What kind of a PSU (in terms of wattage) do I need to comfortably run an AMD 386DX-40 system?
Unlike with today's systems, the CPU played only a minor role in power consumption back then. Over all configuration, like number and size of disks as well as size of RAM was way more important. Hard to give any number without more information - like if it's just the board, or a whole system, or what configuration it is and what peripherals are used.
For an over all size it may be helpful to look at of the shelf systems of back then. Typical PSU sizes in that area ranged between 80 and 200 Watt. Examples:
- DELL 333P (Slimline case), 386/486 @ 33 MHz of 1991 85 Watt
- DELL 333D (Standard Desktop size case), 386 @ 25 MHz of 1991: 177 Watt
- Compaq Proliant 1000 (Server), 486DX2-66 of 1994: 300 Watt
- DELL Power Edge (Server), Pentium/90 of 1994: 300 Watt
- IBM PC Server, 486DX2-66 of 1994: 300 Watt
I had to search hard to find some (mainstream) top end server with more than 300 Watt (*1):
- HP NetFile Server 486/LF, 486DX2/66 of 1994: 350 Watt
- IBM PS/2 Server 95 Array 566, Pentium 66 of 1994: 400 Watt
All servers one being a monstrous huge machines in the 15-20 kUSD range with humongous RAM sizes (like 64 MiB) and disk arrays sucking quitesome power..
So for an average 386 system of that time any 150-200 Watt PSU is more than adequate.
Best source to see what's good/recomended/needed would be a peek into the boards manual.
Eventually I aim to install an ATX power supply and use ATX to AT converter cable.
Should work fine. Don't forget to bridge
Power_On (Pin#16) to
Gnd (#15 or #17) using a wire or some ~100 Ohm resistor.
Would a power rating required for an ATX supply differ from AT one?
Power ratings always differ in the way they are generated. While manufacturers of larger models usually use the 12V wattage, smaller are often advertized by summing up all rails. As usual, a look into a detailed data sheet and/or manual will clear things.
Perhaps, due to different max load distribution across various voltages than AT PSUs were designed for?
That's true, but then again, to be sure you need to check for each (used) voltage. The board itself almost entirely relies on +5V and should stay well below 10A (50 Watt) including reasonable memory. More if there's a lot of I/O cards with much processing and/or RAM. +12V is mainly used for drives and supplied with their own connectors. It may be more important to check if that PSU can work without load on 3.3V. If not, some dummy load (right sized resistor) has to be applied to burn away some juice. A 10 Watt 1 Ohm may already do the trick - again, to be sure check the PSU-manual
In general, chances are good to use some modern (low end) PSU - if you can't find an AT one somewhere
*1 but I'm pretty sure others will find some - everything man can imagine has been done to PCs.