Special resistors Book page 84 87 Syllabus2.12 2.13
Multiple-choice quiz
What am I? LDR, LED or thermistor?
Starter LDR s and thermistors are both types of resistor which can change their resistances. What change in conditions do you think might cause each to change its resistance?
Aim Describe how resistance varies with conditions for LDR s and thermistors, and how this can be investigated experimentally Key words LDR (Light Dependent Resistor) 조명의존형저항기 - diode Thermistor 서미스터, 저항기 Conditions 상태, 조건
How do street lights automatically switch on and off? The big picture Have you ever wondered?
What is a diode? A diode is a component made from a semiconducting material such as silicon. Diodes have the unique property of only allowing current to pass through them in one direction. diode and its symbol The arrowhead on the symbol shows the direction of current flow. Some diodes emit light when they conduct electricity. These are called Light Emitting Diodes (LEDs). LDRs LEDs and Thermistors
resistance (k ) LDRs: light and resistance The resistance of a light dependent resistor (LDR) is not fixed. It is dependent on the intensity of incident light. An LDR has a high resistance in the dark but a low resistance in the light. light intensity LDR symbol The graph shows how the resistance of an LDR decreases as the light intensity increases. This means that LDRs can be used in light sensing circuits, because their output is dependent on the light conditions.
resistance (k ) Thermistors: temperature and resistance The resistance of a thermistor varies depending on temperature. It has a high resistance when cold but a low resistance when hot. temperature ( C) thermistor symbol This is unusual, as resistance normally increases with increasing temperature. Thermistors are useful in the sensor circuits of a thermostat, as their output varies with temperature fluctuations.
LDR Task You have: - 1 LDR - An ammeter - Power packs and wires - 3 pieces of paper of known thickness I have 3 pieces of paper of unknown thickness. You need to tell me: Easy: How the resistance of an LDR changes with light level Harder: The thicknesses of my pieces of paper
Thermistor Task You have: - 1 Thermistor - An ammeter - Power packs and wires - 3 containers of water at known temperatures I have a glass of water of unknown temperature. You need to tell me: Easy: How the resistance of a thermistor changes with temperature Harder: The temperature of my water.
Thermistors and LDRs can work as Sensors: A sensor is a component that allows a physical quantity to change its resistance and hence control the amount of current flowing in a circuit. A A Light Dependent Resistor/ LDR: Its resistance decreases as the light intensity increases. (The higher the illumination, the higher the current Temperature Dependent Resistor /Thermistor: Its resistance decreases as the temperature increases. (The higher the temperature, the higher the current). In this circuit, the LDR can work as a lightmeter. The higher the ammeter reading, the higher the illumination. In this circuit, the thermistor can work as a thermometer. The higher the ammeter reading, the higher the temperature.
Summary Thermistors have a resistance that decreases with increasing temperature Resistance of an LDR decreases as greater levels of light fall on it.
Plenary: Resistance of LDR and thermistor 1. Match the follow electrical symbols for components with their names: cell power supply filament lamp switch fixed resistor voltmeter 2. Draw the symbols for the following components: a) LDR b) variable resistor c) thermistor d) ammeter V 3. As the intensity (brightness) of the light increases, the resistance of the LDR increases / decreases 4. As the temperature increases, the resistance of the thermistor increases. / decreases A
Resistor quiz
which bulb will light up (notice the position of the diodes). Circuit 1 A B C
answer Circuit 1 A & B only A B C
which bulb will light up (notice the position of the diodes). Circuit 2 A B C
answer Circuit 2 None of them A B C
which bulb will light up (notice the position of the diodes). Circuit 3 A B C
answer Circuit 3 A & C only A B C
Key words LDR (Light Dependent Resistor) A type of variable resistor that is sensitive to light intensity. As light intensity increases, its resistance decreases Diode A resistor that only allows current to flow through it in one direction Thermistor A type of variable resistor that is sensitive to temperature. As temperature increases, its resistance decreases Conditions an event will happen if some other thing is taken place. If temperature increases (condition) then resistance will decrease (thing taken place)