View Full Version : Using atx ps for bs or propeller

02-13-2012, 10:17 PM
Hello all.
I recently picked up a very small atx power supply, but it's specs puzzled me. Apparently, this one doesn't do 12 volts, it's rated for both 3.3 and 5 volts, both of which interestingly enough are needed by the propeller and basic stamp.
I'm assuming it was for one of the very small pc type mother boards, since the power supply contains connectors for floppy, and 3 4-pin connectors (the type used for hds and cd-roms). So, my question is, could I somehow rig some sort of connectors to have this thing power my propeller and basic stamps, since it's power outputs match the requirements of said boards?
Does anyone know where to obtain wiring information on atx power supplies, and/or has anyone done this sort of thing before?
Would it work better to just use the floppy 4-pin adapter to power things instead? I've never attempted anything like this before, and I'm just curious if it's possible to do such a thing. If not, I'll just save the ps for a small computer when I run across one that needs it. :)
thanks for any help.

02-13-2012, 10:39 PM
PC power supplies aren't always great as bench supplies. Many need a minimum load on all the outputs, or it'll fold back and won't put out voltage on any output. You could always put a power resistor on any unused tap in order to put a load on it, but that seems like a lot of trouble.

The +/- 12 VDC outputs were typically used for RS-232 serial, which is now mostly gone from PCs. It was also used by some drives.

In any case, the pinouts for ATX-style power supplies is well documented, but your idea of tapping off the floppy drive connector is a good one for at least the 5V source.

-- Gordon

02-13-2012, 11:36 PM
It's important to remember that an ATX power supply is not the same as an AT power supply. In any case I've used both as general purpose supplies with mixed results and have generally ended up giving up on it and switched to other types of supplies. Speaking from memory an ATX shouldn't depend on having a load attached to "turn on" or "stay on". The is an out put line that will supply 3.3 or 5 volts at a "low" current capacity. This is to supply the motherboard with a keep alive voltage to allow the power supply to be turned on or off via a signal from the motherboard.

There is another line which is technically considered an input and is brought to ground to "turn on" the power supply. The info on how that all works is available on the internet via google. If there's interest I can find the information. The problem i have is when dealing with inductive (or capacitive) loads. With motors being the worst offenders. As far as I can tell to much of a phase shift between the voltage and current will cause the power supply to shutdown. I haven't found any rhyme <SP?> or reason which supplies will have problems. I've had good result with expensive supplies and cheap supplies and problems with both types. In the end I've always gone back to All Electronics and have bought their sub twenty dollar switching power supplies. I've never had any problems with them! :smile:

02-14-2012, 12:16 PM
Ok, well, figured it was worth a try. I'll dig a bit, and see if I can come up with something, if not, then I'll work out something else, just wanted to know if this was possible. Apparently it is, but there's issues. (isn't there always) :) Thanks for the info. If I get it working, I'll let folks know, if not, guess I'll post that too. heh. Thanks again, good to know it can be done. Perhaps I'll dig up specs on floppy connectors, and see if maybe I can power the boards that way, might offer some nice alternatives, since there's always a pc or two being used, tapping it for power should be fun and interesting too.

Loopy Byteloose
02-14-2012, 12:21 PM
Since you have it, it is a handy item. I have used an AT power supply with a load resistor at 20% of the output on the +5 and I had to connect an 'enable' to get power.

Using a floppy disk splitter or extension to create modular plug in is also handy.

But the bottom line is that if I were to want to buy a good power supply, it would be one of my last choices. Good switchers have gotten cheaper and smaller - it is difficult to beat using a fresh new supply taylored to your power requirements.

Google for 'ATX power supply' is going to offer you good plug info. And Google for 'hacking ATX power supply' should offer you an explaination of modifications.

12Volts is very handy for driving mechanical relays that are rated for switching up to 12amps of 120/240VAC. Each relay requires about 60ma at 12VDC.

02-14-2012, 12:22 PM
Just buy something like this:


I use 5V and 9V units like those to power most of my stuff. On the bench, I often use one of these: