Page 280 - Mechatronics with Experiments

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in the form of heat by a triac. SCR and triac components are generally used to switch AC
loads. The amount of power that is allowed to conduct is controlled by the timing gate
voltage of SCR and triac relative to the AC voltage applied between terminals A and K.

Example Consider the Zener diode application circuit shown in Figure 5.11b. Let
us assume that V = 24 V, R = 1000 Ω, V = 12 V, and R varies between 1000 Ω and
s s Z L
2000 Ω. Let us determine the currents in each branch of the circuit, voltage across the
load, and power dissipated across the Zener diode. The Kirchoff’s voltage law in the loop
including the power supply and the zener diode gives,

V = R ⋅ i + V (5.107)
s s s Z
24 = 1000 ⋅ i + 12 (5.108)
s
24 − 12
i = = 0.012 A = 12 mA (5.109)
s
1000
Since the voltage drop across the Zener diode is limited to V = 12 V, the same voltage
Z
potential exists across the parallel load resistor,
V = V = R ⋅ i L (5.110)
Z
L
R L
12 = R ⋅ i L (5.111)
L
12
i = (5.112)
L
R L
when R = 1000 Ω, i = 12.0 mA, and when R = 2000 Ω, i = 6.0 mA. Since
L L L L
i = i + i Z (5.113)
s
L
The current across the diode is
i = i − i (5.114)
Z s L
which varies as the load resistance varies. In other words, the current across the zener
diode is i = 0.0 mA when R = 1000 Ω and i = 6.0 mA when R = 2000 Ω. The Zener
L
L
Z
Z
diode provides a constant 12 V voltage across the load resistor. As the load resistor varies,
the Zener diode dumps the excess current while providing the V = 12 V constant voltage
Z
across the two terminals. If R < 1000 Ω, that is R = 500 Ω, the diode does not conduct.
L L
Assume that the diode is not conducting, then, i = 0,
Z
V s 24
i = i = = = 0.016 A = 16 mA (5.115)
s
L
R + R s 1500
L
V = R ⋅ i = 500 Ω ⋅ 16 mA = 8V < V (5.116)
L L L Z
So, the Zener diode acts as a component to dump the excess voltage, but does not make
up for lower voltages. The maximum power dissipated across the Zener diode in this
example is
P = V ⋅ i = 12 V ⋅ 6mA = 72 mW (5.117)
Z
Z
Z
Therefore, a Zener diode with 1∕4 W power rating and 12 V breakdown voltage rating
would be sufficient for this circuit.
Figure 5.12 shows the application of diodes for voltage surge protection due to
inductive loads. Such a use of diodes is called free-wheeling diodes. When a diode is placed
parallel to the transistor in motor control applications, it is called a by-pass diode. When
there is an inductive load (a coil winding of the conductor, i.e., in relays, solenoids, motors),
sudden switching of the current either by mechanical switches or electronic switches