Although in our daily life we use A.C. current devices. But rectifier is a Electronic device which converts A.C. power into D.C. power. The study of the junction diode characteristics reveals that the junction diode offers a low resistance path, when forward biased, and a high resistance path, when reverse biased. This feature of the junction diode enables it to be used as a rectifier. The alternating signals provides opposite kind of biased voltage at the junction after each half-cycle. If the junction is forward biased in the first half-cycle, its gets reverse biased in the second half. It results in the flow of forward current in one direction only and thus the signal gets rectified. In other words, we can say, when an alternating e.m.f. signal is applied across a junction diode, it will conduct only during those alternate half cycles, which biased it in forward direction.
TYPE OF RECTIFIER
Mainly we have two types of rectifier :
1. Half wave rectifier.
2. Full wave rectifier.
JUNCTION DIODE AS HALF WAVE RECTIFIER
When a single diode is used as a rectifier, the rectification of only one-half of the A.C. wave form takes place. Such a rectification is called half-wave rectification. The circuit diagram for a half-wave rectifier is shown in Fig. Principle :
It is based upon the principle that junction diode offers low resistance path when forward biased, and high resistance when reverse biased. Arrangement :- The A.C. supply is applied across the primary coil(P) of a step down transformer. The secondary coil(S) of the transformer is connected to the junction diode and a load resistance RL. The out put D.C. voltage is obtained across the load resistance(RL)
Suppose that during the first half of the input cycle, the junction diode gets forward biased the conventional current will flow in the direction of the arrow-heads. The upper end of RL will be at positive potential w.r.t. the lower end. During the negative half cycle of the input a.c. voltage, the diode is reverse biased. No current flows in the circuit, and therefore, no voltage is developed across (RL). Since only the positive half cycle of the input appears across the load, the a.c. input is converted into pulsating direct current (d.c.).
Disadvantage of Half-Wave-Rectifier :
1. Half wave rectification involves a lot of wastage of energy and hence it is not preferred. 2. A small current flows during reverse bias due to minority charge carriers. As the output across (RL) is negligible. 3. The resulting d.c. voltage is not steady enough for some purpose. The following device is used when a very steady d.c. voltage is required.
JUNCTION DIODE AS A FULL WAVE RECTIFIER
A rectifier which rectifies both waves of the a.c. input is called a full wave rectifier. Principle :- It is based upon the principle that a junction diode offers low resistance during forward biased and high resistance, when reverse biased.
Difference from half-wave-rectifier :- The main difference is that in full wave rectifier we use two diodes. For this when we apply a.c. current to the rectifier then the first half wave get forward biased due to first diode. And when the second half wave comes. Then at that time the second diode comes in action and gets forward biased. Thus output obtained during both the half cycles of the a.c. input Arrangement :- The a.c. supply is applied across the primary coil(P) of a step down transformer. The two diodes of the secondary coil(S) of the transformer are connected to the P-sections of the junction diodes (D1) and (D2). A load resistance (RL) is connected across the n-sections of the two diodes and at centre of the secondary coil. The d.c. output will be obtained across the load resistance (RL).
Suppose that during first half of the input cycle, upper end of (S) coil is at positive potential. And lower end is at negative potential. The junction diode (D1) gets forward biased, while the diode. (D2) get reverse biased. When the second half of the input cycle comes, the situation will be exactly reverse. Now the junction diode (D2) will conduct. Since the current during both the half cycles flows from right to left through the load resistance (RL) the output during both the half cycles will be of same nature. Thus, in a full wave rectifier, the output is continuous but pulsating in nature. However it can be made smooth by using a filter circuit.
REVERSE BIASING ON A JUNCTION DIODE
A P-n junction is said to be reverse biased if the positive terminal of the external battery B is connected to n-side and the negative terminal to p-side of the p-n junction. In reverse biasing, the reverse bias voltage supports the potential barrier VB. (Now the majority carriers are pulled away from the junction and the depletion region become thick. There is no conduction across the junction due to majority carriers. However, a few minority carriers (holes in n-section and electrons in p-section) of p-n Junction diode cross the junction after being accelerated by high reverse bias voltage. Since the large increase in reverse voltage shows small increase in reverse current, hence, the resistance of p-n junction is high to the flow of current when reverse biased.
1. Transformer :
It is device which is used to increase or decrease the alternating current and alternating voltage. For the rectifier, it may be step down or step up. 2. Junction Diode :
It is made up of p-type and n-type semiconductor which conducts when the p terminal of diode to connect to positive terminal of battery and n region is connected to negative terminal of battery i.e. during forward biased and does not conduct during reverse biased.