Converting PC Power Supplies for Amateur Equipment

Modern PCs use well designed, high capacity and reliable switch mode power supplies (SMPS), which ought to be ideal for amateur use. In addition, used supplies from surplus PCs can be had for very little. I had a couple of old obsolete PCs with perfectly good supplies, so I decided to see if I could use one to power a new dual band VHF/UHF transceiver.


PC supplies provide +3.3V, +5V, and +12V at high amperage ratings as well as some low current negative voltages. Since most amateur equipment requires 13.8V +/- 15% some modification would be required. Also, high current for transmitting, up to 20+ amps for a 100W transceiver should be a requirement. Obviously, reliability and fail-safe modes for over-voltage, short circuit protection and cool quiet operation should considered. Recent design PC supplies fit those requirements very well.

Switch Mode Power Supply theory of operation

Most PC switch mode supplies have a circuit configuration similar to the one shown in the simplified block diagram below. 115V AC is rectified to DC, which is them applied to a power modulator. A PCM (pulse-width-modulated) signal converts the DC to a higher frequency (around 30KHz). The high frequency is stepped down for the low voltage DC, rectified and filtered to DC at 5V, 12V etc. The output of the 5V and possibly the 12V is sampled, compared to a reference and used to control the PWM generator to provide feedback to control the voltages. Various other controls for over-voltage, current limiting and temperature control are present, but not shown in the block diagram. It is the feedback voltage that will be used to increase to output to 13+volts for amateur use.

SMPS Block Diagram

Switch Mode Power Supply (click image to enlarge)

Selecting a PC SMPS to modify

Finding a used PC PS should not be too difficult. Lots of folks have old unused PCs lying around and they might be grateful to just get rid of them. PC repair shops and resale vendors may have reasonably priced used items at a fair price. I picked one up recently for $20. The supply you are looking for should have a 12V rating of at least 15A, at least 300W and preferably up to 400W, be in reasonable shape and not too dirty. There are lots of brands out there and no circuit diagrams available - these are meant to be throw-away items, but they are very reliable. If you can open the supply up, look for a variable resistor (trim pot) on the board near the DC supply wires. It may allow easy alteration of the output voltage.

Other items required:
* SPST switch for on-off
* Binding Posts and/or Barrier Strip
* Various resistors, including 10 ohm 10W
* 450 microfarad 15V electrolytic
* Plastic feet for case

Modification procedure

1. Removal and initial preparation

Disconnect cables and remove supply from old PC. Then cut off the cables near the PC connectors, leaving a couple of inches at the connector in case you wish to use them for other purposes. Check to see if any of the wires are connected together on the large connector. These will be connected together later.

2. Check Power Supply

Before opening the case, check out the PS operation. Start by identifying the various output wires. The color coding is as follows:

Ground Black
+3.3V Orange or Violet
+5V Red
-5V White
+12V Yellow
-12V Blue
PS on Gray or Green
PS good  

Connect a 10 ohm 10 watt load to a +5V (red) lead. Short together the PS_on and a ground wire. If any of the pins on the large connector were connected together, such as large and small orange wires, short these together as well. Connect a voltmeter to a +5V wire. Plug in supply and observe that the fan will run and the voltmeter should read 5V. Check the +12V output as well. If the PS starts, then proceed on. If not check connections, especially the PS_on. If it does not work after all of the connections are properly set up, the supply may be bad - discard and find another.

3. Open case and begin modification

If the supply has not been used for a while you can safely open the supply case, otherwise do not touch the circuit board until the state of charge on the large filter capacitors has be determined. See CAUTION below!

**** CAUTION**** High voltages may be present even if the supply is disconnected! Never work on the supply while it is plugged in and check the high voltage capacitors to be certain they are discharged. If in doubt, discharge with a 100 ohm resistor and measure the voltage. Always unplug the supply before handling or working on it.

