Uses Of Diode in a Circuit

In this article, we will discuss the concept behind some of the most frequently asked questions about diode in academics as well as interviews for Electrical Engineer, Electronics Engineer and other related in engineering fields like, How to use a Diode in a circuit and how it behaves for DC supply as well as for AC supply?

This article is specifically designed to provide a practical approach to using a diode in electronic circuits.

A Diode is a very basic and important component in electronics. It has a unidirectional nature of current conduction i.e. current only flows in one direction only. A diode conducts current only when it is forward biased and it does not allow conduction of current into reverse bias.(discussed in detail in working of diode)

In the above statement the conduction of current indicates that a forward biased diode acts as a short circuit so that conduction of current takes place while a reverse biased diode acts as an open circuit.

A forward biased diode only conducts if the voltage applied to the terminals of the diode is greater than the threshold voltage or Knee voltage. This knee voltage offers a small voltage drop in the path of conduction of current hence a forward biased diode acts as a short circuit with a small voltage drop.

Working of Diode in a Circuit

Let us understand the above discussion with the help of a circuit. Consider a circuit as shown in given figure, in which a load is connected to DC supply through a diode.

In the above figure we see that the voltage supplied to the load is V, which positive terminal is connected to the anode of the diode and its negative terminal is connected to the cathode of the diode through the load. Means the diode is forward biased and it is ready to start conduction.

If the applied voltage to the diode is greater than the threshold voltage of diode V_T then this diode acts as a short circuit with a voltage drop V_T and starts conduction as shown in the given figure.

The voltage drop V_T across diode indicates that there is some energy dissipated in the diode, hence a diode is characterized as a passive element.

But if we reverse the polarity of the voltage applied to the circuit then the diode does not allow conduction of current and it behaves like an open circuit as shown in the given figure.

If we change the polarity of the voltage applied to the circuit continuously in every 20 milli-second like AC supply then, how does the diode behave in a circuit?

Working of Diode in AC and DC

In case of DC supply the behavior of a diode in a circuit is the same as discussed above. As DC supply has constant polarity and direction throughout the time until it does not change intentionally.

In case of AC supply the polarity and the direction of supply changes continuously in every 20 milli-second. In that case a diode conducts for that 20 milli-seconds for which the supply is positive and it does not conduct for that 20 milli-seconds for which the supply is negative.

If we analyze this behavior of the diode then we find that it acts like a switch having switching frequency is the same as that of the supply frequency.

For positive half of AC supply (20ms) Diode D is on.

For negative half of AC supply (next 20ms) Diode D is off.

Let us understand the working of a diode in AC by a circuit.

Consider a circuit as shown in the given figure, in which a resistive load having resistance R is connected to AC supply VS through a diode.

In the above circuit, if positive half of the AC supply is given, then the terminal a of the diode is positive with respect to the terminal b. That means the diode is forward biased and it conducts. In this case the load voltage VL is equal to the positive half of the supply voltage Vs.

When negative half of the AC supply is given to the circuit, then the terminal a of the diode is negative with respect to the terminal b. That means the diode is reverse biased and it does not conduct. In this case the load voltage VL is equal to zero.

If a continuous pulse of AC signal is given to the above circuit, then the output of the circuit is shown as in the given figure.

If we analyze the output waveform of the above circuit then we will understand that in this circuit a diode works as a rectifier (A rectifier is an electronic circuit that converts AC signal into pulsating DC). The application of a diode in a rectifier circuit is very basic and the important application of a diode.

Above circuit which we will discuss is the half wave rectifier circuit. As we will see in this circuit only the positive half of the AC supply is rectified or the circuit only works for the positive half of the AC supply.

Apart from the above discussion, there are several most popular applications of diodes, some of which are listed below.