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How much current does an LED take?

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

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  1. Sea Squid

    Sea Squid Guest

    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?

  2. Sea Squid

    Sea Squid Guest

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

  3. Wim Lewis

    Wim Lewis Guest

    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. Look here:

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


    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 Given

    Terry Given Guest

    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.

  6. Rich Grise

    Rich Grise Guest

    ULN2803 - Eight darlingtons in a DIP

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

    Good Luck!
  7. 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

  8. 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
    harm your PC.

  9. dmm

    dmm Guest

    Have a look at

    I also recommend Paul Bergsman's book
    "Controlling Your World With Your PC"
    ISBN 1-878707-15-9
  10. dmm

    dmm Guest

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

    Also,have a look at
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