Build your own Amiga Power Supply - from a cheap ATX PC PSU!

I wrote this email to a twitter follower some time ago after my tweet about how I made my Amiga PSU...

Hello!

As promised here’s my “guide to building your own Amiga PSU”  J

Please be super careful with the electrics – those capacitors in there can hold 400VDC.  My advice is not to power any of this on immediately before pulling them to pieces, as they can hold a charge. 

OK - for me, a PC PSU was going to be the best because they’re cheap, and plentiful, and they output all the right voltages.  Also, the higher output (I think newer Amiga PSUs are only rated up to 22W) would mean I could run a hard drive and accelerator card in the 1200.

I’d seen some kits online, or “pre converted” PSUs around the £30 mark, but they all looked terrible with the ATX harness hanging out, and adapters made to hook up them up.    I wanted something that looked quite neat.

So first of all I hit eBay and looked for a PSU.   Even a 120w one will be enough, FlexATX seems to be one of the smallest form factors.   It’s worth noting here that depending on your willingness to tinker, smaller physical size might not be best.  It was a pretty fiddly job doing all the cutting and soldering and folding all the cut off wires back into the case.

Anyway  - £8 yielded a used 250W FlexATX PSU.   Very cheap, “Bestech” branded, and they don’t have an amazing reputation.  But give it’s going to be running at most a tenth of its rated power, I thought it’s good enough J

So, to the assembly. 

First thing was to open up the Amiga PSU and carefully desolder the cable that goes from the PSU to the Amiga and terminates in the square DIN plug, and put it to one side.

Then I opened up the ATX PSU, and decided which wires I wanted.   The Amiga needs +5, +12, -12 and GND.   On the ATX side of things:

12v = Yellow

5v = Red

-12v = Blue

GND = Black

And you’ll also need POWER = green (sends the signal to the ATX PSU to turn on)

I decided to take the +5 and +12 from the main 24 pin ATX connector.   I might be wrong, but I interpreted the information sticker on the side of PSU to indicate that the 5v rail on the 24 pin connector has more amps available to it than the others, so I took those feeds from there.   On my ATX PSU, there was only one -12v lead.

So,  I took 1 Yellow, 1 Red, the Blue, the Green, and TWO black wires (one for the switch, one for the Amiga power) and cut them right at the ATX connector so the wires would be nice and long.    The rest of the wires (and there were a good 20-odd of them), I cut just short enough so I could push them all back into the PSU case. In my case, that was about 1.5 to 2 inches.   Don’t cut them too short because you’ll need to terminate them so they don’t come into contact with anything they shouldn’t.  I did that by applying heatshrink to the ends of the cut off wires, with an excess of around 5-10mm.   I thought about desoldering the unused wires, but they were soldered into the PCB in bundles and it proved too difficult.

I then soldered the long wires (with the exception of the green and one of the blacks) to the Amiga power lead, using more heatshrink to cover the joins.  The colours on the Amiga lead are different, so be careful.

Red = 5V

Black = GND

Brown = 12V

White = -12v

Here’s a pic with what pin is what:

http://www.ianstedman.co.uk/Amiga/amiga_hacks/Amiga_Power_supplies/power-plug.JPG

Might be worth checking the pins match the colours with a multimeter.  The shield pin can be left disconnected.  Again, I wrapped that in heatshrink to insulate it.

I also then soldered the green wire to the remaining black wire – this will ensure the PSU powers up as soon as it is plugged in.  

Then I tucked all the wires back into the PSU screwed it together and powered it up! Simple as that.

However, after proving everything worked, I opened the case up, split the green and black again, and instead drilled a hole in the case and connected them via a small switch so I could turn the power off at the brick, just like on the original.  Again, using heatshrink to insulate all the soldering.

Hope that helps!  Here’s a link that contains all this information and I found it very useful:

http://www.ianstedman.co.uk/Amiga/amiga_hacks/Amiga_Power_supplies/amiga_power_supplies.html

I also checked to make sure that the metal body of the case had full continuity to ground using a multimeter, and that none of the other pins were shorted to the case, before I switched it on, just to be a little bit more confident. 

When you do switch it on for the first time, probably worth double checking the voltages at the pins with the multimeter.   They will be a bit off, but that’s because they’ll only settle when they’re under load (for instance I was getting -19V on the -12V rail without the Amiga connected) – and make sure you don’t accidentally short any pins to ground while testing.

Once I was sure I wouldn’t be opening it up again, I used some hot glue to secure the power lead grommet into the hole where the ATX loom originally exited the case.   I also stuck some small rubber feet to the bottom J

Good luck!