Capacitors and Inductors
Their responses to changes in voltage

NMSU-Grants, Electronics/Electrical Department

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A sudden change in voltage can occur when a switch is closed or opened. It can also occur when a squarewave is present. (The following properties don't apply to gradual changes, such as occur when there is a sinewave present.) 

Inductors

The following are true when there is a sudden change in voltage across an ideal inductor:
  • Right when the sudden change occurs, the inductor acts like an open circuit.
  • Immediately after the sudden change is over, the voltage across the inductor starts to decrease.
  • If the voltage across the inductor remains constant for a long time, the inductor will act like a short circuit.

Here are some other properties of inductors experiencing sudden changes in voltage:
  • When a sudden voltage change across an inductor is such that the inductor's magnetic field will be reduced, the inductor will react by creating a voltage opposing the change. Sometimes very large voltages can be produced by this reaction, and other components can be damaged.
    • In DC circuits, a diode is often connected in parallel with the inductor to prevent voltage spikes.
    • In AC circuits, a capacitor is often connected in parallel with the inductor to prevent voltage spikes.
  • Inductors are also often used to prevent high-frequency AC from going where it is undesired. External computer cables often have inductors in them. These inductors keep high frequency current from interfering with TVs, etc.

Real inductors have a certain amount of resistance due to the length of wire needed to form the coil. Some inductors have quite large resistances (hundreds of ohms). Others have very small resistances (tenth's of an ohm or less).  To create an inductor that has low resistance, large diameter wire needs to be used. Thus low-resistance inductors with large inductance values are physically large. (To obtain large inductance values, a large number of turns of wire is needed.)
 

Capacitors

The following are true when there is a sudden change in voltage across an ideal capacitor:
  • Right when the sudden change occurs, the capacitor acts like a short circuit (a piece of wire).
  • Immediately after the sudden change is over, the capacitor starts charging up.
  • If the voltage across the capacitor remains constant for a long time, the capacitor will act like an open circuit.
Here are some other properties of capacitors experiencing sudden changes in voltage.
  • Large capacitors can absorb large amounts of current when there is a sudden change in voltage. In some circumstances, these large currents can damage other components (especially semiconductors).
  • Capacitors can supply large amounts of current when there is a sudden change in voltage. Often this is a desireable feature. However, it can also damage other components.
Real capacitors have a certain amount of inductance. This inductance keeps the capacitor from acting like zero ohms when sudden changes occur. Ceramic and mica capacitors have little inductance, and thus they act almost like zero ohms during sudden voltage changes. Tubular capacitors have more inductance, but this inductance is still quite small.

The wires supplying DC power to digital circuit boards have a certain amount of inductance that keeps current from flowing freely whenever the circuitry has a sudden demand for current. On such circuit boards, it's common practice to have two parallel capacitors connected across the DC supply lines. These capacitors are often referred to as decoupling capacitors (because they decouple the circuit board from the inductance present in the DC supply wires going to it). Such capacitors keep the DC voltage more constant than it would otherwise be.  They supply brief bursts of current when the digital circuitry switches on and off.  One of these capacitors is usually ceramic and the other tubular (or some other type that has large capacitance, but also significant inductance). Ceramic capacitors have low capacitance values, but as mentioned earlier, they also have the very desireable low inductance needed. Thus, when a ceramic capacitor is in parallel with a tubular capacitor connected across DC supply lines, the ceramic capacitor is able to supply current quickly. However, because ceramic capacitors have only small amounts of capacitance, they can't supply current for very long. One the ceramic decoupling capacitor has given up it's charge, the tubular capacitor takes over. By this time, voltage across the tubular capacitor will have overcome the capacitor's inductance. Because tubular capacitors can have large capacitance values, they are able to keep supplying current until the sudden demand for current is over. By this time, inductance in the power supply wires will have been overcome by the voltage from the main power supply. The decoupling capacitors are then able to recharge themselves and be ready to supply the next surge in current.
 
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