Resistors in Electronics

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Questions and Answers

What is the primary function of a potentiometer?

Changing the resistance to adjust current (correct)

Voltage regulation only

Enhancing electrical power

Acting as a fixed resistor

How many terminals does a potentiometer have?

Five terminals

Four terminals

Three terminals (correct)

Two terminals

What type of device is a rheostat?

A digital resistor

An amplifier

A variable resistor with two terminals (correct)

A three-terminal device

What is one common use of a potentiometer?

Voltage tapping Signup and view all the answers

Which material can potentiometers and rheostats be constructed from?

Wire or carbon Signup and view all the answers

What does the suffix letter 'K' indicate when coding for resistors?

Multiplier of 1,000 or kiloohms Signup and view all the answers

Which tolerance corresponds to the letter 'C' in resistor coding?

0.25% Signup and view all the answers

How are Surface Mount Resistors characterized in their design?

They are designed for direct surface soldering. Signup and view all the answers

What does the letter 'M' represent in resistor coding for tolerance?

Tolerance of 20% Signup and view all the answers

What are the metal terminals on Surface Mount Resistors used for?

To allow soldering to circuit boards Signup and view all the answers

What is the highest tolerance percentage represented by the code letter 'G'?

2% Signup and view all the answers

Which letter is used to represent a tolerance of 5% in resistor coding?

J Signup and view all the answers

What is a common feature of both Surface Mount Resistors and axial type resistors?

They are printed with a numerical code. Signup and view all the answers

What does resistance oppose in an electrical circuit?

Current flow Signup and view all the answers

What is the formula for calculating heat dissipated by a resistor?

$I^2Rt$ Signup and view all the answers

Which resistor can safely dissipate more heat?

1 watt Signup and view all the answers

What does the maximum current through a resistor depend on?

Its power rating and resistance value Signup and view all the answers

What is tolerance in regards to resistors?

The deviation from the specified value Signup and view all the answers

For a 1000 ohm resistor with a tolerance of ±10%, what could its actual value be?

Between 900 and 1100 ohms Signup and view all the answers

Where is the resistance value and power rating typically found on larger power resistors?

Printed on the resistor body Signup and view all the answers

Which of the following indicates high precision in resistors?

Lower tolerance percentage Signup and view all the answers

What is the primary function of resistors in a circuit?

To limit the amount of current flowing Signup and view all the answers

Which of the following is NOT a type of resistor?

Oscillating resistors Signup and view all the answers

How are carbon composite resistors primarily constructed?

By moulding mixtures of powdered carbon and insulating materials Signup and view all the answers

What is a defining feature of metal film resistors compared to carbon film resistors?

They have a lower power rating Signup and view all the answers

What determines the resistance value of wire wound resistors?

The length of the wire wound around the ceramic body Signup and view all the answers

What does the power rating of a resistor indicate?

The amount of power it can dissipate without burning out Signup and view all the answers

Which of the following resistors typically has a higher power rating?

Carbon composite resistors Signup and view all the answers

What is a common characteristic of carbon film resistors?

They are made by depositing a thin layer of resistive material on ceramic Signup and view all the answers

What does the third digit in a three-digit resistor code represent?

The multiplier for the resistance value Signup and view all the answers

What does a resistor code of '390' indicate?

39 × 1Ω Signup and view all the answers

Which of the following is an example of a temperature dependent resistor?

Negative temperature coefficient (NTC) resistor Signup and view all the answers

When subjected to excess voltage, what happens to a voltage dependent resistor's resistance?

It decreases Signup and view all the answers

What is the purpose of a positive temperature coefficient (PTC) resistor?

To sense temperature in recording instruments Signup and view all the answers

What does a resistor with markings '000' signify?

A zero-ohm resistor Signup and view all the answers

In complete darkness, a light dependent resistor (LDR) has a resistance in the region of how many ohms?

100,000Ω Signup and view all the answers

Which type of resistor is commonly used to control street lights?

Light dependent resistors (LDR) Signup and view all the answers

Study Notes

Resistors

Resistors limit the amount of current flowing or drop voltage in a circuit.
Resistors fall into three groups: fixed linear resistors, non-linear resistors, and variable resistors.

