the dummy load is for the 3.3V rail. No AT motherboards have a 3.3V rail and some modern power supplies don't like being run without any load on the 3.3V rail (stability issues or undercurrent protection IIRC).

In my experience -5V is not critical for pretty much anything outside of early sound cards and exotic cards you likely won't come across anyway..

appiah4 wrote:

I see three kinds of adapters, and can't be sure what kind I would actually need.. […]
Show full quote

I see three kinds of adapters, and can't be sure what kind I would actually need..

One is a simple 20pin to P8/P9 adapter without even a power switch. How does this work? If I connect this to an AT motherboard, how do I even start the power supply, considering there is no PWR_SW header on the motherboard? Is it always on - i.e. you turn the system on/off through the PSU's switch?

The other is the adapter with a switch, and a dummy load. The switch presumably turns on the system by shorting the green/black wires?

Now, there is also a kind of adapter with apparently a dummy load and a -5V rail. My understanding is that the -5V is necessary for *some* sound cards, but I can't seem to find a reliable list of sound cards it's necessary for, nor a definitive answer as to whether it's necessary for system stability, DMA transfers etc. As for the dummy load, I have no idea what it is for.

All I want to do is set up a 486 system inside a cheap ATX case with a simple 300W modern ATX power supply. Which one of these do I need?

Yes the one with just the connectors for the switch when the switch is closed connect PS_ON to GND so the ATX PSU powers up.

Personally i rather buy the one without -5v rail and the dummy load because i mod them for cheaper and add those functionalities, for the -5v rail i use some capacitors( small ceramic and a big electrolytic in parallel to the output pin and gnd) and a LM7905 -5v voltage regulator to step down the -12v present on modern atx psu(i just use this linear regulator because the -5v current draw is usually pretty low, more efficient solutions might be possible), and for the dummy load just dome 10w resistors with the resistance according to the rail.
the dummy load is necessary because the voltage on the psu might not be too stable with extremely light loads(that usually vintage computers do on modern atx psu).

but if you don't wanna do everything and the price difference isn't outrageous, pick the one with the -5v and the dummy load and you are good to go.

Koltoroc wrote:

In my experience -5V is not critical for pretty much anything outside of early sound cards and exotic cards you likely won't come across anyway..

-5v is not exclusive to slots.
It's also used for discrete onboard chips (primarily sound but possibly LAN) that utilize an internal ISA bus.

Given we are talking AT powered mobos the configuration (AT power + OB sound) is rare but it did exist.
The socket 7 PcChips M598LMR is an example.
.

You might want to check out the ATX2AT Smart Converter if you're willing to spend some more on a high-end labour of love from a fellow VOGONS member: ATX2AT Smart Converter - Live on Kickstarter!