First, look at the top of the circuit board to find a trim pot. If it is there, you may be in luck and the adjustment might be sufficient. If the trimmer is there, hook up the supply and measure the +12V while adjusting the trimmer. If the range goes to +13V you are probably OK without any further changes. At this point, with at least 13V, test the supply with sufficient load to draw 6 to 12 amps (2 ohms to 1 ohm load). As long as the voltage stays above +12 at load you should be OK. If the range is not enough or you wish to increase the voltage to 13.8V, proceed on to the feedback circuit modification step.

Once you are certain the capacitors are discharged, work can begin. Remove the circuit card from the case as well as the input power connector. The fan may also have to be removed or disconnected. The PS can be safely run w/o the fan for low power tests. Carefully examine the circuit board and components to look for any signs of heat damage - if there is any, it is probably best to discard this one.

Next, unsolder or cut the unneeded wires:
* All of the 3.3V wires
* All of the +5V wires except the two shorted together above and one additional for a working load.
* -5V and -12V wires.
I like to unsolder the unused wires in order to make more room on the circuit board.

4. Feedback circuit modification - changing the voltage on the +12V line

Now the fun starts. Examine the bottom of the circuit board and look for one or more narrow traces coming from the +5v and/or the +12V area. This is probably the feedback line. The process is to add some resistance to this line to cause the supply to raise the voltage.

First break the narrow trace on the +12V line, if present. If only the +5V line is there use this. Attach wires to allow addition of resistance. Add in a variable resistor (~10K) for test. Hook up the supply as in the previous test and slowly increase the resistance from zero, while monitoring the voltage. Increase up to 13.8V if possible. If the supply shuts down before +13.8 the high voltage protection circuits may be activated. In some cases, both the +5V and +12V lines may need to be modified.

Now remove the variable resistor without changing the setting. Measure the resistance - this will be the value to be added to the circuit. Next add the resistor in permanently. In order to ensure a slow startup and prevent over voltage issues, parallel this resistor with a 450 to 1000 microfarad capacitor (15 working volts). As the capacitor charges, the voltage will be gradually increased from the original set point of 12V to the new 13.8V value. See accompanying photos for examples of my modifications. If the capacitor is too large the supply will be slow to react to load changes. If the response is sluggish, reduce the capacitor value.

PS Modications

5. Final reconstruction

* Mount Binding posts and or barrier strip for output on the case near the wires. Be sure everything will clear inside before drilling any holes. Use grommets to bring wires out for barrier strips.

* Drill holes for on-off switch and LED indicator. Mount switch and attach gray and black wires. Attach LED through 1K ohm resistor to 12V supply wire. The switch will turn on the output power, but does not turn off the high voltage section. Alternatively, a switch in the 120V line could be used, but the gray and black wires then need to be shorted together.

* Attach several +12V (yellow) and ground (black) wires to binding posts and barrier strip.

* Finally, permanently connect a 10 ohm 10W resistor between the remaining +5V line and ground for a permanent load. Use cable ties to attach to the case in a convenient location. Carefully insulate the leads.

* Reassemble the circuit board and power connector to the case.

* Do a final test with load prior to attaching a radio. With an analog meter turn on with load attached to ensure that there is no over voltage at start-up and that the final voltage is OK. It is probably prudent to run the supply under load for a couple of hours to make certain everything is OK and no excessive heat is built up.

* Attach some labels and the supply is ready to go!

Images (click on image to enlarge)

6. Performance

Each of the three supplies I modified was tested by applying various loads and measuring the voltage to give a measure of the regulation. The voltage from no load to full load varied from -5.1% to -7.1%. All of the full load voltages were well above 12V, so they should work properly as transceiver supplies. Output noise was on the order of 40mV, most of which was at 30KHz. A chart of the regulation performance is shown below. (Click on chart to enlarge)


Sources and Links

Considerable additional information and background can be obtained in the links below. I am very grateful to the various authors listed for idaas on how to proceed with this project. I used bits and pieces from several to arrive at my final design and process.

13.8 V / 15 A from a PC Power Supply

200 Watt PS Schematic

Background on PC Power Supplies

Converting Pc Switching PS to 13,5V


Tips on Using PC Power Supply for projects

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