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One of the first things you’ll need after buying an electric R/C car or truck is a quality charger (for my money, the Triton can’t be beat). Resist the temptation to buy the cheapest charger you find, especially if you are serious about this hobby. As with many things, you get what you pay for.
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Most of the really good chargers only work off of a 12 volt DC power supply. Yup, you can’t plug them into a wall outlet. So, if you buy a charger like the Triton you are left with three choices:
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A) Connect the charger to a 12 volt car battery. Many people can live with this and if you only use your R/C vehicle in the field then this isn’t a problem.
B) Buy a 12 volt power supply. This is what most people do. Given that a good high end charger can charge just about any type of battery (12 car battery, AAA, AA, C, D, NiMH, NiCD, etc.) it’s a shame to use it just for your R/C batteries. So charging batteries in the house will allow you to get more out of your charger. However, for $80+, buying a 12 volt power supply when added to the cost of the charger becomes expensive!
C) “Make” your own power supply. Actually, it’s more like modifying an existing supply. You may not realize it, but a perfect 12 volt power supply may reside in an old PC you have kicking around. If you’re not lucky enough to have a spare PC power supply, you can buy a quality PC power supply for under $30. Whether you find or buy a PC power supply, just make sure you get one that provides at least 15 amps on the 12 volt circuit, that way it will pump out enough juice for real heavy duty charging (if you starve your charger for power you will be limited in the number of cells that can be charged at once). Of course you can’t just plug your charger into a PC power supply, that’s where the modifications come in!
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Now two of my favorite things are saving money and building things (my favorite thing is saving money by building things). This 12 volt power supply can be built for about $38 (including the cost of the PC power supply).
Before I go any further, please note that the intention of this article is not to instruct you how to modify a PC power supply. My goal is just to show you that it can be done and more so that I did it. The reason I'm not going to show you is that the capacitors located inside these power supplies (most power supplies actually), can contain enough electricity to KILL. I'm not talking about a little shock that will make you pull your hand back in pain....I'm talking about a jolt that will stop your heart and make you have a very bad hair day. The only people who should even think of attempting such a modification are electricians and other people with similar qualifications.
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Warning: Power supplies generate high voltages internally and are dangerous. Unless you have been specifically trained to work inside power supplies, you should not open one. Even with the unit unplugged dangerous electricity can remain stored within its components for some time.
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Ok, with the warnings in mind, let's see what I used to turn a PC power supply into a very powerful multi-purpose 12 volt power supply.
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1) PC Power Supply: It goes without saying that this is needed! I selected a 400 watt dual fan supply manufactured by Power Up!. Most importantly, this supply has 25 amps on the 12 volt circuit (more than enough for our charging needs!). Here’s a back and side view of the stock supply. Notice that this supply has three fan speeds - nice! The white sticker on the side lists the amps for each circuit. See the big ‘bundle-O-wires??? That’s gotta go!
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2) Resistors: Two 10 ohm 10W 5% wirewound resistors are needed. These are used to place a specific amount of current draw on the 5 volt circuit. Without it, the power supply may not work or may be damaged. I got these at Radio Shack (Model: 271-132, $1.69 for 2).
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3) LED and LED Holder: While not necessary, few things will make your power supply look nicer than a LED (especially a trendy blue one) used to indicate that the power supply is on and all systems are go. Again, you can get these at Radio Shack (5mm Blue LED $2.99, Model: 276-311).
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 4) Bench Top DC Volt Meter: I chose to add this volt meter to the project because it not only provides me with a real time measurement of the voltage being pumped out, it was also on sale ($3.99) and it makes the new power supply look good. If this wasn't enough, it has built in banana jacks. This will produce a nice finished look because the only other alternative was to mount the jacks right on the supply, which doesn't produce a finished look (my charger will plug into these jacks). Of course I had to modify the meter slightly to make it part of the power supply. Since the meter has dual scales (0 - 15 and 0 - 30 volts) and I only was interested in the 15 volt scale I was able to remove the 30 volt jack. This left a hole that was perfect for mounting my indicator LED! I drilled a hole on the side of the meter and relocated the power supply's on/off switch here. This places all of my supply's controls, jacks, and displays in one handy location. I attached the meter to the top of the power supply with 4 screws. Sorry, no model number on this because Radio Shack stopped selling them (guess that explains the sale).
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Wiring
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Once you open up the power supply (unplugged from the wall outlet, of course), you will remove all the power leads except for three ground leads (black wires), one 5 volt lead (red), and one 12 volt lead (yellow). You will also want to leave the “power on” lead intact (either gray or green). Note, these wires may be colored differently depending upon your power supply so make sure you test them with a multimeter to determine not only their voltage but also their polarity.
You will connect one of the ground leads and the 12 volt lead to your external jacks (banana or otherwise). You will also connect one end of the resistor to the 5 volt lead and the other end to a ground lead. Since the resistor will get moderately hot, you’ll need to mount it against an inside wall of the power supply, preferably in the fan’s airflow. The wall will act as a heat sink. It’s a good idea to buy a heat sink for video card RAM and attach that to the resistor as well. If you do both of these things, you won’t need to worry about the resistor getting too hot.
Lastly, to make the power supply think it’s connected to a motherboard, you’ll need to ground the “power on” lead. The power supply will not operate unless you do this. If you want, you can simply connected the power on lead to a ground lead. That way, when you plug in the supply it turns on automatically. I decided to ground the lead across a switch (the rocker switch that’s visible in the picture below). That way I can turn the supply on and off without having to unplug it.
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In order to locate the power on lead on an ATX power supply, use the diagram below. The two darkened pins “should” be the “power on” lead and a ground. These are the two you’ll need to short to make the power supply think it’s connected to a motherboard. Again, use common sense before you do anything because technologies change over time an your power supply’s main lead may differ from what’s shown here.
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Once you have your supply up and running, check it’s output voltage. If you are not getting 12 volts, try using two resistors across the 5 volt line. Try wiring them in series then in parallel (series will double the resistance, parallel will cut it in half). You also want to make sure your power supply stays cool. A hot supply looses as much as 40% of it’s voltage output.
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The Results
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And here's the finished product!!! If you squint you'll see that the supply is producing 12.3 volts. With the charger plugged in and doing it's job (i.e. charging 14 sub C cells at 5 amps) the voltage remains right at 12 volts. More than powerful enough.
This power supply can be used for any item that needs 12 volts and draws up to 25 amps DC.
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