How much current does an LED take?
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Posted by Sea Squid on March 17, 2005, 5:57 pm
Please log in for more thread options 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. | |||||||||||||||||||||||||||||||
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Posted by Wim Lewis on March 17, 2005, 10:14 am
Please log in for more thread options 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). -- | |||||||||||||||||||||||||||||||
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Posted by Terry Given on March 18, 2005, 12:36 am
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Wim Lewis wrote: >
>>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? >
> > 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). > 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 | |||||||||||||||||||||||||||||||
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Posted by Sea Squid on March 17, 2005, 6:00 pm
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I found PP is unable to drive such LEDs, which needs 20mA, but what is the converter chip I shall order? Thanks > 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. > > > > | |||||||||||||||||||||||||||||||
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Posted by Robert Monsen on March 17, 2005, 3:01 am
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Sea Squid wrote: > I found PP is unable to drive such LEDs, which needs 20mA, but what is the
> converter chip I shall order? > > Thanks > 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. | |||||||||||||||||||||||||||||||
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>a cut parallel port cable, then send instructions with Visual Basic
>to create some patterns. Is there any danger to my laptop?