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Originally Posted by Engineer
Converting PC Power Supplies for 12-18V Power
You’ve heard of the St. Louis Switch, now here is my Easy Switcher power supply.
You can make this as simple or complex as you want, but I will tell you how I setup mine and you can make your own custom changes. This is a very easy power supply to build. All we are going to do is float the ground of two of the PC power supplies and hook them up in series. Depending on the power supplies you use, the minimum voltage usually be between 12.0 – 13.5V and the maximum voltage between 16.5 – 18.0V. Remember, this is dependent on the adjustment range of individual PC power supplies you use.
You can do this with just a screw driver, utility knife, and three (3) PC power supplies but a few more tools will make it look clean and professional.
What you need:
1. 3 PC power supplies with a minimum 5V current rating that you want to achieve. I have done this to 20 amp and 30 amp versions.
2. A utility knife for cutting the ground trace on the circuit boards.
3. A Philips screw driver to disassemble the power supply
4. A small regular screwdriver to adjust the voltage pot on the power supplies. (Most have this pot, but some don’t. Don’t sweat it if one or more or your PC power supplies does not have this pot.)
5. Some various sizes of heat shrink.
6. Wire, 12AWG for 20 amps and 10AWG for 30 amps. Red and black are nice.
7. A good soldering iron, some solder, and flux.
8. An appropriate size and type fuse and fuse holder.
9. A board to mound everything too. I used a 30” x 8” x ¾” piece of particle board, but you can configure it anyway you like.
10. Some screws for mounting everything to the board.
11. A 115VAC power strip if you want to use it as a power switch like I did.
12. A volt meter to measure and set the voltage.
Note: You can get PC cases with the power supplies used from many business. Businesses have to dispose of computes as hazardous waste and will usually give them to you or sell them very cheep. Just ask your IT department or have a friend ask the IT department of the business where he works. There are also junk stores that disassemble PC and sell them cheap. Don’t pay more than $5.00 each if you have to buy them. Some of the egghead type computer stores junk old PCs and throw everything in a box and let you take what ever you want, free.
Warning: PC power supplies develop very high and hazardous voltages (250VAC) in their operation. Use care when working with high voltages.
1. Remove the screws and open the PC power supplies (PS). Blow them out with compressed air, they will look like dust bunny farms inside.
2. Now, power up each power supply with the cover off and make sure they are working. Be careful not to touch anything, there are voltages as high as 250VAC inside.
Note: The older power supplies used to switch the 120/240VAC to turn them on and off. The new power supplies usually have a green ‘PW’ wire that that when grounded will switch the power supply on. Some also have a small brown 24AWG wire that needs to sense power or it will shut the power supply down. If your PS does not power up and has this brown wire, solder it to where the red (+5V) wires attach to the circuit board. If you are given the whole PC case, keep the on/off switch connected to the power supply, this will help you to keep from having figure out why your supply does not power up.
3. Connect your voltmeter to the black (Negative) and the Red (+5V) wires from the power supply and measure the voltage to make sure each of your power supplies work. You should see about +5.30V.
4. Now look for a small potentiometer (pot) and turn it to see if the voltage changes. If the voltage changes, you’ve found the voltage adjustment pot. If it does not change the voltage, just set it back to about where it was. Don’t worry, you can’t hurt anything just keep looking. There are some power supplies that do not have voltage adjustments, so you just have to live with it, no biggie either.
5. Adjust the voltage to 1/3rd of what you want the final out put voltage of the system to be. When you hook up all the power supply units, we should have your intended voltage.
Note: Some power supplies units will shut down if the voltage goes outside of a certain range. If this happens the power supply will not restart until you remove power and adjust the pot back to an acceptable level. Turn the pot back to a known good position and then try turning it back on. It will then power up again.
6. From here on we will number the PC power supply units on how they are to be hooked up. Unit #1 will be the first and controlling power supply unit providing the ground reference and +5V. Unit #2 will be the second power supply adding +5V (Total +10V). Unit #3 will be the last in the series providing another +5V for a total of 15V. The total voltage will be dependent upon where you set the voltage of the individual power supplies.
7. Power down and remove the power cords from the power supplies and remove the screws that hold the circuit boards to the chassis on power supply units #2 and #3. Lift the board out of the chassis, you do not need to remove the wires.
8. On the bottom side of the board you will find an area around one (1) or two (2) of the hold down screw holes that is used to ground the circuit board to the chassis. I use the continuity tester on my meter to find them. Use the utility knife and cut under the copper trace area. Lift and cut just enough of this away so that the board is no longer grounded to the chassis. Do this for units #2 and #3 only. This will float the ground of units #2 and #3. Do not cut a trace that connects other component to the ground trace or you will have to solder in a jumper.
9. Replace the circuit boards in units #2 and #3, and fasten the circuit board to the chassis with the screws you removed.
10. Check your work by testing the continuity between the chassis and the back ground wires from the PC power supplies.
11. I wanted my power supply to power up when I turned on the power strip, so I soldered the power switch wires together (from the PC power switch) and put heat shrink over the solder joint on all three power supplies.
Note: Some of the PC power supplies have an output socket for plugging in a monitor. This is usually switched with the PC power supply on/off switch and can be used to switch all the power supplies on and off if you use this plug to power the other two PC power supplies.
12. Since one of my PC power supplies had an output socket, I made a “Y” cable that went from the output socket of unit #1 and spit it off to units #2 and #3. I soldered the cable connections and used heat shrink to finish the job. Now one power cord powers all three (3) PC power supplies.
13. You will have a bunch of 16AWG wires coming out of the power supplies. The Orange wires are +3.3V, the red wires are +5V, the yellow wires are +12V (about 11.3V actually), and the black wires are ground (Negative or +0.0V). There are also some other color wires that we don’t care about unless one is one of the sense wire I described earlier, but you should know which one that is by now.
14. We will use four (4) or five (5) of the red and black wires from each power supply. So cut everything else off and bunch the same color wires together and place a piece of heat shrink over them to keep them from possibly shorting to something. I use some ½” heat shrink on each of the groups of black, red, and yellow wires.
15. Mount your three (3) PC power supplies to a board (if you wish) so that the wires of unit #1 can reach unit #2 and unit #2 can reach unit #3.
16. Connect the four (4) or five (5) of the red wires from unit #1 to four (4) or five (5) of the black wire of unit #2.
Note: I cut the small16AWG wires short in each of the units and soldered in a single 10AWG wire to five (5) of them so I did not have a bunch of wires running between each of the power supplies. I have two 10AWG wires, one into and one out of each PC power supply.
17. Do the same for units #2 and #3. Connect the four (4) or five (5) of the red wire(s) from unit #2 to four (4) or five (5) of the black wire(s) of unit #3.
18. The black wire(s) at unit #1 are your ground/+0.0V and the red wire(s) from unit three will be +15V or what ever you set for you total out put voltage.
19. Place an appropriate fuse on the red wire(s) of unit #3 for protection.
Note: The PC power supplies are thermally and shor/currentt protected and will shut themselves down, but the fuse is for added protection.
20. At this point you may want to go back and adjust the voltage regulation pots of the three (3) units to fine tune your final voltage output.
21. Replace the covers and the cover screws of the three (3) units.
22. You are now finished except for any custom touches you want to add. Now wasn’t that easy.
As always,
Disclaimer:
The author of this document will not be responsible for any out come brought forth by performing the above modifications. This document is for informational purposes only and I will not be responsible for any damage to any equipment or personal injury.
Initial release 6/12/03
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Make your own (more reliable) Peltier/TEC PSU
04-11-06
Make your own (more reliable) Peltier/TEC PSU
