Pico Style PC PSU modules for older AT motherboards

[dexters_lab]

Those ultra small switching 'Pico' ATX power supply modules have been around for a long time now but i was wondering if someone makes one for the older AT style power sockets that have the -5v and -12v rails used on the older motherboards?

i have a small 386 board here that i am going to try and mount into a custom case but an original AT PSU would nearly double the volume of the box needed

[mariush]

You can easily make the -5v and -12v rails with a couple of MC34063 or MC33063 switching regulators. Plenty of example circuits and component calculators out there. Or you can just go on ebay and get a ready made board.  If you need small, I would get a STX power supply and add a tiny circuit board inside it to add the two voltages....  or actually just the -5v as the -12v should still be around.

I would advise against using picoATX style power supplies because typically they're only able to output something like 4-5A on 3.3v and 5v rails, or something fairly small, and those old motherboards usually use 5v in a significant amount (to power the CPU and chipset and maybe even ram)

[dexters_lab]

You can easily make the -5v and -12v rails with a couple of MC34063 or MC33063 switching regulators. Plenty of example circuits and component calculators out there. Or you can just go on ebay and get a ready made board.  If you need small, I would get a STX power supply and add a tiny circuit board inside it to add the two voltages....  or actually just the -5v as the -12v should still be around.

a STX power supply would be overkill and no smaller than the regular AT supply i already have

I would advise against using picoATX style power supplies because typically they're only able to output something like 4-5A on 3.3v and 5v rails, or something fairly small, and those old motherboards usually use 5v in a significant amount (to power the CPU and chipset and maybe even ram)

indeed, which is why i was asking for a AT specific one

TBH i might have a play around with powering it from my bench PSU and take some measurements, it's all going to be on the 5v and other than the RS232 ports i can't see what else would use the negative rails

[rdl]

Mini-ITX (or Flex-ITX) are a slightly smaller form factor if you end up going in that direction.

It's too bad an off the shelf solution isn't available. I have an old 486 and I'd love to have a modern power supply for it. That's the only part making noise now the the HDD has been replaced with CF.

[Ampera]

There ARE modern AT supplies being made, albeit in the same AT/ATX PSU form factor they always have (not much in terms of mounting changed with that X tbh). These are done by companies like Athena Power, which is the brand of my 486's supply, and has lasted pretty well for some years. StarTech also makes a modern AT supply, according to Newegg.

https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&IsNodeId=1&N=100007657%20600014008

If you can at all fit a full sized supply into the case, and want an AT supply, these might be options.

[Halcyon]

Or you could also use an ATX to AT adapter cable which allow you to connect a modern ATX power supply to an AT board. There are a stack on ebay, Amazon etc...

I use these myself and they work perfectly.

[rdl]

Apparently the -5v was part of the spec for ISA expansion slots only and not used by the motherboard. So unless my sound card needs the -5v I would be okay with the adapter.

[dexters_lab]

thanks everyone, those ATX cable adaptors look like a good idea and i had forgotten about the ATX Flex PSUs, i used one a while ago when i replaced the PSU in my old Atari TT so i am not sure why i didn't think of it. In this case it's probably a good option given i also need to power the hard and floppy disk drives.

i've not looked at the actual 5v power consumption of this 386 but the entire system (including floppy & hdd) only draws 33 watts right off the mains. It also seems that the -5v is not being used anywhere, the i/o card does use the -12v though, presumably for the RS232 serial ports.

[tssva]

I know I'm a little late to this party but if later you find you do have a need for the -5v, ATX to AT cables with the -5v are available on eBay. PhilsComputerLab did a review of one on his YouTube channel, https://youtu.be/8QXmxdiNWIs

Links to the eBay seller for the adapter he tested is included in the notes for the video.

Sent from my PH-1 using Tapatalk

[dexters_lab]

i went for a hunt in my parts bin and actually found i had a small FSP branded atx power supply that has the -12 and the -5v looks like it might be a mini-itx size, so all i need is the AT/ATX adaptor cable.

always pays to check your parts bins before buying new stuff!