Convert PC Power Supply

[L82fly]

POWER SUPPLY
Convert a PC Power Supply for Use with an R/C Battery Charger

I'm sure you have seen, heard or read about others modifying an old PC power supply into something that would be beneficial on your R/C workbench. This is the third one I have converted and decided to share the process by a series of pictures just to show members how easy this really can be. Those who enjoy flying any type of R/C aircraft must recharge battery packs periodically and hopefully once you have seen my series of posts you might consider modifying one for your bench.

Note: The last post of the series has a link to download the instructions in one pdf document

[L82fly]

Find a PC power supply, in the photo below you'll see that I had two different models to select from for this project. Here are the power supply 12V specifications.

Left Power Supply: 350Watt 12V output rated at 15amps, According to the name plate. I chose this one for the project!
Right Power Supply: 300Watt 12V output rated at 13amps, again according to the name plate
I'm not sure you can tell by the photo but both units had a switch located on the back to turn the power supply off / on. While not all power supplies have such a switch this does allow you to skip a step in the modification process.

Some Dell computers and older style AT power supplies used nonstandard color codes and connectors. These power supplies may still be modified but the wire colors might vary from this document. If you have a choice, choose a power supply that has a two-row connector similar to the one pictured. The power supplies I’ve modified varied in wattage from 300 to 425. They have all worked fine with my fast charger, cycler and standard charger. But I prefer to use the largest wattage available if that’s an option; since our equipment doesn’t require that much amperage a 250 watt power supply might serve the purpose just as well!

Listed below are both a parts list that I needed to purchase including prices (Radio Shack) and photos showing the items?

1 package of 1 Ohm wire wound resistors, this includes two resistors and of the three power supplies I have converted I only needed one per conversion.
RS part #2710131 $2.19

A small amount of Heat sink compound, this helps transfer some heat from the 1 Ohm resistor to the power supply case. I have read various opinions if this is really necessary or not but I just thought that it couldn't hurt too aid the displacement of heat! The tube you see in the photo below would be enough for at least 50 power supply conversion.
RS part #2761372 $3.29

A method of transferring the power from the power supply to your fast charger, cycler or whatever you need to power. I choose a dual binding post banana jack that you can either plug in a banana jack into it or you can unscrew it and directly connect a wire.
RS part #274-718 $5.29

Notes: Other types of connectors are available, the one I selected just so happens to be one of the most expensive options (go figure)

Tools:
To perform the modifications a few tools are needed, most you should already have. A soldering iron, Phillips screwdriver, wire cutters / strippers, voltmeter, drill and in my case I used a heat gun for the heat shrink.

Miscellaneous Supplies:
The supplies needed for the modification are solder, heat shrink, a few small wire ties, the items listed in my first post and a on / off switch if your power supply doesn't already have one.

[L82fly]

Before we get started with the modifications, I need to warn you that when the cover is off these power supplies have many exposed high-voltage components ready to” ZAP” you if touched. Be sure to put the cover back on the unit before plugging it into the A/C power source. PC power supplies also have overload protection; if the output is shorted or overloaded the unit will simply shutoff without overheating (AKA smoking).

[L82fly]

There are only three major steps in performing this modification and even though I’m briefly listing them here I will later in this document explain each in detail.
1. Remove the excess wires and connectors
2. Perform wiring modifications including the addition of an on / off switch and any other components
3. Add terminal posts for connecting your chargers
Sounds easy…….let’s get to it!

[L82fly]

The first thing to do is to remove the cover off the power supply and cut or desolder most of the wires leading out of the case. Cutting them and using heat shrink to isolate them is much easier and faster than removing the power supply circuit board from the case and desoldering them, but either way you decide to go make sure and leave 1 green, 1 red, three yellow and 5 black wires that will be used later in the project. Many suggest that you only need one yellow and 3 black wires to complete the conversion but my thinking is that the wires are there and since the power supply in this case is rated at handling 15A @ 12V why would you only use one single 18 gauge conductor to handle that load, not that we will ever need that much power. But I just didn’t cut as many wires back and have left three 18 gauge conductors of each color for both the 12V positive (yellow) and negative (black) terminals!

Note: This picture was taken before I trimmed back the wires the last time, later pictures will show this!


Now it’s time to heat up the soldering iron and begin wiring modifications. Just a reminder, be sure to cover all the exposed conductors from soldered connections with heat shrink tubing. In my project example the only soldering required was to install the 1 Ohm resistor. Yes you could solder more if you want to but soldering skills are not my strong suite so I figured ways to keep that to a minimum...You’ll see!

In this picture you will see where I cut back all the unused conductors to their final length and have heat shrunk them together so they can’t short out against other components. If you look closely you will see I left the 1 green, 1 red, three yellow and 5 black conductors as mentioned earlier. Later steps and pictures will depict the conductors clearer.

