PC Power Supply Ham Radio

| enero 4, 2020

Convert PC power supply for Ham radio

Converting PC Power Supplies for Amateur Equipment

Here is some additional information based on our experience that you might find useful if you decide to tackle this project. There is normally a label on one side of the power supply that will tell you its parameters, such as “12 VDC, 6 amps, 200 watts max”

It’s not an exact conversion, as you can see: 200 watts divided by 12 volts equals approximately 16.7 amps, yet the supply says max 6 amps at 12 volts. The difference comes in when you consider that the power supply is designed to put out more than just +12 VDC.

The total wattage rating of the supply is the sum of all of the outputs of all the various voltages, while the 6-amp rating of the 12-volt supply applies only to that portion of the supply. So, that means the maximum output of the 12-volt supply is 72 watts, with another 128 watts available for the other voltages. But note that the pictured power supply specifies a maximum of 125 watts for the combination of both 3.3-volt and 5-volt supplies.

The remaining 3 watts will cover the low-current -12 volt and -5 volt supplies. When you convert one of these supplies, you must ensure that you do not draw more power from each voltage than that particular section of the supply is capable of handling, and that the total power draw of all supplies does not exceed the overall rating of the supply (200 watts in this case).

As this conversion involves only the 12-volt supply, we are limited to 6 amps from the 12-volt line, and this will not exceed the 200-watt limit, so there are no worries. If you decide to expand on this and use the 5-volt and/or the 3.3-volt supplies, be sure to calculate the expected power draw from each voltage and add them all together to ensure you do not overload the supply.

With the newer Pentium and up supplies, more chips have gone to 12 volts, so there is more current-handling on the 12-volt end. Most high-quality desktop supplies are rated around 17 amps, but that rating might be total current. Remember, in a lot of the computer supplies, there can be more than one 12-volt rail. Those specs are usually available for most PSUs online somewhere.

Identify the Wires on the 20 Pin (or 24 Pin) Connector

PC Power Supply Ham Radio

When you get into the supply, the sheer number of wires and connectors can be a little overwhelming until you separate them out and realize you only need to worry about a few of them. In addition to the main 20-pin connector, which you can’t miss, there are also several 4-pin Molex connectors daisy-chained together for hard drives (there may be as many as eight of them); and at the end of this string of Molex connectors, there is usually a small floppy drive pigtail connector (quite tiny).

There is also a square 4-pin block connector on its own. The square block connector has two +12 volt and two ground wires on it as well, and likely comes from the same rail as the 4-pin rectangular Molex connectors.

The wires on the main 20-pin (or 24-pin) connector are color coded. These are the same for all ATX power supplies:

  •     +3.3V wires are orange
  •     +5V wires are red
  •     -5V wires (if they are present) are white
  •     +12V wires are yellow
  •     -12V wires are blue
  •     ground wires are black
  •     “power on” sensor is green
  •     + 5V standby power is purple
  •     “power good” indicator is gray

The green power-on sensor wire is internally connected to 5 volts through a pull-up resistor. If you connect this wire to ground (any black wire), the power supply will turn on. The purple standby power wire puts out +5 volts even if the rest of the power supply has not yet turned on, allowing you to power any circuit that might control the ON/OFF signal. The gray “power good” indicator is at 5 volts if each of the output wires is operating at the correct voltages and could be used to power a LED indicator.

The large 20-pin motherboard connector is made up of two rows of 10 pins each. If you are holding the connector with the guide key on the top (this is a piece of plastic that sticks out from the long side of the connector shell), pin one is the leftmost pin on the bottom row (it’s square), pin 10 is the rightmost on the bottom row, pin 11 is the leftmost on the top row, and pin 20 is the rightmost pin on the top row (also square).

So for reference, pin one is directly below pin 11, and pin 10 is directly below pin 20. The 24-pin connector is arranged in the same way except there 12 pins in each row instead of 10; therefore, the bottom row starts on pin one at the left and has pin 12 on the right end and the top row has pin 13 on the left and pin 24 on the right. All of the pin references in this article will refer to the 20-pin connector, so if you are using a PSU with a 24-pin connector, you will have to make the necessary adjustments.

The only wires that we need to be concerned for our purposes are the green wire to pin 14, and any of the black wires (we used the one from pin 13). You have to tie the green wire to ground or any of the black wires to make the power supply turn on.


PC Power Supply Ham Radio

Power control switch and indicator schematic

Cut the green wire anywhere you like. Then, cut the black wire going to pin 13; it’ll be the pin towards the longer section of pins and wires on the connector, not on the shorter side. Then, strip the green and black wires and tie them together using tape or heatshrink to insulate the splice (a small blue wire-nut will work nicely as well), or the green and black wires may be connected toaSPST switch to allow turning the supplies on/offfrom the switch. Mike used a 12-volt lighted rocker switch for both on/off control and indication (see schematic). This tells the computer power supply to turn on.

