What is the impedance of a capacitor

Impedance / impedance Z

The term "impedance" is often used as a synonym for a complex quantity. This is then the impedance Z. There are also different ways of writing Z with underscores and / or side lines for the impedance.

What is the impedance?

The impedance is a complex resistance consisting of an alternating voltage resistance and an ohmic resistance. The alternating voltage resistance consists of a reactance. In the case of a capacitor, the reactance is called capacitance and in the case of an induction (e.g. coil) it is called inductance.

The impedance differs from normal DC voltage resistance in the phase shift between current and voltage.

With Ohm's law you can calculate how much current will flow when a certain voltage is applied to a purely resistive consumer. The relationship between voltage and current is always constant with an ohmic consumer, regardless of the frequency and the applied voltage. And the phase shift of voltage and current is 0 °. This means that the values ​​of both quantities are maximum at the same time.

But now there are consumers, e.g. B. a loudspeaker that is not a pure ohmic resistor. They also contain capacitances (e.g. capacitors) and inductances (e.g. coils). These consumers have

  • a frequency-dependent resistor (reactance)
  • Phase shift by 90 °

The following applies to a capacity:

  1. The higher the frequency, the lower the capacitive reactance X becomesC..
  2. The current increases if the voltage remains the same.
  3. The alternating current leads the alternating voltage by 90 °.

The following applies to an inductance:

  1. The higher the frequency, the higher the inductive reactance X becomesL..
  2. The current decreases if the voltage remains the same.
  3. The alternating current lags the alternating voltage by 90 °.

The frequency-dependent resistance is called reactance because it provides a resistance to the current, but no real power is converted due to the phase shift. Reactors only temporarily store energy and return it (in a second half-cycle) to the voltage source.

Example: coil

The total resistance of a coil (with alternating current) is made up of the effective resistance R of the coil and the inductive reactance XL. the coil together. It is called impedance Z, impedance or total resistance Z.

The impedance Z can be calculated with the help of Ohm's law and the effective values ​​of voltage and current.

The impedance Z is also the geometric sum of the effective resistance and inductive reactance.
Important: changing sizes must be added geometrically (to the power of 2).

Can you measure the impedance?

If you were to measure the voltage and the current through the capacitive or inductive consumer, it would appear to have a resistance according to Ohm's law. You measure incorrectly because you ignore the phase shift between voltage and current. In this measurement, only the ohmic component is determined, which is always lower than the actually correct impedance.

Why do I need an impedance converter?

There can be different reasons for this. For example, to connect two circuits.

For example, the following circuit should not unnecessarily load the previous circuit. In such a case, the input impedance of the following circuit must be as high as possible, otherwise most of the voltage at the output impedance of the preceding circuit will drop even with the slightest current flow.

If optimum efficiency is to be achieved in the transmission of power, the impedances of the source and receiver must match (and the reactive component must be as low as possible). This is then called performance adjustment.

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