From the V-I characteristic of a junction diode we see that it allows current to pass only when it is forward biased. So if an alternating voltage is applied across a diode the current flows only in that part of the cycle when the diode is forward biased. This property is used to rectify alternating voltages and the circuit used for this purpose is called a rectifier. If an alternating voltage is applied across a diode in series with a load, a pulsating voltage will appear across the load only during the half cycles of the ac input during which the diode is forward biased. The secondary of a transformer supplies the desired ac voltage across terminals A and B. When the voltage at A is positive, the diode is forward biased and it conducts. When A is negative, the diode is reverse-biased and it does not conduct. The reverse saturation current of a diode is negligible and can be considered equal to zero for practical purposes. (The reverse breakdown voltage of the diode must be suffi...
The current through a single resistor R across which there is a potential difference V is given by Ohm’s law I = V/R. Resistors are sometimes joined together and there are simple rules for calculation of equivalent resistance of such combination.
Two resistors are said to be in series if only one of their end points is joined (Fig. 3.13). If a third resistor is joined with the series combination of the two , then all three are said to be in series. Clearly, we can extend this definition to series combination of any number of resistors.
Two or more resistors are said to be in parallel if one end of all the resistors is joined together and similarly the other ends joined together.
Consider two resistors R1 and R2 in series. The charge which leaves R1 must be entering R2 . Since current measures the rate of flow of charge, this means that the same current I flows through R1 and R2 . By Ohm’s law:
Potential difference across R1 = V1 = I R1 , and
Potential difference across R2 = V2 = I R2 .
The potential difference V across the combination is V1 +V2 . Hence,
V = V1 + V2 = I (R1 + R2 )
This is as if the combination had an equivalent resistance R eq , which by Ohm’s law is
R eq V I ≡ = (R1 + R2 )
If we had three resistors connected in series, then similarly
V = I R1 + I R2 + I R3 = I (R1 + R2 + R3 ).
This obviously can be extended to a series combination of any number n of resistors R1 , R2 ....., Rn . The equivalent resistance R eq is
R eq = R1 + R2 + . . . + Rn
Consider now the parallel combination of two resistors (Fig. 3.15). The charge that flows in at A from the left flows out partly through R1 and partly through R2 . The currents I, I 1 , I 2 shown in the figure are the rates of flow of charge at the points indicated. Hence,
I = I 1 + I 2
The potential difference between A and B is given by the Ohm’s law applied to R1
V = I 1 R1
Also, Ohm’s law applied to R2 gives
V = I 2 R2
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