Power Supply

I created this bench top power supply unit using a spare 400Watt ATX PC power supply. It provides 12V, 5V, 3.3V and an adjustable voltage supply from 1.2 to 11V.

The power supply unit uses switching regulators and so far has been very robust. Shorts cause the unit to shutdown and it can be re powered immediately. The unit provides smooth power that is quiet both electrically and audibly. The cooling fan's rpm responds to the amount of current draw from the unit and is fairly quiet even when larger currents are being drawn. The adjustable voltage regulator is a LM317T that has internal current limiting protection.

 

If you have any questions or comments about this project or electronics in general, please email me at cybrown@cp-tel.net

More projects can be found at http://www.pasqualy.com

 

Operation

The power supply has very simple operation, there are only two power switches and one dial. It uses the standard PC power supply 110V AC power cord.
A red Power Ready light indicates that the unit is ready to operate and that 5Vsb is available.
Standard binding post terminals are used for connections. There are 5 colored terminals for the available voltages and 3 black terminals for common ground. A standard PC hardware molex power connector providing 12V and 5V is available on the side of the unit.

Power On/Off: This switch turns on the power supply and energizes the 12V, 5V, 3.3V and Vajust terminals. There is a green Power On light that indicates the power supply is operating.

5Vsb On/Off: The 5Vsb switch energizes the 5 Volt Stand By terminal, this provides the commonly used 5V power without requiring the whole power supply be powered up. This can be used for common low power tasks and allows quiet operation since the power supply's fan is not operating. 5Vsb only provides 2A of current compared to 15A from the regular 5V source.

Vajust dial: The Vajust dial allows the user to set the voltage output on the Vajust terminal from 1.2V to 11V

Available Current:
  • 3.3V (Orange) - 16.7A
  • 5V (Red) - 16A
  • 5Vsb (Pink) - 2A
  • 12V (Yellow) - 10A
  • Vajust (White) - 1.5A

 

Construction
The project started with some simple plans, a project box, and some binding posts.

Here is a scan of my design notes (click image for full size). Use with caution, I haven't double checked it for errors since I finished the project and some changes were made on the fly.

Some basic info for anyone wanting to repeat this project:
The power supply is switched on by grounding the green PS_ON wire on the main connector.

PWR_OK provides 5V for an LED etc to let you know the power supply is ready to be turned on. PWR_OK goes high as soon as the power supply is connected to the AC outlet, the power supply does NOT have to be turned on for PWR_OK to be on.

My power supply unit had an extra brown wire on the main connector bundele that had to be connected to +3.3V for the unit to be able to power up.

+5VSB (Standby) provides a modest amount of current and can be used as I did for a quiet 5V power source. +5VSB is on even when the power supply is off.

I used a LM217T Adjustable Voltage Regulator for my Vajust terminal. A 560 Ohm resistor for R1 gave me the optimal voltage output range. R2 is a 5k adjustable resistor attacked to the Vajust dial.

Below are some useful pin outs for ATX power supplies.
Check the sticker on the side of your power supply unit for more information on current limits etc. Most of the current limits given are totals for each voltage output, so you may not be able to get the listed current from a single output connector unless you tie all of the similar voltage outputs together. Internal power supply design probably varies widely.
First I dremeled out the air vent, power socket, and screw holes.
Then I cut a hole in the project box's lid to make room for the fan intake.
Next I added the power switches and LED indicators.
Here is the bottom view of the binding post installation The binding posts required a little super glue to keep them from slipping during normal use.
Here are the colored posts and the LEDs are working.
I printed up some labels for the posts etc. I wish I hadn't made them quite so gaudy. The label to the right was later removed when I added the adjustable voltage output.

Here is the finished device with the adjustable voltage output and the adjustment dial. You can see that I left two molex power connectors to the right of the device just in case I needed them.

If I ever make another unit, I will use a larger project box and add features like voltage and current meters etc.