Decided to slap together a little board for sinking current from power LEDs.. but it's such a common need, decided to post. I plan to use this for PWM dimming mainly, so I didn't include a spike supress diode on each channel, which you would want if using this for an inductive load like a motor. N-Channel MOSFETS would take more current with less heat, but I was able to get the TIP transistors very cheap a while back. A quick dive in the elcheapo parts bin, and a little time killed dripping solder on the kitchen table, and I cobbled together a little box for sinking up to 24 amps in theory. Total cost was a couple of bucks.
Four channels, each one a Darlington pair with the primary a PN2222A and a TIP41C for the actual power switching. TIP41 transistors are rated for 100v at 6amps maximum, NPN, so we are switching the ground side, the switching transistor after the load. The Base on each pair has a 1k resistor to limit the current.. Since the PN2222 has a gain of well over 100, and the TIP has a gain of at least 20 even at high current, a total switching gain of at least 2000 should make the base current requirement very small. The basic idea can be accomplished with almost any transistor, but it's most useful set up with a very high gain low power transistor, so the load on the digital out lines on the arduino is minimized. I used pn2222, but 2n3904 or whatever cheap NPN you have around... Power transistors tend to have low gain, except for expensive ones...
One point worth noting.. TIP41 has the tab connected to the Collector, so I skip other connection and then put a small machine screw through the tab to create a nice easy mounting nut and connection in one, as well as helping to sink the transistor heat.
Note, I probably wouldn't try to sink more than an amp or so without more heatsinking... I suspect that above, say, two amps that things will get a bit toasty. The plastic box is repurposed from a vonage voip box.. Picked up four of them from a clearance house for about fifty cents each, mainly because they come with a 12v 1.5A switching power supply with each one. Those power supplies will be feeding the LED constant current drivers, just LM317 voltage regulators set up as current regulators. Not the most efficient way to drive an LED, but it works! I use four channels to give me RGB plus white, I am doing a custom lighting thing which may turn out pretty cool, but that's another post. The point is that this setup here switches the ground side, making it possible to change the current driver or the load as desired without needing to change the PWM circuitry or even the code.
For cabling, I got twenty five foot phone extension cables.. Six stranded 24ish gauge conductors, plenty enough for the digital side. A dollar each at the dollar store. Also picked up a hundred foot spool of telephone riser cable, four conductor solid 22ga cable.. Two bucks. Cheap is good!
