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Diode
Diodes have a very high resistance in one direction and very low resistance in the opposite direction. This means that they allow electrical current to pass one way but not the other.
Diodes can be used to convert alternating current to direct current. This phenomenon is called rectification.
Diode as forward biased and reversed biased
Diode as forward biased: The semiconductor diode (PN-junction diode) has the tendency to allow the current to flow in one direction only from P-side to N-side (+ to -) through the PN-junction. When a battery is connectetd accross the PN-junction diode as shown in the figure below, with positive terminal of the battery connected to the P-type and negative terminal to the N-type of the diode. The diode is said to be forward biased a shown below..
The positive terminal of the battery repells holes accross the junction and attracts electrons crossing the junction. The negative terminal repells the electrons accross the junction and attracts holes crossing the junction. Thus giving rise to a large current which can be recorded using an ammeter in a circuit.
Diode as forward biased: If the polarity of the battery is reversed as shown in the figure below, holes and electrons do not cross the junction. Thus no current or a very little current flows through the circuit and the diode is said to be reverse biased.
Diode as a rectifier
A semiconductor diode allows the current to flow only in one direction. Hence a P-N junction diode can be used as a rectifier. A RECTIFIER is a device which converts A.C. in to D.C. and the process is known as rectification.
Diode as half wave rectifier
When A.C. from the supply is applied accross a resistor through the semiconductor diode “D” then current flows through the diode only when the positive half cycle of the A.C. appears at P-side (+) of the diode making it forward biased. But when negative half cycle appears at the P-side of the diode, it makes the diode reverse biased. Thus no current flows through the diode and the circuit during the negative half cycle. This means only positive half cycle will appear accross the resistor R. This type of rectification is called half wave rectification.
Diod as full wave rectifier
This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop “bridge” configuration to produce the desired output. The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below.
Fig.
The four diodes labelled D1 to D4 are arranged in “series pairs” with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below.
The positive half cycle:During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below.
Fig.
The negative half cycle: During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch “OFF” as they are now reverse biased. The current flowing through the load is the same direction as before.
Fig.
In practical circuits the ouput is smoothed by using capacitors of appropriate value.
Rectifier circuits
A circuit which is used to convert A.C. (Alternating Current) to D.C. (Direct Current) is called rectifier circuit. For this purpose we are using a single diode for half wave rectification and a combination of diodes for full wave rectification.
See the following links:
Half wave rectifier
Full wave rectifier