Transcript lecture3
ELEC130 Electrical Engineering 1 Week 3 Module 2 DC Circuit Tools 1 Administration Items Tutorials - Will be held in ES 210 this week. Answers tutorial 1 will be revised Introduction to Electronic Workbench - Revised document Faculty PC’s Rm. ES210 - Go to Diomedes Login: cstudentnumber Password: access keys on students card + daymonth (ddmm) of birth Use Drive u: to save your work Laboratory - THIS WEEK in EE 103(a) Allocation of Laboratory and Tutorial Times NO more changes after Friday 12 March 1999 4 pm If you cannot make your time, please ask for alternative Quiz 1 - THIS WEEK 5 March, 1999 Lecture 3 2 Survey Results Subject Home Page: - through Dept. Pages http://www.ee.newcastle.edu.au/ Then to Undergraduate studies Then to Course Information/Syllabus Then to Subject Web Pages From the web site you have the option to save the file in power point You are expected to read the specified text references to build the background information to the topic areas we are covering. You should think of the lecture as an opportunity to reflect on your reading and clarify difficult concepts. 5 March, 1999 Lecture 3 3 Survey Results (cont.) Current Sources DC power supply, transistors Conductance - Parallel Resistance's Voltage and Current Division Why - Delta - tutorial 1 Question 19 part 4 Floyd pg. 309 Example 8-19 pg. 312 Superposition 5 March, 1999 Lecture 3 4 Conductance Sometimes easier to use inverse of resistance called conductance G = R-1 Symbol: G Units: Siemens S (mhos) NB: Useful when resistors are connected in parallel Geq = G1 + G2 +... +Gn 1/Req = 1/R1 + 1/R2 +... +1/Rn + Case of two parallel resistance's: Req = R1R2 /(R1 + R2) 5 March, 1999 Lecture 3 - Vs R1 R2 I1 I2 5 Week 2 Summary Voltage Division Current Division R2 i1 Is R1 R2 R1 v1 Vs R1 R2 I + Vs R1 R2 5 March, 1999 cont. + V1 Is _ + V2 _ Lecture 3 R1 R2 I1 I2 6 Survey (cont.) Current Sources DC power supply, transistors Conductance - Parallel Resistance's Voltage and Current Division Why - Delta - tutorial 1 Question 19 part 4 Floyd pg. 309 Example 8-19 pg. 312 Superposition 5 March, 1999 Lecture 3 7 Wye Delta Transformations Need to find equivalent resistance to determine current. HOW? (They are not in series, not in parallel) Use Y to transformation 5 March, 1999 Lecture 3 8 Survey Current Sources DC power supply, transistors Conductance - Parallel Resistance's Voltage and Current Division Why - Delta - tutorial 1 Question 19 part 4 Floyd pg. 309 Example 8-19 pg. 312 Superposition 5 March, 1999 Lecture 3 9 Week 2 Summary (cont.) Superposition: If a linear circuit is excited by more that one independent source, then the total response is simply the sum of the responses of the individual sources. Voltage sources - short circuit Current source - open circuit 5 March, 1999 Lecture 3 10 Power Calculations Power is not linear! Superposition will not work directly! With 2 A source opened P’1 = 25 W With 10 V Source shorted P’’1 = 1 W Total P = P’ + P’’ = 26 W (incorrect) Must calculate current by superposition and then work out power I’ = 5 A & I’’ = -1 A Total I = I’ + I’’ = 4 A Power P = 42 R = 16 W 5 March, 1999 Lecture 3 11 Example Week 3 Find I ? Determine VBC ? I What power is delivered by 4V source ? VBC C 5 March, 1999 Lecture 3 12 Week 3 How does the current in the load change if RL is (say) doubled? 5 March, 1999 Lecture 3 13 Thevenin’s Theorem Any linear network with a pair of terminals can be replaced by a circuit comprised of a voltage source in series with a resistor. The observed voltages and currents in the load will be the same using the “Thevenin equivalent” circuit as would be seen using the original circuit. 5 March, 1999 Lecture 3 14 Thevenin’s Components VTh Thevenin Voltage RTh Thevenin Resistance Independent sources inactivated ‘open circuit’ voltage VTh is the voltage which RTh is the total resistance seen would appear across the terminals of the original and equivalent circuit if those terminals are open circuited. when looking into the original circuit with sources inactivated Can also be obtained by observing the short circuit current. RTh = VTh / Isc. 5 March, 1999 Lecture 3 15 Steps to finding the Thevenin Equivalent Step 1 Determine the two points from which the Thevenin is to be found. NB:Polarity – Step 2 Find open circuit voltage across these two points by removing the Load (resistance) VTh = Vo/c Step 3 Find RTh by looking from the two points into the circuit after replacing all independent sources Step 4 Draw the Thevenin Equivalent – Voltage source in series with a resistor 5 March, 1999 Lecture 3 16 Example Week 3 Find I ? Determine VBC ? I What power is delivered by 4V source ? VBC What is the Thevenin Equivalent circuit between A&B? 5 March, 1999 C Lecture 3 17 Norton’s Theorem Any linear network with a pair of terminals can be replaced by a circuit comprised of a current source in parallel with a resistor. The observed voltages and currents in the load will be the same using the “Norton equivalent” circuit as would be seen using the original circuit. 5 March, 1999 Lecture 3 18 Norton’s Components IN Norton Current RN Norton Resistance ‘short circuit’ current independent sources inactivated IN is the current which would appear through the terminals of the original and equivalent circuit if those terminals are short circuited. RN is the total resistance seen when looking into the original circuit with sources inactivated Can also be obtained by observing the open circuit voltage. RN = Voc / IN . 5 March, 1999 Lecture 3 19 Steps to finding the Norton Equivalent Step 1 Determine the two points from which the Norton is to be found. NB:Polarity – Step 2 Find the short circuit current through these two points by putting a short across them IN = Is/c Step 3 Find RN by looking from the two points into the circuit after replacing all independent sources Step 4 Draw the Norton Equivalent – Current source in parallel with a resistor 5 March, 1999 Lecture 3 20 Example Week 3 Find I ? Determine VBC ? What power is delivered by 4V source ? I VBC What is the Thevenin Equivalent circuit between A&B? C What is the Norton Equivalent circuit between A&B? 5 March, 1999 Lecture 3 21 Relationship between Thevenin & Norton A particular circuit can be represented by Thevenin or Norton equivalent. Therefore Thevenin and Norton equivalent circuits must be the same. Hence Req = Rth = RN RTh = VTh / Isc = VTh / IN VTh = RN IN RN = Voc / IN = VTh / IN IN = VTh / RTh 5 March, 1999 Lecture 3 22