With a few modifications and two additional resistors you are able to modify an old AT or ATX PC switch mode power supply unit to a stabilized 13.8 or 5v Volts / 20 Amps power supply.
WARNING: There are high voltages inside the housing, which can be lethal. Before opening the housing of the PC Switch Mode Power Supply unplug the power cable and turn off the switch on the back. Discharge the capacitors of the power supply by attaching a 100 ohm resistor between a black and red wire on the output side. However the high voltage capacitors on the input side can be still charged. The best way to discharge all capacitors is by letting the power supply sit unconnected for a few days. PLEASE NOTE: You do all modifications at your own risk!! I will not be held responsible for any mishaps, lethal shocks or damaged equipment by using the modified power supply. You have been WARNED!!
Now I am not going to "re-invent the wheel" here as there are many articles and videos that explain how one can modify a PC Switch Mode Power Supply for various uses. I will however provide a pictorial view of how I modified a power supply to 5 volt DC 15 Amps. I will also provide information about the spectacular end result of the modified power supply.
In the event that you would like to know more about the modifications, I provide the following two links for further reading:
1. A Cheap Power Supply - The "St. Louis Switcher" by Matt Kastiger N0XEU
2. Building the "St Louis Switcher" by George Heron N2APB
Download articles HERE
Compliments Radio ZS March / April 2003
Construction:
The construction went well but I was doubtful if the used 2002 power supply was still in good conditions. Time will tell!!
After all the modifications were made it was time to switch the power supply on, place a load on it and see if magic smoke escape. First I did not put a load on the supply. Well it switched on and no smoke escaped. There was also no load on the output. Using different 10 Watt resistors I started to test the load. I increased the load as I went along testing the unit. At 10 - 15 Amps the power supply was still happy even though it started to get rather hot. The next test was rather ambitious. I installed a resistor that drew 33 Amps for about 30 seconds. Still no smoke and I decided not to push it any further. The modified supply was working great. I removed all the resistors switched the supply off. After a while I switched the supply on with no load. An instant "bang" was heard with the dreaded magic smoke coming out. I opened the supply but could not see where the smoke emanated from. After unscrewing the PCB from the housing I saw that a PCB track was burned and the housing had a black burned mark on it. Well being lazy I decided to "fix" the burned track by bridging it with a piece of copper wire. Well this sloppy/silly fix resulted in a real Guy Fawkes display. High voltage arc's running between the transformers and high voltage capacitors, clapping and booming so much so that my YL asked what am I doing in the Shack. Funny it took quite a while before the original fuse blew. Upon further investigation afterwards I found that there might have been a short created from the PCB track to the metal housing. How I do not know. Before starting the modifications I also found three electrolytic capacitors with broken safety valves on the top. This must be replaced before final testing. Moral of the story this power supply unit was not a good candidate to modify. It was old and had an issue or two that resulted in it going silent key. Some years ago I modified an old XT power supply which is still running flawlessly today.
End result: Rather look for a good condition PC switch mode power supply before commencing with any modifications. Scratching in my junk room I found a next to new PC power supply which I am keeping for a spare for my desktop computers. So I was not willing to modify it. Scratching more I ran into the ideal 5v Switch Mode Power Supply (See photo.) I forgot about this supply which was salvaged from a instrument that used 5v 20 Amps. Now this find solved the need to modify any other PC power supply.
Finally: I had great fun with this project, However the danger that lies buried in a PC Switch Mode Power Supply was clearly demonstrated by this old supply. Thankfully I applied all the safety measures in modifying the supply with only the supply going SK and not me.
Images (Click on images for larger view.)
