|Dec 06, 2012, 09:24 AM|
12 VDC on the bench from repurposed PC power supply
Iím in the process of choosing a charger that can handle LiFEPO4 batteries. Many donít come with an AC power option, so I made one from an old computerís power supply.
Starting with a how-to thread and the pinouts, I made a trip to the local electronics store for the following:
- Banana jacks, chassis mount, black and red x 2 (point of 12 VDC delivery)
- Banana plugs, black and red x 2 (for chargers that donít have them)
- SPST switch with LED (some power supplies have their own switch; mine didnít)
- 470 ohm resistor (to step down 12VDC for the LED)
The total cost was about twenty dollars.
I used the following tools:
- Linesmanís pliers
- Wire stripper
- Drill bits
- 8mm wrench
- #2 Philips screwdriver
- Soldering iron with stand and sponge
- Heat gun
- Rotary tool
- Heat shrink tubing (various diameters)
- Masking tape
All of the components together:
Once I took the cover off the power supply I looked for mounting locations that would not interfere with the internals. For the banana jacks, I settled on the top edge of one side. I made a template to mark the holes and set the drilling location with the hammer and punch. The cover is pretty flexible so I made sure to brace it first.
Next came the drilling. My banana jacks required a 21/64Ē drill bit. Here are the pilot holes.
|Dec 06, 2012, 09:25 AM|
With the jack holes drilled, it was time to make a hole for the switch. Once I selected and outlined the location, I braced the workpiece for cutting.
With the cover in the vise, out came the rotary tool with a cutting wheel.
Oops, almost forgot. Face protection.
Good thing too. At one point I removed the tool from the work to inspect the progress of the cut and caught an edge with the spinning wheel. The wheel broke and the bulk of it went THOCK on the face shield.
After a little bit of cutting and a lot of filing, the hole was done.
|Dec 06, 2012, 09:27 AM|
With the holes made, I turned my attention to the wires. According to the pinout I wouldn’t need the following:
- Orange +3.3 VDC, 3 wires
- Red +5 VDC, 4 wires
- Grey PWR_OK, 1 wire
- Purple +5 VDC standby voltage
- Blue -12 VDC
Those were clipped short and insulated with heat shrink.
What I did need were these:
- Yellow +12 VDC for the red banana jacks and the switch LED positive terminal. 5 wires.
- Black GND for the black banana jacks, switch LED power negative terminal, switch ground terminal. 6 wires.
- Green PS_ON triggers the power supply to turn on, switch power terminal. 1 wire.
For the banana jacks I doubled up the positive and negative wires to ensure maximum current and minimum voltage drop. Here are the four connections for the red and black jacks - two yellow (+12 VDC), two black (GND).
With the wires ready I soldered them to the lugs and insulated the connections. Remember to slide the heat shrink onto the wire first!
The last step for the banana jacks: Install them in the holes and secure the nuts with the 8mm wrench.
|Dec 06, 2012, 09:28 AM|
Time to wire up the switch. First step was to solder the 470 ohm resistor to the yellow +12 VDC wire and connect it to the positive terminal for the LED. The resistor is needed to impede the voltage so the LED doesn’t burn out.
Second step: Solder the black GND wire to the negative terminal for the LED.
Don’t forget to pass the wires through the switch hole first, and to slide the heat shrink on before soldering!
Next, I soldered the green PS_ON wire to the power terminal, followed by another black GND wire to the ground terminal.
At this point everything was wired up.
Time to put it back together.
|Dec 06, 2012, 09:29 AM|
|Dec 06, 2012, 09:54 AM|
United States, MN
Joined Feb 2011
Looks nice and excellent write-up, but have you tried using it yet? Most desktop power supplies require a load on the 5V line before they will provide stable output on the 12V. You may find that once you start pulling a few amps it will shut down. That's one reason that server power supplies are so popular vs desktop ones.
|Dec 06, 2012, 10:15 AM|
|Dec 06, 2012, 09:20 PM|
Looks good to me!
|Dec 07, 2012, 10:55 AM|
United States, MN
Joined Feb 2011
Here's a thread with more info:
|Dec 08, 2012, 10:34 AM|
Thanks for the link. I left the 5V lines plenty long so I can add a load to them later if need be.
The pack I tested is the biggest NiMH I have. The LiFEPO4s I would be charging are 6.6V 2200 mAh units - not sure what kind of amperage they would draw but I typically have no need for a fast charge.
I'm not interested in LiPO use - I have a tiny one in a park flyer I rarely fly but it has its own charger.
The reasons I went with a PC PSU instead of a server PSU:
(1) I had an old PC sitting around and since this was - at least partly - an experiment, it kept the costs down.
(2) I didn't know the difference at the time. :P
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