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Industrial Electricity
Winter 2017
Name _______________________
DUE 1/24/17 or 1/26/17 (Your next lab day)
Lab 2: Basic Circuit Wiring & Testing
In the space to the right sketch an electrical
diagram (schematic) of a simple electrical
circuit including all required elements. Use
a DC power source and a lamp for the load.
Also include a switch for turning off and on
the current to the load. Label everything.
Before wiring the circuit, measure the
resistances of two of the lamps with you DMM.
Lamp 1 = _____ Ω.
Lamp 2 = _____ Ω (They should be similar in value)
Note that the switch you will be using is a SPDT variety so it will have three (screw) terminals, however
you will only need to use two terminals until the last part of the lab. Use your meter to determine which
two screw terminals to use. You should find that one out of the three terminals is absolutely essential to
use. In the space below, describe clearly and completely how you used the DMM to determine which
two screw terminals to use.
The essential terminal, as described above, is called the “common” terminal. What does that terminal look
like compared to the other two? Keep that in mind for the last part of the lab.
Now wire the circuit using the electrical test board using just one of the lamps. PLEASE DO NOT
CONNECT POWER TO THE CIRCUIT UNTIL YOUR WIRING HAS BEEN CHECKED.
Wiring Check _______
(Lab instructor’s initials)
Adjust the power supply to minimum output. Connect jumper leads from the power supply to your circuit.
Slowly turn up the supply. The lamp should begin to glow. When you get a nice bright glow, use your
meter to measure the following:
The supply voltage: _____V.
Voltage across lamp (on): _____V.
Voltage across the switch terminals (lamp on): _____V.
Voltage across lamp (off): _____V.
Voltage across the switch terminals (lamp off): _____V.
© 2016, David S. Mack. All rights reserved.
1
Turn off the supply. Disconnect one side of the lamp and connect a second lamp to it as shown below.
Screw terminals
Figure 1
Measure the resistance of the two-lamp combination and
record it below.
Resistance of two lamp as shown in Figure 1: _____Ω.
Look back to the resistance reading for a single lamp.
How does this new reading (roughly) compare to the single lamp (half? same? double?)? What can you
infer?
Sketch the new two lamp circuit.
Label everything.
Now reconnect the circuit using your schematic above
Repower the circuit using the same supply voltage as before.
What do you notice about the brightness of the lamps compared to the brightness of the single lamp?
What can you infer?
Measure & record the following voltages:
Supply voltage: _____V.
Voltage across switch when circuit is off: _____V.
Voltage across switch when circuit is on: _____V.
What do you think will happen if one of the lamps
is unscrewed? Talk it over with your lab
partner(s) first and then try it.
If it didn’t turn out like you thought it would, try
to figure out why. Call me over if you’d like my
two cents.
Voltage across lamp 1 (say the upper lamp (lamp on)): _____V.
Voltage across lamp 2 (say the lower one (lamp on)): _____V.
Compare the voltages across the two lamps. What can you say about those two readings? What can you
say about those two readings compared to the supply voltage? Can you infer a simple mathematical
relationship?
© 2016, David S. Mack. All rights reserved.
2
Turn off the supply. Disconnect the lamps and reconnect them as shown below.
Measure the resistance of the two-lamp combination and record it
below.
Resistance of two lamps configured as shown in figure 2: _____Ω.
Figure 2
Look back to the resistance reading for a single lamp.
How does this new reading (roughly) compare to the single lamp (half? same? double?)? What can you
infer?
Sketch the new two lamp circuit.
Label everything.
Now reconnect the circuit using your schematic above. PLEASE DO NOT CONNECT POWER
TO THE CIRCUIT UNTIL YOUR WIRING HAS BEEN CHECKED.
Wiring Check _______
(Lab instructor’s initials)
Repower the circuit using the same supply voltage as before.
What do you notice about the brightness of the lamps compared to the brightness of the single lamp?
What can you infer?
Measure & record the following voltages:
Supply voltage: _____V.
Voltage across switch when circuit is off: _____V.
Voltage across switch when circuit is on: _____V.
What do you think will happen if one of the
lamps is unscrewed? Talk it over with your
lab partner(s) first and then try it.
If it didn’t turn out like you thought it
would, try to figure out why. Call me over if
you’d like my two cents.
Voltage across lamp 1 (say the upper lamp (lamp on)): _____V.
Voltage across lamp 2 (say the lower one (lamp on)): _____V.
Compare the voltages across the two lamps. What can you say about those two readings? What can you
say about those two readings compared to the supply voltage? Can you infer a simple mathematical
relationship?
© 2016, David S. Mack. All rights reserved.
3
Again, switch off power to the circuit.
This last wiring task should be pretty straightforward (and fun?) if you’ve understood the previous “wireups.” What’s shown below are two three-way (SPDT) switches (just like on the test board) controlling a
single lamp. This is a common wiring scheme when you want to control the same light fixture from two
different places, such as the bottom and top of a long staircase. Study the drawing to notice that there are
two different ways to switch on the light and two different ways to switch it off.
The dashed lines surround each switch. The circles (three for each switch) represent the screw terminals
on the (real) switches. One of those terminals is the “common” terminal discussed on page 1 of this lab.
The wires not connected to the common terminals are called “traveler wires” or simply “travelers.” Their
purpose is to tie the two switches together electrically.
Using what you have previously learned, methodically and carefully wire the circuit shown.
PLEASE DO NOT CONNECT POWER TO THE CIRCUIT UNTIL YOUR WIRING HAS BEEN
CHECKED.
Wiring Check _______
(Lab instructor’s initials)
LAMP
Power
Source
Switch 1
Prove your circuit works.
© 2016, David S. Mack. All rights reserved.
Switch 2
Circuit Check: ______
(Lab instructor’s initials)
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Follow up Questions.
1. What are the three physical components that comprise an electrical circuit?
2. What are the three basic electrical parameters (quantities) that DMMs are designed to measure?
3. Of the parameters listed in #2, which one is the:
Electrical analogy to mechanical force or pressure? ______________________
Electrical analogy to mechanical motion? ______________________
Electrical analogy to mechanical friction? ______________________
4. Two identical loads are placed in series as in Figure 1 (page 2). How does the total resistance of the
combination (ideally) compare to the resistance of just one of the loads?
5. Two identical loads are placed in parallel as in Figure 2 (page 3). How does the total resistance of the
combination (ideally) compare to the resistance of just one of the loads?
6. In the space below, sketch the schematic symbols for a A) resistor, B) lamp, C) SPST switch, and
D) SPDT switch. For your sketch of ‘D’ indicate with an arrow the common terminal.
A
B
C
D
7. Consider a powered electrical circuit like the one you sketched on page 1. What voltage would you
expect to measure across a SPST switch if the switch were: A) open and B) closed?
© 2016, David S. Mack. All rights reserved.
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