High 12v amp rating wont create any problem but it wont solve the issue if the 5v amp rating is low either.

The faster P3 CPUs eat about 30W. The maximum power the AGP slot can deliver from 5/3.3/1.5V is about 36W. Add in everything else and you might end up somewhere around 80W total for 5/3.3/1.5V, depending how much stuff you need to power and what GPU you intend to use.

Modern PSUs seem to usually have about 100W total for 5/3.3/1.5V rails which might be ok with less power hungry GPUs, but could be too little for cards like FX 5800. You might want to try to look for a PSU that can deliver 150W combined for those rails just to be on the safe side, unless you are going to use a GPU like 3dfx Voodoo3.

Sombrero wrote on 2022-12-02, 10:15:

The faster P3 CPUs eat about 30W. The maximum power the AGP slot can deliver from 5/3.3/1.5V is about 36W. Add in everything else and you might end up somewhere around 80W total for 5/3.3/1.5V, depending how much stuff you need to power and what GPU you intend to use.

Modern PSUs seem to usually have about 100W total for 5/3.3/1.5V rails which might be ok with less power hungry GPUs, but could be too little for cards like FX 5800. You might want to try to look for a PSU that can deliver 150W combined for those rails just to be on the safe side, unless you are going to use a GPU like 3dfx Voodoo3.

I'm probably going to use a GPU from the GeForce 256- GeForce 2 era so does that mean a normal 100W rail would be enough?

Probably. But which CPU(s) are you exaclty intending to use? The maths is simple: look up the TDP of the CPU(s), look up the TDP of the card. Add maybe 10% for other stuff. That's the minimum your PSU needs to be able to deliver in Watts on the 5V line. Divide that number by five and you get the current in Amps that it needs to support.

Then check you PSU. Note that there are crappy PSUs out there that can't deliver their stated load, so be sure to check (reliable, comparative) reviews of modern PSUs to be sure they do what they're supposed to.

eyalk4568 wrote on 2022-12-05, 06:12:

I'm probably going to use a GPU from the GeForce 256- GeForce 2 era so does that mean a normal 100W rail would be enough?

Well, the GeForce 256 and the GeForce 2 MX/GTS/Pro/Ti are VERY different in terms of power consumption.
The GeForce 256 needs almost 20W of power, while a GeForce 2 MX requires 4 times less (4 - 5 W). The GeForce 2 GTS is also very efficient, with a power consumption of less than 10W.

Having said that, going with a GeForce 256 is definitely not a problem for any good modern PSU that has at least 100W on the 5V & 3.3V rails.
However, it can be a major problem depending on your motherboard: there were many motherboards back in the day (with underpowered/lower quality onboard 3.3V regulators) that died a premature death because of Voodoo 3 / GeForce 256 cards. 😀
Some people even ended up modding their motherboards to fix this problem.
The problem was so well known, that newer revisions of some motherboards actually had "Voodoo 3 jumpers" which basically bypassed the onboard voltage regulator and connected the AGP to the PSU's 3.3V rail directly.

So, if you have one of the motherboards that were affected by the 'AGP power delivery plague' and if you plan on using this PC extensively, I would stay away from GeForce 256 / GeForce 3 /3 Ti 200/3 Ti 500, etc. A GeForce 2 MX or even a GeForce 2 GTS/Pro/Ti should be very safe to use on these motherboards (but I would still add a heatsink on that voltage regulator + a bit of airflow inside the case, especially if you will be using this PC as a "retro daily driver" 😀 ).

Last time I checked power draw of a PIII 866 + GF2 Ultra + 2x 2000RPM 80mm fans + 1x HDD was 75W peak from the wall in 3DMark 2000. Assuming, maybe, 87% efficiency (SSR-550FX) the actual power draw is about 65 watts. Round it to sixty for 5V and that's 12A.

HDDs are an easy way to increase power consumption although they draw from both 5V and 12V. According to Seagate my very late 80GB IDE disk eats 12.20W (0.627A/5V + 0.755A/12V) while seeking (random, 20% idle).

I need to run more tests.

It's mostly a concern when you are building a higher-end Athlon and it lacks the 12V power connector.

swaaye wrote on 2022-12-05, 20:50:

It's mostly a concern when you are building a higher-end Athlon and it lacks the 12V power connector.

ya pretty much that the only time it really matters.

i used a cheap evga 500w that i think does 20a on the 5v and it works fine with a p3

bloodem wrote on 2022-12-05, 07:20:

Well, the GeForce 256 and the GeForce 2 MX/GTS/Pro/Ti are VERY different in terms of power consumption. The GeForce 256 needs al […]
Show full quote

eyalk4568 wrote on 2022-12-05, 06:12:

I'm probably going to use a GPU from the GeForce 256- GeForce 2 era so does that mean a normal 100W rail would be enough?

