How to calculate power draw per rail, to see if Power Supply can handle the load?

Question

I am purchasing this power supply for a computer I am putting together.

http://www.mini-itx.com/store/information/picoPSU-160-XT.pdf

and while in discussion with a rep, they spoke about not overloading any rail, as I could damage the power supply; However it does mention

Overload protection Over load protection will be effected when either of the loads (+5V & +3.3V) exceeds > 150% Max Load.

but not the 12V rail....

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it gives a chart with max current with the voltage which I understand Power = Current * Voltage, but how do I calculate how much each component uses, especially if it uses more than one "rail." i.e., a Molex uses a 5.5v and a 12v pin, so how would I know how much the 5.5v rail is using, and how much the 12v rail is using?

There is only one 3.3v, one 5v, and one 12v rail, correct?

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Here is a breakdown of my components and their estimated power consumption.

Component Estimated Wattage

Intel Core i5-2400 3.1GHz Quad-Core Processor 11W - 95W

Noctua NH-L9i 57.5 CFM CPU Cooler 5W - 10W

Asus P8H61-I R2.0 Mini ITX LGA1155 Motherboard 7W - 30W

G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1866 Memory 9W - 9W

Samsung 850 EVO-Series 250GB 2.5" Solid State Drive 2W - 10W

Noctua NF-A4x10 4.8 CFM 40mm Fan 1W - 5W

Noctua NF-A4x10 4.8 CFM 40mm Fan 1W - 5W

Noctua NF-A4x10 4.8 CFM 40mm Fan 1W - 5W

Total: 37W - 169W

NOTE: This build is not meant for heavy tasks, no gaming, no editing, but maybe light programming, so I HIGHLY doubt I would overload anything, but I 100% want to make sure.

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Any advice on how to calculate my usage, tips to set me in the right direction, or even just a plain answer would be appreciated.

Thank you for your time.

Answer 1

Your fans and CPU cooler will run from +12. All the others will run from various voltages derived from either 3.3 or 5 volts, and without the detailed specs, you cannot tell the current levels at any voltage.

However, since your computed max power is 169 watts and the supply is only rated for 160, regardless of how it's divvied up you're probably in trouble. That's not to say it won't work, necessarily, since it's entirely possible that not all the components will require maximum power and at the same time.

But it's probably not a good idea trust this. Murphy's Law applies.

Answer 2

Yours is a problem faced in the power industry and by me as I try to ensure that I select a suitable power supply for my PC build. What I'm about to say seems like a tangent, but its a very good analogy. Electricity equipment installed in a building must be rated for the peak demand less some demand factors; if all the loads operate at once and is above the rating, then overtime, heat builds up with resultant effects of personnel safety and property damage.

With a PC supply, same thing, if loads exceed rating, bad things can happen: voltage levels drop due to increased current flow. Some devices akin to resisters simply have current also decrease, inductive loads may try to maintain their power level thus current might increase. Will these effects damage your PC components? Maybe, but if your computer shuts down, at the very least you have inconvenience! And, by the way, the power supply protections are meant to protect the power supply; any incidental "protection" to the PC components and the extent of that should really be thought of as incidental.

Now talking about loads not operating fully at the same time is what electrical utility companies ponder. Electrical utilities take into account load diversity, because statistically speaking (and practically speaking), when you have hundreds and thousands of similar loads fed by the electrical system, you're going to have a load profile that is far below merely adding up all the loads. It's economically unfeasible to size utility company lines based on total loads and would be bad engineering practice, but again, you can't do that inside buildings due to risks to humans and threat of fire and other property damage (and you just don't have the same diversification of loads to allow that anyway).

With the PC power supply you have to be practical and utilize risk management. I want to know my total amperage on the 3.3, 5, and 12 volt rails so I know the power supply provides what my computer needs, and not leave it to chance that maybe all the components won't draw power at the same time. If you have 10 USB ports (which I have) there is just no way I will be using them all at the same time; I'll still want to know the power I may need for each, but I'm not going to upsize my power supply now so that 15 years from now I can use 8 of those ports at once instead of the three or four I can imagine using over the next years or two.

In short, unless you know the load profiles of each component (average load, peak loads) and when the loads of each component will or will not coincide). I would think protecting my computer investment by ensuring all loads at each voltage could be accommodated rather than just leaving it to chance. I'm gathering documentation on all devices for my build. Someone stated most power is derived from the 12V rails by the motherboard, so hopefully I can verify the accuracy of that statement. I'm not willing to risk my investment on "what somebody said!"