PSUs 101: A Detailed Look Into Power Supplies

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The objective of this article is to provide detailed information about the most crucial part of a personal computer (PC) system, its power supply unit. Follow us on this journey into PSU territory and we promise that you will gain valuable knowledge.

Measuring PSU's Fan RPM

The easiest way to measure a fan's speed or RPM (rotations per minute) is to use a tachometer. However, in the case of fans with white blades in which the laser beam of the tachometer isn't reflected properly, or when there is no visual contact with the fan blades, there is another way to measure fan speed: you can use the commutation or switching noise that the fan's motor produces. But there is a catch here. In our experience, this method works well only with fans that have increased commutation noise, and these are usually the ones with double ball bearings. In addition, you need an oscilloscope.

Utilizing Commutation Noise To Measure Fan RPM

Before we proceed to our methodology of using commutation noise for fan speed measurements, let's discuss first what produces this noise. The explanation is pretty straightforward: each fan has a number of stator coils (usually four) in its motor assembly, which are responsible for the fan's rotation. In each transition from one of the above coils to the next in line, there is a sudden torque pulse to the fan's rotor right at the time when the next coil switches on. This pulse causes a switching noise that can be measured easily with an oscilloscope. According to Noctua, high levels of commutation noise can cause a minute deformation of the whole fan structure and lead to increased noise output. This means that lower commutation noise leads to a smoother transition between the fan's coils, which in turn can lead to a quieter operation.

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With an oscilloscope, the first thing to do is connect one of the instrument's probes in parallel with the fan's power source and select AC couple, in which only AC signals are allowed to pass through. Afterward, you should configure the volt and timescales in order to have a clear shot of the commutation spikes that the fan's motor produces. After you configure your scope, the only thing left to do is to measure the frequency between two sequential spikes. Every scope can do this automatically, but we chose to do it manually in order to provide you with an example that's easier to follow.

In our sample fan, at full speed, the frequency of the spikes was 160.2Hz. If we multiply this number by 15, we have the speed of the fan, which at the given moment was 2403 RPM. Easy, right? The only thing you have to do is multiply the measured frequency by 15 (60/number of stator coils = 60/4 = 15). If your scope supports math functions, this can be done automatically and provide you with the fan's speed in RPM right away.

Aris Mpitziopoulos
Aris Mpitziopoulos
Contributing Editor

Aris Mpitziopoulos is a contributing editor at Tom's Hardware, covering PSUs.

42 Comments Comment from the forums
  • Alexis Shaw
    In your list of top-tier capacitor manufacturers you missed out on some of the better american and european manufacturers, while these may not be used on many consumer-grade power supplies they are definitely top-tier and if you were to find them you would be happy. I suggest the addition of at least:
    Cornell Dubilier (USA)
    Illinois Capacitor (Now owned my Cornell Dubilier)
    Kemet Corporation (USA)
    ELNA (Japan)
    EPCOS (TDK company) (Germany)
    Vishay (USA)
    Würth Elektronik (Germany)
    Reply
  • Aris_Mp
    Thank you very much for the list you provided. I am aware of almost all cap brands that you mentioned but unfortunately so far I found none of them inside a desktop/consumer grade PSU. I will think about it however (and also make a research on these cap brands), if I should include them as well inside my list.

    Reply
  • InvalidError
    16585466 said:
    Thank you very much for the list you provided. I am aware of almost all cap brands that you mentioned but unfortunately so far I found none of them inside a desktop/consumer grade PSU.
    There is a very high probability you have seen PSUs with several Kemet capacitors in them. You never noticed them simply because SMD capacitors are too small to carry logos, brand name or even value designations.

    The other brands are mostly found in specialty applications such as lab instruments, industrial machines and high-end audio.
    Reply
  • Math Geek
    very interesting read. more in depth than i need to know yet for the most part understandable and with careful reading it did not leave me confused.

    nice article.
    Reply
  • TallestJon96
    I only read 2/3 of it, but it's a good article.

    I basically have committed PC heresy with my cx600m. However I think that I'm in the clear with my 65w CPU and 145w CPU. I'd bet my total power draw is actually below 300w, the supposed highest efficiency point of a PSU.

    As a gamer, not a professional, I think it is better to get low power parts, and get a higher rating than you need, rather than get high power parts and high quality PSUs.

    Additionally, if you compare power consumption of a typical system from today to one from 5 years ago, power draw is considerably lower, with the exception of certain graphics cards. *cough* 390x *cough*
    Reply
  • powernod
    I decided to sign up at Tom's forum, and the only reason was to state how excellent is Aris's article!!!
    Thanks Aris for this very useful article on behalf of us all who want to learn the basic knowledge for PSUs.
    Haven't finished it yet, but i'm very anxious for it !!!
    Reply
  • GoZFast
    Very nice article!!! You made me remember my college physics courses lol
    Reply
  • traumadisaster
    I'm glad there are people dedicated to this but I'm not. I can't even read all of the chapter titles in this article. I disagree with the importance you place on this and all of the references you made to this being crucial knowledge.

    PSU and MB are insignificant to me and I can blindly pick one by reviewing user comments from newegg in about 5 min, and it will last for years. For less than $100 each I'm set for nearly a decade.

    CPU and gfx card now that affects fps and is over $1000, actually the most important part to me.
    Reply
  • Alexis Shaw
    16589602 said:
    I'm glad there are people dedicated to this but I'm not. I can't even read all of the chapter titles in this article. I disagree with the importance you place on this and all of the references you made to this being crucial knowledge.

    PSU and MB are insignificant to me and I can blindly pick one by reviewing user comments from newegg in about 5 min, and it will last for years. For less than $100 each I'm set for nearly a decade.

    CPU and gfx card now that affects fps and is over $1000, actually the most important part to me.

    I heartily dissagree, user are not the best way to judge reliability, and a bad powersupply is at fult most of the time there is a hardware issue. Further a power supply should last more than one system build, and in general I keep mine for a decade at a time at least. So an investment in a good power supply is not a waste, and a bad one will kill that precious $1000 GPU or CPU. The demo dart power supply on the motherboard is a similar story, however in general they are of higher quality than a cheap mains supply.

    Reply
  • Alexis Shaw
    16585679 said:
    16585466 said:
    Thank you very much for the list you provided. I am aware of almost all cap brands that you mentioned but unfortunately so far I found none of them inside a desktop/consumer grade PSU.
    There is a very high probability you have seen PSUs with several Kemet capacitors in them. You never noticed them simply because SMD capacitors are too small to carry logos, brand name or even value designations.

    The other brands are mostly found in specialty applications such as lab instruments, industrial machines and high-end audio.

    As well as SMT ceramic capacitors, Kemet makes through hole aluminium electrolytic capacitors. These are of high quality, though not as well known as their SMT capacitors. They also make high quality polymer SMT capacitors that are used as bulk capacitors on the power distribution circuitry on laptops and other devices.
    Reply