Can that Mitac power supply be specific to a Mitac computer model with AT power connctors but a electronic switch start like a ATX power supply ?

I wonder what that little black thing on the back of the psu, next to the connector for the power cord, is.
that little black thing, rectangular outline, has an I and an O on it, also seems like it could lever a bit.

oh wait. that's a switch.

That's why IBM had them bigger and redder 🤣

My understanding of the OP's question is that he's asking if it's a power supply that has AT style power connectors to the motherboard, but uses an ATX style momentary power switch to power on? Which is apparently what it is; I suppose it's some OEM thing that was used briefly as the transition from AT to ATX was being made?

It isn't. It uses power switch on the back.

PSU that were halfass ATX usually had a third motherboard connector with 3.3V and power good on the same style as P10, and often *cough*Acer*cough* messed up the normal pinout. Then there were some "green" sleepable AT with a powergood 2 pin lead separate, which you could connect to header on supported board so it it turns itself off as well as use momentary on, or just to a momentary on button to control that way.

BitWrangler wrote on 2021-12-16, 19:28:

It isn't. It uses power switch on the back.

PSU that were halfass ATX usually had a third motherboard connector with 3.3V and power good on the same style as P10, and often *cough*Acer*cough* messed up the normal pinout. Then there were some "green" sleepable AT with a powergood 2 pin lead separate, which you could connect to header on supported board so it it turns itself off as well as use momentary on, or just to a momentary on button to control that way.

So there's no way to turn the power on from the front of the computer, just the rocker switch in the back on the power supply? That doesn't seem very end user friendly.
With a whopping 75 watts, I guess this is something from a very early computer?

Well it would even less end user friendly as a soft power PSU as there is absolutely no wire showing that is a power good signal that you could control it with.

BitWrangler wrote on 2021-12-16, 19:45:

Well it would even less end user friendly as a soft power PSU as there is absolutely no wire showing that is a power good signal that you could control it with.

Of course, but my point was what sort of computer was sold that only had a power on switch on the power supply?

well... the IBM PC/XT 5150...

Repo Man11 wrote on 2021-12-16, 20:08:

BitWrangler wrote on 2021-12-16, 19:45:

Well it would even less end user friendly as a soft power PSU as there is absolutely no wire showing that is a power good signal that you could control it with.

Of course, but my point was what sort of computer was sold that only had a power on switch on the power supply?

Apple II. 😜

That power supply looks like it's from a Compaq. If it's anything like the model I'm thinking of, the computer is never in a fully-off state; more like a sleep mode. The PSU fan will constantly run when the rear switch is on, but it won't supply "full on" power to the computer until the sleep button (on the front of the Compaq case) is pushed. Definitely a wonky setup.

weedeewee wrote on 2021-12-16, 20:19:

well... the IBM PC/XT 5150...

Ah, okay, very early indeed. The only computer I'd ever touched back then was a TRS-80 Model One.

duboisea wrote on 2021-12-16, 01:54:

I am reading up on AT vs ATX. I see AT PSU has the P8 and P9 connector, and a push button. ATX just has the connectors.

What is this PSU? https://www.ebay.com/itm/164672892153 it has P8 and P9, but no push button? This is the motherboard download/file.php?id=125676

I was thinking of replacing the PSU with something modern to be safe. I am not sure if I should get a ATX PSU with a convertor, also was looking at https://www.amazon.com/gp/product/B0042P2IIG/

This thread should answer the second part of your question: AT PSU worth it for AT computer?

Also, there is now a hardware work around for the -5 volt issue if that is something that you need: https://www.youtube.com/watch?v=y1-0giyLQIY

For myself, I've so far just been rolling the dice and using the AT power supply that was in the AT case I have when I found it. That's probably not my best choice considering the manufacturing date code on it is "920303" which presumably means it was manufactured in March of 1992. In my case, my motherboard has both AT and ATX power connectors, so I could easily swap in an ATX PSU and use the reset switch as a power on switch instead.

Maybe I'll at least pop the cover off the AT PSU and have a look...

🤣 at "Mary Hsu."

Standard Def Steve wrote on 2021-12-16, 20:20:

That power supply looks like it's from a Compaq. If it's anything like the model I'm thinking of, the computer is never in a fully-off state; more like a sleep mode. The PSU fan will constantly run when the rear switch is on, but it won't supply "full on" power to the computer until the sleep button (on the front of the Compaq case) is pushed. Definitely a wonky setup.

Yes it is a Compaq, Presario 4505 is the exact model! That must be what it is doing. It is a wonky one, it is my childhood computer and showed up on eBay so having fun with it.

Repo Man11 wrote on 2021-12-16, 20:08:

BitWrangler wrote on 2021-12-16, 19:45:

Well it would even less end user friendly as a soft power PSU as there is absolutely no wire showing that is a power good signal that you could control it with.

Of course, but my point was what sort of computer was sold that only had a power on switch on the power supply?

Just throwing another example out there - Pionex / Quantex did this as well. I have a Pionex Pentium PC with the power switch on the back of PSU. Really inconvenient place to put it.

Brickpad wrote on 2021-12-18, 02:38:

Repo Man11 wrote on 2021-12-16, 20:08:

BitWrangler wrote on 2021-12-16, 19:45:

Well it would even less end user friendly as a soft power PSU as there is absolutely no wire showing that is a power good signal that you could control it with.

Of course, but my point was what sort of computer was sold that only had a power on switch on the power supply?

Just throwing another example out there - Pionex / Quantex did this as well. I have a Pionex Pentium PC with the power switch on the back of PSU. Really inconvenient place to put it.

It may not be the only reason they aren't around anymore, but it's probably one of them.

It wasn't too bad on desktop cases, like lay flat desktops (got ppl calling any cased non-laptop non-AIO a desktop these days) and was kinda normal until the 90s although front switch was gaining popularity (Then there's a bunch of quibble-able machines where it was still actually on the PSU but lever or pushrod actuated from the front) . The original AT had it on the PSU still but rotated to the side. But when towers began to sit on the floor nobody wanted to be crawling under the desk so it was more essential to bring them to the front.