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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