Articles – Dreamie PSU guide

Dreamie’s guide to PSU

Here, I will talk about power supply units which are found in computers. These power supplies changes the voltages from AC into DC, which is suitable for sensitive electronics use and is safe. Touching any bare wires from the power supply unit will NOT kill you, but i will not say the same if you are opening up your PSU.
The resistance between the two fingers on each hand is about 500k-10meg ohm, varying from body to body.

This is a diagram for the 20 pin ATX connector found in most PSUs. Dell computers do not follow this rule.

You can make use of this ATX connector to draw more voltages.
I always use this formula when finding voltage which are being supplied to the device.

(V supplied to device) = (V+) – (0V/V-)

0V means ground and V- means negative voltages. I may be wrong but so far, i have been doing my calculations with this formula.
Take for example i have a LED that requires me to supply 24V. I will take the +12V and -12V wire from the ATX connector, connecting the +ve side to the +ve side of the LED and -ve to the -ve side of the LED.

24V = 12V – (-12V)
24V = 12V + 12V

Here you have it, your 24V. You can of course use this ATX connector for many different things, but i would recommend you do a additional connector which splits it into 2, so splicing may not be nessecery.

Next we have the 4-pin MOLEX connector. There are only 4 wires connected to it, Red, Yellow and 2 black. The 2 black wires are connected together inside the PSU at the ground bus. The yellow wire represents +12V and the red wire represents +5V. Fans usually use a red wire, which is connected to the YELLOW wire, so dun mistake fans for 5V.

This 4-pin moelx has got to be one of the most important one. You can easily buy these connectors at SLT. I got a female one, with wires ready crimped to a length for $0.50 each. Couldnt find any males for 0.5$ each though. Search around and you may find it.

There is also the 3-pin fan connector found on your mobo. 2 pins are for power supply and 1 is for tachometer (RPM speed monitoring). Center is +12V, and one side is ground n the other is tachometer. Tachometer’s wire color are usually blue or yellow.

I wont talk about the P4 plug and the funnily shaped straight connector which i have so far found no use for and have decided to 5V mod my mobo so that I get a better 5V reading within my mobo.

Any questions pls post ehre. Pls correct my mistakes too.

– Contributed by dreamie
orginally @
used with permission.
07 March 2003


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