12V Power Supply for your field charger
AT computer power supply
(not ATX)

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NOTE: If you do not have experience working on electronics then this project is not for you. Just go buy yourself a 12V power supply that is already built for you.

Recently a couple of guys at the field began using computer power supplies to power their chargers. To do this they had to load the 5 volt line to keep the 12V up under the load of the charger. One of the guys had to run four 1 ohm 10W resitors in series to get the current from the 12V side that he needed. This seems to be the common technique many others have reported using on the internet, including an article on the rcbatteryclinic.com website. This just plain seemed like the wrong way to go about using the power supply to me. As I was thinking this a fix for it suddenly came to me that would achieve much better results with less wasted energy. The problem is that the power supply was designed to monitor the 5V line. This is great for computers because the 5V is the most critial. However when we use it for our chargers the 12V gets pulled down but the power supply does not respond because as far as it's concerned the 5V is still fine. To get around this and make the power supply pay attention to the 12V line I came up with the idea of removing the 5V source and using a voltage divider network off the 12V to supply the 5V. To begin the project you really should have experience working on electronics. A little info, the plugs that go to hard drives, cdroms, etc. have a yellow which is 12V, two blacks (both are grounds), and a red which is 5V. Inside the power supply you will find diodes mounted to heat sinks. They look like transistors because they have three legs but they are diodes. You can confirm this by looking for two diode symbols on the face and you will see the board designation begins with 'D' such as D5001. The first step is to ohm out the red wire to the cathode (usually the center leg) of each of the big diodes on the heat sinks. Only one should show to be connected to the red wires. Once you find this diode cut the cathode leg. This will prevent the transformer from producing a 5V. Now everything else can be done on the outside of the power supply if you want, but it does make for a cleaner job if you install the resistors on the inside. Connect a 68 ohm 1 watt resistor from the yellow wire to the red. Connect a 47 ohm 1 watt resistor from the red wire to the black wire. That is all there is too it. Now as the 12V varies, so does the 5V from the voltage divider (the two resistors). Now here is a note about the resistor values, I used a low ohm so the 5V feedback circuit would have plenty of current, but there is a scientific way to find the exact values to use for the two resistors. It requires you to measure the current the feedback circuit uses. These were not my power supplies and I didn't want to spend much time on them. However if you do, then here is a good page you can use for the formulas: http://www.tpub.com/neets/book1/chapter3/1-35.htm and here is a calculator so you can find the wattage of the resistors: http://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm