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Return to Blog# Characteristics of Inductors

## Time Constant

When a current is applied to an inductor, it takes some time for the current to reach its maximum value. After reaching its maximum value it will remain in a steady state until some other event causes the input to change. The current could not change instantaneously in an inductor, because for this to occur the current would need to change by a finite amount in zero time which would result in an infinite rate of current change, making the induced electromotive force (emf) infinite as well, and infinite voltages do not exist. High voltages can be induced across the inductors coil if the current flowing through an inductor changes vary rapidly, such as with the operation of a switch (Figure 1).

This inductor circuit has open-switch where no current flows through the inductor coil. The rate of current change in its coil is zero, making zero self-induced emf exist across the inductor. The current will flow through the circuit and slowly rise to its maximum value at a rate determined by the inductor if the switch is closed. Until the current reaches its maximum value and a steady state condition is reached, the self-induced emf across the inductor coil fights against the applied voltage. As the reactance value of the coil has decreased to zero because the rate of current change is zero in steady state, the current flowing through the coil is determined only by the DC or pure resistance of the coil windings.

## Voltage and Current

The amount of voltage that will be produced in an inductor depends on how rapidly the current through it will decrease. The induced voltage will be opposed to the change in current as described by Lenz’s Law. The voltage polarity will be oriented to keep the current at its former magnitude with a decreasing current. As shown in the image below, the amount of induced voltage produced by the inductor depends upon the rate of current change.

The only difference between the effects of a decreasing current and an increasing current is the polarity of the induced voltage. The voltage magnitude will be the same for the same rate of current change over time. High voltage will be produced if current through an inductor is forced to change very rapidly. The more rapidly current is decreased, the more voltage will be produced by the inductor in its release of stored energy to try to keep current constant.

**References**

http://www.allaboutcircuits.com/vol_1/chpt_15/2.html

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