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Old Dec 06, 2012, 08:24 AM
Illegitimi non carborundum
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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
- Drill bits
- Hammer
- Punch
- 8mm wrench
- #2 Philips screwdriver
- Soldering iron with stand and sponge
- Heat gun
- Rotary tool
- File
- Vise

And supplies:

- Solder
- Heat shrink tubing (various diameters)
- Marker
- Paper
- 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.

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Old Dec 06, 2012, 08:25 AM
Illegitimi non carborundum
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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.

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Old Dec 06, 2012, 08:27 AM
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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.

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Old Dec 06, 2012, 08:28 AM
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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.
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Old Dec 06, 2012, 08:29 AM
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Back together - check.

LED on - check.



Voltage on the left jacks - check.



Voltage on the right jacks - check.



Jacks and switch labeled - check.

She are done!
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Old Dec 06, 2012, 08:54 AM
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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.
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Old Dec 06, 2012, 09:15 AM
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Quote:
Originally Posted by grimbeaver View Post
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.
No I haven't - that's a new one on me. I'll do some testing ASAP to verify. Thanks for the information.
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Old Dec 06, 2012, 08:20 PM
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Originally Posted by grosbeak View Post
No I haven't - that's a new one on me. I'll do some testing ASAP to verify. Thanks for the information.
I used my NiMH charger and a 4600 mAh pack for the test. On the power supply it reached 1.85A and stayed there. Another test with the charger's original power pack returned the same results.

Looks good to me!
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Old Dec 06, 2012, 09:00 PM
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My TRUEPOWER 2.0 delivers up to 33Amps. and stays at 11.94V with no load on the 5V rails.

Charles
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Old Dec 07, 2012, 09:55 AM
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Originally Posted by grosbeak View Post
I used my NiMH charger and a 4600 mAh pack for the test. On the power supply it reached 1.85A and stayed there. Another test with the charger's original power pack returned the same results.

Looks good to me!
If that's all you're going to draw off it it then you're fine. I if you are going to have a problem I think it normally it takes more of a load then that to detect it. I wouldn't call it good till you've tried pulling over 10A off it.

Here's a thread with more info:
http://www.rcgroups.com/forums/showthread.php?t=1725626
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Last edited by grimbeaver; Dec 07, 2012 at 10:03 AM.
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Old Dec 08, 2012, 09:34 AM
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GB,

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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