Revision notes
Read the core explanations from Thermistors, LDRs, and Non-Linearity before testing yourself from memory.
1. Why Some Resistors Change
Most resistors have fixed values - a 10Ω resistor stays at 10Ω. But what if you need a resistor that responds to its environment? That's where thermistors and LDRs come in - they're the smart sensors of the electronics world.
These components break the rules. Their resistance isn't constant - it changes based on what's happening around them. This makes them perfect for automatic systems that need to respond to temperature or light changes.
2. Thermistors - Temperature Sensors
Think of a thermistor like a temperature-controlled tap. As temperature rises, the 'tap' opens wider, allowing more current to flow. The resistance drops dramatically - from thousands of ohms when cold to just a few ohms when hot.
This happens because thermistors are made from semiconductor materials. When you heat them up, more electrons break free and become available to carry current. More charge carriers means lower resistance.
Thermistors are everywhere in modern technology. Your car's engine management system uses them to monitor coolant temperature. Central heating systems use them in thermostats to maintain room temperature automatically.
3. Light Dependent Resistors (LDRs)
LDRs work like electronic eyes. In darkness, they have very high resistance - sometimes millions of ohms. Shine light on them and their resistance plummets to just a few hundred ohms.
The science is similar to thermistors but triggered by light instead of heat. Photons hit the semiconductor material and give electrons enough energy to break free. More light means more free electrons, which means lower resistance.
This massive change in current makes LDRs perfect for switching circuits. Street lights use them to turn on automatically when it gets dark. Security lights use them to stay off during the day and activate at night.
4. Practical Applications
The real power of thermistors and LDRs comes when you use them in circuits that make decisions. Here's how they work in practice:
LDR applications work similarly. Garden lights contain LDRs that detect when daylight fades. The changing resistance triggers a switch that turns the LED on. No human intervention needed - the circuit makes the decision automatically.
5. Non-Linear Behaviour and I-V Graphs
Normal resistors give straight-line I-V graphs, so doubling the voltage doubles the current. Thermistors and LDRs are non-linear components, so their I-V graphs are curved. For a thermistor, the curve changes because the component heats up and its resistance changes. For an LDR, the resistance depends on the light intensity falling on it, not simply on current flowing through it.
Here's why: when current flows through a thermistor, it heats up. Higher temperature means lower resistance. Lower resistance means more current can flow. This creates a curved relationship - not the straight line of Ohm's law.
You can use I-V graphs to decide whether a component is linear or non-linear. A straight line through the origin shows constant resistance. A curve shows that the resistance is changing. Where the graph becomes steeper, the current increases more for each extra volt, so the resistance is decreasing. This is how the graph links to the function and properties of thermistors and LDRs.
6. Exam Technique and Common Mistakes
Exam questions on thermistors and LDRs often test whether you understand the direction of change. Many students get confused about which way the resistance changes.
When describing applications, examiners want you to explain the complete chain of events. Don't just say 'the thermistor detects temperature' - explain how the resistance change leads to a voltage change that triggers the switching action.
Understanding these components opens up the world of automatic control systems. From car engines to smart homes, thermistors and LDRs are the sensors that make technology respond intelligently to the world around us.