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## ELEC130 Electrical Engineering 1

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**ELEC130 Electrical Engineering 1**Week 3 Module 2 DC Circuit Tools**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 Lecture 3**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. Lecture 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 Lecture 3**I1**+ - Vs R2 R1 I2 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) Lecture 3**Voltage Division**Current Division + _ + _ V2 V1 I1 Week 2 Summary cont. I R1 + Is R1 R2 Vs - I2 R2 Lecture 3**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 Lecture 3**Wye Delta Transformations**• Need to find equivalent resistance to determine current. HOW? (They are not in series, not in parallel) • Use Y to transformation Lecture 3**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 Lecture 3**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 Lecture 3**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 Lecture 3**I**VBC C Example Week 3 • Find I ? • Determine VBC ? • What power is delivered by 4V source ? Lecture 3**Week 3**• How does the current in the load change if RL is (say) doubled? Lecture 3**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. Lecture 3**VThThevenin Voltage**‘open circuit’ voltage VTh is the voltage which would appear across the terminals of the original and equivalent circuit if those terminals are open circuited. RThThevenin Resistance Independent sources inactivated RTh is the total resistance seen when looking into the original circuit with sources inactivated Can also be obtained by observing the short circuit current. RTh = VTh / Isc. Thevenin’s Components Lecture 3**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 acrossthese 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 Lecture 3**I**VBC C Example Week 3 • Find I ? • Determine VBC ? • What power is delivered by 4V source ? • What is the Thevenin Equivalent circuit between A & B ? Lecture 3**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. Lecture 3**IN Norton Current**‘short circuit’ current IN is the current which would appear through the terminals of the original and equivalent circuit if those terminals are short circuited. RNNorton Resistance independent sources inactivated 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 . Norton’s Components Lecture 3**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 throughthese 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 Lecture 3**I**VBC C Example Week 3 • Find I ? • Determine VBC ? • What power is delivered by 4V source ? • What is the Thevenin Equivalent circuit between A & B ? • What is the Norton Equivalent circuit between A & B ? Lecture 3**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 Lecture 3