Download Document

Electronics
Lab 2
Spring 2015
Today we will investigate the behavior of more devices, light a 12 V light bulb with a
fixed voltage power supply > 12 V, and investigate the behavior of a circuit whose
properties change with time.
1. Switches. I have provided you with a single pole, single throw (SPST) switch (also
known as an “on/off” switch) and a single pole, double throw (SPDT) switch.
a. Identify which switch is the SPST switch and which one is the SPDT switch.
b. How does the SPST switch work? What did you do to figure out how it
works? (Please do not “look it up!”)
c. How does the SPDT switch work? What did you do to figure out how it
works? (Please do not “look it up!”)
2. Powering a 12 V light bulb with a fixed 16 V power supply.
a. Suppose that you have a 12 V light bulb and a fixed 16 V power supply to
light the bulb.
i. Design a circuit that would allow the light bulb to shine as brightly as
possible without exceeding its 12 V limit.
ii. Construct and test your circuit by monitoring the voltage across the
light bulb and gradually increasing the voltage of the power supply.
Show the instructor your working circuit (i.e., 16 V across the
power supply and 12 V across the light bulb). Keep a DMM
connected in such a way that it measures the voltage across the light
bulb.
iii. Add a DMM to your circuit so that you can measure current through
the light bulb.
iv. Add a SPST switch to your circuit so that you can turn the bulb ON
and OFF. What happens to the current and voltage readings when you
turn the switch ON and OFF? Does this result make sense to you?
Why / why not?
v. Use the DMM as an ohmmeter and investigate the internal workings /
behavior of the device with three “ports” and a knob.
vi. Add this device to your circuit so that it will act as a dimmer switch
for your light bulb. What happens to the current and voltage readings
when you turn the knob on the dimmer switch? Does this result make
sense to you? Why / why not?
vii. Show the instructor your completed working circuit.
viii. Measure the current through and voltage across the light bulb with the
dimmer switch in the setting that produces the brightest light and the
on-off switch in the “on” setting.
ix. Leave the dimmer switch in this setting and turn OFF the on-off
switch.
b. Remove the power supply from the circuit from part 2 and replace it with the
leads from the black box.
i. Plug in the black box and turn ON the on-off switch. Does the light
bulb light?
Page 1 of 3
Electronics
Lab 2
Spring 2015
ii. Measure the current through and the voltage across the light bulb.
Compare these values with those you measured in step 2.a.viii. Do
these results surprise you? Why / why not?
3. Time-dependent Behavior.
a. Set-up
i. Connect, in series, a red-red-green resistor represented in the
schematic as a jagged line with a blue cylindrical component
represented in the schematic as two parallel lines (see Figure below).
Note: Connect the long end of the blue cylindrical component to the
switch and the short end to the resistor.
ii. Attach a DMM to measure the voltage
across the blue cylindrical component.
iii. Use a SPDT switch as indicated in the
Figure below.
iv. Set the (variable) power supply to 15 V.
b. Taking Measurements
One partner should write down, or
preferably enter in the computer, the voltage across the blue cylindrical
component every 10 seconds while the other partner announces each 10
second interval.
i. With the switch disconnected from the power supply, turn on the
power supply.
ii. Flip the switch to power the circuit and take voltage measurements
across the blue cylindrical component until the voltage measurements
don’t change much between measurements (take measurements for at
least 10 minutes).
iii. Flip the switch back to disconnect the power supply from the circuit
and take another set of measurements for about the same time period.
c. Plot your two sets of measurements on two separate plots.
d. For discussion in class on Friday: Estimate how long it took for the voltage
across the blue cylindrical component to reach 63% of its final value? How
long did it take to increase to 86% of its final value? 95%?
e. For discussion in class on Friday: Estimate how long it took for the voltage
across the blue cylindrical component to decrease to 37% of its initial value?
How long did it take from this time to decrease to 14% of its initial value?
5%?
4. Response.
a. 1. Explain how the two types of switches work and what tests you did to
reach these conclusions. Feel free to include a diagram to help you explain
what you did and what you learned.
b. 2.a.i. Draw a diagram of the circuit that you used to safely light the 12 V light
bulb with a 16 V power supply? What thinking process did you use when you
designed this circuit to satisfy the light bulb’s voltage limit?
c. 2.a.iv. What happens to the current and voltage readings when you turn the
SPST switch on and off? Does this result make sense to you? Why / why
not?
Page 2 of 3
Electronics
Lab 2
Spring 2015
d. 2.a.v. Describe the behavior of the device that has three “ports” and a knob.
e. 2.a.vi. What happens to the current and voltage readings when you turn the
knob on this device? Does this result make sense to you? Why / why not?
f. 2.b.i. When you replace your original power supply with the “black box”
power supply, does the light bulb light?
g. 2.b.ii. Measure the current through and the voltage across the light bulb.
Compare your measurements of current through and the voltage across the
light bulb with those you measured in step 2.a.viii. Do these results surprise
you? Why / why not? Explain.
h. 3. Provide your plots and superimpose “best-fit” curves (LoggerPro can help)
that you think represent your results.
Page 3 of 3