VCE PHYSICS AOS 2 UNIT 3 Circuit Design and Application
The Components design, investigate and analyse circuits for particular purposes using technical specifications related to potential difference (voltage drop), current, resistance, power, temperature and illumination for electronic components such as diodes, resistors, thermistors, light dependent resistors (LDR), photodiodes and light emitting diodes (LED) 2
Consolidation of theory Dot point provides avenue for examination of content to date Components specifically cited: Ø diodes, resistors, thermistors, light dependent resistors (LDR), photodiodes and light emitting diodes (LED) Current controllers, voltage controllers. Input and output transducers 3
Concept Explorer Need to redo RHS diag please The diode characteristics are shown in the left hand side diagram below. Using the data provided in the right hand side diagram find the current (in ma) flowing in the ammeter. 4
Concept Explorer Redraw circuit: Total resistance = (400 x 400)/(400 + 400) + 200 = 400 Ohms Total current = V / R = (14 0.6) / 400 = 0.0335 A Current equally split: Ammeter reads 17 ma 5
Concept Explorer Design a circuit using a photodiode (characteristics below), variable resistor and a 9.0 Volt DC supply. Clearly show the value for the variable resistor if a voltage drop of 1.5 Volts is needed across the variable resistor, when the light intensity is 10 mw. 6
Concept Explorer Circuit: Photodiode must be reverse biased Voltage defined by current through resistor due to light Current flowing = 4 x 10-3 A from graph V = I R R = V / I R = 1.5 / 4 x 10-3 A = 375Ω 7
Sensor circuits Identify temperature or light language used Identify how resistance changes with input Switch usually on rising voltage Is switch for heating/cooling or lighting 4 point questions not unusual Ø What happened to input Ø What is the effect on the resistance of sensor Ø In order to maintain voltage of, for fixed input PD Ø Need to increase or decrease resistance 8
Concept Explorer Exam paper 1 2007 AOS 2 question 9 needs graph from page 15 also 9
Concept Explorer If room too dark, then light needs to come on earlier when room lighter At higher light intensity then resistance of LDR is less Still require 4 Volts across LDR To maintain voltage ratio, must maintain resistance ratio so as R LDR has reduced so must the R variable Reduce variable resistance 10
Quiz In an incubator, the temperature of the unit must be maintained at 50 degrees. To ensure this, a heater is fitted with a thermistor sensor switch (characteristics below). If the supply voltage is 12 Volts and the switch requires 5 volts to operate. a) Should the switch be placed across the resistor or the thermistor? b) What size of resistor should be placed in series in the circuit? a b 11
Quiz In an incubator, the temperature of the unit must be maintained at 50 degrees. To ensure this, a heater is fitted with a thermistor sensor switch (characteristics below). If the supply voltage is 12 Volts and the switch requires 5 volts to operate. a) Should the switch be placed across the resistor or the thermistor? b) What size of resistor should be placed in series in the circuit? a Thermistor b 4200 Ω 12
Worked solution a) Heating needed for low temperature Resistance increases as temp falls, so rising voltage. Therefore switch placed across thermistor b) Thermistor resistance = 3000Ω Voltage of 5 Volts required across thermistor Voltage divider equation: V out = V in x R 2 /R Total 5 = 12 x 3000/(3000 + R) 15000 + 5R = 12 x 3000 R = (36000 15000)/5 R = 4200Ω Quick check that lower Voltage on smaller resistance 13
Need figure 3 and 5 from 2009 paper 1 AOS 2 pages 14 and 15 Quiz Using the information in the 2 diagrams calculate the size of the current flowing through LED 5. A. 0.20 A B. 0.15 A C. 0.10 A D. 0.05 A 14
Worked solution Current in LED 5 is the same as the current in the resistor. V resistor = 12 9 = 3 V I resistor = V / R I resistor = 3 / 60 I = 0.05 A 15