Project 08 - Computer PSU Conversion.

Project of the Month

Convert a computer power supply to a CB radio power supply.


I have seen people use computer power supplies to power their radio equipment. While these PSU's (Power Supply Unit's) have a 12 volt output, it is the 5 volts that is regulated, not the 12 volts. The outputs usually lack filtering, so they make a buzzing sound in your radio. This project will detail how to add some more filtering, and alter the regulation circuit of the PSU so that the 12 volt output is locked at 13.8 volts. It's unlikely that you will have the exact same model of PSU that I have here, so these instructions will be general, and you will have figure some bits out for yourself. I have performed nearly thirty of these conversions, with only one that I couldn't do, and one PSU that blew up.

A word about safety


While we will not be making any modifications to the high-voltage mains part of this PSU, there is high-voltage mains inside the box. I don't need to warn you about the dangers of mains electricity, but I will urge you to be extra careful with these power supplies because they feature rectified mains, which means not only 240 volts AC, but 340 volts DC as well. DC is the one that makes your muscles clamp (DC fucking hurts.). Never work on the PSU while it is powered up. Wait for at least one minute after unplugging it for the capacitors to discharge (Listen carefully for the faint whistle of the PSU electronics powering down some time after the power is removed). You should have some experience with mains power. If you are not capable of wiring up an extension lead, as a bare minimum, then you should not attempt this project.



You will need:


A computer power supply out of a desktop PC. Most modern computer power supplies are not suitable for this project. This project covers modifying an old 'XT' style computer power supply, the very old power supplies that are switched on and off by an actual mains switch. Make sure it works.

A 1K trimpot.

A 4700uF, 25V capacitor.

Sundry small capacitors.

Step 1


Change the circuit to regulate from the 12 volts out instead of the 5 volts out.

Disassemble the PSU. It will likely be full of dust. I have seen power supplies that looked like they were pulled from the Titanic, but after a dust off with a clean dry paintbrush, they looked like new. Cut all the output wires except for all the yellow wires, and an equal number of black wires. Leave the mains wiring and the fan wiring intact. The picture below shows the three sections of my PSU. Yours will probably look similar.




The computer PSU regulates off the 5 volts, so we have to find where the 5 volt output goes into the regulator section. In most of these PSU's, this is quite easy. On the solder side of the circuit board (PCB), look at the 5 volt output (the red wires). There should be a track running from the 5 volt solder pad down to the regulator section. In my PSU, below, there is a small wire link from the 5 volt output to a track that runs down the edge of the PCB, and to the regulator area. Once you have located this track, cut it. In my case here, it's easy to isolate the 5 volts from the regulator by cutting the wire link up near the red wires.




The drawings below show what we need to do. Now that the voltage control regulator is cut from the 5 volts, we connect it to the 12 volts, through a 1K trimpot, so that we can set the output voltage. Connect one side of the trimpot to the 12 volts, the other side to ground, and the wiper to the track that goes to the regulator section. In my PSU, I drilled 1mm holes in the right places, stretched the trimpot legs,  and soldered the trimpot directly down on the PCB. You do not need to go to that much trouble for yours, the trimpot can be connected to the PCB by short wires if you like.



Stript the ends of the yellow wires and twist them together. Do the same for the black wires and screw them into a terminal block. Connect a 4700uF electrolytic capacitor, a 0.1uF greencap capacitor, and a 0.01uF ceramic capacitor in the terminal block with the wires. The negative lead of the electrolytic capacitor goes to the black wires. This is the additional filtering for the PSU.

Step 2


Get it working.

Set the trimpot to the centre of it's range. Connect a voltmeter to the output, the yellow wires and the black wires. Plug the PSU in and turn it on. If it powers up ok, check the output voltage and adjust the trimpot till you get 13.8 volts. If the PSU fails to start up, the trimpot is probably too far one way or the other. Try different positions until it starts. Remember to wait one minute between turning it off and on, because the the PSU will not fully power down until the main capacitors discharge. You should be able to hear it. If not, just wait the full one minute. If you still cannot get it to start, check your connections, or maybe the problem is that overshoot is triggering the over-voltage shutdown.


A nice cosmetic finish is to add an internal switch to replace the mains switch on the end of a lead, and an LED to show 'power on'. Those small square rocker switches fit nicely into the slot for the output wires. If you add the rocker switch to your PSU, it frees up the slot that was used by the power switch lead. Add an LED and 1K resistor to the 13.8 volt output, wrap it up in insulation tape and stick it into the cord-grip-grommet from the power switch lead. Stick the LED and the grommet back into the power switch lead slot.


Step 3


Check for overshoot.

A problem I have seen quite often with this modification is that the output voltage goes very high at turn-on, for about half a second, before settling back down to the preset 13.8 volts. This is called 'overshoot'. Even though it's only for half a second, overshoot could damage your radio, so we should test for overshoot and correct it before using the PSU. Connect a 12 volt light bulb to the output of the PSU and watch it carefully while turning the PSU on. If the light bulb goes very bright for a second, then you have an overshoot problem. This is caused by the difference in impedance between the old 5 volt feedback, and the new 12-volt / trimpot feedback arrangement.


There will probably be a few small value electrolytic capacitors around the regulator chip, 1uF, 2.2uF, 4.7uF, or such. There is 5 around the regulator chip in my PSU, as can be see in picture #11 below. Remove one of these capacitors and test the PSU for overshoot. Put the capacitor back in if it has no effect, and try another one, and another one, until you find the one that eliminates the overshoot.


If you have no success with that, add a 100uF between the 12 volts and the wiper of the trimpot as shown in the circuit below. If the power supply fails to start up or shuts down, try a 47uF instead.


Shown below is the 10uF capacitor I removed from this PSU to cure the overshoot. It seemed to be between the track coming down the edge of the board and the regulator chip. So tried it first, and hey presto.


Extra Protection


If any of your wires break, or one of your solder joints goes bad, the PSU could lose regulation and the output voltage will go up to somewhere between 22 and 30 volts. This would not be good for your radio gear. One of the good things about these PSU's is that they shut down instantly if they are overloaded or shorted out. That's handy because we can add a simple circuit, called 'crowbar protection', which shorts out the power supply if the voltage goes above a preset level. The crowbar protection consists of an SCR, a 15V zener diode, and 2 resistors.


The crowbar connects to the 13.8 volt output of the PSU as shown in the drawing below. If the PSU voltage goes above 15 volts, the 15 volt zener diode starts to conduct. The zener conducts voltage straight onto the gate (g) of the SCR. When the SCR senses voltage on it's gate, it turns on, that is, it goes dead short from anode (a) to cathode (k). This shorts out the power supply. In other types of power supplies, the crowbar circuit would be used with a fuse so that the SCR blows the fuse. Lucky for us our computer PSU shuts down the instant it detects a short, so no fuse needed. And no fried radio. Use an SCR rated at 8 Amps or more, such as a C122E, Dick Smith part no. Z-4332.


If you decide to build this add-on circuit, don't connect it till after the PSU is finished. It's annoying if the PSU keeps shutting down while you're trying to set the voltage.

And if your PSU won't turn on at all with the crowbar circuit connected, you have an overshoot problem. See step 3 above.

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