yqX wrote: ↑15 Jul 2024, 07:21
You mentioned on a previous page that you have power limiting turned on. Could you elaborate on that setting?
After figuring out that the new PSU basically solved the problem, I wanted to test for what exactly was happening and if I could find a correlation between PSUs and the problem. I have some PSUs laying around here so I just went and tested these with different hardware. As I mentioned in a different post, PSUs can come in a "multi-rail" configuration where they theoretically can output, let's say 550W across the 12V rails, but it can't draw that amount of power continuously from all the rails. I have one at hand here, so I'll use it as an example. It's a beQuiet 700W unit, and it has 2 12V rails. 12V1 can provide 36A and includes the 24-pin power, storages and one of two PCIe (GPU) connectors. The 12V2 rail can provide 30A and supplies the CPU EPS 8-pin and the second PCIE (GPU) connector. You can easily identify this by looking at the PSU label. If it just says "12V" it's a single-rail unit, and if it says "12V1", "12V2" etc. it's a multirail unit. The rail splittings can be found in the handbook or online.
Now, while my PC at worst case draws at most 600W, and the PSU is a 700W part, you would think that i could just plug in everything and call it a day. However, this won't work as this would overload one of the 12V rails. My GPU is a 300W part, it has 2x 8-pin connectors. Let's say, I use the pigtail on one of these connectors (don't do this!) and used it on the 12V2 rail, plugging it into the second position on the PSU. As stated before, the 12V2 rail is responsible for the CPU power and one of the GPU connectors. It can output 30A (30A*12V=360W), and the GPU is already a 300W part. Because we used only one cable to the GPU with the pigtail in our imaginary scenario, this connector alone already has to provide 300W worst-case. This leaves only 60W for the CPU before the 12V2 rail would be overloaded.
You can probably see where this is going. In order to prevent overloading the PSU now, I could either reduce the power limit on the GPU, set a power limit for the CPU or place the GPU connector on the 12V1 rail. In this imaginary case, I would go into the BIOS and punch in a 60W limit.
When using the 12V1 rail for the GPU, this would cause a load of 300W (GPU) plus another 75W for the 24-pin (worst-case) and 50W for the storage devices, fans and so on (worst-case), equating to 425W worst-case. This is still within the specification of 432W (36A*12V), while leaving the 12V2 for only the CPU, giving it 360W (30A*12V) of headroom. If I was really tedious, I could set a 360W limit in the BIOS, but as this CPU would probably return to sand in a nanosecond if it pushed that amount of current throught it, I would leave the power limit to unlimited (4096W), as the CPU will never reach 360W anyways.
For all the PSUs tested, I did the math beforehand and calculated if the 12V rails would be overloaded or not. If they were, I imposed the proper power limits. This was done in the BIOS, and on my MSI BIOS this is just called "Long Duration Power Limit (PL1)" and "Short Duration Power Limit (PL2)". I simply set these to the same value according to the PSU used. Make sure you're not using the "Current Limit" in the BIOS. This current limit is applied on the VRM output side, so if you punch in, let's say 30A here, it will limit the CPU to 30A at its usual operating voltage, let's say 1.2V, giving it only 36W to work with.
On my current PSU, the 1200W one, there is a switch on the back set to single-rail mode, essentially providing 100A on the 12V rail. I therefore have the power limits unlocked (4096W) on the CPU, and I also don't power-limit the GPU any more than what the stock settings would do.
It's a good idea for anybody reading this to have a quick glance at their PSU's label, figure out if it's single or multi rail, and if it's multi-rail quickly check for the rail splittings and do a quick calculation to see if one rail could get overloaded under a worst-case scenario where every component is used to its full potential. The TDPs for CPUs and GPUs aren't a direct power specification, but they come close enough so just use this. For CPUs, Intel and AMD have a database of all their CPUs which lists their TDPs. GPU TDPs can be looked up on TechPowerUp GPU database or similar sites. Labels on PSUs usually give amps, so you can either convert your PSUs amps to watts by multiplying the specified amps by 12, or you can convert your CPU/GPU TDP to amps by dividing TDP by 12.