ENG 214 - Laboratory 3

Introduction to Electrical Measurements Using the Oscilloscope

(Including RMS and Transformers)

Objective: To introduce the use of the oscilloscope for dc and ac voltage measurements;

to provide experience in the measurement of both sinusoidal and of non-sinusoidal

repetitive waveforms, including calculating their RMS levels; and to introduce the

characteristics and properties of electrical transformers.

1. An oscilloscope is at every electrical laboratory test station. Prior to lab

familiarize yourselves with the oscilloscopes operation and controls. The manual

is on the Data Sheet Servers under Manufacturers/Agilent.

2. The oscilloscope allows the measurement of one or two voltage waveforms

(channels) simultaneously. Some specialized oscilloscopes allow the

measurement of more simultaneous channels. Set the scope to channel (CH) 1.

Pressing the Auto button will set the oscilloscope to the approximately correct

setting for measuring a signal connected to the scope. (These settings may not

always be the best for all measurements).

3. Connection to the oscilloscope may be by shielded electrical cables or by special

scope probes. Scope probes usually are available as X1, X10 or X100 voltage

dividers. Investigate oscilloscope probes using the Internet prior to coming to lab.

Select two probes available in the stockroom for use with an oscilloscope during

the lab.

4. Repeat step 5 of Laboratory 1, which dealt with the construction of a voltage

divider. Use the oscilloscope as a dc voltmeter. Set a power supply to 8 V dc.

Construct a 4-to-1 voltage divider and measure the resulting dc voltage. (The dc

voltage level is indicated by the change in level from the ground reference and is

also given by the peak voltage). Measure the voltage with your multi-meter for

comparison.

5. Use the signal generator at your test station to produce a 1 kHz frequency

sinusoidal wave of 4 volts peak (8 volts peak-peak) when connected to the input

of the voltage divider. (The voltage at the output of the generator is not normally

the same as indicated on the generators display). Use the oscilloscope as an ac

voltmeter to obtain the correct voltage levels. (The sinusoidal signal should have

a zero average value, i.e. the maximum value of the sinusoid is + 4.0 volts and the

minimum value is -4 volts. If you cannot obtain 8 volts, try increasing the values

of the resistors used to make the voltage divider to greater than 1 k ohm).

6. Measure the voltage produced at the output of the voltage divider using the

oscilloscope. Turn on CH 2 of the scope and measure the input and output of the

voltage divider simultaneously. Use the scope to measure the frequency produced

by the generator. Record the scope traces using the computer (Scope Control

Sample Application on the desktop

7. Measure the voltage dividers output voltage using your groups multi-meter (ac

scale), using the Agilent (true RMS) voltmeter (ac scale) at the lab stations and

using the oscilloscopes RMS voltage function. Are the measured voltages all the

same? (The RMS voltage of a sinusoidal wave is 0.707 of its peak voltage).

8. Prior to the lab use Matlab to calculate the RMS voltage of a rectangular pulse

shaped waveform with a period of 1 ms. The wave should have a minimum peak

of 0 V and a maximum peak of 4 V. Calculate the RMS voltage for ON time

duty cycles of 20, 50 and 80 percent. Also calculate the RMS voltage for a

triangular wave of the same voltage levels (going from 0 to 4 volts and back to 0

volts at the end of the period).

9. Use the signal generator to produce the rectangular waveforms of step 8. Measure

the voltage of the three waveform using your groups multi-meter (ac scale),

using the Agilent (true RMS) voltmeter (ac scale) and using the oscilloscopes

RMS voltage function. (When you set the voltage from the generator to 4 V p-p,

becareful not to include any spikes at the ends of the waveform in your

measurement. Use the scale on the oscilloscope to insure you have the correct

voltage). Record the waveforms seen on the oscilloscope.

10. Repeat step 9 for the triangular wave. (50% symmetry saw tooth wave).

11. Why are the measured voltages not all the same? (Hint: The oscilloscope gives

the correct RMS voltage). Discusses any differences between the calculated and

measured waveforms. Discuss how you might design a true RMS voltmeter

(search the web for ideas).

12. Prior to lab research transformers on the Internet. Become familiar with the

relationship between Vin/Vout, Iin/Iout and Zin/Zout and transformer turns ratio.

13. Obtain a test transformer from the stockroom. Connect the output leads (black) across a 1 k ohm load resistor. Connect the input leads (red) to a signal generator set for a

sinusoidal signal at a frequency of 60 Hz. Adjust the signal generator for a voltage of

about 10 V peak-peak (or as high as you can get) across the input of the transformer.

Measure the output voltage across the 1 k ohm output load resistor. Measure the current

flowing into the input of the transformer. (Use the oscilloscope for all measurements.

To measure input current connect a resistor of 1 ohm is series with the ground side

of transformers input, and measure the voltage across this resistor to obtain the

current. Be careful to use a common ground for the voltage measurements, or else

you will short out the voltage. Also be sure to subtract the voltage across this

resistor from the input voltage to get the input voltage to the transformer.)

14. Calculate the ratio of the output voltage to the input voltage. Calculate the current flowing through the output load resistor. From the Vout to Vin ratio estimate the turns

ratio of the transformer. What is the impedance (resistance) seen at the transformers

input (Vin/Iin)? Calculate the power flowing into the transformer and the power flowing

out of the transformer into the load resistor. What is the approximate power loss of the

transformer? What the power transfer efficiency of the transformer?

15. Repeat the last two steps for frequencies of 600 Hz and 6 kHz.

16. Measure with your ohm meter the dc resistance of the input and output winds of your transformer. Is the dc resistance of the transformer windings a significant factor

compared to the input and output resistances?

17. Qualitatively observe the input and output waveforms of the transformer? What happens when you switch to a triangular wave and a square wave? (Record the waveforms.)

18. What is the ratio of Vout/Vin of the transformer as a function frequency? Graph this ratio versus frequency. Compare the performance (efficiency) of the transformer at

different frequencies? Graph efficiency vs. frequency. What would happen if you

applied dc to the input of the transformer? (You can try 1 V dc.) What is the highest

frequency you would recommend for use of the transformer? Can the transformer be

used with triangle and/or a square wave?