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I want to power something using the parallel port of on an old Dell Dimension 8200. How much current can be drawn from each of the 8 the data pins? I have pulled a maximum of 78mA/pin so far, but what is the upper limit? Can I short pins together to raise this limit?

EDIT: So far I see that powering things directly is not a possibility and that putting pins together is a no-no. However, it would be nice to be able to turn things on and off using the data pins, which would mean a relay. These things draw current too, what should be the amount of current that they are allowed to draw?

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You shouldn't do this at all. Parallel ports aren't designed to provide any power. Their circuitry is of the transistor to transistor type and are designed to basically provide sufficient energy only for this function. The normal specs that I've seen only talk about 2.5ma to 14ma max.

http://www.fapo.com/1284elec.htm As an example.

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Generally I agree, but just want to clarify one point. While legacy port hardware on recent computers can provide up to hundred milliamps per pin (because semiconductors are much better nowadays than when all these ports were developed), it is surely not good for your south bridge and will make it eventually fail. Also if the OP will connect data pins together when some driver will probe an LPT printer with non-uniform bit pattern it will instantly kill the port. –  whitequark May 11 '10 at 23:31
    
Is it OK to use a relay to switch higher-powered things on and off? What would the specs for that be? –  marcusw May 11 '10 at 23:49

The parallel ports are not designed to provide any stable amount of power, they use just enough current to send data between two devices. Even if there were enough power on the parallel port you couldn't guarantee that it would work on all or even most computers. Since this is NOT a standard it would be spotty at best. You have a much better bet using a USB port which has a standard of 5 volts supplied through it.

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Is it possible to toggle these on and off? Low-latency on/off is important for my use case. –  marcusw May 12 '10 at 22:52
    
Rather than toggling the power supply (which will always be slow) why not simply have a switch which you can allow power through or not (on whatever you're building). For a power supply of any kind to turn on or off it takes a while. –  Daisetsu May 13 '10 at 19:42

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