Topic 6 – Resistive  Transducers

In the exam you are expected to:

Know about resistive transducers;

Be able to interpret and sketch characteristic graphs for thermistors and LDR;

Describe their use in a bridge circuit and in potential dividers.

A resistive transducer is a device that senses a change in the environment to cause a change in resistance.  Transducers do NOT generate electricity.  Examples include:

These are called passive devices.  (Active transducers do generate electricity from other energy sources.)

Question 1 What is meant by a passive device?  How does it differ from an active device?

ANSWER

Some examples of resistive transducers are shown in the photograph:

The photograph below shows the strain gauge in an electronic balance.

LDR and Thermistors

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. 

Question 2  What is the mechanism by which an LDR changes its resistance with changing light levels?  ANSWER

The majority of LDRs respond to light of about 500 nm, which is yellow to green in colour.

Question 3  Explain whether a cadmium sulphide LDR could respond to infra red light.

ANSWER

We can show the way the resistance varies with light level as a graph:

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:

The determination of a relationship from such a graph is not easy.  We can use a logarithmic scale instead.  A logarithmic scale is based on powers of 10.  So instead of the scale going 0, 1, 2, 3, 4, etc, it goes 10⁰, 10¹, 10², 10³, 10⁴ (0, 1, 10, 100, 1000, 10000).  Such a scale allows us to use a very large range of values in a reasonable space.

Both the scales on the graph are logarithmic, and this gives a straight-line relationship.

The LDR can only take a limited current, otherwise it can get hot and burn out.  At high light intensities, the resistance may be only a few ohms, therefore a large current can flow.  This will have a significant heating effect.  We need to add a protection resistor in series.

Question 4  An LDR has a resistance of only 15 ohms at a certain very high light level.  What value of protection resistor is needed if a current of no more than 10 mA is to flow when the supply voltage is 9.0 V?   ANSWER

 LDR’s are used for:

·        Smoke detection

·        Automatic lighting

·        Counting

·        Alarm systems.

Thermistor

The thermistor responds in much the same way as the LDR; instead of light levels, its response is to temperature changes.  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.

Below is a picture of typical thermistors and their symbols:

The resistance can be plotted against temperature:

The resistance on this graph is on a logarithmic scale, as there is a large range of values.  This gives a straight line.  On ordinary graph paper, the relationship is not linear.

The LDR is most commonly used in a potential divider circuit.  

Potential Divider Circuit

We have met potential divider circuits before, but it’s worth revising the principles:

·        Although it is simple, the potential divider is a very useful circuit.  In its simplest form it is two resistors in series with an input voltage V_s across the ends. 

·        An output voltage V_out is obtained from a junction between the two resistors.

·        If the output current is zero, the current flowing through R₁ also flows through R₂, because the resistors are in series.  So we can use Ohm’s Law to say:

            Now V_out = IR₂ =

Þ       

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 5 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 V_out would rise.  If the output were connected to a transistor, the transistor would switch on if the V_out were above 0.7 V.

If we used a thermistor in a simple potential divider, the current flowing through the thermistor will cause a heating effect which will alter the resistance as well as the temperature change.  This is known as self heating.  The thermistor gets hot due to the increasing current through it.  This can lead to a false reading of the resistance at a given temperature and in extreme circumstances cause thermal runaway.  The thermistor gets so hot that it burns out.

Thermistors are normally set up in a Wheatstone bridge circuit, which is essentially two potential dividers in parallel

The thermistor R₁ is the one that senses the temperature, while R₂ is known as a dummy thermistor.  Both will have the same current flowing through them, so that the heating effect (hence the change in current) will be the same for both.  When the ratios of R₁ to R₂ and R₃ to R₄ are the same, the circuit is balanced.  Each end of the voltmeter is at the same potential, so there is zero potential difference.  In other words the voltages across R₂ and R₄ are the same.  We can sum up the condition for a balanced bridge circuit in the following expression:

The voltage through the voltmeter is 0.  When the resistance of the thermistor R₁ is altered due to a change in temperature, the circuit is no longer balanced and the voltmeter has a reading.  Thermistors are nowadays widely used in electronic thermostats and thermometers.

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