Well, the GeForce 256 and the GeForce 2 MX/GTS/Pro/Ti are VERY different in terms of power consumption.
The GeForce 256 needs almost 20W of power, while a GeForce 2 MX requires 4 times less (4 - 5 W). The GeForce 2 GTS is also very efficient, with a power consumption of less than 10W.

Having said that, going with a GeForce 256 is definitely not a problem for any good modern PSU that has at least 100W on the 5V & 3.3V rails.
However, it can be a major problem depending on your motherboard: there were many motherboards back in the day (with underpowered/lower quality onboard 3.3V regulators) that died a premature death because of Voodoo 3 / GeForce 256 cards. 😀
Some people even ended up modding their motherboards to fix this problem.
The problem was so well known, that newer revisions of some motherboards actually had "Voodoo 3 jumpers" which basically bypassed the onboard voltage regulator and connected the AGP to the PSU's 3.3V rail directly.

So, if you have one of the motherboards that were affected by the 'AGP power delivery plague' and if you plan on using this PC extensively, I would stay away from GeForce 256 / GeForce 3 /3 Ti 200/3 Ti 500, etc. A GeForce 2 MX or even a GeForce 2 GTS/Pro/Ti should be very safe to use on these motherboards (but I would still add a heatsink on that voltage regulator + a bit of airflow inside the case, especially if you will be using this PC as a "retro daily driver" 😀 ).

Oh is that why canopus spectra GeForces had the molex connector on the gpus pcb?

To stop motherboards from failing by providing the power directly?

Some early AGP boards weren't well designed and couldn't supply enough power to the AGP slot to power cards like GeForce and Radeon.

I've been messing with my P3 recently and hooked up a killawatt.

It pulls about 65-75 watts when gaming. 1.0 ghz, 815 chipset, 3x ram sticks, sd-ide, geforce2 pro (agp). no extra case fans or spinning rust.

bloodem wrote on 2022-12-05, 07:20:

Well, the GeForce 256 and the GeForce 2 MX/GTS/Pro/Ti are VERY different in terms of power consumption. The GeForce 256 needs al […]
Show full quote

eyalk4568 wrote on 2022-12-05, 06:12:

I'm probably going to use a GPU from the GeForce 256- GeForce 2 era so does that mean a normal 100W rail would be enough?

Well, the GeForce 256 and the GeForce 2 MX/GTS/Pro/Ti are VERY different in terms of power consumption.
The GeForce 256 needs almost 20W of power, while a GeForce 2 MX requires 4 times less (4 - 5 W). The GeForce 2 GTS is also very efficient, with a power consumption of less than 10W.

Having said that, going with a GeForce 256 is definitely not a problem for any good modern PSU that has at least 100W on the 5V & 3.3V rails.
However, it can be a major problem depending on your motherboard: there were many motherboards back in the day (with underpowered/lower quality onboard 3.3V regulators) that died a premature death because of Voodoo 3 / GeForce 256 cards. 😀
Some people even ended up modding their motherboards to fix this problem.
The problem was so well known, that newer revisions of some motherboards actually had "Voodoo 3 jumpers" which basically bypassed the onboard voltage regulator and connected the AGP to the PSU's 3.3V rail directly.

So, if you have one of the motherboards that were affected by the 'AGP power delivery plague' and if you plan on using this PC extensively, I would stay away from GeForce 256 / GeForce 3 /3 Ti 200/3 Ti 500, etc. A GeForce 2 MX or even a GeForce 2 GTS/Pro/Ti should be very safe to use on these motherboards (but I would still add a heatsink on that voltage regulator + a bit of airflow inside the case, especially if you will be using this PC as a "retro daily driver" 😀 ).

How do you know which motherboard was affected by the "AGP power delivery plague"?

eyalk4568 wrote on 2022-12-06, 17:13:

How do you know which motherboard was affected by the "AGP power delivery plague"?

A voltage regulator that gets very hot (over 70 - 80 C) after 10 - 15 minutes of gaming (usually even less). It will get even hotter than that when using a more power hungry card.
In most cases, it will be somewhere close to the AGP slot, but there are boards where it can be in a more weird location (such as near the DIMM slots).

If you tell us what board (make / model / revision) you have, we might be able to say if it's affected or not.

bloodem wrote on 2022-12-06, 18:27:

A voltage regulator that gets very hot (over 70 - 80 C) after 10 - 15 minutes of gaming (usually even less). It will get even ho […]
Show full quote

eyalk4568 wrote on 2022-12-06, 17:13:

How do you know which motherboard was affected by the "AGP power delivery plague"?

A voltage regulator that gets very hot (over 70 - 80 C) after 10 - 15 minutes of gaming (usually even less). It will get even hotter than that when using a more power hungry card.
In most cases, it will be somewhere close to the AGP slot, but there are boards where it can be in a more weird location (such as near the DIMM slots).

If you tell us what board (make / model / revision) you have, we might be able to say if it's affected or not.

I currently don't have any board, I'm just planning to build a Pentium 3 PC, but its a good thing to know before buying any motherboard from that era.
so thank you for your help 😀