Varistor (Nonlinear Resistor)

It is a voltage-dependent metal-oxide material whose resistance decreases sharply with increasing
voltage. The relationship between the current flowing through a varistor and the voltage applied
across it is given by the relation : i = kenwhere i = instantaneouscurrent,e is the instantaneous
voltage and 11is a constant whose value depends on the metal oxides used. The value of 11for
silicon-carbide-based varistors lies between 2 and 6 whereas zinc-oxide-based varistors have a value
ranging from 25 to 50.
The zinc-oxide-based varistors are primarily used for protecting solid-state power supplies from
low and medium surge voltage in the supply line. Silicon-carbide varistors provide protection against
high-voltage surges caused by lightning and by the discharge of electromagnetic energy stored in the
magnetic fields of large coils.
1.20. Short and Open Circuits
When two points of circuit are connected togetherby a thick metallic wire (Fig. 1.38),they are said
to be short-circuited. Since 'short' has practically zero resistance,it gives rise to two important facts :
(i) no voltage can exist across it because V =IR =I x 0 =0
(ii) currentthroughit (calledshort-circuitcurrent)is very large(theoretically,infinity)
Two points are said to be open-circuitedwhen there is no direct connectionbetweenthem
(Fig. 1.39), Obviously,an 'open' representsa break in the continuityof the circuit. Due to this break .
(i) resistance between the two points is infinite.
(ii) there is no flow of current between the two points.
1.21. 'Shorts' in a Series Circuit
Since a dead (or solid) short has almost zero resistance, it causes the problem of excessivecurrent
which, in turn, causes power dissipation to increase many times and circuit componentsto bum out.In Fig. 1.40 (b), 3-Q resistor has been shorted out by a resistanceless copper wire so that RCD=O.
Now, total circuit resistance R= 1 + 2 + 0 = 3 Q. Hence, / = 12/3 = 4 A and P =:42X3 = 48 W.
Fig. 1.40 (c) shows the situation where both 2 Q and 3 Q resistors have been shorted out of the
circuit. In this case, 2
R = 1 Q, / =12/1 = 12 A and P =12 xl = 144 W
Because of this excessive current (6 times the nonnal value), connecting wires and other circuit
components can become hot enough to ignite and bum out.
1.22. 'Opens' in a Series Circuit
In a nonnal series circuit like the one shown in Fig. 1.41 (a), there exists a current flow and the
voltage drops across different resistors are proportional to their resistances. If the circuit becomes
'open' anywhere, following two effects are produced:
(i) since 'open' offers infinite resistance, circuit current becomes zero. Consequently, there is
no voltage drop across RI and R2.
(ii) wholeof the appliedvoltage(i.e. /00 V in this case) isfelt across the 'open' i.e. across terminalsA and B [Fig. 1.41 (b)]. The reason for this is that RI and R2 V}become negligible as compared to the infinite resistance of the 'open' which has 100V practicallly whole of the applied voltage dropped across it (as per Voltage Divider
Rule of art. 1.15). Hence, voltmeter in Fig. 1.41 (b) will read nearly 100 V i.e. the supply voltage. 'Opens' in a Parallel Circuit .
Since an 'open' offers infinite resistance, there would be no current in that part of the circuit
where it occurs. In a parallelcircuit, an 'open' can occur either in the main line or in any parallelbranch.
As shown in Fig. 1.42 (a), an open in the main line prevents flow of current to all branches.
Hence, neither of the two bulbs glows. However, full applied voltage (i.e. 220 V in this case) is
available across the open.