Notes on using ATX power supplies to power transceivers

There are many articles on the Internet about modifying ATX-style PC power supplies for use as 12 volt bench supplies. This instructables article is a good example. I won't repeat most of the details here, but I will give some notes about amateur radio use of these supplies.

Advantages:

• Compact and lightweight
• Extremely cheap and plentiful. I paid $2 for my supply at a local computer recycler.
• Internal protection against shorts and overloads makes for a bulletproof supply.
• Wide input voltage and frequency tolerance. Most supplies are switchable between 120 and 240 volts, and will operate on 50 or 60 Hz power. Some of the better ones don't require switching and will operate on anything from 90 to 240 volts.

Disadvantages:

• Max output voltage is usually 12.0 volts, which isn't enough to get full rated power out of many mobile transceivers.
• 12V regulation is fairly loose; my supply drops almost 1 volt under full load. (The ATX spec allows up to 10% variation from no load to full load.)
• May create audible "birdies" in your receiver, especially if you're using an indoor antenna.
• The fan noise may be annoying in some situations. However, in a "go kit" type setup the fan is actually an advantage, since it can help exhaust hot air from inside the box.

Rated 12V output varies widely even among supplies with the same wattage rating. Look for the highest 12V output you can find. 10A is pretty common but I managed to find one rated for 18A.

Basic connections to make the supply run:

• The green PS-ON wire must be connected to one of the black ground wires to take the supply out of standby and turn on the outputs. This is a handy place to put an on/off switch if your supply lacks one.
• There must be a load on the red +5V line to get proper regulation. A 10 ohm, 5 watt wirewound resistor makes a good load. Sometimes a higher load on the 5 volt line will cause the 12V supply voltage to rise a few tenths of a volt, so you may want to experiment.
• On some supplies, there is a brown 3.3V sense wire to detect voltage drop in the wiring harness. It will be wired to the same pin as the orange +3.3V wire on the multipin connector. If you cut the connector off, you need to connect the brown and orange wires together or the supply may not run.
• +12V will be on the yellow wires. They're generally all connected together internally. For high-current use I bundle them all together, since they're usually fairly light gauge.

RFI issues:

• My supply would go into shutdown when transmitting into an antenna, but not when transmitting into a dummy load, indicating RF was getting into the supply and confusing its overload shutdown circuit.
• Cutting off all unused output wires inside the supply's metal case helped somewhat.
• Putting bypass capacitors between the yellow and green wires and ground was enough to cure the rest of the problem on my supply. These should go inside the case, as close to the circuit board as you can reasonably accomplish. I used two ceramic disc capacitors from my junkbox. The value isn't critical; I think I used 0.01 µF. This may also reduce "birdies" in your receiver.