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Thread: Difference between XT and AT power supplies

  1. #1
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    Default Difference between XT and AT power supplies

    Since the question came up, and there are few good search results, for the sake of any eBayer who has obtained a PC/XT or AT form power supply to sell or is looking to buy one, but is not sure which is which (scale can be hard to determine from some pictures), here are some examples.

    First, both XT and AT power supplies use a connector like this:
    AT Power Plug.jpg
    Clearly it is not an ATX connector. An AT power supply may connect to and power an XT motherboard and and XT power supply may connect to and power an AT motherboard, but the two have different form factors and will not fit in a case designed for the other kind.

    Generic XT Power Supply.jpg
    This is an example of a generic clone "XT" power supply. It may be used in an IBM PC (5150), an IBM XT (5160), or a generic "XT" clone that uses the same size case.

    It measures about 8 1/4" long, 5 1/2" deep, and 4 3/4" high, not including the switch or the raised shielding around the switch.

    Note that many clones used their own form factor and proprietary power supplies.

    Generic AT Power Supply.jpg
    This is an example of a generic clone desktop "AT" power supply. It may be used in an IBM AT (5170) or a generic "AT" clone that uses the same size case. Desktop AT clones may contain 286, 386, 486, Pentium 1, and even a some AMD-K6 motherboards.

    It measures about 8 1/4" long, 5 7/8" deep, and 5 7/8" high, excluding the switch, but including the "overhang". This style of power supply leaves an area at the bottom that allows a full sized AT motherboard to extend underneath it.

    Note that power supplies for later tower-style systems do not have an attached switch. Again, note that many clones used their own form factor and proprietary power supplies.

    Third party power supplies were common upgrades and replacements in genuine IBM equipment, and generic form XT and AT power supplies were common in full-sized XT and AT desktop clones.

    Thankfully, neither have rainbow-LED lit fans.

  2. #2
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    Electrically there was no difference between XT and AT PSU, they all followed this standard until the end of the (Baby-)AT Mainboard formfactor was no more used. The difference is usually watt power output and psu-chassis formfactor, amount of connectors for drives. At least there were four important chassis formfactors. You displayed two of them, the first is the IBM 51xx XT desktop case like one, like in first picture. The 2nd was the one in ATs, not much different from the first but a big bigger and having this edge spared out for the mainboard. The 3rd was used in early bigtowers, it reminds a bit on your 2nd, but it has power switch at a long cable to place it at the front panel of the tower chassis. 4th type was more compact one, used in later more compact desktops, large and minitowers, also with power switch at a long cable. Besides that there were special formfactor PSU, mostly compact ones, for pizza box PC and whatever, all using the same power connector. Some brands like Commodore, Olivetti, Compaq, HP, ... used to have their own formfactors and connectors for their own PC chassis designs, often with own style connectors.

  3. #3
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    Thanks @SomeGuy!
    The PSU dimensions are especially hard to find (I didn't). As a novice IBM 5150 owner, I'm not used to seeing a 63W power supply almost anywhere. Even my laptops have bigger PSUs.

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    Quote Originally Posted by 1ST1 View Post
    Electrically there was no difference between XT and AT PSU,....
    There is at least one difference electrically that I can think of. The wiring of the motherboard power connector P8. See the following links to minuszerodegrees.net thanks to our member MODEM7.

    P8 and P9 5160

    P8 and P9 5170

  5. #5
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    Practically speaking, the addition of the +5 line to the 5170 connector makes no difference. If you plug a 5170 PSU into a 5160 motherboard, it will still work as pin 2 doesn't even have a physical connection. If you plug a 5160 PSU into a 5170 motherboard, again, there's no practical difference that I know of, as all +5 lines on P1 and P2 are tied together.

  6. #6
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    I remembered ~25 yeas back to have upgraded someone's already not in original condition beeing 5150 with a 486-VLB mainboard and VGA card. I did not swap the PSU. It was running nice.

    Most difficult was to get the four half height drives in the mounts of the two full height drive bays.

  7. #7

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    Is anyone aware of a source for new production PC/XT type power supplies?

  8. #8
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    Quote Originally Posted by maxtherabbit View Post
    Is anyone aware of a source for new production PC/XT type power supplies?
    You can buy new AT power supplies on Amazon, but those are in the standard AT (square) size. I do not think anyone makes them in the old 5150/5160/5170/5162 form factor. I just bought a new one and transplanted all it's innards to a dead 5150 power supply box and it worked out quite nicely, though did not some soldering and I had to engineer a way to mount it in the old PSU chassis.

    IBM 5160 - 360k, 1.44Mb Floppies, NEC V20, 8087-3, 45MB MFM Hard Drive, Vega 7 Graphics, IBM 5154 Monitor running MS-DOS 5.00
    IBM PCJr Model 48360 640kb RAM, NEC V20,, jrIDE Side Cart, 360kb Floppy drives running MS-DOS 5.00
    Evergreen Am5x86-133 64Mb Ram, 8gb HDD, SB16 in a modified ATX case running IBM PC-DOS 7.10

  9. #9

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    Quote Originally Posted by lutiana View Post
    You can buy new AT power supplies on Amazon, but those are in the standard AT (square) size. I do not think anyone makes them in the old 5150/5160/5170/5162 form factor. I just bought a new one and transplanted all it's innards to a dead 5150 power supply box and it worked out quite nicely, though did not some soldering and I had to engineer a way to mount it in the old PSU chassis.
    Soldering doesn't bother me in the least but I'm super picky about mounting things securely. In order to be happy with such a transplant I'd probably have to fabricate mounting adapters from sheet metal or similar and I'm really not willing to put in that kind of effort to be honest.

  10. #10
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    Heh, when we were doing a proof-of-concept project back in the day that involved multiple PC-XTs, we bought Taiwanese clone boards and accessory cards cheap and used some PSUs scavenged from some old Fujitsu terminals (no keyboards). All done on bare 5/8" plywood--not even a case.

    Basically, you can buy bare PSUs with +5 and +/- 12 pretty easily; the challenge is putting them into some enclosures.

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