Resistors

Learning Objectives

By the end of this page, you should be able to:

• Define resistance and explain what a resistor physically does to electric current
• Apply Ohm's law (V = IR) to calculate voltage, current, or resistance in a circuit
• Distinguish between fixed, variable, and specialised resistors and select the appropriate type for a task
• Interpret resistor colour codes to read resistance values
• Explain the significance of power rating and temperature coefficient in practical designs
• Describe at least three common circuit applications of resistors

Quick Answer

A resistor is a passive two-terminal component that opposes the flow of electric current, converting some electrical energy into heat. Its resistance value is measured in ohms and follows Ohm's law: voltage equals current times resistance. Resistors are used everywhere in electronics — to limit current through an LED, set the gain of an amplifier, divide a voltage, or bias a transistor. They come in fixed, variable, and specialised forms (thermistors, photoresistors), with each variant responding to different environmental inputs. The three most important ratings to check before using a resistor are its resistance value, power rating, and tolerance.

Introduction

Resistors are one of the most fundamental components in electronic circuits. They play a crucial role in controlling the flow of electric current and are essential for designing various electronic devices.

What is a Resistor?

A resistor is a passive electrical component that opposes the flow of electric current. It converts some of the electrical energy into heat energy through resistance. The main function of a resistor is to reduce voltage or limit current in a circuit.

Key Properties of Resistors

1. Resistance: Measured in ohms (Ω), it represents how much opposition the resistor offers to the flow of electrons.
2. Power rating: Indicates the maximum power the resistor can handle without failing.
3. Temperature coefficient: Determines how the resistance changes with temperature.

Types of Resistors

There are several types of resistors available, each with its own characteristics and uses:

1. Fixed Resistors

Fixed resistors have a constant resistance value throughout their operating range.

• Wire-wound resistors: Made by winding wire around a core
• Film resistors: Consist of carbon or metal oxide films deposited on an insulating substrate
• Metal film resistors: Similar to film resistors but use metal films instead of carbon

2. Variable Resistors

Variable resistors allow adjustment of resistance within a certain range.

• Potentiometers: Used for volume control in audio equipment
• Trimmers: Small variable resistors used for fine-tuning circuits
• Rheostats: Large variable resistors used for controlling current in low-voltage circuits

3. Specialized Resistors

• Thermistors: Resistors whose resistance varies significantly with temperature
• Photoresistors: Resistors whose resistance varies with light intensity
• Carbon composition resistors: General-purpose resistors made from carbon particles mixed with a binder

How Resistors Work

The behaviour of a resistor is described by Ohm's law:

V = I × R

Where:

• V is the voltage across the resistor (volts)
• I is the current flowing through the resistor (amperes)
• R is the resistance of the resistor (ohms)

Applications of Resistors

Resistors are used in countless electronic devices and circuits. Some common applications include:

• Voltage division: Two resistors in series split a supply voltage proportionally
• Current limiting: Placed in series with an LED or other component to prevent excess current
• Impedance matching: Match source and load to maximise power transfer
• Biasing transistors: Set the operating point of a transistor amplifier
• Creating RC circuits: Combined with capacitors for timing and filtering

Practical Examples

Example 1: Simple Voltage Divider

A voltage divider uses two resistors in series. If R1 is 10 kΩ and R2 is 10 kΩ connected across a 10 V supply, the output voltage taken at the midpoint is 5 V. If R2 were 5 kΩ, the output would drop to approximately 3.3 V, demonstrating how ratio determines the output.

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Key Terms

Term	Definition	Related Concept
Resistance	Opposition to current flow; measured in ohms (Ω)	Ohm's law
Ohm's law	V = I × R; relates voltage, current, and resistance	All resistive circuits
Power rating	Maximum power (in watts) a resistor can dissipate safely	Heat management
Tolerance	Allowable percentage deviation from the stated resistance value	Colour code
Thermistor	Resistor with resistance strongly dependent on temperature	Temperature sensing
Photoresistor (LDR)	Resistor whose resistance decreases as light intensity increases	Light sensing
Potentiometer	Three-terminal variable resistor used as an adjustable voltage divider	Audio volume control
Voltage divider	Two resistors in series used to produce a fraction of the supply voltage	Biasing, level shifting
Temperature coefficient	Rate at which resistance changes per degree Celsius	Precision circuits
Wire-wound resistor	Resistor made by winding resistance wire on a ceramic core; good for high power	Power electronics

Common Mistakes

Misconception: A resistor with a higher resistance always dissipates more power. Why it's wrong: Power dissipated is P = I² × R or equivalently P = V²/R. At constant current, higher resistance does mean more power. But at constant voltage, higher resistance means less current and therefore less power. Context — constant current vs. constant voltage — determines which way the relationship goes. Correct understanding: Always identify whether current or voltage is held constant before reasoning about power; use P = I²R when current is fixed and P = V²/R when voltage is fixed.

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Misconception: The power rating of a resistor is the power it normally operates at. Why it's wrong: The power rating is the maximum safe continuous dissipation. Operating a resistor near its maximum rating causes overheating. Good practice is to derate to 50% of the rated value in typical designs. Correct understanding: Choose a resistor whose power rating is at least twice the power you calculate it will dissipate in normal operation.

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Misconception: All variable resistors work the same way. Why it's wrong: A potentiometer is a three-terminal device used as a voltage divider; a rheostat is a two-terminal device used as a series resistance to control current. They are both variable resistors but serve different circuit functions. Correct understanding: Use a potentiometer when you need a variable voltage output; use a rheostat when you need a variable series resistance.

