ATX Bench Power Supply

An ATX power supply repurposed as a bench supply for 5V and 12V.

The project involves little more than just installing connectors into the case, but there are some precautions that must be observed if the conversion is going to work properly.

ATX Power Supply

The ATX power suply is pretty much standardized for both electrical and physical features.   Therefore the example used here will apply to most power supplies that can be recovered from old ATX-format PCs.  Although it is here described as a bench power supply it will only have fixed 5V and 12V supplies, and there is no voltage or current control: It is simply a 5V and 12V supply at very high current.    In addition to general benchtop use, the conversion will also apply to dedicated usage, such as with a model train layout.

The case is quite crowded, so this example has been restricted to tapping into the 5V and 12V supplies. Adding 3.3V, -12V or (where available) -5V terminals is possible, but the interior would become difficult to access.   Displays or level controls would have to be added separately, and considering that the device would still be restricted to a maximum 12V it would likely be preferable to start from scratch and build a proper bench supply with a larger voltage range: the required components are readily available.  

Cautions

The conversion basically consists of cutting off the existing harness connector, collecting the 5V, 12V and ground wires together into large bundles and connecting them to screw terminals mounted on the front panel. But there are several considerations that must be taken into account, some of which may not be obvious.

Switching. If the PS is fitted with a power switch then this can be used as the main switch.  In that case the green power control wire from the harness must be connected to ground.  If the PS has no power switch then one can be added.  This can switch the mains, so that the green wire is then earthed as above.  A simpler option is to have the mains input permanently live and the power switch configured to switch the green wire to ground. This arrangement makes the standby 5V available, which can be used for a mains power indicator.

Default Load.  Like any high-current switch-mode power supply, the unit might not switch on, or might provide poor regulation, if there is no load attached.   A bench supply is likely to spend considerable time powered up but with no load attached. It is critical that a dummy load is provided.   Options include a low-value high-wattage resistor, or a small incandescent lamp.  It can be applied across the 12V or 5V and can be anywhere from 1W to about 3W.  The 'best' option depends on the particular unit, and there is no easy way to find out what is best - experimenting is the only method.  But for most units the load is not critical.   If the unit fails to start, or clicks or turns itself off at any time, then the load is insufficient. (Note: Many ATX power supplies can be recovered from the junk pile by testing them with a load applied (and the green wire earthed).  Often a unit written off as faulty will prove to be perfectly functional when properly loaded.)

3.3V Sense.  The 3.3V sense wire is a brown or orange wire connected to an orange (3.3V) wire at the plug end of the harness.  It will likely have a connection point on the PCB located away from the other 3.3V wires, and will likely be a thinner wire than the others on the harness.   This wire MUST be connected to 3.3V, regardless of whether or not 3.3V is being used in the supply.   Without this connection the voltage control circuit for 3.3V will see zero volts on the sense, and might try to ramp up the 3.3 volt supply to bring it up to spec.  If that happens the power supply will explode.  Do not operate the unit without the 3.3V sense wire connected to a 3.3V lead. Newer power supplies are usually protected from this sort of over-regulation, but it should not be relied on.

Also:   Unused leads should have their ends sealed with heatshrink. Just cutting them off short is likely to cause a problem in the future.
            Be sure to leave enough leads out of the bundles connected to the terminals for the purposes mentioned above.
            The hole where the cable bundle exited the enclosure should be filled with a suitable plug.  If a connector is added as mentioned above it could be mounted on a plate that screws over the hole.

Construction

The important point about the construction is finding a suitable location for the terminals and routing the cables from the PCB to the terminals.   The details will vary between units. 

For this example the considerations were:

• The fan took up almost all of the lid, so nothing could be placed above the level of the heatsinks
• There was space for the terminals behind the front panel, but they had to be placed towards the top to make wiring access practical
• Cable routing for the terminals is a mixture of straight to the front and round the back. Wires should be cut to length: there is very little room for excess wire.
• Separate negative terminals were provided with separate cabling: a single negative terminal, or two terminals with one set of cabling would be acceptable alternatives if space is a problem.
• There were two holes for indicator LEDs already drilled at the bottom of the front panel. The main PCB was already cut away immediately behind these holes, providing space for a small protoboard for mounting the LEDs.
• The load resistor was fastened across the ventilating mesh at the rear.  This put it in the cooling air-stream without any significant impact on the flow.
• Mounting points were not used for the other connections (3.3V sense, power control): they were just soldered together and wrapped in heatshrink.
• Unused 3.3V wiring was bundled together and tied against the back panel, allowing for a possible future expansion to include additional terminals.

The terminals used here are 4mm dual banana/screw terminals.  The spade connectors are crimped and soldered.  About 5 wires is the maximum they can accommodate, but that is ample.  Note that sufficient exposed end must be allowed - when twisted together the bundle of bare wires becomes tapered, and it is easy to leave one or two short of the crimped section.   If crimping is used the wires must be twisted - the crimping is not even across the width of the terminal.  An alternative is to leave them untwisted and solder without crimping.

The image shows the mounting of the LEDs on their piece of protoboard at the base of the front panel.  Note that this board requires one of the spare earth leads.

The image shows the load resistor clamped across the ventilation mesh at the rear of the case.  Note that this requires one of the spare 12v wires plus one of the spare earth leads.

Indicators

The indicator LEDs are mounted with their resistors on a scrap of perfboard.   Stakes are then added for terminating the wires from the harness. The module is then positioned with the LEDS protruding through the holes already drilled in the front panel, and held in place with a small dab of hot glue.  This places it well out of the way of the power terminals and their associated wiring.

The red LED is powered by the purple wire (+5V standby) and the green LED is powered by the gray wire (power good). 

A possible enhancement is to bring one +12V wire, the -12V (blue) wire and one ground wire to a low-current 3-pin connector for use as a bipolar supply. As a low current output, the connector would be small enough to find space on the front panel.