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I was just reading in Capacitors in DC Circuits that "Capacitors do not play an important role in DC circuits because it is impossible for a steady current to flow across a capacitor". I think it means that a capacitor doesn't allow current to flow when it's charged. What role does it have in circuits like motherboards, graphics cards, soundboards, etc. which works on DC current?

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That's a very narrow view, where you only look at the capacitor and not the surrounding circuit. – starblue Jun 10 '11 at 11:38
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most every answer will discuss a number of things like decoupling, the important note, computer motherboards ideally have a DC supply, but they are not simple DC machines. There are billions of pulses of power consumption a second on a modern processor. – Kortuk♦ Jun 10 '11 at 19:51
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A purely DC circuit would be one that has always been on, and always will be. – Nick T Jun 11 '11 at 15:51
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@Kortuk's comment is probably the most important take away from this question. In reality there really is no such thing as a purely DC circuit. – Mark Jun 12 '11 at 2:14

4 Answers

up vote 15 down vote

Capacitors in DC circuits have many roles, such as:

  • Decoupling - small reservoirs of power for rapid power responses
  • Noise suppression - reduce EMI by filtering it
  • Timing circuits - RC networks for clock signals, etc
  • and many, many more.
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I think the confusion here is that if you need decoupling it is not what a textbook in circuits means by a DC circuit. – Kortuk♦ Jun 10 '11 at 20:25
up vote 14 down vote

Decoupling capacitors are like an energy reservoir. The distance from the power supply can be rather long and when a component suddenly needs extra power the inductance of the PCB traces prevent this power to come quickly enough from the power supply. If you don't have decoupling capacitors this may cause a dip in the supply voltage. The nearby decoupling capacitor bridges this dip.

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up vote 11 down vote

That's a very misleading statement, so I suspect that it's context was important. It's only true for DC in the theoretical sense, when there is no change, ever to the voltage or current. All practical circuits are switched on at some point and have pulsed and transient currents. That's when the capacitor acts as local storage to supply current to the ICs quickly, before the power supply can act.

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up vote 1 down vote

The presupposition of your question is incorrect. Motherboards and graphics cards are about as far away from DC as you can get in home electronics (OK, your microwave oven might be a little farther, but not by much). Motherboards and graphics cards generate and use RF energy. That is 'RF' as in "Radio Frequency" AC. Yes, they are supplied by a DC source, but that is about where the DC part ends.

Most of those caps are there to keep the RF noise that your MOBO or GPU generate from traveling to places it shouldn't and thereby destroying the functionality of the MOBO/GPU.

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Microwave is a wide band so modern cpu clock rates definitely fall within. I guess the PC isn't so far way from the microwave after all. ;). @Sachin Think of a flashlight as a DC circuit - as soon as the power is applied it remains constant (until the chemical reaction in the batter falls off and your voltage drops). Stick a multimedia probe (don't really do it - just a mental exercise) on any tp on a mobo. You will most likely NOT read a steady DC value. – SRM Jun 10 '11 at 22:19
@SRM --> I was particularly thinking of the (IIRC) 2.45GHz at high power for microwave ovens; but agreed in any case. – Vintage Jun 14 '11 at 22:16

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