The Ultimate ATX Power Supply Mod With USB Charging Ports

Step 5 Wire Up The Power Supply

The old connectors need to be removed and replaced with some butt splices. Because the wires are thin and it will put out a lot of current we need to bunch up several wires so it will handle the current. I left the 4 pin molex connectors on so i could use them to test old computer drives.
ATX power supplys have a minimum load which should be listed on the power supply. Mine requires .3A on the 3.3v line, 1A on the 5v line, and 1A on the 12v line. I used resistors for the 3.3 & 5v lines and a mini fridge for the 12v line.
The formula to calculate the the resistor size is Resistance = Volts / Amps.
I needed 11 Ohms for the 3.3v line and the closest resistor avalable was 10 Ohms which gives .33A and a 5 Ohm resistor was needed for the 5v line.
The formula to calculate the wattage is Watts = Volts * Amps which gives just over 1W for the 3.3v line & 5W for the 5v line.
You should use a resistor that is twice the wattage you need to help keep the temp down. I used 10W resistors for both lines. They are mounted between two large TO-220 heatsinks which was then superglued to the case which keeps the resistors below 110o F.

Comments

[Theinventor94]

On the specifications of my computer power supply it says "+5 VDC minimum load is 0.2 A when there is a minimum load of 0.3 A on the +12 VDC and 0.42 A on the +3.3 VDC outputs simultaneously. +5 V minimum load is 1 A for load transient tests." After reading this I still have no clue if I need a Power resistor. How do I know what I need?

[rocketman221]

3.3V - 5 ohm - .66A - 2.2W
5V - 5 ohm - 1A - 5W
12V - 20 ohm - .6A - 7.2W

The 12V resistor will get very hot. It should be a 20W resistor or two 10 ohm 10W resistors in series. The 5v resistor should be 10W and the 3.3v should be a 5W.
Using a resistor rated for twice the power dissipated keeps them from getting burning hot.

[Theinventor94]

Thanks!