Power supplies/ identifying what voltage(s) each component needs

carrierPigeon

Limp Gawd
Joined
Sep 22, 2017
Messages
162
Background: I was about to upgrade the power supply on an old computer from 250W to 450W. The reason that I was doing this is because I had installed a new video card a couple of months ago to test something, it solved the problem and I forgot how weak the power supply was. According to a review website the GPU uses 65 Watts at idle and 130W at full load. Interestingly, everything seems to be working perfectly fine without installing a new power supply. Since the video card was PCIe 3.0 8x and the motherboard slot was for PCIe 2.0 16x, perhaps that reduced its abilities and power consumption.

Anyway, the graphics card manufacturer gives no information about how many watts the graphics card uses. Only a recommendation about what size power supply you should have. Seems strange to me; is that typical?

Comparing the 250 watt power supply to the 450 watt power supply, the 250 watt power supply has higher specs on the 5V rail (around 14 amps on the 250 watt, and 10 amps on the 450 watt). So, it's possible that the 250 watt is actually more suited for the job.

I also looked at the motherboard manual, and I could find no data as to what conversions the motherboard is doing (if it's converting any of the voltages from the 20/24 pin connector to something else).

Is there a way to figure out the voltage rail/rails that our hardware actually uses and what conversions our motherboards are doing? Or do you just have to go insanely overboard on the power supply, and waste money on electricity?
 
Last edited:

Nenu

[H]ardened
Joined
Apr 28, 2007
Messages
20,162
Background: I was about to upgrade the power supply on an old computer from 250W to 450W. The reason that I was doing this is because I had installed a new video card a couple of months ago to test something, it solved the problem and I forgot how weak the power supply was. According to a review website the GPU uses 65 Watts at idle and 130W at full load. Interestingly, everything seems to be working perfectly fine without installing a new power supply. Since the video card was PCIe 3.0 8x and the motherboard slot was for PCIe 2.0 16x, perhaps that reduced its abilities and power consumption.
PCI-e gen has no bearing on how much power a gfx card uses.
It can affect how much is drawn over the PCI-e bus vs power socket, a minimum PCIE-e spec may be mentioned.

Anyway, the graphics card manufacturer gives no information about how many watts the graphics card uses. Only a recommendation about what size power supply you should have. Seems strange to me; is that typical?
Yes it is fairly typical to ensure people are not bogged down with specs other than total power requirement.
It is slightly over specced to account for less than stellar PSUs and the drop in power output over the years of use.

Comparing the 250 watt power supply to the 450 watt power supply, the 250 watt power supply has higher specs on the 5V rail (around 14 amps on the 250 watt, and 10 amps on the 450 watt). So, it's possible that the 250 watt is actually more suited for the job.
The major power rail shifted from 5V to 12V a long time ago but some PSU for a while after continued to have high spec 5V lines.
I dont look at the 5V rails max power output these days because it isnt used by much.
It would help to know what hardware you are talking about.

I also looked at the motherboard manual, and I could find no data as to what conversions the motherboard is doing (if it's converting any of the voltages from the 20/24 pin connector to something else).
Anything needing high power will use the 12V rail with VRMs to bring the voltage down to the required level.

Is there a way to figure out the voltage rail/rails that our hardware actually uses and what conversions our motherboards are doing? Or do you just have to go insanely overboard on the power supply, and waste money on electricity?
For PSU selection, assume modern hardware uses the 12V rail.
A few things will use 5V but they are so low in power they are easily accommodated.
 

carrierPigeon

Limp Gawd
Joined
Sep 22, 2017
Messages
162
I hadn't heard the term VRM before. But, now I have another search word for learning more :)

The 450 watt power supply that I have is a Corsair CX450. It's ATX v2.4. It seems that ATX 2.4 is pretty modern. Here is its output:
+3.3V 20A
+5V 20A
+12V 37.4A
-12V 0.8A
+5Vsb 3A

But, I don't know how much of that is going through each of the wires. I found this info about ATX v2.2:
https://www.lifewire.com/atx-24-pin-12v-power-supply-pinout-2624578
 
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