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On several circuit simulators I have seen capacitors with a "Power" figure.

Also, I have wondered how capacitive power supplies work.

Does an ideal, zero resistance, zero leakage, zero inductance capacitor dissipate any power?

See image:

cap power

Link to simulation of this schematic

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  • \$\begingroup\$ Can you show this "Power" figure? \$\endgroup\$ Oct 19 '10 at 11:56
  • \$\begingroup\$ See attached image. \$\endgroup\$
    – Thomas O
    Oct 19 '10 at 12:00
  • \$\begingroup\$ Note that in the simulation, the P value alternates between negative and positive \$\endgroup\$
    – endolith
    May 14 '14 at 14:26
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No, they don't dissipate energy, but they do store it. So energy can flow into a capacitor, and remain there, and then flow back out. Since power is the rate at which energy is used or moved, the power will be non-zero whenever energy is moving. But it's not being dissipated (converted into heat). It's just being moved around and stored.

It's important to remember the difference between instantaneous power and average power. A 1 ohm resistor with 1 VRMS AC across it will dissipate an average power of 1 W, for instance, but the instantaneous power will vary with the waveform:

average vs instant power for a resistive load

With a reactive element (capacitor or inductor) as the load, the power will fluctuate between positive and negative as energy flows in and out, but the average will be zero:

average vs instant power for an inductive (i think) load

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  • \$\begingroup\$ In the image you posted, the average power seems to be non-zero... so where is all this power going? \$\endgroup\$
    – Thomas O
    Oct 19 '10 at 14:54
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    \$\begingroup\$ That's a resistive load. The electrical energy is being converted into heat energy. Energy flows, power doesn't. Power is the rate at which energy flows. \$\endgroup\$
    – endolith
    Oct 19 '10 at 14:55
  • \$\begingroup\$ amasci.com/miscon/energ1.html \$\endgroup\$
    – endolith
    Oct 19 '10 at 15:34
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    \$\begingroup\$ So do you mean If we make circuit using Ideal Capacitors and Inductors there wont be any power consumed at all ? \$\endgroup\$
    – rahulb
    May 14 '14 at 10:31
  • \$\begingroup\$ What about filter circuits then ? \$\endgroup\$
    – rahulb
    May 14 '14 at 10:31
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An ideal capacitor would not dissipate any power. Real capacitors dissipate a small amount of power whenever current flows through them, due to ohmic losses. Also, when operated under continuous AC there are dielectric losses, which are minor at power line frequencies but can get significant at higher frequencies, depending on the type of cap.

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    \$\begingroup\$ So the power figure is the power the capacitor is supplying? What happens when the power figure is negative, is that the power it is consuming? \$\endgroup\$
    – Thomas O
    Oct 19 '10 at 11:46
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    \$\begingroup\$ On average, a capacitor can't supply power. In an AC circuit, a capacitor can supply power during part of the cycle, but only because it absorbed power in the previous half of the cycle. \$\endgroup\$
    – JustJeff
    Oct 19 '10 at 16:39
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    \$\begingroup\$ In the case of your example, yes, the power is fluctuating between positive and negative because the capacitor is alternately absorbing and then supplying energy. \$\endgroup\$
    – endolith
    Oct 19 '10 at 19:21

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