1 PHYSICS 360 - LAB #1 Laboratory Equipment & DC Circuits ...

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Summer 2010

1

PHYSICS 360 - LAB #1
Laboratory Equipment & DC Circuits

Objective:

• To become familiar with the electronic equipment in the laboratory, to learn proper
operational procedures, and to know the capabilities and limitations of the equipment.
• Study simple DC circuits (Kirchhoff's laws and Thevenin's theorem).
• Understand the voltage divider circuit operation and resistive transducers.
• Study voltage divider circuits involving resistive transducers.


Introduction:

Each laboratory work station has 4 instruments:

(a) Function generator -- Generates a 0-5 volt square wave at its transistor-transistor logic (TTL) output terminal
and a sine, square, or triangular wave at its analog output terminal. The frequency can be varied by a rotating
knob and by selecting a range push button. The amplitude and a zero Vdc offset of the analog output are also
adjustable between 0 - 10 volts. The source impedance is 50 Ω.

(b) Oscilloscope -- Probably the most versatile instrument in the electronic lab. It can be used to view waveforms
and to measure the amplitude (in volts) and time/frequency of the waveforms. Input impedance is 1 MΩ shunted
by 10 pF.

(c) Digital multimeter (DMM) -- Provides digital readings of voltages, currents, and resistance values. In
resistance mode, it can also be used to find polarities of diodes and transistors. Typical input impedance of 10 MΩ
shunted by 10 pF.

(d) Design Workstation -- Permits one to construct analog, digital and microprocessor circuits utilizing resistors
capacitors, transistors, diodes, and IC's with connecting wire jumpers instead of soldering wires for connections.
A real time saver in the lab. It consists of 8 logic probes circuits, a function generator (source impedance 600 Ω)
with continuously variable sine, square, triangular wave forms plus TTL pulses, a triple power supply, audio
experimentation speaker, potentiometers etc.

Experiments:

Part 1: Introduction to the oscilloscope, the function generator and DMM.

1-1. DC voltages measured by the oscilloscope and DMM.

Display a 10 volts DC signal (with zero ac amplitude) on the oscilloscope. Use the oscilloscope
to measure the value of the signal with the vertical amplifier set to dc coupling. Test with ac
coupling of the vertical amplifier. Notice that any dc voltage in the input signal is cutoff. Use
the DMM to measure the DC signal you displayed on the oscilloscope.

V (Oscilloscope, dc coupling) = ____________ V (DMM) = _____________
V (Oscilloscope, ac coupling) = ____________
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1-2. AC voltages measured by the oscilloscope and DMM.

Display a 1 kHz sine wave (with zero V dc offset) on the oscilloscope with an amplitude of 10-
V
p-p
. Use the calibrated setting of the vertical amplifier input (dc coupled) of the scope to
measure the peak-to-peak value of the signal. Then use the DMM to measure the rms (root-
mean-square) value. Convert the peak-to-peak value to a rms value to see if the two
measurements agree. Repeat the preceding for several other frequencies.

Frequency Waveform Amplitude,Vp-p Calc RMS DMM reading

10 Hz sine 10 V _________ _________
100 Hz sine 10 V _________ _________
1 kHz sine 10 V _________ _________
10 kHz sine 10 V _________ _________
50 kHz sine 10 V _________ _________
100 kHz sine 10 V _________ _________
1 kHz square 10 V _________ _________
1 kHz triangle 10 V _________ _________

On the basis of your observations, is there a frequency range or cut-off frequency where the
DMM measures sinusoidal (sine wave) voltages accurately? Does the DMM measure a true
RMS voltage value for non-sinusoidal waveforms?














1-3. Resistance measurements with the DMM.

Even though the DMM is functioning as an ohmmeter, it is still using its digital voltmeter.
However, in the Ω function mode, the DMM supplies a current to the external load resistance
when connected to its output terminals and measures the corresponding voltage at these
terminals. Set the DMM function dial to Ω and measure 4 different (ranging in values from
few Ω to few kΩ) carbon resistors. Compare the measured values to the nominal values as
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determined by the color code. Is the agreement within the indicated resistor tolerance (4th
band)?

Color Code Nominal Resistance Measured Resistance % Error

_________ _____________ ________________ _________

_________ _____________ ________________ _________

_________ _____________ ________________ _________

_________ _____________ ________________ _________




1-4. Introduction to the design workstation.


The breadboard is a convenient, time-saving way to construct and test electrical circuits. The
breadboards in the lab have their own power supplies with a +5 Vdc terminal for TTL circuitry,
two variable outputs: 0 to ±15 Vdc for CMOS and op amp circuitry, potentiometers etc.
Measure these voltages with respect to GND using the DMM in the DC V (voltage) mode with
an appropriate voltage scale setting (i.e. voltage to be measured ≤ voltage scale) and write
down the minimum and maximum voltages that you can obtain from these terminals. Measure
the resistance of the potentiometers between one of the end terminals and one of the variable
terminals in the middle. Write down the range of the resistance that you can get between these
terminals (i.e. minimum and maximum).

