Computing stuff tied to the physical world

TD – PC Power Supply

In Hardware on Apr 3, 2012 at 00:01

Welcome to the Tuesday Teardown series, about looking inside the technology around us.

Well, not a very “deep” teardown, just opening it up and looking inside a conventional 400W PC power supply:

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When turned on, but not powered up, the power cunsumption is a substantial 2.8 W. IOW, that’s your computer when turned off. But the nasty surprise was that even with the mechanical switch in the off position, it still draws 0.04 W? Oh well, the sticker says “2006”, so let’s assume things have improved since then.

Here’s the top view inside:

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Two large heatsinks with two fans blowing air across, the bottom fan is on the outside of the case.

These caps scare me, I had it powered up briefly, so I’d probably get a jolt if I were to touch them now:

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Two small transformers in there, on the right. And here are three more:

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One last toroidial one in where the main circuitry appears to be – note the one-sided PCB with jumpers:

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And that board at the right is filled with varicaps, etc – noise and surge suppression, no doubt:

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Found a schematic of a 200 W supply on this website:


Go to the website for the full-size view. Looking at the number of transformers, this supply is probably similar. The basic idea is simple: generate a high-frequency AC signal, and feed it through some transformers for galvanic isolation and to produce the much lower voltages at much higher currents. A high frequency is used i.s.o. 50 Hz because transformers are more efficient that way. There’s a feedback mechanism to regulate the output voltages.

The TL494 chip (which is not necessarily the same as used in this particular supply) is the heart of a PWM Power Control Circuit, which seems to drive it all. It generates pulses, and varies the ON-time as a way to regulate the generated output voltages. I think.

What I never understood is how you can regulate multiple voltages with what looks like only one feedback loop. In the schematic, the +12 and +5 V outputs are brought together as a single regulating signal through 2 resistors. What if the power draw from those 12V and 5V sections differ widely over time?

Anyway, go to that website mentioned earlier to read more about how it all works. I’m sure it does since there must be hundreds of millions of these on the planet by now…

Update – This particular unit will turn on without adding 10 Ω resistors, as sometimes suggested for lab use of such PSU’s. Voltage unloaded is 3.39V, 5.18V, and 11.99V, so close enough – with a little extra to compensate for wire losses. Big downside for lab use of such a “raw” PSU, is the nearly unlimited current that will flow with a short-circuit – guaranteed to vaporize lots of things! One solution would be to add basic current sensing and MOSFETs to switch off when pre-set values are being exceeded. With proper dimensioning, the added current drop need not be more than perhaps 100 mV, so the generated voltages would still be “in spec”. The + and – 12V would make a nice ±10V supply for analog experiments with dual-supply op-amps, for example.

  1. I have wondered about the possibility of using a PC PSU for projects at home. As a bench PSU but also to provide some 12v, 5v, and 3.3v (through the SATA power connectors) around my house for sensors, LED strip lighting and USB charge points. Is this a silly idea?


    • Not at all, go to for several examples on how to do this. But there is one issue to be aware of: these power supplies can supply very large currents. That means that any mistake may cause several amps to flow, which can easily vaporize thin copper wires, etc.

      A lab supply has adjustable current limiting, which lowers the voltage once the limit is reached. So an adjustable supply set to say 100 mA will not supply more than 100 mA no matter what mistakes and short-circuits you run into (happens all the time, comes with the territory). A nice low-power current-limited supply is very useful for electronics experimentation (and it could draw its power from a PC PSU, of course).

      For use around the house, a PC PSU might be ok, but here too there’s a caveat: low voltages at high currents lead to fairly substantial power losses if the wiring isn’t really thick copper. You’ll see this as getting less than 12V on a LED strip if the distance is too large. I’ve been thinking about such a low-power net myself, but perhaps not one net for the entire house, more like a few connected across one or two rooms. And perhaps send out say 14V iso 12V, to compensate for losses.

  2. Probably you went already once into this, but anyway: How do you measure power consumption at such small levels as 0.04W on mains power? I have a cheap power meter, but if connected anything under 20W it is used better as a random generator than a measurement instrument.

    • Look for a unit with a limited high range… then the low values might actually be meaningful.

      I use this meter from Conrad – (Dutch). It’s the only low-range unit I found so far and even then, 0.04 might well be off by a factor of 2. But still, better than nothing.

  3. Your PSU carries a “CE” sticker, and from the date you mention, IEC61000-3-2 is in force – this requires the designer to make the mains load appear almost unity power factor. A common solution is to have a power supply (wave shaping to force the input current envelop to look like the sine wave input voltage) feeding a further power supply that acts as a DC – DC convertor producing the high current/low voltage outputs.

    Hence the complexity and multiple inductors involved. Note that the heavy output currents require special handling – the multiple yellow/red /orange/black wires clustered on the PCB are in parallel to reduce the voltage drop in the flying cable and the get around the current carrying limitation of the standardised “Molex” connector.

    Don’t blame the designer for the quiescent power consumption – this comes from the ATX spec which insists on an unswitched 5Vsb pin. This is typically used to power LAN cards that stay alive ready to bring the main PC out of sleep remotely (Wake on LAN functionality).

    The residual ~40mW comes from the mains filter/surge protection that is often put before the main switch to minimise radiating connection length. The surge protector MOV is directly across the mains supply; these devices are amazingly good – absorbing several joules of mains spikes when triggered, but their “off” resistance is not infinite, a leakage current in the hundreds of uA is typical. Also the blue ceramic line filter caps (not Varicaps) and the yellow X2/Y1 cap hiding in there are also not pure capacitance – some power loss in the dielectric is inevitable.

    This is typical of the compromises constraining a designer when working towards a general product release; meet the mandatory specifications first, then try to optimise after (but build cost will rank higher than maximum efficiency/minimum energy consumption). An individual can do better since the environment is often more controlled. For example, my own mains distribution system centralises power surge protection at the mains distribution board; this has several advantages including paying the leakage cost once only and protecting appliances that did not bother to include individual surge circuitry.

    To complete the process, in theory, the individual varistors can be removed if you know the appliance is hard wired to that environment. But it’s worth remembering the rule of diminishing returns – saving a continuous 40mW is about the same power consumption as boiling the water once every six months to make an extra cup of coffee to help figure all this out !

    • Thanks for all the insightful comments, as always.

      saving a continuous 40mW is about the same power consumption as boiling the water once every six months to make an extra cup of coffee to help figure all this out !

      Excellent point! So stay away from the coffee, eh? ;)

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