Calculations

So lets suppose we have a power supply with a minimum current of 0.5 A on the +12Volts output only. So we need a resistor of 12/0.5= 24 Ohms maximum. A good approximation would be a 22 Ohm resistor. The power dissipated would be 12V*0.5A= 6 Watts so a good compromise would be a 22 Ohm resistor of 15 Watt at least.

If the power supply has a minimum current on other outputs we need to calculate the resistors for the other outputs using the same method.

Opening the power supply

You may get an electrical shock even if the PSU is disconnected from the mains, so be careful, or you may not need to follow the next steps. One measure of caution that may not work, but may diminish the danger, is to leave the PSU disconnected one full day before opening it.

List of wires and colors

Black Wires -Ground- connect to ground (Ground Output)

Green Wire - Connect to one black ( to place the power supply always on )

Grey - Power Ok - unconnected

Purple - +5V standby - unconnected

White - -5v ( not used in recent motherboards) unconnected

Blue- -12V unconnected

Orange Wires (+3.3v) - leave opened ( no connection )

Red Wires (+5v) - leave opened ( no connection )

Yellow Wires (+12Volts) connect to Positive Output

Resistors to connect

As only the +12V output has a minimum current requirement we only place one load resistor ( previously calculated ) between the yellow and black wires. The resistor (or resistors) should be placed inside the PSU to benefit from the PSU fan.

Practical resume:

Black = Ground = Connect to Black wire of the radio

Yellow = 12 Volts = Connect to Red wire of the radio

Place the Resistor between Yellow and Black

Orange = +3.3V = Leave open (unconnected)

( Place a resistor between Orange and Black if needed, if the +3.3V Output has a minimum current requirement)

Red = +5V = Leave open (unconnected)

( Place a resistor between Red and Black if needed, if the +5V Output has a minimum current requirement)

Green = connect to Black

All the other wires are unconnected

Practical experience:

I got one power supply (460W nominal with a big 120mm fan) and modified it. Acoustically it is very quiet. Electrically with a VHF/UHF FM rig the RF noise is not detectable. So to operate a VHF rig the PSU is suitable.

One good thing about it (and many others) is an adjustable resistor inside to control the output voltage and that way I could adjust the output voltage up to 13.5 Volts instead of usual 12Volts (with a lot of care because of the deadly voltages inside of the power supply).

To test the RF noise I compared the PSU with a linear PSU, and with a Gel-Cell battery, using one hf radio on several amateur bands, and a 20 meter horizontal long wire antenna.

On the first test, inside the house, the PC modified PSU was a noisier than the linear PSU but not much worse, and the battery was comparable to the linear supply.

Then outside, I placed the radio under the antenna and yes, the "computer" PSU, is usable on VHF and UHF FM, but on HF gives a lot of noise. Surprise, the linear PSU also makes noise, and the battery is very quiet, because almost all the QRM I hear is from the coaxial cable routing from the inside to the outside of the house.

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