Electricity Tutorial 6 - Passive Transducers

Passive transducers  are often referred to as resistive transducers.  They detect a change in the environment (e.g. light level) by changing resistance (hence resistive).  They do NOT generate an electric current.  That is why they are called passive.  The table shows some passive devices.

 

Active transducers generate a current.  A pick-up cartridge on a record deck is one such.

 

 

 

Here are some passive devices:

 

Device

Action

Where used

Light Dependent Resistor

Resistance falls with increasing light level

Light operated switches

Thermistor

Resistance falls with increased temperature

Electronic thermometers

Strain gauge

Resistance changes with force

Sensor in an electronic balance

Moisture detector

Resistance falls when wet

Damp meter

 
The picture below shows examples:

 

 

This picture shows a strain gauge, which is found in an electronic balance:

 

 

Light dependent resistor (LDR)

The light dependent resistor consists of a length of material (cadmium sulphide) whose resistance changes according to the light level.  Bright light releases electrons so that the material conducts better.  Therefore the brighter the light, the lower the resistance. 

The characteristic graph of the LDR is shown below:

The graph shows us the variation using a linear scale.  However, the measurement of light intensity is not an easy scale to work with. 

Here is a list of typical intensities:

Light Source
Illumination (lux)

Moonlight

0.1

60 W light bulb at 1 m

50

1 W MES bulb at 0.1 m

100

Fluorescent lighting

500

Bright sunlight

30 000

 

 LDRs are used for:

Thermistors

The word thermistor comes from the mixture of thermal and resistor.  So it changes its resistance in response to a temperature change.

The most common type that we use has a resistance that falls as the temperature rises.  It is referred to as a negative temperature coefficient device.  A positive temperature coefficient device has a resistance that increases with temperature.

Question 1

Explain the difference between a positive temperature coefficient thermistor and a negative temperature coefficient thermistor.

Answer

Below is a picture of typical thermistors and their symbols:

We can use apparatus like this to measure the way that the resistance of the thermistor changes with different temperatures:

The oil is heated with the 50 W heater and the temperature recorded.  The multimeter shows the resistance.  The data are plotted as a graph that looks like this:

If we plot the vertical axis as log10 (resistance) we get a straight line, indicating that the temperature and resistance are linked by a logarithmic function:

You are not expected to know about logarithmic functions until A2.

Question 2

A thermistor has a negative temperature coefficient.  At 10 oC it has a resistance of 170 ohms.  It is connected to a 20 V supply. 

a.       What current passes through the thermistor?

b.      What is the power dissipated by the thermistor?

c.       Explain what could happen to the thermistor if it were to be left connected to the supply.                                                                                                      

Answer

 
Potential Divider Circuit

Resistive transducers like LDRs or thermistors are often put into a potential divider circuit.

 

Now:

 

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This result can be thought of as the output voltage being the same fraction of the input voltage as R2 is the fraction of the total resistance.


 

 

Question 3   

What is the output voltage of this potential divider? 

Answer

 

If the light level rose, the resistance of the LDR would fall.  Therefore the voltage Vout would rise.  If the output were connected to a transistor, the transistor would switch on as Vout rose above 0.7 V.

 

Question 4

At a certain light level, an LDR has a resistance of 200 ohms.  It is connected  to a 1000 ohm resistor in a potential divider circuit, as shown:

The output voltage is 0.6 V.  What is the input voltage?

Answer

 

Summary

Resistive transducers are passive devices.

 

LDR has a resistance that falls with increasing light levels;

 

Thermistor has resistance that falls with increased temperatures;

 

These are arranged in a potential divider.