Comparison and Connections

Feature	Fixed Resistor	Potentiometer	Thermistor	Photoresistor
Terminals	2	3	2	2
Resistance set by	Manufacturing	Manual adjustment	Temperature	Light level
Primary use	Current limiting, biasing	Volume/position control	Temperature sensing	Light sensing
Typical tolerance	±1% to ±5%	±20%	Varies with curve	Varies with illuminance
Common package	Axial, SMD	Rotary, slide	Bead, disc	LDR disc

Practice Questions

Recall

1. What is the unit of resistance, and which law relates voltage, current, and resistance? Answer guidance: The unit is the ohm (Ω). Ohm's law states V = I × R.

2. Name three types of fixed resistors. Answer guidance: Wire-wound, film (carbon or metal oxide), and metal film resistors.

Understanding

3. Why is it important to check the power rating of a resistor before using it in a circuit? Answer guidance: Exceeding the power rating causes the resistor to overheat and fail. Power dissipated is P = I²R; if actual dissipation exceeds the rated wattage, the component is damaged.

4. A thermistor has a negative temperature coefficient (NTC). What happens to its resistance as temperature rises? Answer guidance: Resistance decreases as temperature rises for an NTC thermistor — opposite to a standard resistor's modest increase.

Application

5. A circuit requires a current of 20 mA through an LED from a 5 V supply. The LED has a forward voltage of 2 V. What resistor value should you use? Answer guidance: Voltage across resistor = 5 V − 2 V = 3 V. R = V/I = 3 V ÷ 0.02 A = 150 Ω. Choose the nearest standard value, typically 150 Ω or 160 Ω, and check power: P = 0.02² × 150 = 0.06 W, so a quarter-watt resistor is fine.

6. Two resistors of 10 kΩ and 20 kΩ are connected in series across a 9 V supply. What is the voltage across the 20 kΩ resistor? Answer guidance: Voltage divider: V = 9 × (20 / 30) = 6 V.

Analysis

7. A student connects two identical resistors in parallel and measures less resistance than either resistor alone. Explain why this is correct and derive the combined resistance formula. Answer guidance: Parallel paths provide more routes for current, reducing total opposition. For two equal resistors R in parallel: 1/Rtotal = 1/R + 1/R = 2/R, so Rtotal = R/2. This is always less than either individual resistor.

8. A carbon film resistor has colour bands: brown, black, orange, gold. What is its value and tolerance? Answer guidance: Brown = 1, Black = 0, Orange = ×1000 (multiplier), Gold = ±5%. Value = 10 × 1000 = 10 kΩ ±5%.

FAQ

Why do resistors get hot? When current flows through a resistor, electrons collide with atoms in the resistive material. These collisions transfer kinetic energy to the lattice as heat — this is Joule heating. The power dissipated equals I²R watts. This is intentional in some applications (like heaters) but a design constraint in most circuits. If a resistor feels very hot, it is probably operating close to or beyond its power rating.

What does tolerance mean on a resistor, and why does it matter? Tolerance is the maximum allowed deviation from the marked resistance value. A 10 kΩ resistor with ±5% tolerance could measure anywhere from 9.5 kΩ to 10.5 kΩ. In most digital logic and signal circuits this spread is acceptable. In precision analog designs — like a feedback network setting amplifier gain — tighter tolerance (1% or 0.1%) resistors are needed to keep the actual gain close to the designed value.

What is the difference between a potentiometer and a rheostat? Both are variable resistors, but they are wired differently. A potentiometer uses all three terminals: two ends and a wiper. The wiper slides between the ends, so you get a variable fraction of the total resistance — effectively a voltage divider. A rheostat connects only one end and the wiper, giving a two-terminal variable resistance. Potentiometers are used for setting voltages (volume knobs, sensor calibration); rheostats are used for controlling current (old lamp dimmers, motor speed controllers).

Can I use any resistor as long as it has the right resistance value? Not quite. You also need to check the power rating and, for precision work, the tolerance. A 100 Ω resistor rated at 0.25 W will burn out if you pass enough current to dissipate 0.5 W through it. Additionally, at high frequencies, wire-wound resistors behave inductively and are unsuitable. Always match the type and rating to your application, not just the nominal value.

How do I read a four-band resistor colour code? The first two bands are digits, the third band is a multiplier (power of ten), and the fourth band is tolerance. For example, red–violet–orange–gold: 2, 7, ×1000, ±5% = 27 kΩ ±5%. A fifth band (for precision resistors) adds a third digit before the multiplier. There are mnemonic devices to remember the colour sequence (black, brown, red, orange, yellow, green, blue, violet, grey, white = 0 through 9).

Quick Revision

• A resistor opposes current flow and dissipates energy as heat
• Ohm's law: V = I × R
• Power dissipated: P = I²R = V²/R = V × I
• Fixed resistors have a constant value; variable resistors (potentiometers, rheostats) are adjustable
• Thermistors change resistance with temperature; photoresistors change with light
• Power rating is the maximum safe dissipation; derate to 50% in practice
• Tolerance is the percentage deviation from the stated value
• Series resistors add: Rtotal = R1 + R2
• Parallel resistors combine as: 1/Rtotal = 1/R1 + 1/R2
• Voltage divider output: Vout = Vin × R2 / (R1 + R2)
• Colour code order: black(0), brown(1), red(2), orange(3), yellow(4), green(5), blue(6), violet(7), grey(8), white(9)

Related Topics

Prerequisites: Ohm's law; electric current and voltage; atomic structure; basic circuit concepts

Related Topics: Capacitors (RC filters and timing circuits); transistor biasing; Thevenin's theorem; signal filtering

Next Topics: Capacitors; voltage divider networks; transistor biasing circuits; operational amplifier feedback networks