PDA

View Full Version : AT adapter for ATX power supply



Casey
February 19th, 2017, 11:38 AM
I've followed the various discussions here about using an adapter on modern ATX power supply for older AT motherboards. Of interest of course is the idea that one must find an ATX which supplies a -5v line for proper operation.

First I present a YouTube channel wherein the speaker has said that he has encountered no problems in that respect except for very specific circumstances, usually involving older sound cards. Don't have a url offhand, but I can find it again if someone wishes.

Second I present this ebay sale (http://www.ebay.com/itm/251025150787?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT). Seller says the adapter includes a dummy 3.3v as well as a -5v line.

I invite comments, especially on the latter. It sounds like a good solution to the problem.

Chuck(G)
February 19th, 2017, 12:05 PM
Dunno. There are a lot of the ATX-to-AT connectors available. I'm not sure about the need for a 3.3V dummy load--usually the +5V load is sufficient to prevent undercurrent tripping.

The missing voltage on an ATX PSU is, of course, -5V. It doesn't matter for most things--and if you really need it you can purchase a DC-DC converter for cheap to supply it. (posted on another thread by me).

Malvineous
February 20th, 2017, 04:33 AM
It's probably only a good solution if you need it. If you're not running a Sound Blaster 2.0 or some other card that needs -5V then having a DC-DC converter or regulator isn't necessary and is just wasting power. Probably best to try it with the passive cable adapter and see if it works - you're not going to break anything and they are only a few bucks from China. There's a YouTube video demonstrating what a -5V card sounds like without -5V (https://www.youtube.com/watch?v=8QXmxdiNWIs) if you're curious what happens.

The easiest solution is to probably just build the machine without any cards that need -5V, which shouldn't be hard unless you specifically want one of those cards in it. You can look at the ISA connector (the 'golden finger') on many cards and see whether the pad for -5V has a trace on it or not. Many cards just omit that pad entirely, making it easy to tell whether the card needs -5V or not.

I've always wondered why Creative ditched -5V so early on and instead ran their own -5V regulator off the -12V line. Did they know -5V was going to be phased out? Or was -5V poorly regulated on most power supplies and it was an attempt to improve audio quality?

Druid6900
February 20th, 2017, 04:41 AM
Why don't you just buy an AT power supply?

I have a bunch of tested ones, even the old "BRS" ones.

I can see adapting something if an item is so scarce that getting an original unit is prohibitively expensive, but, that's not the case with AT PSUs

Malvineous
February 20th, 2017, 04:58 AM
Personally I prefer ATX supplies because they switch off if there's a short circuit. The AT supplies I've used all lack short circuit detection, so if you get a short (e.g. capacitor failing closed) the system appears dead until something suddenly goes bang in a shower of sparks, which I dislike immensely...

You can also buy 3-4 ATX supplies for the price of one AT supply in my part of the world, so it's definitely the cheap option (especially as a number of them are older ATX models with -5V present, giving you the best of both worlds.)

Chuck(G)
February 20th, 2017, 09:03 AM
II've always wondered why Creative ditched -5V so early on and instead ran their own -5V regulator off the -12V line. Did they know -5V was going to be phased out? Or was -5V poorly regulated on most power supplies and it was an attempt to improve audio quality?

The -5V was used on the 5150 primarily because three-rail (4116) 16K DRAM was used on the first model. As it's a substrate bias signal, the current available is pretty low. By the time the second version of the 5150 came out with single-rail (4164) 64K DRAM was employed, the -5 was superfluous. Some PC clone supplies didn't even include the line. It was obvious that the PSU voltage didn't have a future. The -12V line was employed primarily for the async cards and so had a brighter future.

Casey
February 21st, 2017, 05:13 PM
Why don't you just buy an AT power supply?

It's more of a general planning-ahead question than anything else at this point. I'm inclined towards a new ATX mainly because they're new, but if you have reliable re-builds available, that sounds like a valid option as well.

My biggest concern is about "classic*" AT power supplies at this point if I lose another one. The p/s went flakey in my XT and I replaced that a few months ago with a spare. Have an extra 200w unit on the shelf but not sure it it's up to snuff. Alas, my helpful friends tossed my voltmeter when they moved me.

...Looking around, looks like one can pick up an inexpensive one for ~$8-$10 online. Does minuszerodegrees include a "how to test power supply" page? :)

[edit] Ooops. Missed the "BRS" bit before I saved this the first time. :)


*=fits in an XT or AT case with the Big Red Switch built in.

Casey
February 21st, 2017, 05:16 PM
That YouTube channel is how I discovered the -5v adapter in question... :P

But, yes, It gave me pause to wonder if a -5v adapter is necessary absent a particular piece of hardware.

lutiana
February 21st, 2017, 05:22 PM
Why don't you just buy an AT power supply?


Fry's had one for $20 or so, and I bought it. It did not have the -5V rail, so I returned it. But yeah, there are plenty of new AT PSUs available. Most for under $50. This one (https://www.amazon.com/StarTech-com-Replacement-Computer-Power-Supply/dp/B00005133U)for example, and Startech is a quality brand and it does have the -5V rail.

modem7
February 21st, 2017, 11:44 PM
By the time the second version of the 5150 came out with single-rail (4164) 64K DRAM was employed, the -5 was superfluous.
The second version of the 5150 motherboard still had a -5V requirement, for chip U1 in the cassette interface. If there is no -5V to the second version motherboard, the POST displays a 131 error.


Does minuszerodegrees include a "how to test power supply" page?
None planned. There are too many unpublished specs, and you need those to see if the power supply is fully conforming to its specifications.

Specs such as regulation performance, minimum loading (what amount on which lines), maximum slew rate of output at power off/on (e.g. to cater for MC146818 chip).

Chuck(G)
February 22nd, 2017, 04:30 AM
The second version of the 5150 motherboard still had a -5V requirement, for chip U1 in the cassette interface. If there is no -5V to the second version motherboard, the POST displays a 131 error.

I stand corrected. An interface that almost nobody used and IBM was very sketchy about supporting critical to a PSU voltage that was completely superfluous. One wonders why IBM didn't just include a TO-99 79L05 off the -12 line to meet that need. Of course, -5 was completely irrelevant tot he 5160.

Druid6900
February 22nd, 2017, 06:32 AM
It's more of a general planning-ahead question than anything else at this point. I'm inclined towards a new ATX mainly because they're new, but if you have reliable re-builds available, that sounds like a valid option as well.

We can't afford to sell anything unreliable and each PSU is tagged with the full-load measured voltages.

Casey
February 22nd, 2017, 12:56 PM
Thanks! Always nice to have a reliable vintage dealer around.

Look around. Oh, hey, I have that drive! And that one, too! :)