Capacitor Quirks

Discussion in 'Reactance and Impedance - Capacative' started by Tony R. Kuphaldt, Jun 24, 2010.

  1. Tony R. Kuphaldt

    Tony R. Kuphaldt

    Joined:
    Jun 21, 2010
    Messages:
    394
    Likes Received:
    3
    As with inductors, the ideal capacitor is a purely reactive device, containing absolutely zero resistive (power dissipative) effects. In the real world, of course, nothing is so perfect. However, capacitors have the virtue of generally being purer reactive components than inductors. It is a lot easier to design and construct a capacitor with low internal series resistance than it is to do the same with an inductor. The practical result of this is that real capacitors typically have impedance phase angles more closely approaching 90o (actually, -90o) than inductors. Consequently, they will tend to dissipate less power than an equivalent inductor.

    Capacitors also tend to be smaller and lighter weight than their equivalent inductor counterparts, and since their electric fields are almost totally contained between their plates (unlike inductors, whose magnetic fields naturally tend to extend beyond the dimensions of the core), they are less prone to transmitting or receiving electromagnetic "noise" to/from other components. For these reasons, circuit designers tend to favor capacitors over inductors wherever a design permits either alternative.

    Capacitors with significant resistive effects are said to be lossy, in reference to their tendency to dissipate ("lose") power like a resistor. The source of capacitor loss is usually the dielectric material rather than any wire resistance, as wire length in a capacitor is very minimal.

    Dielectric materials tend to react to changing electric fields by producing heat. This heating effect represents a loss in power, and is equivalent to resistance in the circuit. The effect is more pronounced at higher frequencies and in fact can be so extreme that it is sometimes exploited in manufacturing processes to heat insulating materials like plastic! The plastic object to be heated is placed between two metal plates, connected to a source of high-frequency AC voltage. Temperature is controlled by varying the voltage or frequency of the source, and the plates never have to contact the object being heated.

    This effect is undesirable for capacitors where we expect the component to behave as a purely reactive circuit element. One of the ways to mitigate the effect of dielectric "loss" is to choose a dielectric material less susceptible to the effect. Not all dielectric materials are equally "lossy." A relative scale of dielectric loss from least to greatest is given in Table below.

    Dielectric loss

    MaterialLoss
    VacuumLow
    Air-
    Polystyrene-
    Mica-
    Glass-
    Low-K ceramic-
    Plastic film (Mylar)-
    Paper-
    High-K ceramic-
    Aluminum oxide-
    Tantalum pentoxidehigh
    Dielectric resistivity manifests itself both as a series and a parallel resistance with the pure capacitance: (Figure below)

    [​IMG]

    Real capacitor has both series and parallel resistance.

    Fortunately, these stray resistances are usually of modest impact (low series resistance and high parallel resistance), much less significant than the stray resistances present in an average inductor.

    Electrolytic capacitors, known for their relatively high capacitance and low working voltage, are also known for their notorious lossiness, due to both the characteristics of the microscopically thin dielectric film and the electrolyte paste. Unless specially made for AC service, electrolytic capacitors should never be used with AC unless it is mixed (biased) with a constant DC voltage preventing the capacitor from ever being subjected to reverse voltage. Even then, their resistive characteristics may be too severe a shortcoming for the application anyway.
     
    Last edited by a moderator: Apr 10, 2014
    Tony R. Kuphaldt, Jun 24, 2010
    #1
    1. Advertisements

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Similar Threads
  1. boki
    Replies:
    0
    Views:
    1,047
  2. David Farber

    Small capacitor across a big capacitor.

    David Farber, Aug 21, 2003, in forum: Electronic Repair
    Replies:
    5
    Views:
    1,111
    Sam Lloyd
    Sep 27, 2003
  3. Robowang

    Capacitor Start, Capacitor Run Motor

    Robowang, Oct 15, 2006, in forum: Electronic Repair
    Replies:
    13
    Views:
    2,889
    Robowang
    Oct 18, 2006
  4. jack1981
    Replies:
    3
    Views:
    1,492
  5. Glenn Pure

    Li ion battery quirks

    Glenn Pure, May 2, 2004, in forum: Hobby Electronics
    Replies:
    21
    Views:
    924
    TonyP
    May 7, 2004
  6. Cheers
    Replies:
    8
    Views:
    2,168
    Malcolm Smith
    Nov 27, 2007
  7. Tony R. Kuphaldt

    Inductor Quirks

    Tony R. Kuphaldt, Jun 24, 2010, in forum: Reactance and Impedance - Inductive
    Replies:
    0
    Views:
    2,713
    Tony R. Kuphaldt
    Jun 24, 2010
  8. Tony R. Kuphaldt

    BJT Quirks

    Tony R. Kuphaldt, Jun 28, 2010, in forum: Bipolar Junction Transistors
    Replies:
    0
    Views:
    4,096
    Tony R. Kuphaldt
    Jun 28, 2010
Loading...