Introduction: Yet Another ATX to Bench PSU Conversion

About: The nickname is because I couldn't spell "Frobscottle". Loving getting back into electronics as a hobby after a break of many years. Now I work as an EPOS engineer, so I spend my days fixing tills in…

Warning: Never operate an ATX power supply with the case off unless you know exactly what you are doing, they contain live wires at lethal voltages.

There are a few projects around to convert an ATX psu to a bench psu, but none of them were really what I wanted, so I decided to do my own version with a little help from some cheap buck converters (which can be modified to buck-boost mode to produce a negative output) to get some voltages other than the ATX standard ones. The nice thing about using the converters is that they waste very little power.

The things I found wrong with the ones I've looked at are:
* Too big - large external case
* No external case - I wanted to keep my ATX's case intact!
* Under-use of outputs
* Limited outputs
* Lack of flexibility.
* Underuse of the power available from an ATX PSU.

That said, there are some beautiful designs here on Instructables, you should definitely check them out before proceeding with this one.

An ATX psu has a lot of wires for a reason - it can deliver a lot of amps. Admittedly most of those amps come at one voltage, 5v or 12v, but they are very useful voltages you have to admit. Because more power is available at those voltages than I'm ever likely to use in my experiments, it makes sense to turn some of it into different voltages. I used second-hand KIS3R33 converters for the non-ATX voltages.

"rc", below means "rated current for the ATX PSU you are using"
So the voltages from this psu will be:
+2.5v, 0, -2.5v @3A ...... useful if you want to run 5v op-amps on a split supply
+3.3v, 0 @ rc, ...... I was going to add -3.3v, but there isn't really any point
+5v, 0, -5v @ rc ...... If -5v is available, why not use it. You could add a more powerful -5v output using one of the modified converters.
+5v, 0 via a USB socket (removed from an old PC)
+9v, 0 @ 3A ...... I wanted to be able to use it in place of a 9v battery
+12v, 0, -12v @ rc

The 3A outputs will have a peak rating of 4A.

After this the voltages available depend on the complexity you are prepared to deal with:
* Adjustable + and - outputs up to +11, 0, -11 volts @ 3A using the KIS3R33 modules
* These can be made to track, somewhat badly, with the addition of an op-amp and some resistors
* Voltages higher than the ATX maximum, going up to whatever you want, really. These can be adjustable and they can track, but you need to build a boost and a buck-boost circuit using a couple of MC34063 switching ic's. I got these for one reason - they are cheap. A strip of 10 surface mount packages cost only £1. The caveat of this approach is the input current can reach very high peaks.

After much experimentation I discarded the idea of tracking + and - adjustable outputs using 2 of the KIS3R33 converters, with one modified for buck-boost operation, because the tracking isn't accurate enough nor the range great enough to be really useful. However I have included a circuit - hopefully you can improve upon it.

Of course, you can mix and match to get whatever outputs you want.

The -12v output of the the ATX psu is quite limited for current, I discovered that mine was a bit short on the voltage too. If you want -12v with more grunt you will have to add a more powerful buck-boost converter. If you don't want to build a MC34063 circuit, It is possible to daisy chain the modified KIS3R33 modules.

3A is specified because that is the maximum rated current for the buck converter modules. It may be little less for the negative voltages

0v is the point from which all the other voltages are measured - it refers to the black wires from the psu. But of course you knew that...

Other voltages may be obtained by using a non-zero voltage for one side, eg, if you use -5v as 0, +12v will give you 17v, however the "real" 0v line will now be at +5v in relation to your new 0v. Also the current will be limited to the lowest rated supply being used in this arrangement.

The basic version of this supply does not have current limiting beyond the rather high limits of the ATX PSU. Addition of foldback limiting is not within the scope of this instructable.

What you need:

* An old ATX psu, commonly extracted from an old PC.
* Some KIS3R33 buck converters. You can buy these on eBay and other places very cheaply. Don't be caught out by those "conversion kits". The converters themselves contain a MP2307 chip, an inductor and some other components. They are set at 3.3V but have an adjust pin so you can set any voltage you want, and are easy to convert to negative output.
* Some 4mm binding posts in various colours, or other termination of your choice.
* Some sheet metal for the case
* Some sheet plastic for the front panel
* Some chipboard for the base
* A small piece of wood to mount the switch and LED's
* Some blind rivets (aka pop rivets)
* Some wood screws
* A switch and some LED's, preferably one red and one green. (NB since writing this instructable I have changed the switch for a new design, see here:

* Some crimp terminals

I used these materials because they are what I happen to have. Recycle what you have, my friends, and produce something unique!

