Sensing Circuits
Sensing Circuits
- A sensor changes resistance. A potential divider converts that change into a changing output p.d.
- The direction of the output change depends on which resistor the output is measured across.
Locate the output before predicting it
sensor R = 1.99 kΩ·Vout = 3.99 V of the 6.0 V
- For an NTC thermistor: temperature increases → resistance decreases.
- For an LDR: light intensity increases → resistance decreases.
- If the output is across the sensor, its decreasing resistance makes the output decrease.
- If the output is across the fixed resistor, the same sensor change makes the output increase.
Use a four-link reasoning chain
- State how the sensor resistance changes.
- Identify the resistor across which the output is measured.
- Use the divider fraction to state the output change.
- If the source has internal resistance, continue through source current, lost volts and terminal p.d.
Your turn9702/21/O/N/25 Q5(b)(ii)
An NTC thermistor is in an external circuit supplied by a cell with internal resistance. Its temperature increases. Explain why the terminal p.d. of the cell decreases. [3] (9702/21/O/N/25 Q5)
Show worked answer
The NTC resistance and total external resistance decrease, so the source current increases. The lost volts increase, so the terminal p.d. decreases.