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Lab 10: RL Circuits Student name: Student ID: I. Introduction On this laboratory assignment, you will perform a practical analysis of a circuit that contains two of the three most common passive elements, resistors and inductors, this means that you will analyze an RL circuit. For this assignment, it is important that you remember, review, and apply all theoretical knowledge about capacitors that you learned on your Electric Circuits I course. Also, the use of simulation software for the analysis of circuits will be an important part of this assignment II. Objectives After completion of this laboratory assignment, learner will be able to: 1. Setup square waveform and RL network to measure RL time constant. 2. Simulate RL network transient solution using simulation software (PSpice/ LTSpice). III. Equipment Required • Digital Oscilloscope • Function Generator • Digital Multimeter Breadboard • Power Supply • Miscellaneous cables IV. Material Required • 1 Inductor: 100 mH 4 watt resistors: (1) 20k52 Jumper wires
V. Development 5.1 Measurement of RL Time Constant L2 100mH V1 20K R2 Figure 1. RL Circuit 1. Set up the oscilloscope to measure the function generator. Set up the function generator while monitoring it with the oscilloscope to produce a 0 to 5.0 Volts +/- 0.1 Volts square wave using the DC offset. The square wave should have a frequency of 20 kHz or pulse width of 50 us. The pulse should be 50% duty cycle: high for 25 us and low for 25 us. RECORD the waveform labeling both the vertical and horizontal scales showing at least two cycles and include it in your report 2. Build the RL network as in Figure 1 with the function generator in place of the DC voltage source. Use a 20k2 resistor and a 100 mH ceramic capacitor for the R and L components. 3. Set up channel 1 to measure the function generator input. Set up channel 2 to measure the voltage across the resistor. RECORD the waveform labeling both the vertical and horizontal scales showing at least two cycles and include it in your report. 4. Maximize the displayed voltage by setting the OV reference on the screen bottom line and increasing channel 1 and 2 gain V/div to the minimum volts/div for which the waveform is still on the screen grid. Measure (using time cursors) the time required for the resistor voltage to decrease from 5 volts to about 1.84 volts. Discharge Time Record this value and compare it with the calculated RL time constant using the R and L component values. RL Constant
For each inductor voltage from 5.0V to 0.5 V in 0.5 V steps of the transient waveform record the voltage and time in a table in your laboratory handout. USE CURSORS. Voltage [V] Time [ms] Voltage [V] Time [ms] 5.0 0 2.5 4.5 2.0 4.0 1.5 3.5 1.0 3.0 0.5 5. Measure the voltage in the inductor, use one channel of the scope to measure the inductor voltage. Note that the voltage waveform for the resistor is continuous but the voltage waveform for the inductor is not. Explain why the waveform can be discontinuous. RECORD the waveform labeling both the vertical and horizontal scales showing at least two cycles and include it in your report. 5.2 PSPice/LTSpice simulation of waveforms L1 tClose=0 1 U1 2 100mH R1 20k M Figure 2. Source-free RL Circuit 1. Simulate Figure 2 circuit using PSpice or LTSpice. Simulate a capacitive discharge using the circuit in Figure 1 setting the Inductor initial condition to 0.25 mA and a tclose t=0. Also simulate the circuit in Figure 1 setting the capacitor IC initial condition to 0 mA and a tclose 1=0. Use PROBE to obtain a graph of the capacitor voltage and current with one time constant about mid-point on the horizontal axis. Print out your graphs. 2. Use VPULSE from the source library and set up the VPULSE source DC= (blank), AC=(blank), V1=0V, V2=5V, TD=0, TR=ln, TF=ln, PW=25 us, PER= 50 us, for no DC, no AC, and a OV to 5V pulse with 0 delay time to start, with Trise 1 nanosec, Tfall 1 nanosec, Pulse Width 25 us, and Period 50 us to do a VPULSE simulation of your circuit. Use PROBE to obtain a plot like Figure 3.
3. When you are ready to simulate your circuit, go to Analysis>Setup and select Transient. You will then see a window called “Transient where you will be able to specify the print step and the final time of your simulation. Some reasonable values are 10ns for your print step and about 40 us for your final time. You can try these values to see what you get and then you can change them to get the output you want. VII. After the Lab Find information about the RL time constant in your book or online to help you answer the following questions. 1. What is the RL time constant? What is its significance? 2. From your PSpice simulations, how can you find the time constant of the circuit you did during the laboratory? Does your answer agree to the time constant you found in the lab? Show how you would use the output waveforms from PSpice to find the time constant and the steps and formulas you would follow to accomplish this QUESTIONS. 3. You have a square wave input of 333Hz frequency, 50% duty cycle, and a 4V peak to peak voltage used as an input for an RL circuit with a resistor value of 15k12. If you know the time required for the inductor to charge through the resistor to 63.2% full of charge is 0.15ms, what would be the value of the inductor? 4. What would be the current in the inductor when this has been discharged to about 36.8% of its peak current? 5. If the inductor starts charging up after the Os mark, then after what time would the inductor reach this discharge current (from question 4)? Include all your measurements performed during the lab procedures as well as your circuit simulations, schematics, and solutions to the after the lab section in your report write up. Make sure you keep track of the correct units and show your work.