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Discussion Starter #1 (Edited)
Hi All,

thought I'd post some pics of the DIY DC-DC converter I "built" from the regular PC power supply ($40 or so on Amazon or your local electronics store). As always, YMMV (your power supply might be differently wired, might use a different IC, etc, etc. but generally they are all similarly constructed)

You need:
1. PC power supply - I used 350W which is rated for 12A on 12V output. Should work similarly for any more powerful supply, as well
2. 20kOhm trimpot
3. Soldering kit
4. Multimeter

Here's what you do (for information on the color coding of the wires, see something like http://en.wikipedia.org/wiki/Molex_connector):
1. Open the box
2. Take out (unsoldering is best, but I just cut them off) most of the output wires connected to the PCB (normally, you'd want to just keep one black wire (ground), one yellow (+12V), and a green wire (this is your output voltage trigger - see below). You might want to replace a thin stock wire with a thicker gauge while you're at it
3. Unscrew the PCB, turn over and find two lines on the PCB - (1) ground line (your black wire goes into that), and (2) line that connects to a pin 1 on the IC chip (the top pin on the left side of the key on the IC package if you look from the top of the PCB). See the picture for my reference point - YMMV
4. Solder one short (3-4") wire to each of the lines you found in the previous step. Solder the other ends of these wires to the trimpot. Set the trimpot to max resistance (20K)
5. Take a green wire we talked about earlier and connect it to the ground line on your PCB. I connected it to one of the jumpers linking ground lines on the top of the PCB (you can see that soldered on on one of the pics)
6. Tape / ziptie all the lose ends
7. Power up the thing from the regular outlet. Measure the voltage between yellow and black wires you have left. Should be more than 12V. On my unit it was 13.6. Adjust the trimpot until you get something between 13.5-13.8 (good voltages for feeding into the car battery / accessories).
8. Close the case.

I have tested my supply to 15Amps at 13.6V (there IS a bit of a sag at that current but doesn't go below 13 I think). If you have a more powerful power supply, yours will handle even higher currents. The unit does not shutdown - I just did not test it beyond that amperage (just had a 0.8 Ohm resistor around). Will test to shutdown point later and post. Also, the particular unit I used was rated for 12A on 12V line so I did not want to overstress it too much. If you want 40Amps, just get a 800W unit at your electronics store (I could find one for my PC for $90 minus $40 mail-in rebate so pretty much the same cost...)

Congrats, you have just saved yourself a couple of hundred bucks ;-)

Feel free to ask questions if anything is unclear.

EDIT: generally, you don't need to change anything on the input side to make it work with DC - see the schematics attached (NOT from my power supply but all of them are similarly wired). You can see that the input is directly connected to the rectifiers so your DC will pass right through.

Attached inline are: (1) a representative schematics of a power supply (borrowed from one of the threads on some RC forum), and (2) my beautifully designed faceplate for the converter ;-). Rest of the photos are in the zip file due to image size limitations of the forum.
 

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Excellent work. I'll give that a try. One question. Does it shutdown or just current limit on an overload condition?
 

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That's cool.

15A is going to be a bit marginal for most conversions though.

Also, PC power supplies are not exactly designed for vibration and moisture rich environment so make sure you conformal coat the board and glue down any large heavy components like inductors, transformers and capacitors.
 

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If you check ebay regularly on:

13.8V Switching Power Supply

You often find an offre of more than 200W for less than 40 euro. New, and complete. I wouldn't go this way.
 

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Discussion Starter #5 (Edited)
Excellent work. I'll give that a try. One question. Does it shutdown or just current limit on an overload condition?
Thanks. No shutdown - I just had a test resistor of about 0.8 Ohm (well, more like a long piece of wire that just worked out to 0.8 Ohm :). So it might work for more than 15A.

@peggus: yes, there's that. But I am planning to build a connections box anyway - this will just go there. The box will be mounted on rubber mounts to the frame so hopefully that will help deal with vibrations. Also, I am planning to build a DIY charger jackbauer and Simon built on http://www.diyelectriccar.com/forums/showthread.php/200-build-your-own-intelligent-charger-36627.html - will also go into the connections box. I project total savings of ~$2000 from not going the 'buy new 80% margin products'...

@Jan: yes, you may be right. Although: (1) I can pick any PC power supply (say, 800W I just picked up at my local electronics store for $50 ($90 minus $40 mail-in rebate)) and get better current rating immediately, and (2) this IS the same switching power supply. All you do is modify the calibration resistor voltage divider that helps the IC control the voltage. There does seem to be the limit on how high you can crank the voltage - due to hardware protection by zeners in the circuit (I believe you can't go past 14.x because of that) but until you hit 14V you are fine.

More than anything, I just really like doing stuff like this and figuring how things work. With the DC-DC, you might say the gain after you spend all this time etc is marginal but to me, it's just fun ;-) Now, when you start talking about $2000 chargers, replacing them with jackbauer/Simon's $300 solution, that's the real deal. I am planning to do that, too - a bit more involving than soldering on a couple of resistors, though ;-)) More on that in some other thread later.
 
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