DMM reading
+ 5 Vdc terminal _______________
0 to +15 V dc terminals _______________
0 to -15 V dc terminals _______________
0 to 1 k Ω terminals _______________
0 to 10 k Ω terminals _______________



The breadboard consists of a series of metal clips that are interconnected in columns and rows
with a plastic housing on the top, which permit connections by wire jumpers. Using the DMM
as an ohmmeter, determine which sockets (holes) are interconnected in columns and which
ones in rows. Sketch a diagram of the interconnections below. Note: only a small portion of
the breadboard area is shown in the diagram on the next page.

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Part 2: DC Circuits

2-1. Verification of Kirchoff’s Laws

Express the Kirchoff's laws in words:








Construct the circuit shown in Fig. 1-1 and measure the currents and voltages using the DMM.
Use the following values for the resistances: R
1
= 1 k Ω, R
2
= 2.2 kΩ , R
3
= 4.7 k Ω




Fig. 1-1 A simple DC circuit


I
1
= ____________ V
A
= _______________
I
2
= ____________ V
B
= _______________
I
3
= ____________ V
C
= _______________

Calculate the above quantities using Kirchhoff's laws (show the calculations) and compare
with your measured values. Comment on your observations.









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2-2. The Voltage Divider

Voltage dividers are found everywhere in electronic circuits. You should understand the
concept thoroughly and be able to design one. Give it a try.

Design a voltage divider that meets the following criteria:
• It must be built with no more than 3 standard carbon composition ¼ W 5%
resistors.
(Minimizing part count and using lowest-possible-power-rating components is usually
cheaper and saves space. Although these considerations might seem secondary to circuit performance, they
are important design considerations, especially in consumer portable electronics.)

• When driven by an ideal 15 V source, its no-load output should be about 10V.
• The divider’s output shouldn’t droop by more than about 5% when driving a
load that varies from a low of 6 k to a high of 100 k.
Draw a diagram with component values to show your solution. Justify your choices. Then
build and test your circuit to see if it meets the design goals.
(Hints: Don’t fry the resistors and remember that 5% resistors are limited to a standardized set
of stock values. Consider the worst case scenario when choosing components.)



Fig. 1-2 A Voltage Divider Circuit


When finished, describe the tradeoff between minimizing droop and minimizing power
dissipation when selecting resistors in this design. (Tradeoffs when making component choices
are ubiquitous in circuit design.)









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2-3. Thevenin's theorem

Explain Thevenin's theorem.








Use your 2/3 voltage divider from the previous activity to explore Thevenin’s theorem.
Experimentally measure V
Th
and R
Th
and compare with the calculated values.


V
Th
(measured) = _________ I
sc
(measured)

= _________

R
Th
(measured) = (V
Th /
I
sc
) = _________

V
Th
(calculated) = V
in
R
2
/ (R
1
+R
2
) = _________

R
Th
(calculated) = R
1
R
2
/ (R
1
+R
2
) = _________

Draw a Thevenin equivalent circuit. Now, if you connect a load resistor R
L
= 6.2 kΩ between
points A and B, what is the value of V
AB
? Calculate and measure it. For the calculation use
the Thevenin equivalent circuit (much easier than using the real circuit).


V
AB
(measured) = _______________ V
AB
(calculated) = _______________
















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2-4. The photoresistor

Photoresistor: Many physical quantities can be transformed into resistance by resistive
transducers. The photoresistor is a resistance made of a semiconducting material that
changes its resistance in response to the intensity of light falling on it. We will be using
a cadmium sulfide (CdS) cell, which is a photoresistor. The varying resistance can be
converted into a varying voltage by using the cell as one of the resistors in a voltage
divider circuit.

Use a DMM to measure the resistance of a CdS cell I) in room light, ii) in the presence of an
incandescent light close to the cell and, iii) in darkness. Report the results. Connect a
voltage divider circuit shown in Fig. 1-3 using the CdS cell as R
1
and a small incandescent
light bulb as R
2
. Observe the effects of room light, light from an external incandescent bulb
and darkness on the incandescent light bulb in the circuit. Report your observations.





Fig. 1-3. Voltage Divider with a Photoresistor

• Can you think of situation where such a circuit could be used?


R
1
= photoresistor R
2
= light bulb V
in
= 15 V


Resistance of CdS cell in the room light =

Resistance of CdS cell in the presence of an external incandescent light =

Resistance of CdS cell in the darkness =


λ
V
in
R
1
R
2
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2-5. The thermistor

Thermistor: Thermistor is a resistor made of a semiconducting material whose
resistance depends on the temperature. Resistance of a thermistor decreases with
increasing temperature.

Using a DMM measure the resistance of a thermistor at 0
o
C, room temperature and at boiling
point of water. Connect again a voltage divider circuit, shown in Fig. 1-4, with the thermistor
in place of R
1
and keeping the bulb as R
2
, as shown below. Observe the effects of
temperature (thermistor at 0
o
C, room temperature and at boiling point of water) on the bulb in
the circuit. You may need to use long connecting wires to the thermistor to enable you to dip
the thermistor in boiling and ice waters. Report your observations.





Fig. 1-4 Voltage Divider with a Thermistor



R
1
= Thermistor R
2
= light bulb V
in
= 15 V


Resistance of thermistor at 0
o
C =

Resistance of thermistor at room temperature =

Resistance of thermistor at boiling water =

Temperature of boiling water =


• Can you think of situation where such a circuit could be used?