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EXPERIMENT 3 Parallel Circuits & Voltage Divider Rules for Series Circuit 1. OBJECTIVE: 1.1 To investigate the characteristics of a parallel circuit. 1.2 To examine the relationship between combinations of voltage drops and combinations of resistance values in a series circuit using voltage divider rules. 2. INTRODUCTION: 2.1 Parallel Circuits: A Parallel circuit has certain characteristics and basic rules summarized here: A parallel circuit has two or more paths for current to flow through. Voltage is the same across each component of the parallel circuit. The sum of the currents through each path is equal to the total current that flows from the source. You can find total resistance in a Parallel circuit with the following formula: 1/Rt = 1/R1 + 1/R2 + 1/R3 +... Rt = R (total) If one of the parallel paths is broken, current will continue to flow in other paths. 2.1.1 A parallel circuit has two or more paths for current to flow through. This is self explanatory. Simply remember that PARALLEL means two paths up to thousands of paths. The flow of electricity is divided between each according to the resistance along each route. Figure 3.1 Rule 1 for parallel circuit. -24- 2.1.2 Voltage is the same across each component of the parallel circuit. You may remember from the last section that the voltage drops across a resistor in series. Not so with a parallel circuit. The voltage will be the same anywhere in the circuit. Figure 3.2 2.1.3 Rule 2 for parallel circuit. The sum of the currents through each path is equal to the total current that flows from the source. If one path is drawing 1 amp and the other is drawing 1 amp then the total is 2 amps at the source. If there are 4 branches in this same 2 amp circuit, then one path may draw 1/4A (.25A), the next 1/4A (.25), the next 1/2A (.5A) and the last 1A. Don't worry, the next rule will show you how to figure this out. Simply remember for now that the branch currents must tally to equal the source current. Figure 3.3 Rule 3 for parallel circuit. -25- 2.2 Voltage Divider The voltage across one resistor equals the ratio of that resistor's value and the sum of resistances times the voltage across the series combination. This concept is so pervasive it has a name: voltage divider. Referring to Single Loop circuit in Figure 3.4, which yields Vin = Vout1 + Vout2, where, Vout1 = R1i Vout2 = R2i Combining these equations, we find And Vin = R1i + R2i i = Figure 3.4 V / ( R1 + R2 ). Single loop circuit So finally the equations for voltage divider for both resistors are, Vout1 = R1 Vin R1 + R2 Vout2 = R2 Vin R1 + R2 -26- 3. COMPONENT AND EQUIPMENT: 3.1 Breadboard – 1 unit 3.2 DC power supply – 1 unit 3.3 Digital Multimeter – 1 unit 3.4 Wires 3.5 Resistors: 3.5.1 1.2 kΩ resistor - 1 pcs 3.5.2 1.8 kΩ resistor - 1 pcs 3.5.3 2.2 kΩ resistor - 1 pcs 3.5.4 3.3 kΩ resistor - 2 pcs 3.5.5 5.6 kΩ resistor - 1 pcs 3.5.6 33 kΩ resistor - 1 pcs 4. PROCEDURE: 4.1 Parallel Circuit: 4.1.1 Part 1(a) - Voltage characteristic in a parallel circuit. a) Connect the circuit in Figure 3.5. Adjust the voltage source to a value of 12 volts (with the circuit connected). b) Using the DMM, measure the voltage across each resistor. Record your measurements in Table 5.1 Figure 3.5 Schematic diagram of circuits. -27- 4.1.2 Part 1(b) - Current relationships in a parallel circuit. a) Connect the circuit in Figure 3.6. Make sure that the source voltage is properly set to 12 volts with the circuit connected. b) Using a current meter, measure the current through each resistor and the total current. Record your measurement in Table 5.1. Figure 3.6 4.1.3 Schematic diagram of circuits. Part 1(c) - Resistance relationship in a parallel circuit. a) Connect the circuit in Figure 3.7. Note that there is no source voltage connected. b) Using a DMM, measure the total resistance. Record your measurement in Table 3.1. Figure 3.7 c) Schematic diagram of circuits. Remove each resistor from the circuit. Using the DMM, individually measure R1, R2, and R3. Record your measurement. -28- 4.2 Voltage Divider for Series Circuit. 4.2.1 Connect the circuit in Figure 3.8. 4.2.2 Measure and record below the voltage drop across each resistor. When measuring VAB, the voltmeter probe should be connected to point A and the common lead to point B. This would be expressed as VAB. Note that in the subscript “AB”, the first letter “A” is the point to which the probe is connected and the second letter “B” is the point to which the common lead is connected. Therefore, the expression VAB means the voltage at point “A” in respect to point “B”. 4.2.3 Properly label these measured voltage drops on each resistor in Figure 3.9. Mark the polarity (use a + and a - to indicate polarity) of the voltage drop on each resistor. 4.2.4 Measure the voltage, VCE, between point C and point E. When measuring, the voltmeter probe should be connected to point C and the common lead to point E. This would be expressed as VCE. Note that in the subscript “CE”, the first letter “C” is the point to which the probe is connected and the second letter “E” is the point to which the common lead is connected. Therefore, the expression VCE means the voltage at point “C” in respect to point “E”. Record this voltage in Table 3.2. 4.2.5 In a like manner, measure and record the following in Table 3.2. VAC = VDG = VCA = (note opposite polarity!) VEA = VBF = -29- VCG = _______ Figure 3.9 Schematic diagram of circuits. -30- Name : ______________________________ Matrix No : ______________________________ 5. Date: ______________ RESULT: EXPERIMENT 3: Parallel Circuits & Voltage Divider Rules for Series Circuit. 5.1 Information: 5.1.1 Always use the measured value of resistance for all calculations. 5.1.2 Always adjust the power supply voltage with the circuit connected. 5.1.3 When measuring voltage, the voltmeter must be connected across the circuit element of interest. 5.1.4 When measuring current, the current meter must be inserted into the “break” in the circuit (in series). TABLE 3.1 15 TABLE 3.2 PROBLEM (1) PROBLEM (2) MARKS 10 10 50 15 % Table 5.1 Voltage, current and resistance measured in Part 1 Part 1 (A) Voltage (V) Part 1 (B) Current (mA) Part 1 (C) Resistance (ohm) VR1 = IR1 = Rtotal = VR2 = IR2 = R1 = VR3 = IR3 = R2 = Itotal = R3 = -31- Name : ______________________________ Matrix No : ______________________________ Date: ______________ Table 3.2 Voltage measured in Part 2 Part 2 VR1 = VAB VR2 = VBC VR3 = VCD VR4 = VDE VR5 = VEF VR6 = VFG Voltage Measured (V) VCE VCA VAC VDG VEA VBF VCG -32- Name : ______________________________ Matrix No : ______________________________ Date: ______________ 6. EXERCISE: 6.1 Find the equivalent resistance seen by the source and current i. -33- Name : ______________________________ Matrix No : ______________________________ Date: ______________ 6.2 In the voltage divider shown the power delivered by the source is 9mW and Vi = V/4. Find R, V, Vi and i. -34- Name : ______________________________ Matrix No : ______________________________ 7. DISCUSSION: The current and voltage values is .. 8. CONCLUSION: The conclusion for this lab is… -35- Date: ______________