Electronics & Software – Tips & Guide


What is a displacement current in a capacitor

What is a displacement current in a capacitor?

consider a capacitor consisting of two plates with nothing between them, with air as dielectric between them and now consider that a battery is connected through the capacitor . the capacitor starts to charge and the charge starts to accumulate on both plates .

one of the plate receives a positive charge and the other plate receives a negative charge at the respective terminals of the battery and the two plates are separated by air as indicated previously.

now, due Positive and negative charges accumulated on the plates, an electric field is established between the two plates thus ionize the air between the two plates and produce the current called current of displacement . it is to note that the real electron flow does not happen here … the ionization of the air causes the displacement current.

in a capacitor, you always have a displacement current and never a conduction current under normal conditions (ie you apply a potential difference lower than the specified maximum voltage).

Conduction currents are when the electrons actually move. but in the displacement current, no load carrier is involved. it is only the variations of the electric field which are supposed to be equivalent to a current.

The capacitor contains an insulating material (called dielectric) sandwiched between two conductors. Since insulators can only carry an electric field but not moving media, the above paragraph is justified.

However, if you apply a huge voltage on a capacitor, it behaves differently. under sufficiently large potential differences, many insulators cease to isolate. that is, they conduct electricity. thus, if you apply a significant voltage beyond the specified limit, the dielectric behaves like a conductor. so that you get a conduction current in the capacitor. it happens exactly like a thunderbolt, when the potential difference between the clouds and the earth becomes so great that the atmosphere is forced to behave and the electric flash hits the ground. this is what is called breaking a capacitor

when a capacitor fails, it no longer carries the displacement current. because he’s now a conductor! like any other conductor, the electric field inside it is zero, as is the displacement current. these days, you get very compact capacitors. because of their compactness, their dielectric decomposes quite easily. For example, many current miniature capacitors are manufactured to work properly up to 60 volts, beyond which their failure occurs.

how can there be a displacement current but no conduction current in a capacitor?

Unlike resistors and inductors, electrons can not pass through the dielectric medium between the plates of a capacitor under normal conditions. but a capacitor will be considered a conducting current when used in a circuit. Indeed, instead of going directly from one plate to another through the dielectric medium, the electrons accumulate or leave one of the capacitor plates. the plate that loses electrons will develop a net positive charge and the other plate will develop a net negative charge (as the electrons accumulate there). this process of moving the electrons from one plate to the other will be considered as a normal electric current when in reality no electron was crossing the capacitor.

this displacement occurs until the potential difference between the capacitor plates is equal to that of the input voltage. such a displacement of electrons will cause the formation of an electric field in the dielectric medium which will retain the electric field even after the suppression of the input voltage because load between the pins.

This is how the displacement current across a capacitor differs from the conduction current through other passive devices. under normal conditions. A conduction current can occur during a dielectric failure.