What is Impedance

The net effect of the combination of resistance and energy storage and release is what we call impedance. The units for the resistance and the impedance are the same, the ohm with the symbol 




Some parts like Resistor do not react with frequency whether current is AC or DC, its resistance remains same because resistors do not store energy, and the flow of current through them is fairly straightforward but some components are affected by frequency.


The two main components which is effect by frequency is Capacitor and Inductors (Coil) also called reactive components. This device not only has some resistance to current flow, but it can store energy from what comes in, and release energy too. Capacitor store energy in the form of electric field and Inductor stores energy creating magnetic field. So this adds complication to the flow of current. Capacitors have high impedance at low frequency and low impedance at high frequency while Coils (inductors) have low impedance at low frequency and high impedance at high frequency. Both have opposite property.

AC wave reaction with impedance

                                A signal is partially reflected back where the impedance changes.

To calculate the impedance of a capacitor and coil the formula is given below :

Why Impedance calculation is important ?


If the impedance of the source, cable and load are not all equal, then a fraction of the signal is reflected back to the source, wasting power capacity and creating interference.


AC Motor designer or speed controller have to keep impedance in mind.


 You may have noticed, when old fan rotates slowely then electrician replaces fan capacitor (most ceiling fan uses 2.5MFD/440V capacitor). This happens because after many years of using capacitor becomes weak (capacity decreases) and flow of AC decreases. You can slide small value of capacitor and notice speed difference.


 In a normal speaker (Mid-range) connect 10Mfd/25V capacitor in series  you will hear trible sound again replace with 1 Mfd/25V capacitor then you will hear high frequency music only (high trible sound). You will also notice adding a coil in series with speaker filters bass. Low frequency instrument like drum beat will be loud. You will find crossover network kit in DJ speaker system. A bass trible circuit in amplifier uses capacitor to pass or remove (divert) high frequency.

Crossover network kit use in sound box

                                  fig. Crossover network kit

   You will see coil is connected in series with woofer and capacitor is connected in series with dome tweeter. You will observe that piezo speaker gives high frequency, same character as capacitor. In many circuit volume is increased by adding a step up transformer.


A speaker with a lower impedance is like a bigger pipe in that it lets more electrical signal through and allows it to flow more easily.


When indicator and capacitor are connect in parallel or series (also called RC Circuit) in a circuit then a resonant (very sensitive to specific frequency) is created.