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Lesson 5

Basic Laws of Electric Circuits. Equivalent Resistance. Lesson 5. Basic Laws of Circuits. Equivalent Resistance:. We know the following for series resistors:. Figure 5.1: Resistors in series. R eq = R 1 + R 2 + . . . + R N. 1. Basic Laws of Circuits. Equivalent Resistance:.

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Lesson 5

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  1. Basic Laws of Electric Circuits Equivalent Resistance Lesson 5

  2. Basic Laws of Circuits Equivalent Resistance: We know the following for series resistors: Figure 5.1: Resistors in series. Req = R1 + R2 + . . . + RN 1

  3. Basic Laws of Circuits Equivalent Resistance: We know the following for parallel resistors: Figure 5.2: Resistors in parallel. 2

  4. Basic Laws of Circuits Equivalent Resistance: For the special case of two resistors in parallel: Figure 5.3: Two resistors in parallel. 3

  5. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. By combination we mean we have a mix of series and Parallel. This is illustrated below. Figure 5.4: Resistors In Series – Parallel Combination To find the equivalent resistance we usually start at the output of the circuit and work back to the input. 4

  6. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. Figure 5.5: Resistance reduction. 5

  7. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. 6 Figure 5.6: Resistance reduction, final steps.

  8. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. It is easier to work the previous problem using numbers than to work out a general expression. This is illustrated below. Example 5.1: Given the circuit below. Find Req. Figure 5.7: Circuit for Example 5.1. 7

  9. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. Example 5.1: Continued We start at the right hand side . of the circuit and work to the left. Figure 5.8: Reduction steps for Example 5.1. Ans: 8

  10. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. Example 5.2: Given the circuit shown below. Find Req. Figure 5.9: Diagram for Example 5.2. 9

  11. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. Example 5.2: Continued. Fig 5.10: Reduction steps. 10

  12. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. Example 5.2: Continued. 10  resistor shorted out Req Fig 5.11: Reduction steps. 11

  13. Basic Laws of Circuits Equivalent Resistance: Resistors in combination. Example 5.2: Continued. Req Fig 5.12: Reduction steps. Req 12

  14. Basic Electric Circuits Wye to Delta Transformation: You are given the following circuit. Determine Req. Figure 5.1: Diagram to start wye to delta. 13

  15. Basic Electric Circuits Wye to Delta Transformation: You are given the following circuit. Determine Req. Figure 5.13: Diagram to start wye to delta. 14

  16. Basic Electric Circuits Wye to Delta Transformation: We cannot use resistors in parallel. We cannot use resistors in series. If we knew V and I we could solve V Req = I There is another way to solve the problem without solving for I (given, assume, V) and calculating Req for V/I. 15

  17. Basic Electric Circuits Wye to Delta Transformation: Consider the following: Figure 5.14: Wye to delta circuits. We equate the resistance of Rab, Rac and Rca of (a) to Rab , Rac and Rca of (b) respectively. 16

  18. Basic Electric Circuits Wye to Delta Transformation: Consider the following: R2(R1 + R3) Eq 5.1 Rab = Ra + Rb = R1 + R2 + R3 R1(R2 + R3) Eq 5.2 Rac = Ra + Rc = R1 + R2 + R3 R3(R1 + R2) Eq 5.3 Rca = Rb + Rc = 17 R1 + R2 + R3

  19. Basic Electric Circuits Wye to Delta Transformation: Consider the following: Eq 5.4 Eq 5.5 Eq 5.6 18

  20. Basic Electric Circuits Wye to Delta Transformation: Observe the following: Go to wye Go to delta Eq 5.4 Eq 5.5 Eq 5.6 We note that the denominator for Ra, Rb, Rc is the same. We note that the numerator for R1, R2, R3 is the same. We could say “Y” below: “D” 19

  21. Basic Electric Circuits Wye to Delta Transformation: Example 5.3: Return to the circuit of Figure 5.13 and find Req. a c b Convert the delta around a – b – c to a wye. 20

  22. Basic Electric Circuits Wye to Delta Transformation: Example 5.3: continued Figure 5.15: Example 5.3 diagram. It is easy to see that Req = 15  21

  23. Basic Electric Circuits Wye to Delta Transformation: Example 5.4: Using wye to delta. The circuit of 5.13 may be redrawn as shown in 5.16. a c b Figure 5.16: “Stretching” (rearranging) the circuit. Convert the wye of a – b – c to a delta. 22

  24. Basic Electric Circuits Wye to Delta Transformation: Example 5.4: continued a a c c b b (a) (b) Figure 5.17: Circuit reduction of Example 5.4. 23

  25. Basic Electric Circuits Wye to Delta Transformation: Example 5.4: continued Figure 5.18: Reduction of Figure 5.17. Req = 15  This answer checks with the delta to wye solution earlier. 24

  26. Basic Laws of Circuits circuits End of Lesson 5 Equivalent Resistance

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