More Potential Divider Circuits

1
22 August, 2014
More Potential Divider Circuits
Starter

• Calculate the p.d. across the 200 W resistor

800 W
12 V
200 W
2.4 V
Vout
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22 August, 2014
More Potential Divider Circuits
Learning Outcomes
At the end of this lesson you should
ALL Describe how the resistance of a light
dependent resistor (LDR) depends on the intensity
of light MOST Explain the use of Thermistor and
light dependent resistors in potential divider
circuits SOME Evaluate the advantages of using
data-loggers to monitor physical changes
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22 August, 2014
More Potential Divider Circuits
Keywords
intensity, LDR, light, p.d., resistor, rheostat, p
otential, switch, temperature, thermistor, transis
tor, voltage.
Tasks

1. Make notes on LDRs, thermistors and their uses in
   potential divider circuits.
2. Complete the worksheets on More Potential Divider
   Circuits.

Extension
Evaluate the advantages of using data-loggers to
monitor physical changes.
4
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
LDRs and Thermistors
NTC Thermistor
Light dependent resistor (LDR)
5
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
The Light Dependent Resistor (LDR)
The circuit symbol for a light dependent resistor
(____) is
LDR
The ________ of an LDR depends upon the light
intensity. In bright light the LDR has a ___
resistance. In the ____ it has a ____ resistance.
resistance
low
dark
high
6
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
The Light Dependent Resistor (LDR)
The graph shows how the resistance of a LDR
changes with light intensity
7
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
Using a LDR for a Light Sensor
The resistor, R, has a value so that in daylight
the p.d. across it is almost equal to the supply
voltage.
Hence Vout is very small
8
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
Using a LDR for a Light Sensor
When it becomes dark the resistance of the LDR
________. The p.d. across it also ________.
increases
increases
Hence Vout increases
9
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
Using a Transistor
A transistor can be connected to Vout which will
switch on a light when the p.d. is bigger than
0.7 V.
10
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
Using a LDR for a Light Sensor
If you want the light to switch on during the day
time, this can be achieved by swapping the
positions of the resistor and the LDR.
What is the effect of changing the fixed resistor
for a variable resistor?
11
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
The Thermistor (NTC)
The circuit symbol for a thermistor is
The resistance of a NTC (_______ __________
_________) thermistor depends upon the
temperature. When hot the thermistor has a ___
resistance. When ____ it has a ____ resistance.
negative
temperature
coefficient
low
cold
high
12
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
The Thermistor (NTC)
The graph shows how the resistance of a NTC
thermistor changes with temperature
13
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
Using a Thermistor for a Temperature Sensor
The resistor, R, has a value so that at room
temperature the p.d. across it is almost equal to
the supply voltage.
Hence Vout is very small
14
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
Using a Thermistor for a Temperature Sensor
When it becomes colder the resistance of the
thermistor ________. The p.d. across it also
________.
increases
increases
Hence Vout increases
15
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
Using a Thermistor for a Temperature Sensor
How could you make a fire alarm
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22 August, 2014
More Potential Divider Circuits
Plenary
Now you should
ALL Describe how the resistance of a light
dependent resistor (LDR) depends on the intensity
of light MOST Explain the use of thermistors and
light dependent resistors in potential divider
circuits SOME Evaluate the advantages of using
dataloggers to monitor physical changes

• ? ?

17
Learning Outcome Explain the use of thermistors
and light dependent resistors in potential
divider circuits
Homework
Deadline