* Tin snips
* Drill + drill bits
* Step cutter (to get neat large holes)
* Centre punch
* Compass
* Square
* Ruler & pencil
* Saws (I actually found an electric jigsaw to be useful when cutting thicker steel sheet)
* Riveting tool
* Screwdriver
* Spanner to fit nuts on the binding posts (though you can use pliers)
* Soldering iron
* Crimping tool

Afterword: I have since had to replace the ATX PSU in this conversion as the first one died. I think it may have been due to not having a resistor connected to the output.

Step 1: ATX to Go...

So you've found yourself an ATX power supply. Depending on when it was made, it may have various extra connectors, but the standard ones are the motherboard connector and daisy-chained molex connectors. Unless it is very old it will have an extra 4 pin connector with 2 x 12v and 2 x 0v wires. It may also have a white 6 pin connector.

Depending on when it was made, it may or may not have a -5v output. If it does, most of the power is also provided on the +5v output, however newer supplies deliver most of the power to the +12v output. Check the label for details.

A good source of information is - I pulled the technical drawings from their documents.

The particular PSU I used is 250W unit, with the following outputs:
3.3v, 15A
5v, 25A
5v standby, 1A
-5v, 0.3A
12v, 7A ..........On a modern supply, this is where most of the power is available. 84W on this one, not too bad.
-12v, 0.8A

Find the 4 pin 2x12v connector. If the supply is to the 2.0 specification or later (read the label for this), you need to keep the 12v wires to this as a pair, because it is a separate supply to the rest of the 12v outputs and has it's own current protection, so tape this pair of yellow wires together. If in doubt keep them as a pair anyway.

I got the above information from this wikipedia entry:

Examine the motherboard connector, refer to this chart At pin 13 (on a 24 pin connector) there are 2 wires going into the pin, one orange and a thinner one which may be brown or orange (the thinner one is a sense wire) You will need to connect them together again, so tape them together. Identify the "power good" indicator wire on pin 8, it will be either grey or white, and mark it. If there is a -5v supply on pin 18 it will be either white or blue, so mark that too (but you won't have two white wires). So now you chop the connector off. Leave enough length of wire to reach the front panel sockets. Note which is the -12v wire, usually blue, but could be brown.

Next chop off the molex connectors. I did consider leaving one attached in case I want to run a hard drive or something, but then decided if I need to do that I can just connect it to the front panel sockets, so off it came. Again, leave enough wire to connect to your front panel connectors.

Find the green and purple wires from the motherboard connector. The green one you are going to connect to a switch to switch it on. The purple one will power the standby LED. The "on" LED can be powered from the "power good" wire. Bundle these together for later. You will also need some extra wire for the 0v return for the LED's and "on" switch, and the USB socket

Now might be a good time to count the wires, make a note of how many you have of each colour.

Step 2: Make the Case

I made a case 11cm wide by 15cm high and 15cm deep, which is just big enough to hold the PSU with room for air to circulate and to make the front panel connections. With hindsight it should probably be a bit deeper to allow for the wires and extra PCB's.

Sides. These measure 19cm x 20.5cm. I cut pieces from an old microwave oven casing which I'd dismantled for something else. Allow about 8mm flange at the front, top and back edges, so each piece will measure 16.6cm x 15.8cm

I bent the edges over by clamping the pieces between two pieces of steel racking and whacking the edges with a hammer. You can bend the edges by clamping them in a vice, or even bend them with pliers, but you get a bit of a wavy edge with those methods.

I made the top out of some thicker steel cut from an old PC case, already with a nice black finish. It is only bent at the front and back. The bend at the front is part of the original shape.

The back piece is another piece of thin steel. Measure your psu to find out exactly where to make the holes, but allow a bit of "wiggle room". Use the drawing from as a basic guide, but modify it to suit the supply you actually have.

The whole thing just slides onto the chipboard base and is held in place with screws.

Cut a piece of wood in which to screw the front panel mounting screws and also to mount the LED's, switch, and USB socket. Glue this into the top front of the case.

Ventilation holes.
Find the centre of each side piece and mark it with a centre punch. Draw concentric circles with a compass. The size of each circle is judged by eye to get a more "natural" looking spacing. The holes are spaced out with 6 per circle. When you've drawn each circle, mark a spot on it anywhere and use the compass to divide it into 6. In case you don't know how to do this, place the point of the compass on your starting spot and use it to make a mark to either side. Place the point of the compass on each mark you made and make 2 more marks. Place the point of the compass on each of these, and hopefully the last marks will be in the same place. When you've done this on both side pieces, set the compass for your next size up and do the next one. Again, pick any random spot around the circle for your start in order to get a more natural look.

I drilled out the holes using a step cutter because it makes nice round (and large) holes, but you can just use increasing sizes of drill bit, however expect your holes to be slightly triangular in this case. Drill small pilot holes to ensure the larger size doesn't wander.

