Interpret an NTC thermistor in a sensing circuit
Connect temperature to resistance for the specified NTC type and distinguish that trend from a guaranteed output voltage.
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Recognize a resistor symbol.
- State the fixed-p.d. current comparison.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
At the same p.d., what happens to current when total positive resistance falls?
What you will learn
- Recognize and draw an NTC thermistor symbol.
- Predict its resistance trend and a simple fixed-p.d. current trend.
Use the specified temperature response
A thermistor is a temperature-sensitive resistor. This syllabus specifies the negative temperature coefficient, NTC, type: its resistance decreases as its temperature increases over the stated operating range. The symbol uses a resistor with a temperature-related diagonal mark; label NTC in a drawing when the type needs emphasis.
Temperature refers to the thermistor itself, which can be affected by its surroundings and by heating from its own current. The NTC trend alone supplies neither a numerical resistance nor a universal graph. Use the supplied measurements or calibration for exact values.
Predict the circuit response conditionally
For a simple fixed-p.d. path in which the NTC change reduces total resistance, current increases as the thermistor warms. Other components and source behaviour must be considered in a more complicated circuit. A component label does not show which terminals an output measurement uses.
In a potential divider, warming an NTC can make the measured output rise or fall depending on whether it occupies the upper or lower resistance and which p.d. is measured. That Extended account comes later. Here, practise the component symbol and its resistance behaviour without applying an unsupported universal voltage rule.
Pause and explain
Which conclusion follows from warming an NTC without a circuit diagram?
Worked example
An NTC sample warms from 20 °C to 35 °C. What qualitative resistance change is expected?
Show the worked solution
- Identify the type as NTC.
- The temperature increases.
- Negative temperature coefficient means resistance decreases in its operating range.
Answer Resistance decreases; an exact value needs calibration.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw and label an NTC thermistor symbol and state its warming response.
Give me a hint
Use a resistor symbol with the temperature-related diagonal mark.
Compare my reasoning
- Draw its resistor body and terminal wires.
- Add the thermistor mark and an NTC label.
- State that warming decreases its resistance in the operating range.
An NTC-marked thermistor; resistance falls as temperature rises.
Look for these in your work
- Recognize the mark.
- State the specified type.
An NTC cools while a fixed-p.d. simple path retains its other conditions. Describe resistance and current changes.
Give me a hint
Reverse the warming trend, then apply the fixed-p.d. relation.
Compare my reasoning
- Cooling increases NTC resistance in the stated range.
- Total path resistance therefore rises.
- At fixed p.d., current decreases.
Resistance increases and current decreases under these conditions.
Look for these in your work
- Name both quantities.
- Retain the fixed-p.d. assumption.
An unlabelled output from a circuit containing a warming NTC is said to always fall. What evidence is missing?
Give me a hint
The sensor resistance trend does not identify an output pair.
Compare my reasoning
- Warming lowers the NTC resistance.
- The connections and measured terminals are not given.
- A divider placement and output definition are needed to predict voltage direction.
The circuit connections and output terminals are needed; resistance alone does not settle the voltage trend.
Look for these in your work
- Keep the valid NTC trend.
- Reject the unsupported universal output claim.
Common mistakes
- The specified type is NTC, not every thermistor type.
- Warming concerns the component temperature.
- Output-voltage direction depends on the circuit connections.
What can you explain now?
How does an NTC resistance change as its temperature rises?
Compare with the explanation
It decreases.
The negative temperature coefficient gives this resistance trend over the stated range.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw an NTC thermistor and distinguish its warming-resistance trend from a circuit-specific output trend.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.