(1) At time (t1) the voltage with respect to ground will be positive (+)

and an electron current will be flowing upward through the resistor.

(2) At time (t3) the voltage with respect to ground will be negative

(-) and an electron current will be flowing downward through the resistor.

(3) Here we can see that the direction of the voltage is controlling the

direction of electron current through the resistor. So the current first flows in

an upward direction; then, as time passes, it reverses. It alternates once each

cycle or revolution of the rotor.

c. The amount of current through the resistor is determined by its value of

resistance. With a very large resistive value, only a small current will flow.

With a very small resistive value, a large current will flow. The values of

resistance lay between two extremes. The maximum value of resistance is an open

circuit; the minimum value of resistance is a short circuit. These extreme values

of resistance are approached in the rectifying diode. When the diode is forward

biased, it exhibits nearly a short circuit. When it is reverse biased, it

approximates an open circuit.

d. Figure 28 shows an AC potential being applied to a resistor and diode

connected in series. Since the voltage potential is alternating above and below

ground or zero potential, the diode will experience times of forward bias at (t1)

and reverse bias at (t2). The diode becomes forward biased any time after the

voltage potential leaves zero and goes positive. The diode becomes reverse biased

any time after the voltage potential leaves zero and goes negative.

Figure 28.

Half-wave rectifier.

(1) When the diode is forward biased, maximum current will be allowed to

flow through the resistor thereby developing a positive half cycle.

(2) When the diode is reversed biased, minimum current will be allowed

to flow through the resistor. If the minimum current approaches zero, then the

voltage developed across the resistor will also approach zero. For all practical

purposes it is said to be zero, since the value of positive voltage is much greater

than negative.

(3) This circuit is called a half-wave rectifier because only half of

the entire cycle of AC voltage is allowed to be developed across the resistor (fig

29).

OS 010, 1-P15

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