ATX Bench Power Supply

 

This is the first project post I make on here. Despite I had way cooler and more difficult projects I want to start with a simple project. I chose to use my Bench Power Supply converted from a ATX power supply as a fist post.

Converting a ATX power supply is a easy project to do. On the web you can find many tutorials on how to do this (for example from Jumper One or Great Scott). It’s a great and simple way to give yourself an extra power supply. It’s cheap, can deliver a lot of power (can be a con) and has all the usual voltages.

Finding a ATX power supply

I started out with picking a suitable ATX power supply. Like all makers I had a fair bit laying around. But I wanted to be able to draw a lot of current at the 12V rail. The older ATX supplies have most of there power at the 5V rail rather than the 12V rail. And most of the supplies I had laying around are not so new. But in the and I found a ATX supply that fitted my needs. It a Q-tec supply that can deliver 17A @ 12V. Because it’s not a top brand PSU I don’t expect it to deliver all the power is says but if it can deliver halve of it at a single rail I’m more than happy. I want to use all the voltages, including the negative voltages. This can come in hand when working with opamps or other stuff that needs a dual rail voltage.

Full specs

  • 17A @ 12V
  • 35A @ 5V
  • 20A @ 3,3V
  • 0,5A @ -5V
  • 0,5A @ -12V
  • 2A @ 5V Standby
BPS1
The candidate, already stripped from fan and switch

 

 Stripping and layout

Now it was time to take the PSU apart. The main PCB was taken out and all connections are desoldered. I has some terminal binding post laying around so this was the time to use them. They accept the normal banana plugs but also a wire. Only problem, they are all just grey. To make clear what’s positive and what’s negative I’m going to paint the enclosure. I made a layout to include all the voltages, multiple common connections, two switches and two LEDs. The first switch will switch the main power and the second switch will turn the BPS on. The to LEDs indicate if main is on (connected to 5V standby) and if the BPS is on (connected to 5V).

Because the common is connected to mains earth it’s hard to connect another non-floating supply in series to get the desired voltage or the use the BPS as a negative voltage while measuring with my scope. A solution would have been to disconnect mains earth but I didn’t want to do that. So I traced out the connection between common and main earth. The connection was made with the screw that hold the main PCB to the enclosure. I cut the trace to make the common of the BPS floating. This works to make the BPS floating but unfortunately it makes the output float high and noisy with respect to mains earth :( But I can always ground it if it’s not workable.

P1180895
Connection between common and mains earth removes

Holes were drilled for the posts and LED and the posts. Unfortunately I forgot to give them the default  19mm (3/4 inch) spacing. But even without that they work just fine. The hole where all the cables ran through was just big enough for the power switch (that used to be on the back). The hole was made square and a second switch was added (to turn the BPS on).

P1180882
Holes drilled and sanded

Then if was time to start sanding the case. I wanted to make the BPS look good next to the Lab Power Supply on my desk. After sanding and cleaning I applied 3 coats of primer and 2 layers of white. To make clear distinction between the common and the voltages I painted the case red behind the voltage posts. After some coats of red I took a fresh permanent marker and a ruler and started tracing the borders and writing down the voltages. I should have finished it off with a clear coat but I didn’t. Why I know? While I assembled the BPS I damaged once side of the enclosure :(

P1180891
First layer of primer. Sanded again after it was dry because of the dripping :(

Wiring

Now the fun part started, the wiring! First the mains switch was wire. The mains fuse was desoldered and replaced with a wire. No worries, I did put a fuse holder on the back of the BPS to replace the original. Because the ATX supply also had a mains power output I wired this as well. It’s turned on with the mains switch on the front.

Next was to wire the on switch and the LEDs. One (orange) LED was connected to the 5V standby to indicate the mains switch was on. A second (green) LED was wired to the 5V to indicate the BPS was on.Also the fan was placed back into the enclosure.

Because a ATX supply can be unstable is there is no load present I attached some power resistors. I used what I had which was 10R 10Watt resistors. I used 2 in series parallel at the 5V line (to draw 1A / 5W) and 2 in series at the 12V line (to draw 600mA / 7,2W). Even though the resistors are used well within the power specs they get warm. So they are mounted on a small PCB. Ths PCB is mounted on top of a heat sink to place it in the stream of air from the fan.

I wanted to be able to load the BPS so I didn’t want to cut the wires down to one per voltage. Instead I used all existing wires to the post, multiple wires to each post.¬† All wires were cut to length and I used crimping ring terminals to connect the wires to the posts. It took some time to get all the wires connected and tucked away with cable ties.

Result and conclution

All in all it took me way long than I planned because of all the painting. Even though I had some setbacks the end result looks good and is easy to use.

If I would do it again I would use the proper 19mm spacing between the posts and I would apply a clear finish to it to make it more durable.

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