So one of my projects I've been thinking about doing is making a workbench power supply. By make I mean I'm not looking at making the power supply itself, just using a ready made power supply, then using buck converters to give the output I want. I've made one using an old ATX psu which is working well but only gives up to 12v. As I am looking at starting making my own PCBs I've bought a 30000rpm motor to use as a drill press for PCB making, which can run at either 12v or 24v. So I started looking at getting a 24v power supply for my new workbench supply.
In my travels I came across a method of using 2 server PSU's linked in series to provide a 24v power supply, and it just so happens I had a few of those lying around, emphasis on the "had".

So after reading up for a day or two to make sure I wasn't going to kill myself straight away or burn my freaking face off I decided to give it a go.
I've removed the PSU PCBs from their metal casings and mounted them on MDF using Nylon standoffs. I wired a switch to both PCB's to turn them on and off together. Tested and was working. Then I added some solder to each PCB's 12v and GND outputs, there were 6 pins for each so I soldered them all together, PSU's still isolated from each other apart from the switch at this point, tested and working still.
So this morning I thought I'd wire the 2 in series and see what happens. I got some 16awg wire and ran 2 lengths between PSU1 12v out to PSU2 GND, PSU1 gives 12v, PSU2 24v, both use PSU1 GND. Had some dutch courage (Amsterdam courage if you will) and decided to turn it on. It turned on but after about 3 seconds there was a lovely little white puff of smoke from PSU2 by the outputs, and the smell we have all come to love, so I unplugged it. Looked at the PCB and couldn't see any real signs of anything so thought I'd plug it back in. Well after about another 3 seconds same thing, only this time from PSU1 and was a bigger puff of smoke. So I've just ditched the whole lot.

I'm wondering why my attempt didn't work where there is quite a few examples of people doing it, the remote control community seem to do it a lot.
It looks like it was a couple of surface mounted resistors that have blew, again up near the outputs. Could it be a case of the PSUs not being designed for this sort of thing in the first place so it would never have worked, I'll add that I haven't seen one using the specific model of PSU I have. Or could it be because I removed the PCB's from their case?? Removing a GND/Earth connection along the way?? Again I haven't seen any examples of people removing them from their case, they just don't earth PSU2.

So I found an old server on ebay for a tenner with 2 PSU's that are hopefully are the exact ones some guy uses and sells in this configuration. Should have them by the end of the week and going to give it another go with those.

Also, can you use this method with the LED style Switched power supplies??

Cheers
Iain

 

All the computer power supplies I have come across have the 0V (generally the black wires) connected to mains Earth.
So when you connect the 0V of one supply to the +12V of the other supply then you have a short circuit across the second supply output via the earth connection. This means that you cannot connect them in series.

Do not even think about disconnecting the mains earth connections. If you try that it may well be you that goes up in a puff of smoke and, like electronic things, humans don't work when their magic smoke is released

 

Cheers Albert, in the RC community, they disconnect the mains earth connection from the second PSU to overcome the short across it to run them in series but leave them in the metal housing.

All the computer power supplies I have come across have the 0V (generally the black wires) connected to mains Earth.

AC side via a cap?? I ask because the only earth connections made were from IEC Socket Earth to case. IEC Socket Earth to Negative via what looks to me like a cap, and then the same to IEC Earth to Positive.
If DC side could that be done via heatsink or screw-holes to case as that was the only other "connection" that could be made.

Cheers Iain

 

Those power supplies were designed to be earthed. They are not safe to use with the earth disconnected or with the case removed.

Last words:

IF YOU DISCONNECT THE EARTH YOU RISK SERIOUS INJURY OR DEATH FOR YOURSELF OR ANYONE ELSE THAT MAY COME ACROSS THEM.

 

You should look around for a charger for a laptop.

Most run about 19.5 volts and it would be an excellent dedicated supply for your motor. Add a cheap 10 $ PWM eBay controller and you have variable speed.

 

Message received. They went in the bin the second the smoke came. I am looking for more info to educate myself as with everything on the internet there is contradicting information everywhere. I've asked questions in multiple places on the subject leading up to doing this and have had conflicting info.