ELEC 241 Lab

Experiment 1.1

Resistive Voltage Dividers

Part 1: Measuring AC Voltage with the DMM

observe freq response limits and scale factor.


Step 1:

 Connect your BNC T connector to CH1 of the scope.

Step 2:

 Use a BNC patch cord to connect one end of the T to the function generator output. Connect a BNC clip lead to the other end of the T.


Step 3:

 Set the function generator to produce a 2 V p-p (i.e. 1 V peak), 100 Hz sine wave.

Step 4:

 Set the DMM to AC Volts and connect the probes to the clip leads. What is the voltage reading on the DMM?

Step 5:

 Set the function generator to produce a square wave. Readjust the Amplitude control if necessary to maintain the 2 V p-p amplitude. What is the voltage reading on the DMM?

Step 6:

 Reset the function generator to sine wave. Vary the frequency from 5 Hz to 5 kHz.

Question 1:

 What is the useful frequency range of the DMM for measuring AC signals?

Diversion:

 The AC voltage function of the DMM is calibrated to read (approximately) the RMS value of the waveform. RMS stands for root-mean-square i.e. the square root of the mean value of the square of the function:

We'll see the importance of this when we study power. For now, just remember that for a sine wave, .

Part 2: The Basic Voltage Divider



Step 1:

 Wire the following circuit, using 10k (brown-black-orange) resistors for both and :

What is the divider ratio ( )?

Step 2:

 Set the function generator to produce a 2 V p-p sine wave.

Step 3:

 Using the oscilloscope, measure . Is it what you expect?

Step 4:

 Without disturbing the function generator settings, replace the two 10k resistors with two 47 (yellow-violet-black) resistors. What is the output voltage now?

Step 5:

 Again leaving the function generator alone, replace both resistors with 1M (you're on your own) resistors. Measure the output voltage.

Moral:

 No circuit exists in isolation. To be useful its input or output (or both) must be connected to some other circuit. Unfortunately, the interaction between the circuit and a non-ideal source or load causes it to behave differently than it would in an idealized situation. With careful design, this interaction can be minimized or accounted for. If ignored it can reduce the performance of the system, or keep it from working altogether.

For example, for the measurements we made in this part, we would have the following model for the complete system including circuit, source, and load:



Question 2:

 Based on your measurements and the above model, what are the output resistance ( ) of the function generator and the input resistance ( ) of the scope?

Part 3: The Potentiometer (Volume Control)

part: volume control

A potentiometer (or pot for short) is a fixed value resistor with a third, movable contact or slider which may be positioned anywhere along the resistive element. The slider is connected to a screwhead which allows it to be ... If we represent the position of the slider by alpha , where alpha varies between 0 (fully counterclockwise) and 1 (fully clockwise), then the resistance between the lower end of the resistor and the slider will be and between the slider and the upper end will be , where R is the total resistance of the potentiometer.

If we connect the two fixed contacts to a voltage source and measure the output between the movable contact and one fixed contact, we get a variable voltage divider:

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Then the output is



Step 1:

 Select a 10k potentiometer from your parts kit. It will have three short wires sticking out the bottom in a triangular pattern. The center terminal is the slider contact and the two outer terminals are the fixed contacts.


Note
Figuring out the value of a pot can be tricky. Some pots are labeled directly with the value (e.g. "100" or "10K"). Others are labeled using the same code as for fixed resistors, execpt that numbers, rather than colors, are used. For example, a 10k resistor would have the bands brown-black-orange. The values of these colors are 1, 0, and 3, so a 10k pot would have the label 103"


Step 2:

 Wire the following circuit:


Step 3:

 Set the function generator to produce a 2 V p-p 100 Hz sine wave.

Step 4:

 Set the potentiometer adjustment screw to mid scale and measure .

Step 5:

 The potentiometer has a scale divided into 10 equal divisions, presumably representing 10 equal divisions of resistance. Set the potentiometer to each of these 10 divisions and measure . Was this presumption correct?

Part 4: Resistive Transducers

part: carbon mic microphone is pins 42 and 43 In Lab 2 we looked at active or generating transducers, i.e. they converted acoustical or optical energy to electrical energy and produced a voltage (or current) directly. Some transducers are passive, they don't produce voltage directly, but vary some electrical parameter (e.g. resistance) and must be connected to an external source to produce an output.

If in the volume control circuit above, were fixed and varied with time, we would have , i.e. we would have a resistive transducer. More common is where we have just a single resistor with R = f(x) , where x is the physical parameter being measured.

For example, if p(t) represents the acoustical pressure in a sound wave and we have a resistance which varies with pressure, , then in the following circuit we would have .

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The output consists of the desired signal, K p(t) , superimposed on a constant DC offset, . We saw this last week with the photodiode, and we know how to deal with it: just switch the scope to AC.


Step 1:

 Unscrew the cover from the mouthpiece of the telephone handset. (The mouthpiece cover has 37 holes.) Carefully remove the microphone cartridge.
Measure the resistance between the two contacts on the back of the microphone. This is of microphone.

Step 2:

 Shake or tap the microphone and measure again. Hold the microphone in a vertical position and measure again. How consistant is the resistance?

Step 3:

 Replace the microphone in the handset and replace the cover.

Diversion:

 Since we don't have any current sources in the lab, we'll have to build an approximate current source the way we talked about in class: by connecting a voltage source in series with a large resistor. When we do, we get the following circuit (which looks suspiciously like a voltage divider):
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Step 4:

 Connect the green and red binding posts on the breadboard to the top bus strip. Connect the gaps at the center of the bus strip to form two full width power buses.


Step 5:

 Set the Meter Selector switch on the power supply to +20V. Adjust the 0 to 20V voltage control to product 10 volts.

Step 6:

 Make sure the green banana plug from the interface board is plugged into the green binding post on the breadboard. Use a red banana patch cord to connect the 0 to +20V terminal to the red binding post on the breadboard. With a green cord, connect the Common terminal to the green binding post.

Step 7:

 Plug one end of a handset coil cord into the handset and the other end into P8 of the interface board.

Step 8:

 Connect one side of the microphone to ground by connecting pin 42 of the interface board socket strip to pin 41. Connect the other side (pin 43) to the free end of the 10 k resistor.

Step 9:

 We now should have wired the circuit shown in the figure above. Connect CH1 of the oscilloscope to .


Step 10:

 Speak into microphone and observe . What are the DC offset and the peak to peak signal amplitude?

Question 3:

 How does the signal amplitude of the carbon microphone compare with that of the microphone we used last week? Could we connect this directly to the speaker or handset earpiece and produce an audible sound?

Step 11:

 Leave this circuit assembled. We will use it in the next experiment.