Understanding reactance, capacitors explanation.

Discussion in 'General Electronics Chat' started by danielb33, Oct 24, 2012.

  1. danielb33

    Thread Starter Member

    Aug 20, 2012
    105
    0
    I do not understand reactance. What is the resistance to? A capacitor exhibits a certain resistance to certain frequencies. If I have a 1uF cap, how can I tell the resistance to straight DC, a 1KHz, a 100KHz, and 1 MHz signal? I know the equation is 1/(s* PIE * capacitance * F). Does this mean that at any given F, the R is to relative to that F???

    Thanks for clarifying.
     
  2. mbohuntr

    Active Member

    Apr 6, 2009
    413
    32
    Reactance is an AC issue. Try this visualization, A sine wave on a scope is displaying the AC voltage. There is another invisible wave form that follows the waveform in lock step. (current) In a purely reactive circuit, the invisible sine wave is off (left or right) by 90 degrees.

    This will negatively affect the amount of current available at the peak of the voltage sine save and is lost power. Does this help?

    http://www.allaboutcircuits.com/vol_2/chpt_4/2.html
     
    Last edited: Oct 24, 2012
  3. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Yes, the reactance (not resistance) of a capacitor (or inductor) is a function of frequency. The reactance of a capacitor goes down with frequency, being infinity at DC and near zero at a high frequency. The inductor is opposite, with a reactance of zero at DC and a high value at a high frequency.

    Reactance is different from resistance, although they both impede current. Resistance is caused by a resistive component in the circuit and dissipates power when a current goes through it. Voltage and current are in phase.

    Reactance does not dissipate power when carrying current. That is why the voltage and current are out of phase with a reactance. If you multiply instantaneous current through by instantaneous voltage across a reactor to get its power, the result will always be zero due to the phase shift between voltage and current.

    The combination of resistance and reactance in a circuit is impedance and it is, in general, a function of frequency.
     
  4. electron_prince

    Member

    Sep 19, 2012
    93
    3
    do you know about capacitor? capacitor is a device which can store charge. There is a dielectric between capacitors plate and so current can't flow through it. If you connect the capacitor with the battery (dc source) the electrons from the negative plate of battery will go to the plate of capacitor. an equal but of opposite polarity charge will induce on the other plate of the capacitor. Because of equal and opposite charge on both plates of capacitor there built a voltage difference. As the capacitor charges the voltage across capacitor increases exponentially.

    because of increase in capacitor voltage, the charging current decreases exponentially and when the capacitor is at voltage equal to the battery, the charging current vanishes completely and no current flows in the circuit.

    This whole process take place almost instantly and so we say that the capacitor provides infinite resistance to dc current. But actually current flows at the start which gradually decreases to zero.

    Now when you connect an ac supply, for one cycle it will charge the capacitor and for the other cycle it will discharge the capacitor. hence the current can flow. but it the frequency is low, i.e if it takes time to change its polarity then the capacitor will get slightly charge and hence we'll get less current.

    but if the frequency is high, i.e, if the source changes its polarity rapidly, then capacitor can't get charge for that particular half cycle and we get high current.

    current through capacitor is given by the formula

    i = C dV/dt

    Sorry for bad english. I tried my best to explain why reactance of capacitor is low for high frequency and high for low frequency.
     
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