

I also recommend Paul Bergsman's book
ISBN 1-878707-15-9

10. ### dmmGuest

I highly recommend a book by Paul Bergsman
"Controlling THe World With Your PC"
ISBN 1-878707-15-9

Also,have a look at
http://www.boondog.com
..