Front panel.
I had some red perspex from a piece of old shop sign I found, so I cut out a piece of that. You can use any material so long as you can mount the binding posts on it. When marking out the front panel you have to bear in mind that the mounting nuts for the bottom row of terminals must clear the chipboard base. The nuts for the terminals at the sides must clear the flanges on the side panels. There must be space at the top for the switch and LED's, and the piece of wood they are mounted on.

If you are using different dimensions to those in the drawing, you need to decide how many terminals will comfortably fit in the width you have available, divide the width by the number of terminals. That is your spacing between them. Divide this amount by 2 to get the distance from each edge. You may have to tweak this a bit to make everything fit. To fit the height, determine where the top and bottom rows must fit, then divide up the space between them, again decide how many terminals will fit, and divide up the space accordingly. One or more of the terminals will be replaced by a control knob, so you need to ensure there is enough space at this position.

If I was making this again I would have cut out a section of the wooden fillet at the top in order to raise the USB socket.

Step 3: Fit the Terminals

I chose to use cheap binding posts available in packs of 5 colours on eBay from various vendors. If using these, shop around, the prices are quite variable, and I have seen at least 2 styles, however the colours seem to be limited to red, black, green, blue and yellow. I also bought extra red and black binding posts of the same type.

Depending on the power supply you have, it is likely that you will choose a different scheme. A modern one should have the emphasis on 12v outputs. This one is quite old so it has more 5v outputs.

The particular terminals I used have 2 nuts to make the connection, as well as a solder terminal. One of the nuts secures the metal core in the plastic body. I tightened this nut before mounting the post in the panel to strengthen it before tightening the main mounting nut, in order to reduce the chance of breaking the plastic body.

Drill small pilot holes in the panel before drilling the full size holes for the terminals. This ensures more accurate positioning. All drills "wander" before biting into the material being drilled, and bigger drills wander more. A pilot hole ensures they can't do this. The holes should be 7mm for these particular terminals. Ideally, since the posts have flat sides on the threaded part, the holes would be oval to stop the posts being able to turn (maybe 5.5mm across the flats), however I was happy just to drill plain round ones.

Insert the terminals into the holes, starting with a row of black ones at the bottom, then (for an older psu) a row of red ones above these. These will be the 0v and 5v terminals.

Pair the wires from the PSU according to colour, but also try to match them by length. Try to sort them out a bit so they don't twist and cross so much. Again, your number of each type of wire and number of terminals may be different, so some combination other than pairs may be more appropriate to you.

So. strip about 5 - 7mm off the end of each wire and fit them with a small ring crimp terminal. Fit an additional thinner black wire into 2 of the black pairs, and an additional thinner red wire into one of the red pairs. Also add an extra full-thickness wires a 12v pair and a 5v pair. These must be long enough to reach the switch and LED's, USB socket and KIS3R33 regulators. The longer pairs go to the terminals furthest from where the wires come out of the PSU. Fit each ring terminal to a terminal post, but don't fully tighten the nuts yet, because the wires need to be able to move a bit whilst you work on it. It also makes them easy to undo if you need to change things or remove the panel. If you have them it is also a good idea to fit an anti-shake washer between the ring and the top nut

Of course you can solder the wires, but this is harder to dismantle if you need to do so.

Even though you don't have all the voltages ready yet, this gets some of the wires out of the way.

Step 4: Switch, Lights and USB Power

I used a scrap of circuit board from something I dismantled for this, because it already had a switch on it and some holes to mount the LED's in. I simply screwed it to the bit of wood at the top of the case and measured where the holes needed to be. I extended the push on/push off switch using a bit of plastic tube from a soap dispenser and fitted some kind of button to it. You could use a panel mounting switch and panel mounting LED's (it would certainly be easier). The nice thing about fitting an extension to a push switch like this is it enables you to locate the switch well back from the panel.

Connect the cathodes of the LED's and one of the switch terminals together, connect a 470 ohm resistor to the anode of each LED, and connect the other end of one of these to the purple "standby" wire and the other one to the grey (which might be white in your case) "power good" wire. I have a green LED for standby and a red one for power good. Connect the green wire to the switch. You might find you need different value resistors for your two LED's to get them the same brightness.

Connect one of the thinner black wires you added from front panel to the common connection of the switch and LED's. Connect the other one to the 0v terminal on the USB socket. Connect the thinner red wire you added to the 5v terminal on the USB socket.

Connect the USB socket shield to ground, and the two data pins together, but don't connect them to anything else. Some USB power supplies have a resistor between data and V+ or V-, but the actual specification doesn't mention it.

USB power supplies should be limited to 500mA output. You can add a foldback limiting circuit or a fuse to achieve this, but I just left it as is, since it's just for me.

