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**electronics fundamentals**

circuits, devices, and applications THOMAS L. FLOYD DAVID M. BUCHLA Chapter 5 – Parallel Circuits

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**Resistors that are connected to the same two points are said to be in parallel.**

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**A parallel circuit occurs if:**

more than one current path (branch) is connected to a common voltage source. The voltage between the two points appears across each of the branches.

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**Draw the parallel circuits shown below as schematics**

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**Connect the groups of resistors to make parallel circuits.**

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**Connecting resistors in parallel**

reduces total resistance and increases total current. 60 amp 120 amp 24 v 24 v

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**From Chapter 2, what is Conductance and its unit**

Conductance (C) is: ease with which current can pass the reciprocal of resistance, Siemens

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**Parallel circuit rule for resistance**

The total resistance of resistors in parallel is the reciprocal of the sum of the reciprocals of the individual resistors. For example, the resistors in a parallel circuit are 680 W, 1.5 kW, and 2.2 kW. What is the total resistance? 386 W

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**The total resistance of resistors in parallel is**

the reciprocal of the sum of the reciprocals of the individual resistors. GT = G1 + G2 + G3 + … + GN

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**What is RT (REQuivalent) for this circuit**

200 3500 50 RT = ohms

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**Special formula for calculating resistance of two parallel resistors**

The resistance of two parallel resistors can be found by either: or Total resistance of two resistors in parallel is equal the product of the two resistors divided by the total resistance. What is the total resistance if R1 = 27 kW and R2 = 56 kW? 18.2 kW

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**Special formula for calculating resistance for equal resistors in parallel**

500 500 500 500 500 What is the value of RT 100 ohms

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**Special formula for calculating resistance for equal resistors in parallel**

500 500 500 500 100 What is the value of RT 55.56 ohms

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**Application of a parallel circuit for the exterior light system of an automobile.**

What happens if one headlight burns out? What happens to the current drawn by the headlight circuit?

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**Parallel circuits are used in residential wiring.**

WHY?

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**Draw the schematic for this circuit board.**

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**Voltage across parallel branches is the same.**

Parallel circuit rule for voltage Voltage across parallel branches is the same.

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**Parallel circuit rule for voltage**

Because all components are connected across the same voltage source, the voltage across each is the same. The source voltage is 5.0 V. What will a volt- meter read if it is placed across each of the resistors?

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24 v What is the voltage reading for R1 and R2

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**What is the voltage reading for R1 and R2**

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**Application of Ohms law**

Calculate VS, I1, I2 and I3. Calculate IS if R3 is shorted, if R3 is open and voltage remains the same Vs = volts, RT = ohms, I1=0.023a, I2=0.009a, I3=.00505a Ishort = Error; RT = 158 ohms, IS = 32mA

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**Kirchhoff’s current law (KCL)**

The sum of the currents into a node equals the sum of the current out of that node.

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**Kirchhoff’s current law (KCL)**

The algebraic sum of the currents entering and leaving a node equals zero.

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**Proving Kirchhoff’s current law.**

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**Parallel circuits are know as current “dividers”**

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**Parallel Circuit is a Current divider**

When current enters a node (junction) it divides into currents with values that are inversely proportional to the resistance values.

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**The current IX through any branch equals**

The total parallel resistance RT Divided by the resistance RX for that branch Multiplied by The total current IT in the branch

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**Current divider For two resistor parallel circuits and**

Notice the subscripts. The resistor in the numerator is not the same as the one for which current is found.

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The branch with the lowest resistance has the most current, The branch with the highest resistance has the least current.

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Current divider

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Solving for Current in a parallel circuit when the voltage is unknown and the IT and the values of R1 and R2 are known.

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Current divider Assume that R1is a 2.2 kW resistor that is in parallel with R2, which is 4.7 kW. If the total current into the resistors is 8.0 mA, what is the current in each resistor? 5.45 mA 2.55 mA Notice that the larger resistor has the smaller current.

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**Power in parallel circuits**

Power in each resistor can be calculated with any of the standard power formulas. Just like a series circuit PT = P1 + P2 + P3 + … + PN What is the total power if 10 V is applied to the parallel combination of R1 = 270 W and R2 = 150 W? 1.04 W

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**What is the maximum power that the amplifier must be able to deliver?**

Power for one speaker = watts

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Assume there are 8 resistive wires all with the same resistance that form a rear window defroster for an automobile. (a) If the defroster dissipates 90 W when connected to a 12.6 V source, what power is dissipated by each resistive wire? (b) What is the total resistance of the defroster? (a) Each of the 8 wires will dissipate 1/8 of the total power (b) The total resistance is 1.76 W x 8 = 14.1 W What is the resistance of each wire?

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**When an open condition occurs in a parallel circuit:**

Open branches in parallel circuits When an open condition occurs in a parallel circuit: the total resistance increases and the total current decreases and the current through all other branches (i.e. R2) remains unchanged.

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When a lamp filament opens (or the light is turned off), the total current decreases by the amount of current in the lamp that opened. The other branch currents remain unchanged.

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**Finding an open path by current measurement.**

Troubleshooting parallel circuits Finding an open path by current measurement.

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**Know how to determine if there is a open resistor**

Troubleshooting parallel circuits Know how to determine if there is a open resistor

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**Practice Exercise Complete the parameters listed in the Table.**

I1= R1= 0.68 kW V1= P1= I2= R2= 1.50 kW V2= P2= I3= R3= 2.20 kW V3= P3= IT= RT= 386 W VS= 5.0 V PT= 7.4 mA 5.0 V 36.8 mW 3.3 mA 5.0 V 16.7 mW 2.3 mA 5.0 V 11.4 mW 13.0 mA 64.8 mW

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Key Terms The relationship in electric circuits in which two or more current paths are connected between two separate points (nodes). Parallel Branch Kirchhoff’s current law Node Current divider One current path in a parallel circuit. A law stating the total current into a node equals the total current out of the node. A point or junction in a circuit at which two or more components are connected. A parallel circuit in which the currents divide inversely proportional to the parallel branch resistances.

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**1. The total resistance of parallel resistors is equal to**

a. the sum of the resistances b. the sum of the reciprocals of the resistances c. the sum of the conductances d. none of the above

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**Quiz 2. The number of nodes in a parallel circuit is a. one b. two**

c. three d. any number

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**3. The total resistance of the parallel resistors is**

a kW b kW c. 5.1 kW d kW

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**4. If three equal resistors are in parallel, the total resistance is**

a. one third the value of one resistor. b. the same as one resistor. c. three times the value of one resistor. d. the product of the three resistors

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**Quiz 5. In any circuit the total current entering a node is**

a. less than the total current leaving the node. b. equal to the total current leaving the node. c. greater than the total current leaving the node. d. can be any of the above, depending on the circuit.

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**6. The current divider formula to find I1 for the special case of two resistors is**

b. c. d.

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**7. The total current leaving the source is**

a. 1.0 mA b. 1.2 mA c. 6.0 mA d. 7.2 mA

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8. The current in R1 is a. 6.7 mA b mA c. 20 mA d mA

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9. The voltage across R2 is a. 0 V b V c V d. 4.0 V

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**10. The total power dissipated in a parallel circuit is equal to the**

a. power in the largest resistor. b. power in the smallest resistor. c. average of the power in all resistors. d. sum of the power in all resistors.

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Quiz Answers: 1. d 2. b 3. a 4. a 5. b 6. c 7. d 8. b 9. c 10. d

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