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Computer Power Supplies

(Author: Roger Thorpe)

Computer Power Supplies

Written by Roger Thorpe


I have been asked by several Meccanomen recently about using computer power supplies for powering Meccano models. The following notes may be of use to others.

All computer power supplies that we are likely to encounter today are of the switch mode variety. This keeps the cost (and weight) down as there is no heavy and expensive step down transformer. There are transformers inside but they work at high frequency and are thus much lighter and compact.

A standard power supply will provide fixed voltages of 12, 5, 3.3, -12 and 5VSB. In more modern versions there are very often two 12 volt outputs to cope with the demand for power. All of the outputs are DC. You may be surprised to learn that a 400 Watt PC power supply can typically provide high current on various outputs:

+12 Volts - 14 Amps
+12 Volts - 15 Amps
+ 5 Volts - 28 Amps
+3.3 Volts - 30 Amps
-12 Volts - 0.3 Amps (300 mA)
+5 Volts SB - 2 Amps

The 5 VSB output is 5 Volts Standby. This means that 5 volts is available when the unit is switched off (but still connected to the mains).

Some of these power supplies do not regulate properly with output currents of less that 1 Amp and I have come across some units that give no output unless the 5 Volt rail has a demand of 1 Amp. In these cases a 5 Ohm, 7 Watt resistor can be connected across the 5 Volt rail or a 12 Ohm, 15 Watt resistor across the 12 Volt rail.

These power supplies have a range of connectors on the end of the output cables. The Molex type of 4 pin connector (normally used for the larger hard disc drives and DVD ROM drives) carries 2 X 0 Volts cables (black), a 12 Volt cable (usually yellow) and a 5 volt cable (Usually red). The more modern power connector used on Serial disc drives carries 0 Volts, 3.3 Volts, 5 Volts and 12 Volts. The smaller Molex connector is for use on 3 ½" floppy disc drives and carries four cables the same as in the larger Molex connector. There is also a multi-pin connector that connects to the computer motherboard and a smaller 2 x 2 pin or 2 x 4 pin connector that also connects to the motherboard.

The multi-pin motherboard connector comes in either a 20 or 24 pin variant. The extra 4 pins on the larger connector carry an extra 0 Volt, 3.3 Volt, 5 Volt and 12 Volt cable. Most of the power supplies that I have encountered follow the same colour coding of wires:

0 Volt - Black
+3.3 Volt - Orange
+5 Volt - Red
+12 Volt - Yellow
+5 VSB - Mauve
-12 Volt - Blue
Power OK - Grey
PS On - Green

The mains input is via an IEC type connector (kettle type lead). There is sometimes a mains on/off switch on the back of the power supply. When you connect the unit to the mains and switch on the switch at the rear (if fitted) the only output will be the 5 VSB. To get the power supply to wake up and provide all the other outputs you will have to connect the PS On connector in the multo-pin connector to 0 Volts. That is connect the Green cable to one of the black ones. The mains plug should have either a 3 Amp or 5 Amp fuse fitted.

You must use a fuse with the outputs given the very high current that these units can deliver. I would suggest that 5 Amps in each circuit used is suitable for most users. If the power supply detects a short on any of its outputs then it will shut down and one must remove and reconnect the power to restore normal operation.

Obviously all of the outputs are fixed voltage and 12 volts is the maximum. It is not possible to use the +12 Volt and -12 Volt outputs in series in order to get 24 volts unless small currents of less than 300 mA are required. This output should be protected with a 250 mA fuse.

The metal case of the unit is connected internally to the earth pin of the mains plug. The 0 Volt connections are also connected to the unit case and thus it is not possible to use the power supply with a floating 0 Volt line.

If variable voltages are required then the 12 Volt or 5 Volt supplies can be used with linear variable voltage regulators. If one uses two voltage regulators, one on the 12 Volt rail and one on the 5 Volt rail then a useful bench power supply can be constructed at low cost. It is best not to use a switching regulator as there can be interference between the switching frequency of the power supply and that in the regulator.

If the power supply is to be used for powering motors or relays or other inductive loads then a reverse biased diode should be connected across the output to prevent reverse EMF from damaging the circuitry inside the power supply.

The unit should be housed inside a suitable case in order to prevent foreign objects (such as screwdrivers and axle rods) passing through the ventilation holes. A nasty shock can be received from these units. Make sure that your case contains adequate ventilation.

Some of these units are quite cheap. In my experience the cheaper ones are less reliable and suffer with more ripple and poorer regulation on the outputs. Whereas this is undesirable in a computer it matters very little for the low duty cycle required for powering a Meccano model.

These power supplies can be PAT tested but beware that false readings can be obtained due to the nature of the internal circuitry.


Roger      (at 12:57pm, Sat 2nd Jan, 16)

Hello Bob,

Thank you for your comments with regard to this article. In response to your question about fuses: You must always fuse each supply that you use individually in the positive rail, this is for a couple of reasons:

Each supply line will have a different current capacity and also your loading on each rail may be different so you must use the appropriately rated fuse.

If you fuse a negative rail which is common to more than one supply rail and that fuse blows then you will still have a circuit between the positives rails. For example, if you are using the 3.3 volt and the 12 volt rails and the negative fuse blows there may be a current flowing from the 12 volt to the 3.3 volt supply. This is undesirable and may damage your circuits and the power supply.

I hope that this helps, if you require more specific information then please do not hesitate to contact me.


Bob Duck      (at 8:55am, Sat 2nd Jan, 16)

Hi Roger, This is a brilliant article on the Computer Power Supplies, well done, full marks here. I have just taken an old computer to pieces, which a friend had given me. The wiring was exactly as stated in your article for the Power Supply. It just needs tidying-up a bit and then it's ready to drive a Meccano Model or similar. Will fit fuses or cutouts before use. Could they be in the common return (Black Cable)? Or do I need one fuse/cut out for each supply, 3.3v, 5v and 12 volts? Keep up the good work.

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