Q1. What is a Thermistor?

A1. A thermistor is a resistor whose resistance changes significantly with temperature.

The name comes from: THERMal + resISTOR

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Thermistors are commonly used for:

• Temperature measurement

• Temperature compensation

• Over-temperature protection

• Inrush-current limiting

• Electronic control systems

 

There are two principal types.

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a) NTC Thermistor

NTC = Negative Temperature Coefficient

For an NTC thermistor:

Temperature increases => Resistance decreases

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NTC thermistors are widely used for temperature sensing.

A simplified conceptual relationship is: R ↓ as T ↑ 

 

Example

Suppose an NTC thermistor has: R = 10 kΩ at a particular temperature.

As its temperature rises, its resistance may fall significantly.

This change can be detected by an electronic circuit and converted into a temperature measurement.

 

b) PTC Thermistor

PTC = Positive Temperature Coefficient

For a PTC thermistor:

Temperature increases => Resistance increases

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PTC devices can therefore be used for:

• Overcurrent protection

• Resettable protection

• Motor protection

• Temperature sensing

• Circuit protection

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Important distinction

A normal metallic resistor also generally has a positive temperature coefficient, but a thermistor is specifically designed to have a much stronger and more useful resistance-temperature relationship.

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Q2. What is a Varistor?

A2. A varistor is a voltage-dependent resistor. Its resistance changes according to the voltage applied across it.

The most common type is the MOV — Metal Oxide Varistor.

Varistors are primarily used for transient voltage protection.

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Normal operating condition

Under normal voltage, a varistor has relatively high resistance and carries only a small leakage current.

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Voltage surge

When the voltage rises significantly above its normal operating level, the varistor's resistance decreases sharply. It then provides a path for surge current and limits the voltage appearing across the protected equipment.

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Conceptually: R ↓ as V ↑ during the varistor's nonlinear conduction region.

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Example: Surge Protection

Consider a sensitive electronic control circuit supplied at a normal voltage.

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A voltage transient occurs due to:

• Lightning

• Switching operations

• Inductive load switching

• Fault clearing

• Other transient events

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The MOV can conduct heavily during the transient and help clamp the voltage to a safer level.

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Important point

A varistor is not simply a normal resistor whose value is manually changed. Its resistance changes nonlinearly with applied voltage.

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Q3. What is Short Circuit?

A3. A short circuit occurs when two points having different electrical potentials become connected through a very low impedance path.

An ideal short circuit is represented by: R = 0Ω.

Therefore, from Ohm's law: I = V/R, an ideal zero-resistance path would theoretically produce infinite current for a non-zero voltage.

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However, real short-circuit currents are not infinite because the source, conductors, transformers, generators and other parts of the system have finite impedance.

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A more realistic expression is: ISC = V / Zsystem

where Zsystem represents the total impedance limiting the fault current.

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Q4. What is Open Circuit?

A4. An open circuit occurs when the electrical path is interrupted.

An ideal open circuit has: R → ∞

 

Therefore, from Ohm's law: I = V/R = 0A, an ideal open circuit allows no current to flow through the open path.

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Note:

Open circuit current = 0A but voltage may exist

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Q5. What are effects of open/short conditions on series and parallel circuits?

A5. Followings are the details on how open and short circuits effect series and parallel networks.

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a) Open Circuit Condition in a Series Circuit

Consider a series circuit consists of: R1 - R2 - R3

If one component becomes open: R2 → ∞

then the complete current path is interrupted.

Therefore: I = 0A through the series circuit. 

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Example:

A control circuit contains 24 V supply → fuse → switch → relay coil → return.

If the switch becomes open, the relay coil cannot receive current.

The relay does not energize.

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b) Open Circuit Condition in a Parallel Circuit

Consider a parallel circuit consists of: R1 || R2 || R3

If one component becomes open: R2 → ∞, I2 = 0A.

But the other branches may continue operating.

Therefore: An open branch does not necessarily interrupt the entire parallel circuit.

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Note:

This is one reason parallel connections are commonly used for independent loads.

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c) Short Circuit Condition in a Series Circuit

Consider a series circuit consists of: R1 - R2 - R3

If one component becomes short: R2 → 0Ω, the component effectively becomes bypassed.

But the other components may continue operating.

Therefore: A short component does not necessarily interrupt the entire series circuit.

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d) Short Circuit Condition in a Parallel Circuit

Consider a parallel circuit consists of: R1 || R2 || R3

If one component becomes short: R2 → 0Ω, 

the equivalent resistance becomes approximately: RT → 0Ω.

Therefore: a very large current can flow through the circuit, limited by the source and system impedance.

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Example:

If a 24 V supply has a 0.1 Ω effective source/loop resistance and its output is accidentally shorted:

ISC = 24 / 0.1 = 240A.

The power supply or protective device must be capable of safely handling or interrupting this fault condition.

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Q6. What is Relative Potential?

A6. Relative potential describes the electrical potential of one point compared with another reference point.

Voltage is fundamentally a difference in electrical potential.

If point A has potential: VA = 15 V, VB = 5 V.

then the voltage of A relative to B is: VAB = VA - VB = 15 - 5 = 10 V.

This means point A is 10 V higher in potential than point B.​​

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Changing the Reference Point:

VAB = VA - VB = 15 - 5 = 10 V but VBA = VB - VA = 5 - 15 = -10 V.

The negative sign simply indicates that B is 10 V lower in potential than A.

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Ground or Reference Potential

In circuit analysis, one point is often selected as the reference node, commonly assigned: 0V.

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