Picking The Right Power Supply: What You Should Know

How does a power supply work? Why is it important to choose a sufficiently powerful and efficient model? We guide you through discussions of efficiency and tips for getting the best deal before we go on to explain why less can be more in the PSU market.

This piece is for the folks who'd like to learn more about the facts, technologies, and terminology behind PC power supplies.

Keeping The Explanation Simple

Don't worry; this won’t be complicated or boring. We’ll just quickly explain how a switching PSU works, then use examples to illustrate some of the most common technical issues. We’ll explain what efficiency, loss, and reactive power mean, and why those words are relevant to you. Then we’ll look at the possible and (more importantly) necessary protective measures before applying theoretical knowledge to practical examples.

Practical Examples

Big versus small, efficient versus high-performance; we're going to examine three different PCs based on a trio of different usage models, calculate the power supplies they really need, and then explain the right class of PSU to use in them based on quality and long-term environmental impact.

The Frequency Trick

Remember those ancient radios with the vacuum tubes? They were massively built and tended to be clunky, heavy, very functional-looking things. However, it wasn’t just the wooden frame contributing to their weight. The large, massive transformers added their part as well.

The point is, even back then, clever engineers were taking advantage of a neat trick of physics that would later come to be used in every modern switching power supply. In order to convert a high alternating current into a low one and achieve galvanic separation of currents, they used normal, albeit powerful, transformers with a core made of iron plates.

While a mains frequency of 60 Hz required a comparatively large transformer, the so-called output transformers that delivered much higher low-frequency signals between 100 Hz and 16 kHz could be built much smaller while handling the same power. By aggressively capping the frequencies at the lower end of the spectrum, it was possible to increase the power that could be handled by a transformer of the same size. With the invention and subsequent introduction of new components, such as powerful switching tubes, and later, semiconductors using the same underlying physical principle, this advantage was carried over into other fields, opening up new possibilities.

And How Does That Apply To My PC?

Due to the high overall power requirements of modern computers, a conventional transformer-based PSU is no longer capable of converting the mains power into the low voltages required by PC components. The transformer required for the job would be too large and consequently far too heavy. Instead, we use switching power supplies that employ the same frequency trick as the good old tube radio. Their job is to provide the required voltages and currents as efficiently as possible, while also reliably maintaining those levels. Analogue (linear) solutions are no longer viable solutions. Instead, we now rely on transistors as switches to convert the mains power into higher frequencies, allowing us to use smaller transformers to transmit high power levels. Indeed, this is where the term “switching power supply” comes from. We’ll cover how those work in more detail in the next chapter. Don’t worry, it’s all simpler than you might think.

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  • 13thmonkey
    Great article, since you mentioned power draw at 'off' could you please include it? and perhaps go to the next stage as below and work out the power drawn per day/year and the cost of that? I've included an example of what I mean below, assuming off=10% of idle. There's £9/year spread from least to most efficient, the Be Quiet seems to be the be value if you are going to keep the PSU (not the PC) for 3.5 years or more, I'm not sure about Be-Quiet as a brand as I've not seen reviews and such for them.

    The addition below would turn this from a great article into an incredible article
    -------------BQ---------RA-----------SF-------------LC------------HM-----------Hrs
    Idle----------53.2---------56.7---------60.8---------68.2---------72.1---------4
    Normal-----91.6---------90.4---------92.2---------98.9---------102.5-------4
    Load--------125.6-------124.8-------122.2-------131.7-------136.4-------1
    Off------------5.32---------5.67---------6.08---------6.82---------7.21---------15
    Watt-hrs-per-day
    ----------------213----------227----------243----------273----------288
    ----------------366----------362----------369----------396----------410
    ----------------126----------125----------122----------132----------136
    ----------------80------------85------------91------------102----------108
    Units-per-day
    ----------------0.78---------0.80---------0.83---------0.90---------0.94
    Units-per-year
    ----------------286----------291----------301----------329--------344
    Cost-@-15p/unit
    --------------£42.96-----£43.70-----£45.19-----£49.41-----£51.63


    Apologies for the formatting - WSIWIG please please pleaseits 2011 for crying out loud, just spent 5mins putting spaces instead of tabs and its still looses formatting
    2
  • bobbyp86
    Which model is that Corsair case at the end? Looks nice!!
    1
  • bobbyp86
    aha it's the Obsidian 800D, better get saving :)
    1
  • flaminggerbil
    Yet to have a PSU go pop on me (touch wood), great article though, very interesting.
    Wonder how long that 450w superflower could have kept running on full load.

    But glad I've always stuck with Corsair & Enermax.
    1
  • silver565
    Corsair have been good to me. My 550vx is still going strong
    I only use the cheaper cooler master ones(something like this http://tinyurl.com/3wxcs3t) for my file server. No high drain, just a few HDDs.
    0
  • Solitaire
    flaminggerbilYet to have a PSU go pop on me (touch wood), great article though, very interesting.Wonder how long that 450w superflower could have kept running on full load.But glad I've always stuck with Corsair & Enermax.


    A long time, although its overall lifetime would likely have been shortened running it at 110% load for extended periods. Their last two generations have been fantastic, just wish you could actually get them that cheap in Europe! :p

    And saying all current-gen quads use the same amount of power is just silly Toms! The top-end Phenoms can creep past 130W at full speed, but the i5-2500k uses barely half as much, not 125W!
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  • shanky887614
    i had a 600watt winpower supply

    it ran fine for 4 years

    then there was an electronic whine coming from it under load it frove me crazy

    it was a 5 or 600 watt i beleive i know a core 2 duo and hd 4670 cant draw that much power

    could get a decent brand one at a local shop so had to go for a gigabyte 550 (450 watt continuos power supply)

    im not kidding ive had quiter hair dryers!!!!
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