momaka wrote on 2026-04-17, 21:01:

Yes, but their whole power draw is very small, so there's just no need for *any* rail to be very strong. Just look various CPU T […]
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MattRocks wrote on 2026-04-17, 20:40:

I don't think that is right. My understanding is that both Socket 7 systems are almost entirely powered by the 5v rail: CPU, motherboard, and the PCI cards. When swapping HDD to SSD, there's almost no 12V demand in those platforms.

Yes, but their whole power draw is very small, so there's just no need for *any* rail to be very strong.
Just look various CPU TDPs for these and you will see.

A 166 MHz MMX, for example, is rated for about 16 Watts tops. Add another Watt or two for worst case VRM inefficiencty, and you're still at under 20W - that's less than 4 Amps on the 5V rail. A K6-2 500 is rated a mere 5 Watts more than the 166 MMX. Now add another 5W for RAM, and 10W tops for all other cards. You should be looking at 35-45 Watts max... which is still under 10 Amps from the 5V rail. Therefore, just about any PSU should technically be able to power these. The problem comes with certain group-regulated ATX v2.x PSUs, which just might not regulate properly due to the unbalanced 5V/12V rail loads. For such units, it won't matter how high you go with the power rating - the 5V/12V rail imbalance will always throw them off. FSP ATX v2.x compliant units tend to fall in that category. Delta PSUs tend to handle cross-loading much better, so that DPS-400WB of yours may not have any problems. Ditto for the Antec Truepower.

And this is why most socket 7 stuff can be powered with a PSU under 100 Watts easily.
If you don't believe me, buy a wall plug-in power meter and see for yourself. The draw from the wall (AC) will be slightly higher due to inefficiency from the PSU. In any case, you may just be surprised at how economical those old PCs are. Then again, if you look at the heatsink size in these old systems (or rather, how few chips have heatsinks), it shouldn't come as a surprise at all that their power draw is so low.

Everything is coming off the 5V rail, not just the CPU. It would be drawing ~100W on 5V rail when most new PSUs max out ~20W on 5V rail.

I'll look into the power draw at the socket after they are doing stuff. It will take me a while to go through the motions - I'm more a thinker than a doer 😉

cyclone3d wrote on 2026-04-17, 20:55:

Also, remember back in the day, the PSU requirements were given for how bad most of the trash PSU were.

Very old PSUs were rated for max temporary wattage, while any decent quality newer PSUs are rated for max continuous wattage.

For ATX PSUs I buy, I generally look for at least 80+ Bronze from good companies such as Seasonic. Other brands generally use various OEMs to manufacturers their PSUs so you need to find out who the OEM that made the specific model, and sometimes revision of that model is.

Those premium ATX PSUs have 120mm fans, which are great in ATX cases but..

I hit the AT case snag in the opening post that locks my MMX build to 80mm fans.

Any ATX PSU is upside down in an AT case, so the big internal ATX PSU vent originally designed to sit above a Pentium II slot will instead face off to a steel lid and any ATX PSU with a 120mm fan is going to experience airflow restrictions it wasn't designed for.

The Aopen has a 80mm fan at the rear. Its ATX vents still point unexpectedly upwards, but the surface area of the vents looks large compared to its 80mm fan so I imaging airflow will be fine. And, when the lid is on the air inside the case will need to follow the original AT path.

So tell me exactly how an 80mm fan that also extracts air from the case with the PSU vents pointing upwards is going to be better than an ATX PSU that also extracts air from the case with the vent / fan intake pointing upwards?

As already stated before, it looks like there is plenty of space for the fan to pull air from.

I've run power supplies like this where the space is not very much and they work just fine.

I've also got lian-li fan cages that mount above the slots. Because of the limited space, the fan doesn't have that much of a gap between the fan and the box it is in. Those work just fine as well.

Also, the PSU with the 120mm fan is going to be spinning the fan at a pretty low rpm and will still move significantly more air than the punt little 80mm fan will.

cyclone3d wrote on 2026-04-17, 22:21:

So tell me exactly how an 80mm fan that also extracts air from the case with the PSU vents pointing upwards is going to be bette […]
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So tell me exactly how an 80mm fan that also extracts air from the case with the PSU vents pointing upwards is going to be better than an ATX PSU that also extracts air from the case with the vent / fan intake pointing upwards?

As already stated before, it looks like there is plenty of space for the fan to pull air from.

I've run power supplies like this where the space is not very much and they work just fine.

I've also got lian-li fan cages that mount above the slots. Because of the limited space, the fan doesn't have that much of a gap between the fan and the box it is in. Those work just fine as well.

Also, the PSU with the 120mm fan is going to be spinning the fan at a pretty low rpm and will still move significantly more air than the punt little 80mm fan will.

This is about the distance between fan and obstruction that starves the intake.

When I installed my RM850x into my upside-down Antec P180 case I encountered a similar concern, in so far as the 120mm fan would be facing off to a solid lump of case. In reading around that, I found a statement from Corsair specifying the minimum clearance needed. The Antec case provided the minimum clearance. The AT case lid provides about half that clearance.

120mm fan: Fan is inside the case. If the vent is pointing upwards then the fan is pointing upwards, about 10mm from the metal obstruction, and trying to suck the case lid into itself.

80mm fan: Fan is pointing outside the case. Even when the vent is pointing upwards the fan is still pointing outwards, and the fan remains over 100mm from the metal obstruction. It's not sucking air straight off the lid, but more at angles across the lid.

You can feel the effect by sucking air into your lungs: When you put your hand further from your mouth air flows inward quietly and easily. And, when you put your hand closer to your mouth the air wraps around your hand noisily and breathing gets harder. That's a lot like what you are doing to the fans when you position the case lid closer.

I still think that the tiny amount of heat output by the system internals as well as the very low power draw on the power supply will make it work just fine.

Those specs from Corsair are going to be for the fan running at full tilt with the power supply being loaded at 100%.