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SYSTEMS PREPARATION QUESTIONS 2007 - 5
 
V. Ryan 2007-2017
 
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A railway track has sets of points that allow trains to transfer from one track to another. This allows them to change direction. However, in winter points can freeze and this is extremely dangerous as a train can be easily derailed.
The block / systems diagram shows how a heating system works. This warms up points in freezing weather ensuring that they operate safely.
 
 
The ice sensing part of the circuit is shown below.
 
 
 
 
1. Which component acts as a sensor in this circuit ?
 
 
ANSWER
 
Component B - Thermistor
 
2. What does the component sense ?
 
 
ANSWER
 
The component senses a change in temperature
 
3. Which component is used to adjust sensitivity of the circuit ?
 
 
ANSWER
 
Component A - Variable resistor
 
4. The ice sensing circuit shown above can be altered to sense heat. In the summer the points can expand due to extreme heat. This can also be dangerous. To counter this problem a cooling unit is to be fitted to the points. Complete the next circuit by adding the heat sensor.
 
 
ANSWER
 
 
 
 
5. What is the name of component C shown in the heat sensing circuit above?
 
ANSWER
 
Component C is a diode
 
6. What is its function ?
 
 
 
 
ANSWER
 
To protect the circuits components form back EMF. When current to a relay is removed the relay can emit current, sending it round the circuit in the opposite direction. This can damage sensitive components such as transistors. The diode prevents this happening.
 
 
 
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