# What is Unilateral and Bilateral Elements? - Examples & Differences

An electrical element or component is the basic building block of any electrical circuit. An electrical circuit is the interconnection of several electrical components. Electrical elements will generate, consume, transfer, or control the electrical energy in an electrical circuit.

Depending upon the properties and behavior electrical elements are classified into, active and passive elements, linear and non-linear elements, unilateral and bilateral elements, time-variant and time-invariant elements, and lumped and distributed elements.

In this article, let us learn about unilateral and bilateral elements with examples and differences, also let us learn about what is unilateral and bilateral circuits.

## What is a Unilateral Element?

An electrical element that has different properties and characteristics for the two possible current flow directions is called a unilateral element. In simple words, the V-I characteristics of a unilateral element depend upon the direction of the supply current or voltage. When the direction of the supply voltage or current changes the properties of the element also changes.

A unilateral element allows current only in one direction i.e., when the polarity of the voltage across the element changes the magnitude of the current flowing through the element gets affected.

### Examples of Unilateral Element :

Examples of unilateral elements are diodes, SCR, transistors, etc.

A diode is a two-terminal semiconductor device that allows current only in one direction. When the diode is forward biased i.e., the anode terminal of the diode is connected to the positive terminal of supply and the cathode to the negative terminal of supply, it will be in an ON state allowing current through it.

But when the diode is reverse biased i.e., the anode terminal is made negative with respect to the cathode, it will be in an OFF state where it blocks the current flow. Thus when the polarity of the diode is changed its properties or characteristics changes.

An SCR conducts current in forward conduction mode i.e., when its anode is made positive with respect to the cathode provided a positive gate pulse. But in reverse blocking mode i.e., when the anode terminal of SCR is made negative with respect to the cathode, no current flows through it. Thus when the polarity of SCR is changed its characteristics change making them a unilateral device.

## Unilateral Circuit :

An electrical circuit that consists of one or more unilateral elements is called a unilateral circuit. In simple a circuit having at least one unilateral element is regarded as a unilateral circuit. A diode rectifier is an example of a unilateral circuit. The below shows the operation of a half-wave rectifier with a PN junction diode connected to ac supply.

During the positive half cycle of the supply, the diode is forward biased and it conducts current connecting resistive load to the supply. Now when the negative half cycle of the supply starts the polarity of the supply reverses due to which the diode will be reverse biased. Thus, during the negative half cycle current flow in the circuit will be zero.

## What is a Bilateral Element?

An electrical element that has same properties and characteristics in both directions of current flows is called a bilateral element. Unlike unilateral elements, the V-I characteristics of a bilateral element are independent of the direction of the supply current or voltage i.e., even when the direction of the current or the polarity of the supply voltage is reversed, the characteristics of the element are not affected.

A bilateral element allows the flow of current in both directions. Bilateral elements are mostly used to control current, voltage, and power in the circuit.

### Examples of Bilateral Elements :

Examples of bilateral elements are resistors, inductors, capacitors, traic, etc.

The properties of a resistor, inductor, and capacitor remain same even when the direction of current flow through the element or the polarity of the voltage across the element changes.

A TRAIC is a 3-terminal semiconductor device that has similar characteristics for both the polarities. It is a bidirectional switch that conducts current both in the forward and reverse direction. Thus a TRAIC is also considered a bilateral element.

## Bilateral Circuit :

A bilateral circuit is an electrical circuit in which all the passive elements are bilateral. Thus a circuit made up of a resistor, inductor, capacitor, or other bilateral elements is called a bilateral circuit.

A transmission line is the best example of a bilateral circuit because the circuit characteristics remain same even if power is supplied in the reverse direction. A voltage divider circuit and a magnetic circuit are other examples of bilateral circuits.

## Difference Between Unilateral and Bilateral Elements :

Unilateral ElementsBilateral Elements
The properties or characteristics of a unilateral element change when the direction of supply voltage or current changes.The characteristics of a bilateral element remain same even if the direction of supply voltage or current changes.
Unilateral elements allow current flow in one direction only.Bilateral elements allow current in both directions.
The impedance of unilateral elements varies when the direction of the current changes.The bilateral element almost offers the same impedance in both directions of current.
If an electrical circuit has at least one unilateral element then it is called a unilateral circuit.If all the elements are bilateral in a circuit then it is called a bilateral circuit.
Examples of unilateral elements are diode, SCR, FET, BJT, etc.Examples of bilateral elements are resistors, inductors, capacitors, traic, etc.

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