Step 5: Extra Voltages

The KIS3R33 buck converter modules are available as a used item, cheaply in quantity from various vendors on eBay and other places. I bought a pack of 10 to experiment with. They contain an MP2307 buck converter chip, an inductor and some capacitors and resistors. With no connection other than V+ and 0v the output will be around +3.3v. If you connect a 100k potentiometer with the wiper to the adjust pin, one end to the output and the other end to 0v, you can adjust the output between around 1v and near the supply voltage.

Negative output

Using a small screwdriver, pop the bottom off one of the modules' case. In the corner where the on/off pin is located, there are 2 vias (these are small holes plated through with copper that connect the two sides of the circuit board). Using a small drill bit held in your fingers, carefully cut away the copper around these. You are only removing copper, don't drill through the board!

On the other side of the board, the two vias you just cut are connected to a capacitor, and you need to connect a wire to it. You can either push the wire into one of the holes and solder it from this side using a fine tipped iron, or you can pop the board out of the case and solder the wire on the other side. Be careful you don't short it to ground or the on/off connection. You can of course connect the wire inside the case, which leaves room to put the bottom back on.

Cut the wire to length and connect the other end to the output of the converter. The connections are now:
input: unchanged
ground: the original output
output: the original ground.

The voltage is still adjusted in the same way. The difference between 0v and the most negative extent of the output will now be greater than the difference between 0v and the most positive extent of the output of an un-modified converter, however you probably shouldn't run it at the most negative extent. There must not be more than 23v between the -V output and the +V input

You can make a circuit board to put the converters on, or mount them on a piece of matrix board, or because the circuit is quite simple you can wire everything "rats nest" style. It doesn't really matter as long as there is enough room for air to circulate. If taking the "rats nest" option, glue the converter cases directly to the metal case. I drew a design directly onto a piece of scrap copperclad SRBP using an OHP pen. I surface mounted everything and used super strong double sided foam tape to stick the the other side of the board into the case

Variable outputs.

It is simple to make an adjustable 3A regulator using one of the KIS3R33 modules, both for + and - outputs. I experimented with circuits to adjust a negative regulator in track with a positive one to produce mirrored outputs.

Tracking can be achieved using the op-amp circuit shown, with one of the modules modified for negative output, however the result is less than satisfactory. The circuit works because the op-amp wants to keep both its inputs at the same voltage. Since one input is connected to 0v, and the other input is connected in a summing configuration, it should cause both outputs to be equal in magnitude and opposite in polarity.

however I encountered some problems:
* The outputs do not track accurately, there can be 0.5v or more mis-match
* The extents are limited to around +/- 11.5v and +/- 1V
* There is a big question as to how useful this actually is when the extent is only +/- 11.5V

I did try removing the voltage-setting resistors from a pair of the modules, however found that the result was very non-linear and the tracking even worse than before.

Step 6: Other Voltages

A major limitation of ATX PSU's is the upper voltage of 12v. Suppose I want 13.8v, or 18v, or 24v? Or some other voltage?

This is where a boost converter comes in. This is a little circuit which works by switching a current on and off through an inductor, which produces a higher voltage at the output than at the input. Very useful in this situation.

I quickly learned that to get a significant amount of current from the output of a boost converter demands a large peak current at the input, therefore for any significant output current, the amount of voltage boost needs to be limited. Using an MC34063 converter chip with an external pass transistor, to get a 25v output at 1A from a 12v supply causes a peak current of around 4.5A - quite a hefty demand.

Another thing I learned about boost converters is that they don't make good wide-range variable supplies. It's far better to use a linear regulator for that. However a few volts of adjustment is fine.

So the big question is: is it worth it?

Well, it depends what you want it for. Suppose I wanted to make a car batter charger. It would need to be able to deliver 4 amps at 13.8 volts - only a 1.8 volt increase from the input. And yet the current the poor old inductor and transistor and diode would have to pass is 10.35 amps. So in this case it's definitely not worth it.

If on the other hand I'm only interested in using low currents, with a plain MC34063, no external transistor, an output of 24V at 320mA is possible, and at 15V 520mA is possible. So in this case, yes, it's worth doing.

The range of 13 to 24 volts is one that can be adjusted over without any problem, however the current limit is provided by a fixed resistor, and the limit this sets will vary as the output is changed. The resistor will also become very hot if any significant current draw is required. For the range described above the resistor needs to be 0.43 ohms.

On balance, I would say it's best to build a dedicated supply if you need higher voltages.

Step 7: At Last... It Lives!

Ok, moment of truth. You've clipped, crimped, soldered and bolted, drilled, sawn, snipped, riveted and screwed. Time to test your creation.

Plug in and switch on at the back if the ATX psu has a switch. There might be crackle or a loud pop, but this is normal especially on older units due to the primary capacitors charging. Your "standby" LED should be lit. Push the button, the "on" LED should light.

Check the voltages. Check the extra voltages - adjust if necessary. Check the adjustable outputs, make sure they track correctly.

Enjoy your new psu!