Variable Benchtop ATX Power Supply

This is my second benchtop power supply. At the time of making Mini ATX benchtop power supply

Mini ATX benchtop power supply

i had already decided to make a variable benchtop power supply and had ordered parts from online store for it. It took lot of time for the parts to arrive as i had ordered them from overseas supplier. For this project i am using an old ATX power supply which has been salvaged from an DELL Optiplex desktop. This power supply is rated at 350W and has two 12V rails unlike the mini power supply which had only one 12V rail. This variable power supply can produce an variable voltage from 12V to ~1.3V with the help of a DC-DC step down converter.

At the time of construction of this PSU i had only DC-DC step down converter the DC-DC step-up-down converter which i had ordered came very late by that time the PSU was almost ready and i did not wanted to modify the circuit. I had tested the step-up-down converter on 12V rail and with this converter i was able to get voltage between 24V to ~1.3V this converter also has a short circuit protection and current limiting circuit which comes in handy as high Amp can destroy the circuit when probing. I will create a separate extension wing for power supply in future using this new converter.

Step up down converter

Step down converter

Wiring

ATX power supply follow proper colour coding of wire, You can also use the ATX connector to trace the proper wire. There are 2 types of ATX connectors depending on your power supply you may have one or the other.

Wire colour code

• Red +5V
• Orange +3.3v
• Yellow +12V
• Blue -12V
• Black COM/GND

I am using a single unit digital voltmeter ammeter. Ammeter can measure maximum current of 30Amp  and voltmeter can measure maximum voltage of 100V that is enough for PSU. Voltage is shown in red colour and current in blue colour. There are 2 micro pots on backside of the unit to calibrate it. Ammeter is connected in series to GND banana post terminal this way the ammeter can measure the current drawn across all the terminals(12V, 5V, 3.3V and VAR). Voltmeter is connected in parallel between the output of DC-DC converter and GND this gives the reading for only VAR output terminal other terminals already have fixed output voltage so they do not require reading check. I have taken out the small pot on the DC-DC converter and have replaced it with a 10 turn 10k pot. The DC-DC converter which I have does not have a short circuit protection so I have connected a fuse in series to the output +ve terminal of converter. Other terminals do not require fuse as the ATX PSU has short circuit protection inbuilt. A PSU can be switched on by connecting green PS_ON# and black COM togrther so i have placed switch between them.

There is a separate 5V rail in every ATX power supply which is always powered on. Color code of this wire is purple +5VSB. I have connected a green LED to this lead as an indicator of main power. 

There are two 12V rails +12V1DC and +12V1DC. The color code is yellow for 12V. One rail is available on the ATX connector and other rail is available to the molex Hdd power connector. One 12V rail is used by DC-DC converter and the other rail is directly available as 12V output in the panel.

Case Construction & Finishing

Have extensively used my homemade table saw for this project and very happy with cut quality, accuracy and ease of cutting small pieces. Body of the power supply is made from 12mm plywood. For front and back panel i have used panel that i have made by gluing two 1mm laminate sheets. To make the case more sturdy i am using rabbet joints. Front and back plates slide into the grooves cut in the case. 

Bottom piece of the case is removable and the ATX PSU is also mounted on it, this allows the case to be opened later for some repair or extension work. Front edges of the case have been rounded with router and round over bit.

To allow proper ventilation in the case i have drilled 23 holes on each side of the case and 41 holes on the back plate.

Finishing is the last and most important step in any project. If not done properly the whole look of the project can be spoiled. The wooden case of the power supply is painted.

FINISHING STEPS

1. Sand the surface smooth.
2. Apply a coat of wood primer with brush.
3. Fill all the holes and imperfections with putty.
4. Sand again with a fine grit sand paper.
5. Apply 2 coat of primer by spray.
6. Apply 3 coats of acrylic spray paint.
7. Apply a layer of clear coat spray.
8. Finally polish the surface.

This is the first time I have used spray paints and I am really impressed with the quality of finish.

Primer applied

I have designed the front panel of the PSU on sketch up, initially for this project I had tried using other free cad software but found them to be challenging to use so in the end I reverted bk to sketch up. Sketch up also allows 1:1 scaled printing of the plans. After printing the plan I have laminated the printout by putting a transparent tape on top of it. Have used Dremel to cut out the holes for attaching various components, you can read my detailed post on this tool Dremel 4000 Review. I have applied a carbon fibre pattern vinyl film on the front plate and then glued all the graphics on top of it.

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About apoorv chaudhary