Builders
How To Use A PC Power Supply
To Run An RC Battery Charger
NOTICE
As with modifying anything electrical, this is a potentially dangerous procedure that has the potential to kill you and/or burn down the structure you're doing this in, and/or cause any number of disasters that may be attributed to working with electricity, including death.
I make no guarantee that this will work as stated and I must caution the reader that I have not tried this and have not verified that it works as I have had this e-mailed to me and it's origin is from an unknown site over the Internet. With that said, feel free to experiment at your own risk.
All parts except PC PS from Radio Shack. PC PS was "new", UL listed (costs a little more, but probably worth it; most PC PS are not UL listed). A used, *free" PC PS is always good.
Parts list: 1 x 250W PC Ps. 6 x 10ohm 10W power resistors. 1 x 2"x3" circuit board. 1 x DPDT rocker switch. 2 x banana jack pairs. 2x banana plug pairs.
Notes: A 250W PS is the minimum. In theory, a 250W PS gives some 20A @12v; this is only true if all power were converted to 12V output. Actual output is split between 5V and 12V. Expect at least 7.5 amps hope for more. Older PS are supposed to give higher amps for 12V output and require less resistance on the 5V taps. A 300W PS would be better, but is hard to find and expensive.
The PS is a designed to be super "safe" (No exposed wiring or components). "Clean" (minimal extra wiring, unused wiring internally), and "handy" (banana jacks, for any output). The banana jacks keep the size down, for transporting.
Some guys recommend automotive bulbs. Bulbs are glass, breakable, very hot, and would cause an internal short if they broke. Just say no.
Some guys say you can do it with fewer resistors. My testing shows 2 x 10 Ohm 1OW resistors gives about 3.5A continuous 4 x 10 ohm 10W resistors gives about 4.5A continuous 6 x 10 ohm 10W resistors gives about 7.5A continuous; more might be better, but the Millennium only charges up to 7.0A. The PC PS would not power the Millennium continuously with fewer than 4 resistors, but it would with 6.
- Attach grounding strap and unplug power supply. Drain the capacitors by running bulbs across the various +12V, +5V, -5V, -12V taps. Use a multimeter to verify there is no juice in the system. 60hz AC and HIV caps can hurt (or kill) Open the PC PS case (voiding warranty).
- Replace that ugly AC push-button switch with the DPDT rocker switch. The basic cord is too long, so you can cut it to length. Solder your connections.
- Remove the PS circuit board from the PS case.
- Remove the wires that would normally go to the PC motherboard. There are 2 sets of these, each set ends in a 6x plug end have all sorts of funky colors. We don't need or want these in the case they will only restrict airflow. Leave the fan wiring alone. You should only have +12V yellow wires, +5V red wires, and grounding black wires left; all going in pairs to various 4x1 plugs. Mine has the unnecessary wires unsoldered, but just cutting would be adequate.
- Cut out the 4x1 plugs and separate wires into a +5V red/back group and a +12V yellow/black group. Put the PS circuit board back into the PS case.
- Save 2 +5V red/black pairs for your 5V output. Keep the remaining (at least 2) +5 red/back pairs for your 5V load. Use at least 3 yellow/black pairs for your 12V output. We're going to be moving a lot of current across the wires, so we need enough wires to carry it without too much internal resistance. You can use 12 ga. wiring for the outputs.
- Solder the 6 power resistors to the 5V load wires. The power resistors should be wired in parallel (side by side). Best is to mount the resistors onto the circuit board, and to mount the circuit board in the case in front of the fan, The power resistors will generate heat, but if you keep them in front of the fan, all will be well. My resistors are in 2 groups of 3 resistors each, each group drawing from 2 +5V red/black pairs. It may be nice to place a switch in line with the resistors, so the fan can just cool them when you're done charging for the day.
- Attach the circuit board to a vertical heat sink element or the side of the case. I used screws with aluminum standoffs into one of the vertical heat sinks (keeping airflow around the heat sink). Make sure no 5V wiring touches the screws.
- Drill 2 pairs of holes into the case for the banana jacks to fit into. Label them +5V and +12V.
- Take the yellow/black 12V pairs and solder them to the +12V banana jacks. Make a set of banana plugs to power your charger.
- Take the red/black 5V pairs and solder them to the +5V banana jacks. Make a set of banana plugs for any accessory.
- Close the case, you're done.
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