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Lab Session — Lab 3 - Lamps Part I: Observing and Measuring Lamp In-rush Current At Turn-on 1. Return the oscilloscope to its default settings by pressing the SAVE/RECALL hard key, and then the DEFAULT SETUP soft key. 2. Connect the circuit shown in Figure 1. For the switch shown, use a normally open (RED) push button switch. Connect Channel 1 and Channel 2 as indicated in the schematic. 3. The power supply button should be ON and the Output On/Off button should be in the ON position. Press the + 25 V button and adjust the voltage to 12 volts. The current reading of the power supply should be 0 Amps and the light bulb should be off. 4. Press the 2 button on the oscilloscope, then select the Invert soft key. Turn the Channel 1 and 2 vertical position knobs (below the 1 and 2 buttons) to position the channel 1 waveform in the top half and channel 2 waveform in the bottom half of the screen. 5. Set the Volts/Div control for channels 1 and 2 to 5 V/Div. Do this by turning the V/div knobs just above the 1 and 2 buttons. 6. Turn the Horizontal position knob to position the triggering start at the left side of the screen. 7. Press the Mode/Coupling hardkey, and then use the Mode soft key to select the Normal triggering mode. Turn the Triggering Level knob to select a triggering level of about 5 volts. Press the Edge button and press the soft keys to select Channel 1 and negative slope. 8. Adjust the Time/Div control on the front panel for 5 ms/div. Press the Main/Delayed hardkey, and the Time Ref soft key to locate the reference time at the left. The oscilloscope should now have the following setup: 9.Momentarily press the ON/OFF pushbutton switch. The sweep of the oscilloscope should be triggered once, and a new trace recorded (see Figure 3 for a typical display). Adjust the position controls for Channels 1 & 2 to separate the traces. If more than one trigger occurs for a single press of the ON button, choose the trigger mode hardkey, and then the single trigger mode softkey. You will have to press the Run hardkey after each trigger to “arm” the sweep. Invert the channel 2 signal as follows: Press the Math hard key, then the 1-2 soft key. Press the Math hard key, then the settings soft key. Adjust the 1-2 settings scale to 5 volts. Adjust the 1-2 offset to align the 1-2 waveform with a horizontal grid. Save the screen shot to Floppy disk. Saving a screen shot: Place floppy in the laptop drive. Open My computer and navigate to the floppy. Delete all the files on the floppy. Place floppy into the oscope. Press save/Recall button followed by the formats soft key. Select TIF format. Then press the Quick Print button. 10. In Table 1, record the initial and steady state Lamp voltage. Also record the initial and steady state resistor voltage. Calculate all of the remaining values in Table 1. 11. Replace the 10 ohm resistor with a 100 ohm resistor . Replace the Lamp with and LED. Repeat steps 1 through 10 for a 100 ohm resistor and an LED instead of a lamp. Record results in Table 2. Table 1. Lab Measurements and Calculations of Lamp and Source Electrical Quantities Electrical Quantity Symbol Channel 1 voltage VLamp Channel 2 voltage V10 Current I Lamp resistance RLamp Lamp power dissipation Voltage across Source Resistance PLamp Source Resistance RS Initial Value Steady State Value VRs Table 2. Lab Measurements and Calculations of LED and Source Electrical Quantities Electrical Quantity Symbol Channel 1 voltage VLamp Channel 2 voltage V10 Current I Lamp resistance RLamp Lamp power dissipation Voltage across Source Resistance PLamp Source Resistance RS VRs Initial Value Steady State Value Post Lab — Lamps Theory section: 1 Explain figure 2 and the voltage divider rule. Using the voltage divider rule, how does one compute the load resistance given the ideal source voltage and the source resistance. If the source resistance increases, will the terminal voltage increase decrease or remain the same? 2 Explain the theory of figure 1. Given these same voltages how does one compute the current? How does one compute the lamp resistance? Given the lamp voltage, the resistor voltage, and the ideal source voltage, how does one compute the voltage of the source resistance? 3 Explain the circuit setup of figure 1 and the resulting oscilloscope screen shot in Figure 3. What circuit voltage is represented by channel 1? What circuit voltage is represented by channel 2? From which channel voltage can one compute current? How does one compute the current? How does one compute Lamp resistance? How does one compute Lamp power? How does one compute the voltage across the internal source resistance? How does one compute source resistance? What is meant by initial and steady state values of voltage and current? What is meant by the time to reach steady state? Date/Analysis section: 1 Briefly explain what you did in lab. Why did the instructions ask you to invert channel 2? 2 Discuss the results of the Tungsten lamp circuit. Compare initial lamp resistance with steady state lamp resistance. Compare in-rush (initial) current with steady state current. Is in-rush current higher or lower than steady state current? Explain why in term of initial and steady state lamp resistances. In other words can you explain how the change in lamp resistance produces the change in current from initial to steady state? 3 Discuss the results of the LED circuit. Compare initial and steady state values. What do the results tell you anything about the thermal and electrical properties of the LED? 4 Compare the power dissipation of the LED with that of the Tungsten lamp. Using the steady state power dissipation values of the LED and the lamp, and assuming that the two lights were about the same brightness, approximately how many times more does it cost to operate a Tungsten lamp in comparison with an LED? 5 Using the curves obtained in the lab session, estimate the time to reach steady state for the Tungsten lamp and the LED. Lab Report due Monday, October 5