ELECTRICITY 4: Circuit devices and how are they used? (e.g. thermistor, LDR and diode)

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

What is an LDR?

Describe and explain a practical use of an LDR

What is a thermistor?

Describe and explain a practical use of a thermistor.

What is the function of a diode?

What is an LED?

Introduction

Some components are designed to change resistance in response to changes in the environment e.g.

the resistance of an LDR varies with light intensity,

the resistance of a thermistor varies with temperature,

and these properties used in sensing systems to monitor changes in the environment.

These kind of circuit components can be used to turn systems on and off, increase or decrease power to control the output depending on the ambient (surrounding) conditions.

Thermistor - temperature dependent resistor

Circuit 42 shows how you can investigate the resistance of a thermistor.

The voltmeter is wired in parallel with the thermistor, the p.d. V is measured in volts (V).

The variable resistor allows you to vary the p.d. and current flow.

The ammeter, wired in series, gives you the current I reading in amps (A).

You must decide on the initial p.d. and see how the current varies.

You calculate the resistance of the thermistor from Ohm's Law equation: V = IR, so R = V/I

Somehow you need to vary the temperature of the thermistor resistor e.g. dipping it into a beaker of water of varying temperature, making sure the circuit is insulated from the water.

You can make measurements from 0 to 60oC by using ice and then warm-hot water and try to get measurements for every 5 or 10oC incremental rise in temperature.

You should find that the resistance falls with increase in temperature because a thermistor is a temperature dependent resistor.

The higher its temperature, the lower a thermistor's resistance (e.g. tens of ohms) and much higher at low temperatures (e.g. thousands of ohms).

High resistance in a cool environment and low resistance in a warm environment.

You can see this trend clearly in the resistance - temperature graph for a thermistor.

Thermistors can therefore respond to changes in temperature.

Thermistors can act as temperature detectors and are used in thermostats, temperature sensors - cooling systems in car engines etc.

Circuit 32 shows in principle how to control a cooling fan in a room.

(real thermistor circuits are more complicated)

The fixed resistor and cooling fan are wired in parallel. This means they always have the same potential difference across them.

However, the thermistor is a variable resistor.

The p.d. of the power supply is shared out between the thermistor and the 'loop' consisting of the fixed resistor and fan wired in parallel.

The output component (fan) and the thermistor are wired in series.

(I've indicated this with blue arcs - not meant to be wires!)

The greater the component's resistance, the greater proportion of the p.d. it takes.

If the room gets hotter, the resistance of the thermistor decreases, so it takes a smaller shared of the p.d.

Therefore the p.d. across the fixed resistor and fan rises (V1 increases, V2 decreases).

The fixed resistor and cooling fan motor are wired in parallel, so have the same p.d. V1 across them.

The greater the p.d. across the fan, the faster it goes as the power output can increase (P = IV).

If the room cools, the thermistor's resistance increases and the process reverses and the fan slows down or stops.

Thermistors are used as temperature detectors e.g. electronic thermostats in heating and cooling systems in the home or electric kettles (relatively low temperatures), or in high temperature situations like a car engine.

Footnote on the I-V graph for a thermistor  (graph (2) on the right)

The graph of current versus voltage for a thermistor is similar to that of a filament bulb.

Its a non-linear graph and the phrase non-linear component may be used.

The passage of current heats up the filament and the rise in temperature causes the resistance to increase. So a thermistor is a non-ohmic conductor.

As the current increases, more heat energy is released and the filament gets hotter and hotter, so further increase in temperature further increases the resistance.

This decreases the rate at which the current increases with increase in potential difference.

LDR - light dependent resistor

Circuit 44 shows how you can investigate the resistance of an LDR in varying light conditions.

The voltmeter is wired in parallel with the LDR, the p.d. V is measured in volts (V).

The variable resistor allows you to vary the p.d. and current flow.

The ammeter, wired in series, gives you the current I reading in amps (A).

You must decide on the initial p.d. and see how the current varies.

You calculate the resistance of the thermistor from Ohm's Law equation: V = IR, so R = V/I

Somehow you need to vary the light intensity shining on the LDR resistor e.g. using a lamp working of variable resistor and taking a reading with a light meter as well as the p.d. and current readings.

An LDR is a light dependent resistor and you should find ....

the higher the light intensity, the lower an LDR's resistance.

i.e. high resistance in darkness and low resistance in bright light.

You can see this trend clearly in the resistance - temperature graph for an LDR.

Therefore an LDR can respond to changes in light intensity e.g. daylight/night time.

The circuit system will sense the presence of light

LDR resistors are used automatic control of lights at night - outdoor lighting, burglar alarm circuits, light intensity meters.

Circuit 33 shows in principle how to control the output of a lamp bulb.

(real LDR circuits are more complicated)

In this case the 'active' component, the bulb, is wired in parallel with the LDR response resistor.

In this case the p.d. across the LDR and the lamp bulb is the same, though the LDR is a variable resistor.

In dim light or darkness, the p.d. across the LDR and bulb is high because LDR's resistance is high.

The greater the p.d. across the lamp the greater the power output (P = IV), so the bulb lights up - glows more brightly as the surroundings get darker.

If the surroundings e.g. a room or a garden path gets brighter, the LDR's resistance decreases, the p.d. decreases, so the power output decreases and the lamp glows dimmer or 'goes out'.

As well as automatic night lights, an LDR can be used in a burglar alarm circuit.

A small and constant beam of light is shone on an LDR (it can be from an invisible infrared emitting LED). If the shadow (of the burglar) crosses the light beam, the intensity of light falling on the LDR is reduced. Therefore the resistance of the LDR is reduced and this triggers an alarm.

The diode

The current through a diode flows in one direction only

This is because a diode has a very high resistance in the reverse direction.

A diode is a special device made from a semiconducting material based on silicon (classed as a semi-conductor).

Since the current only flows one way through a diode, it can be used to convert an ac current into a dc current.

With alternating current (ac), the current changes direction in a cycle, but with direct current (dc) there is no reversal in current direction, it flows one way with a constant voltage.

Oscilloscope traces comparing ac and dc current signals - showing the alternating + <=> - oscillation of the alternating current p.d. and the constant p.d. of a direct current.

Diagram 2. shows what happens if pass an ac current through a diode - 'before and after' trace after the dc output has been smoothed.

In some devices in the home the output from e.g. the transformer in your computer power supply, is rectified to convert it from ac to a dc supply.

Diodes are used as rectifiers, signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulators, and oscillators etc. etc. in other words - rather useful in electronic circuits.

An LED emits light when a current flows through it in the forward direction.

You should know that there is an increasing use of LEDs for lighting, as they use a much smaller current than other forms of lighting.

They have a much greater efficiency in converting electrical energy into visible light.

• Practical work to help develop your skills and understanding may have included the following:

• investigating potential difference/current characteristics for LDRs and thermistors,

• planning and carrying out an investigation to find the relationship between the resistance of thermistors and their temperature,

• investigating the change of resistance of LDRs with light intensity.

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