Next, you will need one of the yellow+12 volt DC wires. We used a 12-volt wire from one of the four Molex connectors, as these have a higher current rating than those on the ATX connector. Holding the 4-pin Molex connector (of which there will be several daisy-chained together on each computer PSU) with the key guide on top of the shell, the right pin is +12 volts, the two center pins are ground and the left pin is +5 volts.

Cut a yellow wire off the 4-pin Molex connectors (remember, it’s the right-hand pin and wire when holding the Molex connector with the key guide on top). Then cut the wire right next to that for the ground. Now, you can just strip these and tie them straight to your radio’s DC cord, or you can use some banana terminals, binding posts (as Mike did), Anderson Power Poles, or whatever you want to make quick and easy connections.

You can cut the ATX connector right off and tape up the ends of the wires at this point as you will not need anything else for a plain 12-volt PSU. If you want to build something with USB charging power etc., you can certainly get the parts easily enough and use any +5 volt wire as per the pin out table to create a charging port for your phone or whatever. In fact, a typical 250-watt PSU can provide 2-amp charging to probably 3 or 4 devices at once if you use the high current +5 volt wires found on the 4-pin hard drive Molex connectors. If you want a secondary lower-current 12-volt line, use the +12 volt wire from pin 10 on the ATX connector as well … great for shack accessories, etc.

Check Your Work

We used a multimeter throughout to check for ground continuity and for 12 volts once I tied the green and black wires together. The green wire does show 5 volts when not shorted to ground, but it uses a pull-up resistor so, once you ground it, it just turns the supply on and off. As noted earlier, you can put a switch between the green wire and ground if you want to be able to turn the PSU on and off if it doesn’t have its own switch on the back.

This is a very rough conversion, and you can get as fancy and tidy as you want. Possible enhancements include mounting binding posts and/or USB ports on the PSU chassis, cutting all unnecessary wires inside the chassis to make a nice clean build, adding meters, etc. Mike made a panel to attach to his operating position with binding posts and a switch

A Few Preautions

An article appeared in QSTa few years ago that included a warning that you should use good-quality, well-shielded supplies because the usual computer does not supply much current at 12 volts; so cheaper supplies may get hot when converted and subjected to heavy loads. Also, the older supplies on XT and ATX boards had a more limited 12-volt supply capability; most of those chips ran at 5 volts, so the 12 volts was just for powering the fan and disk drives. In addition, cheap unshielded supplies may emit loads of RF, causing birdies that can get into your receiver. These are all valid and should be taken into consideration.

Another caution is that some computer power supplies don’t regulate all their output voltages independently. What this does is cause the 12-volt line to change voltage if the load on the 5-volt line is heavy. Switching power supplies do what they do by sending a variable-width pulse through the primary of their power transformer.

This pulse width controls the output voltage of the whole transformer, so taps off of the secondary may not show the proper voltage if the transformer sees a high load on the 5-volt line because that is where the voltage regulator sits. The 5-volt line regulates very nicely under a heavy load, but the other voltages may go too high as the comparator adjusts the pulse width to make the 5-volt output happy.

Unless you add a 5-volt USB charging port or something, load restrictions aren’t really an issue when using just the high-current 12-volt rail supplying the 4-pin Molex connectors. With a good quality, shielded supply, this makes a small, cheap and useful shack power supply.

Computer PSUs tend to be very well-regulated and clean and for usually quite a bit less, even new, than your typical 23-amp supply from Samlex, Pyramid, etc. A local shop has a 500-watt computer PSU for $39. This would conservatively provide 15 amps at 12 VDC, more than enough to run a couple dual-banders at 50 watts forever.

These supplies are not great for full 100-watt HF rigs, as those are looking for 22 amps minimum at 13.8 volts, but any mobile rig is going to run fine at 12 volts as that is what the batteries in our cars put out. If you can scavenge a couple of these PSUs for free, you could certainly run them in parallel for more current capability. We would suggest running a good, high-current, 12-volt regulator and balancing resistors between the supplies if you are going to parallel them.

We hope this helps and motivates you, as it did us, to dig into those old PSUs sitting around collecting dust. They can be used for far more than ham radio stuff. They can power 12-volt LED lighting, motors, and plenty of the new kit-style devices that are out there.

ATX power supply for ham radio – Putting a Computer Power Supply to Work in Your Ham Shack
BY RON MILLIMAN,* K8HSY AND MIKE MILLIMAN,# WB5VQX

More.. Check out this online guide to start: <https://tinyurl.com/y94dhs8w>

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