Application of a Junction Diode as a Rectifier

The concept of Direct Current and Alternating Current is not alien to many. Devices, invariably have clear instructions about using AC/DC current. However, there are times when you might have an AC power point but need to connect a device that requires a DC. This is when a Rectifier steps in. In simple words, a Rectifier converts Alternating Current into Direct Current.

The Volt-Ampere characteristics of a junction diode explain how current is passed through the diode only when it is ‘forward biased’. Hence, if an alternating voltage is applied across a junction diode, then the current will flow only in the part where it is forward biased. This property of a junction diode can be used to rectify alternating voltage/ current. The circuit used for this purpose is a Rectifier.

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A junction diode can be used as a rectifier in two ways:

Half-wave Rectifier

Look at the following diagram:

In the above circuit, an alternating voltage is applied across a junction diode connected to a load in a series connection. In this case, a voltage will appear across the load only during those half cycles of the AC input when the diode is forward biased. This circuit, which rectifies only one half of the input current is a Half-wave Rectifier.

The alternating current is supplied at points A and B. During the alternating cycle, the diode is forward biased when the voltage at point A is positive. This is when the diode conducts. On the other hand, when the voltage at point A is negative, the diode is reverse biased and it doesn’t conduct. For all practical purposes, the reverse saturation current can be considered zero since it is negligible.

Hence, an output voltage is available only through one half of the input cycle. Also, no current is available in the other half. Hence, the output still varies between positive and zero but the negative cycle is cut off and the output voltage is said to be rectified.

Full-wave Rectifier

Look at the following diagram:

In the above circuit, two junction diodes are connected to a load. This circuit gives out in both positive and negative halves of the AC cycle. Hence, it is a Full-wave Rectifier. In this circuit, the p-sides of both the diodes are connected to the input. And, the n-sides are connected together and connected to the load.

Also, the mid-point of the transformer is connected to the load to complete the circuit. This mid-point of connection is also called Center tap and hence, the transformer is called Center tap transformer.

The reason two diodes are connected is very simple. One diode rectifies the voltage for one half of the cycle while the other diode rectifies it for the other half. Therefore, the output between their common terminals and the centre-tap of the transformer becomes a full-wave rectifier output. Let’s see how this works-

If the voltage at point A is positive, then that at point B is negative. In such a scenario, the diode D1 is forward biased while D2 is negatively biased. Hence, D1 conducts while D2 blocks the current. Hence, during the positive half of the input AC cycle, we get output current.

Subsequently, the voltage at point A becomes negative and that at point B becomes positive. In such a scenario, D2 conducts while D1 blocks the current. Hence, we get an output current in the negative half of the input AC cycle too.

Since the circuit rectifies both the halves of the input voltage, it is called Full-wave Rectifier. But, the output is pulsating and not steady. To derive a steady DC output, a capacitor is connected across the output terminals (parallel to the load).

Role of a Capacitor

A capacitor filters out the AC ripple and provides pure DC output. Here is how it works:

As can be seen in the circuit above, a capacitor is connected parallel to the load. When the voltage across the capacitor rises, it gets charged. It discharges only when a load is connected to it. On discharge, the voltage across it falls.

As the AC cycle changes and the second diode kicks in, the capacitor charges again to its peak value. This is again discharged due to the presence of the load.

It is important to note here that the rate of discharge depends on the inverse product of Capacitor and Resistance (or load). To increase the discharge time and get a steady DC output, large capacitors are connected. The idea is to obtain an output voltage close to the peak voltage of the rectified current. You can find such filters in most power supplies.

Solved Examples for You

Q1. What is a half-wave rectifier?

Ans. A half wave rectifier is a circuit which converts one half of the alternating input voltage into direct voltage. This is achieved by using a junction diode in series with a load. During the positive cycle of the input alternating voltage, the diode is forward biased and conducts current. Whereas during the negative cycle it shuts off and no current is passed.

Q2. What is a full-wave rectifier?

Ans. A full wave rectifier is a circuit which converts the entire alternating input voltage into direct voltage. This is achieved by using two junction diodes. The p-sides of the diode are connected to the input while the n-sides are connected together and along with the centre, tap form the output.

During the positive cycle of the alternating voltage, one diode conducts while the other doesn’t. This is reversed during the negative cycle. Hence, both positive and negative cycles are converted into a direct voltage.