Building a bench top power supply using a ATX power supply.

I followed this guide because the wires were identical to my mini-ATX box, which I bought at 16,000 KRW.
ATX to Lab Bench Power Supply Conversion

Micro or mini ATX power supply has too small space. It fits on slim computer case. I recommend get a ATX power supply if you are planning to build a bench top power supply. This experimental project had high and low for last two days. Everything turned out worked fine. I was feeling great fifteen some hours of hard work was paid off till a disaster hit it hard. I explained it later part of writing.

Requirement:

A proper work bench or improvised secure table that can take medium duty(?) work.

A ruler
Marker or pencil
(Vernier caliper is optional)
Philips and flat head screw driver: to disassemble the case.
Power drill with various bits: to make enough holes for binding posts or banana jacks.
Medium duty clamp: to secure the case when you drill holes.
Round file or grinder with small sanding blade: you will face to increase the diameter of holes for few times.
Center punch is optional.
Digital multimemter
Test wires
Soldering iron, solder.
AWG 20 wires: you can salvage wires from an ATX power supply you will convert.
Wire cutter, plier, precision wire cutter.
Wire stripper
Crimping pliers
Butt connector with heat shrinking tube and heat gun Or electrical tape with paitence
Helping hand or secure clamp that hold small parts, i.e wire or resistor.
Perforated circuit board is optional.
Fuse holder and fuse – it will save you one day.
Schematic diagram or wire connection information.
A power resistor: 10Ohm 10 Watt x1
Heat sink for the power resistor is optional (I haven’t enough chance to measure the resistor temperature. Next time.) I installed an old heat sink for the resistor by the way.
Simple on/off switch x1
LED x2: one for power on, one for stand by. I used two
Current limiting resistors. I used 5mm red and green LEDs.
A pickup or magnet
Vacuum cleaner or indoor broom and trash can
Safety goggle: should wear it for metal work or soldering

Building procedure:

Removed the case. Most ATX power supply is composed of two metal pieces, one covers the left and right, and the top. The other one support the circuit board, and front and back side.

Make space for metal work.
Remove screws that hold the circuit board. There are four screws in each corner of case. Once screws are gone, lift the circuit board to make enough space between the frond side where you will drill. Put a strong plate that stops drill bit is optional. I only have a 700W power drill. I missed twice to punch a hole or damage capacity. This part is physically demanding, dangerous, and noisy.
– Mark hole positions
– Drill holes one by one, use a small bit than change to bigger on
– Plug parts and correct the sizes of holes

Secure the circuit board again.

Install binding posts or banana jacks, power switch.

Cut power and ground wires. ATX power supply has many wires. Use precision wire cutter to remove extra wires. For example the zoom in photo of the P/S hows four or three orange wires for 3.3V output. You need only one or leave another one for future use.

Connect wires and parts along the schematic or wire diagram you have.
Please install at least one fuse for protection. I mean it.

Do test run to check the bench top power supply works.
Use the digital multiplier first then connect machines that run under 12V, 5V, 3.3V, etc.

Assuming everything works fine, secure all connections, organize wires, then put other case back.

Five binding posts installed

All the wiring marks on the board

ATX power supply specification

Initial test – 3.3V

Initial test – 5V

Initial test – 12V

Initial test – -12V

The power supply was shorted when I hooked up a 12V USB charger in revers polarity. I took the shot for future reference.


I spent yesterday afternoon til today morning at six o’clock to build a bench top power supply. The power supply died shortly in the afternoon when I was testing for a 12V automotive USB charger. Everything was running good until the power supply stopped running and triggered power down entire apartment. Woops!

I made few mistakes during the testing. First of all, I did not pay attention to the USB charger. I should open it up and see how the polarity is set. Only one side clip was connected to ground wire. The round tip with a fuse was positive. When I hooked up the USB charger, the LED light wasn’t turned on. So I connected the ground wire to the round tip, positive wire to the side clip. I heard pop noise. I saw smoke coming out of the charger and put it away. It shorted the USB charger, then I suspect it damaged the power supply too.

I did not install a fuse on the power supply. I kept all the circuit as is except connecting wires to convert for a bench top power supply. The circuit do not have a fuse on it. Anyway I just anxious to see actual working of the exciting machine. I picked up a 12V computer fan, connected it to 12V. The fan was running for few minutes till it stopped suddenly.

The power supply had green light on, which is standby, the red light would stay dark. Disconnecting the fan cause I knew the power supply was working fine, I turned on and off the power supply without any load. Suddenly the power of entire apartment went down. I knew I was the one who triggered it, reset the circuit breaker no problem.

The ATX power supply is aroud 16,000 KRW. I’ll better buy another unit to build a bench top power supply again. Next time I’ll put short protection circuit for sure with more confidence. For last over sixteen hours I learned a lot.

How to enlarge a hole in a metal case
Smaller drill bit does better job than bigger one. I think manual round file or grinding blade are the right tool.

How to install binding post
It was first time. The correct installation order is:
Binding post rod, round plastic spacer, rear plastic cap, metal washer, a nut, wire connecting ring?, a nut. It’s hard to solder a wire on the rod.

How to connect wire on circuit board

AWG 20 wire is right size. Strip the rubber tube, apply a layer of soldering iron, plug in a hole till the rubber sits on surface. Multi strand wire is easy to broken off. One wire fell off while I was moving around.

How to solder parts on a perforated circuit board

Check the polarity of part especially diode, transistor. When I tested the power supply before the final soldering, wire housekeeping, the red LED was dead. It turned out I soldered the wires in wrong way, anode was on ground wire.

Applying soldering iron directly on the iron tip won’t work. While heating up part’s leg or surface apply soldering iron.

Perforated circuit board is fragile. To reuse left over area, use a razor or cutting blade of a grinder. Wire cutter easily create damage, unwanted effect.

Organize wires

I think it’s coming from experiences. Choose clean, shorted path, and keep wires tidy. One tutorial on Youtube, uses shrink tube too much. I was going to follow his method but it looks stupid. For example, the ATX power supply I used had a lot of wires. The number of ground wires, color black, were fifteen to twenty. How could I make these to one big wire? The fact is all the ground wires share one ground. One or two ground wires are sufficient for bench top power supply. I checked power rating of AWG 20 wire, which withstand up to hundreds amps. Considering most electronic components use less than 1A, making a big wire same thickness as batter cable is overkill.

When connecting two wires use butt splice with clamp tool. I wish I had have them. I think I wasted good hours just to cut and reconnect all wires by cutter, soldering iron, electrical tape. Doing one or two for emergency situation is Okay.

Prepare worst case scenario

Why did not add fuses on the power supply? I bought six fuses from 1A to 2A yesterday on a store spending two hours back and forth by walking. The store didn’t carry fuse holders though. If I really wanted to install fuses on the power supply, I could just solder them on to five wires. Over confidence or ignorance again proved me I was naive.

I found information about how to repair power supply here.

Have an isolation transformer

I’ll definitely have an isolation transformer.

PS: This is probably the last project I’ve done in my apartment room. I’ll have secure, quiet, and spacious workshop soon. Improvised workbench is done by today period. I think modern apartment is a living prison for builders. Most of them slowly give up creative activities as they get growing older.

About janpenguin

Email: janpenguin [at] riseup [dot] net Every content on the blog is made by Free and Open Source Software in GNU/Linux.
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