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3C : oltage divider checkpoints


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CHECKPOINT A

1. Write down the formula for calculating Vout from a voltage divider.

2. What is the alternative name for this kind of circuit?

3. Does the resistance of the LDR go down or up in bright light? Explain your answer.

4. Calculate Vout for the circuit shown below, assuming the LDR has a resistance of 2 kW.

5. In this circuit, does Vout increase or decrease in bright light?

6. How could you reverse the action of the circuit?

to ANSWERS A Back

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CHECKPOINT B

1. Distinguish between ntc and ptc thermistors. What happens to the resistance of each type as temperature increases?

2. A greenhouse fan is to start automatically if the temperature exceeds 40C. Estimate the thermistor resistance at 40C from the characteristic curve:

3. Draw a voltage divider circuit to give an increase in Vout as the temperature rises. What should be the value of the fixed resistor for maximum sensitivity at 40C?

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CHECKPOINT C

1. Calculate the value of RX in the Wheatstone bridge circuit shown below:

2. What are the advantages of sensor subsystems using Wheatstone bridge circuits?

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CHECKPOINT D

1. A reed switch is closed when a magnet is placed nearby. In a burglar alarm system, the magnet is fixed to the moving part of a door or window, while the reed switch is fixed to the frame. Vout is to be LOW when the door or window is closed and is to become HIGH when the door or window is opened. Which of the voltage divider circuits shown below will give this result?

2. Suggest a value for Rtop and Rbottom in these circuits.

3. How could several reed switches be included in the same voltage divider circuit, giving a HIGH voltage when any one of the reed switches is opened?

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ANSWERS A

1. voltage divider formula:

2. alternative name: potential divider

3. LDR resistance decreases in the light because light energy increases the number of charge carriers available to transfer current.

4. calculating Vout :

5. Vout increases in the light.

6. The action of the circuit is reversed by placing the LDR at the bottom of the voltage divider.

back to CHECKPOINT A


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ANSWERS B

1. In ntc or negative temperature coefficient thermistors, resistance decreases as temperature increases. In ptc or positive temperature coefficient thermistors, resistance increases as temperature increases. ntc types are more commonly used.

2. At 40C, thermistor resistance is close to 10000 W, or 10 kW

3. The circuit required is:

Selecting the fixed resistor as 10 kW gives maximum sensitivity at 40C because 10 kW is equal to the thermistor resistance at this temperature.

back to CHECKPOINT B


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ANSWERS C

1. Substituting in formula:

2. When the bridge is in balance, only differences in temperature from the set point are detected. With two thermistors in the temperature-sensing voltage divider, the bridge automatically compensates for changes in external conditions. The Wheatstone bridge circuit provides similar advantages using other types of sensor.

back to CHECKPOINT C


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ANSWERS D

1. Circuit A gives Vout HIGH when the door or window is opened.

2. 10 kW is a suitable value for any switch/resistor voltage divider.

3. To include several reed switches in the same voltage divider, they should be connected in series:

If any reed switch opens, Vout becomes HIGH. This works because the reed switch sensors are normally closed. Normally open switches such as pressure pads would be connected in parallel.

back to CHECKPOINT D


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