[L82fly]

The green wire that you have left longer in the earlier step is normally used to turn the power supply on and off (granted your power supply had a two row connector similar to this picture.

If your power supply has a case mounted power supply next to the fan on the back like the one (picture below) just solder one black wire to the green wire and this will allow the power switch on the back to operate the power supply. If your power supply doesn’t have a case mounted switch and you want to add one, solder the green wire to one switch terminal and the a black conductor to the other terminal. Then drill a hole in the case and mount the switch.

If you really look closely you can see the gray wire nut at the bottom of the picture (remember I hate to solder) on the green wire and one black in the unit I converted.

The next step is to install a load on the 5 volt circuit. The purpose of this load is to bring the yellow conductors (12V positive) up to over 12 volts. The photo below is from my installation and only required one resistor to reach my target voltage of around 12.3 volts but some power supplies require two resistors wired in series to achieve 12 to 12.5 volts. You just have to test your particular supply prior to soldering the connections

As you can see I decided to solder the resistor in between the one red conductor left in the earlier step and one black conductor, then I used some clear heat shrink to cover the exposed conductors on the resistor. Wire nuts work here as well but you need to make sure and cover all the exposed conductors with heat shrink so they don’t touch anything! I then used (1) small wire tie to hold the resistor to the back of the power supply case but not before running a small bead of heat sink compound between the case and resistor to dissipate some heat. FYI this resistor gets hot, don’t ask how I know!

Note: I have later learned that due to heat from the resistor that the plastic wire tie might break down in a very short amount of time. The easy solution is to replace it with one or two wire bread ties (with the plastic or paper removed) instead of the plastic ties. The metal from the bread ties can easily handle whatever heat is released from the resistor!

[L82fly]

The remaining yellow and black wires (three each in my example) can now be tied to the terminal posts that were purchased. But first I needed to drill the required mounting holes in the front of the case to mount the binding post. Included on the back of the binding post packaging was a template to use for drilling the holes, see picture below for what it looks like transferred to the case. One of the drawbacks of the power supply that I selected is that it had a giant fan in the top instead of the back like most power supplies. This fact really limited where I could mount my banana clip / binding posts as you can see below!

Once I had drilled my holes in the case and mounted the binding posts I used crimp connectors on both the three yellow and black connectors as pictured below. This really fit the particular binding posts I bought better than soldering them on and of course as I previously mentioned I don’t like to solder!!

[L82fly]

Now it’s time to see if the supply operates properly. Replace the cover, plug in the 110v power cord and turn on the switch. The fan should start; some have a delay of as much as a second. Be careful not to touch the case at the location where the resistors are mounted it can get quite warm in this area. Now check the voltage between the terminal posts. It should read between 12 to 12.5 volts. If the voltage is too low, less resistance is needed this can be accomplished by using a lower value resistor or putting two resistors in parallel. If the voltage is too high, add a resistor in series. See picture below on how the sample supply worked out!

Note: I later found a plastic knock out plug to fill the hole left in the case by the original wires and also located some stick on rubber feet for the power supply.

[L82fly]

Using the power supply is simple. Attach the red connector from your charger to the positive terminal on the power supply, in this case the red banana clip. Which since I used binding posts for this particular installation can support alligator clips bare conductors or banana clips. Do the same thing for the black conductor from your charger; connect it to the negative or in this case the black binding post on the power supply. ! Since I used a 350 watt power supply for this project I could have easily installed at least two sets of binding posts thus giving me the ability to connect two different types of chargers at the same time, unfortunately due to the oversized fan space was at a premium and really all I needed was the one 12v connection.

I’ve read that others have had issues with their power supply’s shutting down when the charger is connected due to a voltage spike and they overcome this by disconnecting the charger then turning the power supply off then on again. This according to what I’ve read from others lowers the spike on the second attempt and the power supply stays on. I have been using a unit similar to the one in this example for a few months and have never seen this behaviour before; of course I connect the charger to the power supply than turn on the power supply than the charger. I’m not sure if the sequence I have been using minimizes the spike mentioned above or not but so far I haven’t witnessed their issue

[L82fly]

This is an extremely easy and affordable way to construct a regulated 12v power supply for your work bench capable of supporting almost any variation of chargers that we require in the R/C hobby. The time required for this project build was about 1.5 hours, mind you this is my third one to complete and when I work in my shop I usually have a football game going on the TV so it could take you a little more time or maybe you can accomplish it in less time than it took me! The associated costs for the build was slightly less than $11 and if I were to build a second one the extra parts from the first build would lower the second build to less than $6! I hope this document helps if you decide to build your own workbench power supply!