Fixed Linear Resistors

Carbon composite resistors are made by molding carbon and insulating materials into a cylindrical shape.
- They offer mechanical and electrical protection.
- Resistance is varied by changing the mixture of carbon and insulating material.
Carbon film resistors are made by depositing a thin layer of resistive material on a ceramic body and cutting it to form a helix or spiral.
- Carbon film resistors have a lower tolerance than carbon composite resistors but cost more.
Metal film resistors are made by spraying a thin film of metal onto a ceramic body until the desired resistance is achieved.
- Metal film resistors have a lower power rating than carbon composite resistors.
Wire wound resistors are made by wrapping a length of wire around a ceramic body and adding terminals for connection.
- They are low value resistors and tend to be high current and power resistors.
- They are quite large in size.

Power Rating of Resistors

The power rating is the amount of power a resistor can dissipate as heat before burning out.
You can calculate the heat dissipated using: I² * R * t or (V² / R) * t
Larger resistors can dissipate more heat than smaller resistors.
The maximum current a resistor can handle without overheating is: I = P / R where:
- P = Power rating of the resistor in watts.
- R = Resistance value of the resistor in ohms.

Tolerance

Tolerance is the deviation or error from the specified value.
For example, a 1000-ohm resistor with a tolerance of ±10% could have an actual value between 900 and 1100 ohms.
Higher precision requires resistors with a lower tolerance.

Resistor Values

The resistance value, tolerance, and power rating are printed on the resistor body.
British Standard BS 1852:
- "K" = thousands or kilohms.
- "M" = millions or megaohms.
- "R" = multiplier is equal to or less than 1.
Be careful not to confuse the resistance letter k for kilohms with the tolerance letter K for 10% tolerance.
Be careful not to confuse the resistance letter M for Megaohms with the tolerance letter M for 20% tolerance.

Surface Mount Resistors (SMD Resistors)

Surface mount resistors are small, rectangular shaped metal oxide film resistors that are soldered directly onto a circuit board.
SMD resistors are highly accurate, up to 0.1% tolerance.
SMD resistors are marked with 3 or 4 digit numerical codes:
- 3-digit code: The first two digits are the first two numbers of the resistive value, and the third digit is the multiplier (x1, x10, x100, etc.).
- For example: "103" = 10 × 1,000 ohms = 10 kΩ
- Surface mount resistors with values less than 100Ω’s are written as: “390”, “470”, “560” with the final zero representing a 10 xo multiplier, which is equivalent to 1.
- For example: "390" = 39 × 1Ω = 39Ω´s or 39RΩ
- "000" or "0000" markings indicate zero-ohm (0Ω) resistors or shorting links.

Non-Linear Resistors

The resistance of non-linear resistors is determined by an outside influence, such as light, voltage, or heat.
There are three types: temperature dependent resistors (TDR), voltage dependent resistors (VDR), and light dependent resistors (LDR).

Temperature Dependent Resistors (TDR)

NTC = Negative temperature coefficient. Resistance decreases with temperature.
PTC = Positive temperature coefficient. Resistance increases with temperature.
- Thermistors are PTC resistors used for temperature sensing in electric motors and recording instruments.

Voltage Dependent Resistors (VDR)

These resisters have falling resistance when an excessive voltage occurs.

Light Dependent Resistors (LDR)

The amount of light falling on an LDR governs its resistance.
- Greater the light, the lower the resistance.
- Resistance in complete darkness is around megaohms, and in bright light reduces to ohms.
- They are used in street light control and alarm systems.

Variable Resistors

Variable resistors are user-adjustable resistors.
There are two main types: potentiometers and rheostats.

Potentiometers

Have three terminals: one connection to the rotating arm and the other two to each end of the resistor.
Can be constructed from wire or carbon.
Resistance is varied by moving the wiper along the resistor.
Uses: voltage tapping, current limiting, and volume control.

Rheostats

Have two terminals: one terminal via the rotating arm and the other to one end of the resistor.
Can be constructed from wire or carbon.
Operation is similar to potentiometers but with a focus on current limiting.

Review Questions

The physical size of a resistor indicates how much heat can be dissipated in the form of heat.
The tolerance of a resistor tells you the amount the resistor could deviate from the specified value.
The most common type of fixed linear resistor is the carbon composite resistor.
A thermistor has a positive temperature coefficient.
A rheostat is a two-terminal device.
A potentiometer is a three-terminal device.

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Description

This quiz covers the fundamentals of resistors, including their role in controlling current and voltage in circuits. It explores the different types of fixed linear resistors such as carbon composite, carbon film, metal film, and wire wound resistors. Test your knowledge about their construction, characteristics, and applications.