• Please review our updated Terms and Rules here

Converting ATX to AT PSU

F

fargo

Experienced Member
Joined
Dec 6, 2019
Messages
82
The PSU for one of my old PCs is dead. It's an AT PSU with 5v, 12v, -5v and -12v outputs.

I'm thinking of adapting a relatively new ATX PSU and convert it to work as an AT PSU. The problem is that ATX PSUs, at least newer ones, don't output -5v. Any suggestions please on how to generate the missing voltage from the ATX PSU?

Thank you
 
In order to make sure the computer works, I tested with another PC's power supply that natively has -5v. It worked as you can see in the picture below.
IMG_3658.jpg
 
Old stuff on a well-worn path. I recommended a 79L05 (small package) as I believe the original PSU was spec-ed for only 100 mA max. Depending on what you have in your PC, you may not even need it. Early rev. 5150s (16-64K) needed it for the DRAM substrate bias; later ones don't use it. Some early SB cards may use it; I don't recall.
 
Admittedly something of a long shot, but you could search surplus outlets or ebay for something that might fit the bill. Someone had nos at p/s's in the last year or two. But what I was thinking was some generic switcher, not specifically meant for pc's.
 
Good grief! If that's not gilding the lily, I don't know what is. I used to buy clone XT boards, mount 'em on a hunk of plywood and use scavenged terminal power supplies for them. Worked fine as headless servers; had both floppy and hard disk.

This is old technology and pretty darned rugged, not your dainty 3.3V1.85V logic. Recall that the Sanyo MBC500 series used a plain old linear supply.
 
Not to mention you can get a chinesium ATX to AT passive dongle on ebay for like 60cents or something. Older ATX supplies with -5V aren't exactly rare either.
 
You can indeed, but it all depends on how much you care about your boards.

Do you use junk PSUs on your servers and primary desktops, too? I learned my lesson about that many years ago.

- Alex
 
Strange--the salvaged terminal PSUs were far better than most XT power supplies of the time. For example, 4-lead power rails (2 supply+2 sense). I think you underestimate the quality of industrial gear.
 
maxtherabbit,

Rereading my post, it came over as overly critical, which I didn't intend. Sorry about that!

Anyway, my point was about the "chinesium" (though, since power supplies, which I have had bad experiences with. I had a 386sx board killed by a cheap ATX supply which blew up and went significantly overspec on 12Vdc. I've also had that happen a couple of times, over the years, with then current machines and cheap power supplies. It's been enough to make me only buy quality supplies, for anything that I care about.

- Alex
 
It's my understanding that (as Chuck(G) pointed out) unless you have an older sound card that requires -5v it's not an issue. AT to ATX adapters are common & inexpensive.
 
ajacocks said:
maxtherabbit,

Rereading my post, it came over as overly critical, which I didn't intend. Sorry about that!

Anyway, my point was about the "chinesium" (though, since power supplies, which I have had bad experiences with. I had a 386sx board killed by a cheap ATX supply which blew up and went significantly overspec on 12Vdc. I've also had that happen a couple of times, over the years, with then current machines and cheap power supplies. It's been enough to make me only buy quality supplies, for anything that I care about.

- Alex
Click to expand...

Fair enough, I'm not advocating cheap garbage supplies. The chinesium I'm advocating is nothing more than some wires, contacts, and plastic. Pretty hard to screw that up
 
I did a little bit of digging last night on this topic, since I have an upcoming project where I would like to power an AT motherboard. I found that the ATX V1.0 spec apparently includes the -5V line, but the V2.0 removes it. I also noticed that the cheap adapters seem to include the wire in the correct location for the -5V. So if you have an old ATX V1.0 power supply, with the -5V line, it would be ideal. I'll dig through my ATX supply pile later today to see if any of them actually include the -5v line.

I'm intending to power up an old board, that may have some shorted tantalum capacitors. To prevent damage, I'm going to cut the ATX socket off one of these adapters, and add some banana plugs. I'll bring the board up on the bench supply with a reasonable current limit before connecting to an ATX supply.
connector_atx_pinout.GIF
 
schenkzoola said:
I did a little bit of digging last night on this topic, since I have an upcoming project where I would like to power an AT motherboard. I found that the ATX V1.0 spec apparently includes the -5V line, but the V2.0 removes it. I also noticed that the cheap adapters seem to include the wire in the correct location for the -5V. So if you have an old ATX V1.0 power supply, with the -5V line, it would be ideal. I'll dig through my ATX supply pile later today to see if any of them actually include the -5v line.
Click to expand...

Yes, ATX V1.0 used to require -5V. I have working ATX supplies with -5V. After it, it wasn't removed from the spec, but they said it was optional to provide it. You might find later supplies with it too. ISA slots existed in PC for some time even with the phase out in the standard and motherboards, so it sort of can still be used, even though, not very much uses it. It's not a big deal though, and unless you really need it, you can just use a straight ATX->AT adapter on many ATX power supplies -- just check what your needs are and if the supply satisfies them.
 
You must log in or register to reply here.
Back
Top