Comparing AC and DC voltages with an oscilloscope A Direct Current voltage (DC) the current and voltage remains constant over time and the voltage pot...
Comparing AC and DC voltages with an oscilloscope A Direct Current voltage (DC) the current and voltage remains constant over time and the voltage potential is measured from a known reference point commonly referred to as ground. An Alternating Current voltage (AC) the current and voltage varies with time. The peak to peak voltage is measured from the maximum positive peak to the maximum negative peak. A sinusoidal waveform is a common form of AC voltage. This is the type of voltage supplied by TXU to your household. The 110 V generally associated with the wall outlet is the Root Mean Square voltage, RMS, for the supplied voltage. The peak to peak value would be higher. The relationship between the peak to peak voltage and the RMS voltage is V RMS = V p to p / 2√2
The RMS value is important because this value can be used to evaluate a circuit and obtain accurate results in the same manner as if the circuit was using a DC voltage source. Most AC voltages that vary with time has a fixed period, which is the time it takes to complete one cycle. One cycle is akin to one lap around a race track. The period would be the time it takes if measured from its positive maximum peak to go through the zero reference point, then through the maximum negative peak, back through zero and ending at the maximum positive peak. Generally an AC voltage is given in terms of frequency, which is found by taking the inverse of the period. The units of frequency are s-1 most commonly referred to as Hertz (Hz).
An oscilloscope is a device used to measure and observe AC or DC voltages. The basic operation of an oscilloscope is that an electron beam is deflected horizontally across a Cathode Ray Tube (CRT; the screen of a television is a sophisticated CRT). The amount of time it takes the beam to traverse the screen is controlled by a Time/Div (time per division) knob. When a voltage is connected to one of the input channels, this voltage cause the electron beam to deflect in the vertical direction, the amount of deflection can be controlled using the Volts/Div knob. To make the oscilloscope a useful device the screen is sectioned off into a 10x8 grid with 1 cm major divisions forming a checker board. Using the Time/Div and Volts/Div knobs the signal input can be displayed and a measurement taken to determine the voltage and period. Another way to control the horizontal and vertical deflection is to set the oscilloscope into X-Y mode. In this mode the Time/Div knob is disable and the voltages into the two input channels control the horizontal and vertical deflections. This can lead to some interesting patterns displayed on the CRT. In this lab we will refer to numbers inside of parenthesis (12), which indicate the knob shown on the picture of the oscilloscope.
Procedure
Initial oscilloscope settings
CW – Clockwise
CCW - Counter Clockwise
Time/Div. (22) 5ms Horiz. Position. (26) set to mid point Trigger Level (30) set to mid point Trigger Slope (30) + (pushed in) CH 1 coupling (11) AC CH 1 Var. gain (15) Fully CW Vertical Mode (19) CH 1 Intensity, Focus (3),(4) set to mid points
Horiz. Variable. Trigger Hold Trigger mode Trigger Source CH 1 Volts/Div CH 1 position Power switch
(25) (29) (27) (28) (13) (17) (1)
fully CW CCW to Norm Norm CH1 5V set to midpoint pushed in
The illustrations used in the procedure are an example to show positioning and where to measure. Your waveform will be similar but not necessarily exactly the same. Alternating Current, AC Voltages 1.) Set up the oscilloscope for the initial settings. Connect wires from the transformer to the channel 1 input (9) and ground (37). Turn on the oscilloscope (1) and plug in the transformer. It may be necessary to adjust the Trigger Level (30) to get a stable signal display. Use the horizontal position (26) and vertical position (17) knobs to position one of the peaks to be located at where the two main lines intersection, as illustrated to the right. Measuring the period 2 Along the horizontal line count the number of horizontal divisions from one peak to the other peak. One division is one square of the checkerboard pattern. Along the main line it is divided further into fifths of a division. The dark vertical lines represent the measurement to be made. This is one complete cycle. Record the measurement onto the data sheet. Record the Time/Div setting(22) on the data sheet. The small m on the knob is for milli or 10-3 seconds. Determine the period for one cycle by multiplying the Time/Div setting by the measured number of divisions. Record your calculations on the data sheet. Next determine the frequency of the sinusoidal waveform using frequency = 1/seconds. Record this value.
Measuring the peak to peak voltage 3.) Reposition the waveform using the horizontal position (26) and vertical position (17) knobs so that the bottom of the waveform lies upon a horizontal line and one of the top peaks is bisected by the center vertical line. See diagram. Count the number of vertical divisions from the bottom peak to the top peak. This time use the Volts/Div setting(13) and multiply it by the number of divisions to determine the peak to peak AC voltage of the signal. Record your findings. Measure the AC voltage using the Digital Multimeter. Calculate the RMS voltage using your measured peak to peak voltage. Different AC waveforms 4.) Unplug and disconnect the AC transformer. Turn on the function generator and select the sine wave function. Select the 500 Range. Connect the generator to the channel 1 input (9) and ground (37). Adjust the Time/div knob and Volts/div knobs to obtain a sign wave on the oscilloscope. Vary the Course knob on the Function generator and observe what happens to the waveform on the oscilloscope. Describe the effect that increasing the frequency has on the period of the waveform. Change the waveform type to a triangular wave and also a square wave. Sketch each waveform onto your data sheet. Direct Current DC voltages 5.) Turn off and disconnect the function generator the oscilloscope. Connect the unknown emf to the Channel 1 input ( 9) and ground (37). Set the coupling switch (11) to gnd. Set the Trigger mode switch(27) to Auto and set the Time/Div knob to 50us. Use the position knob(17) to position the trace on the center horizontal line this will be the ground reference point. Turn the coupling switch (11) to the DC position. By toggling between the gnd and DC position on the coupling switch (11) observe the effect it has on the trace. With the coupling switch set to DC adjust the volts/div switch until the trace disappears off the screen. Then adjust the volts/div switch back until you see the trace again. This will give you the most accurate reading. Count the number of divisions from the ground reference point to its maximum vertical deflection. Record the value and calculate the DC voltage
Data Sheet Step 2 AC Period & Frequency Time/Div setting _______ measured number of Horiz. divisions _____
calculated period ________ calculated frequency ________
Step 3a. AC Volts peak to peak Volts/Div setting _______ measured number of divisions _____
measured voltage peak to peak________
Step 3b. AC Volts RMS Measure the AC voltage using the DMM. _______ Using your measured peak to peak voltage from step 3a calculate the RMS voltage _______
How does the RMS voltage measured with the DMM compare with the RMS voltage determined using the peak to peak voltage?
Step 4: Different AC Waveforms Describe the effect that increasing the frequency has on the period of the waveform.
Sketch the waveforms Sine:
Triangular
Step 5: DC volts Volts/Div setting _______ measured number of divisions _____
