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ZENER DIODE THEORY:-A zener diode is heavily doped p-n junction diode, specially made to operate inthe break down region. A p-n junction diode normally does not conduct whenreverse biased. But if the reverse bias is increased, at a particular voltage itstarts conducting heavily. This voltage is called Break down Voltage. High currentthrough the diode can permanently damage the device

To avoid high current, we connect a resistor in series with zener diode. Once thediode starts conducting it maintains almost constant voltage across the terminalswhat ever may be the current through it, i.e., it has very low dynamic resistance.It is used in voltage regulators.

CHARACTERSTICS OF P-N JUNCTION DIODE THEORY:-A p-n junction diode conducts only in one direction. The V-I characteristics of thediode are curve between voltage across the diode and current through the diode.When external voltage is zero, circuit is open and the potential barrier does notallow the current to flow. Therefore, the circuit current is zero. When P-type(Anode is connected to +ve terminal and n- type (cathode) is connected to veterminal of the supply voltage, is known as forward bias. The potential barrier isreduced when diode is in the forward biased condition. At some forward voltage,the potential barrier altogether eliminated and current starts flowing through thediode and also in the circuit. The diode is said to be in ON state. The currentincreases with increasing forward voltage.When N-type (cathode) is connected to +ve terminal and P-type (Anode) isconnected ve terminal of the supply voltage is known as reverse bias and thepotential barrier across the junction increases. Therefore, the junction resistancebecomes very high and a very small current (reverse saturation current) flows inthe circuit. The diode is said to be in OFF state. The reverse bias current due tominority charge carriers.

HALF -WAVE RECTIFIER THEORY: -

During positive half-cycle of the input voltage, the diode D1 is in forward bias and

conducts through the load resistor R1. Hence the current produces an outputvoltage across the load resistor R1, which has the same shape as the +ve halfcycle of the input voltage.During the negative half-cycle of the input voltage, the diode is reverse biasedand there is no current through the circuit. i.e., the voltage across R1 is zero. Thenet result is that only the +ve half cycle of the input voltage appears across theload. The average value of the half wave rectified o/p voltage is the valuemeasured on dc voltmeter.For practical circuits, transformer coupling is usually provided for two reasons.1. The voltage can be stepped-up or stepped-down, as needed.2. The ac source is electrically isolated from the rectifier. Thus preventing shockhazards in the secondary circuit.

FULL WAVE RECTIFIER THEORY:-The circuit of a center-tapped full wave rectifier uses two diodes D1&D2. Duringpositive half cycle of secondary voltage (input voltage), the diode D1 is forwardbiased and D2is reverse biased.

The diode D1 conducts and current flows through load resistor RL. Duringnegative half cycle, diode D2 becomes forward biased and D1 reverse biased.Now, D2 conducts and current flows through the load resistor RL in the samedirection. There is a continuous current flow through the load resistor RL, during

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both the half cycles and will get unidirectional current as show in the modelgraph. The difference between full wave and half wave rectification is that a fullwave rectifier allows unidirectional (one way) current to the load during theentire 360 degrees of the input signal and half-wave rectifier allows this onlyduring one half cycle (180 degree).BRIDGE RECTIFIER THEORY: -

The Bridge rectifier is a circuit, which converts an ac voltage to dc voltageusing both half cycles of the input ac voltage.

The circuit has four diodes connected to form a bridge. The ac inputvoltage is applied to the diagonally opposite ends of the bridge. The loadresistance is connected between the other two ends of the bridge. For thepositive half cycle of the input ac voltage, diodes D1 and D2 conduct,whereas diodes D3 and D4 remain in the OFF state. The conducting diodeswill be in series with the load resistance RL and hence the load current flowsthrough RL. For the negative half cycle of the input ac voltage, diodes D3 andD4 conduct whereas, D1 and D2 remain OFF. The conducting diodes D3and D4 will be in series with the load resistance RL and hence the currentflows through RL in the same direction as in the previous half cycle. Thus abi-directional wave is converted into a unidirectional wave.