Lab 4: Using the Digital Multimeter (DMM) Introduction: Parts of the DMM (to be explained by Instructor or T/A) •
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Leads Function selector o DC Volts o AC Volts o DC Current o AC Current o Resistance o Scales: 0‐2mV, 0‐200mV, 0‐2.0V, 0‐20V, 0‐200V Readout DC vs. AC Fuse How the DMM Works •
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Measuring voltage o Connecting the DMM o A/D conversion o Scaling o Readout Measuring current o Connecting the DMM o Converting current to voltage (high current) o Converting current to voltage (low current) o Scaling Measuring resistance o Connecting the DMM o Converting resistance to voltage o Diode check function o Continuity testing Other functions •
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Transistor testing Capacitance meter Diode tester Battery tester Frequency counter Breadboards and their use Resistor Color Codes Resistor values can be determined by “reading” the bands of color printed on the resistor. Resistors of different wattages showing color code bands. Bottom = 0.25W. Color bands give resistance and tolerance. Gold tolerance band is +/‐ 5%, silver is +/‐ 10%. Read values by taking first two digits and multiply by 10d3 where d3 is the number represented by the third color band. Chart is below. For example the resistor in the middle is a 1W, 47 x 103 Ω or 47kΩ resistor. The one above it is 2W, 27 x 102Ω or 2700Ω or 2.7kΩ. Potentiometers Potentiometers are resistors whose value can be varied, depending on the position of a shaft or screw. The picture below shows some examples of different styles. Potentiometers. Circular ones work like the diagram on the left. The center terminal is connected to the “wiper”, which conducts a piece of carbon, plastic or coil of wire. The rectangular “pot” is constructed as shown on the right, with a tiny lead screw for adjustment. Experiment 1: Resistor tolerances and DMM accuracy Equipment: DMM, five x 1kΩ resistors. Procedure: • Label resistors with your lab station number and a letter, a,b,c, d or e • Lab partner 1 measure the resistance of each resistor and record • Lab partner 2 measure the resistance of each resistor and record • Repeat, so that each person measures each resistor three times. • Trade multimeters with one of the other groups at your bench and repeat, noting the new DMM in your records. Compare the results by analyzing the differences between the measurements to see if you can detect the influence of the DMM, the influence of the operator, and the tolerance of the resistors. Writeup: Write up this experiment as instructed in the Lab Report Guide, posted on eCampus under the “Labs” folder. Pay special attention to the analysis section. Experiment 2: LED Current and output intensity Equipment: DMM, LED, 4 x 1kΩ resistor, breadboard, power supply, wires Procedure: • Construct a circuit as shown in Figure 1. The LED has two leads and a flat spot on one side of the case. The Anode is usually the longer of the two leads, and the cathode has the flat spot. The Anode goes toward the “+” side of the circuit and the cathode toward the “–“ side. Measure the resistance of the resistor you use, or use one with known resistance from Experiment 1, but record the value. • Turn on the power supply and switch the DMM to measure DC Volts. Measure the voltage across the LED. Record the reading. Measure the supply voltage and record the reading. Note that these two readings, plus the knowledge of the resistor value allow you to compute the current through the circuit. Do that now and record it. • Measure the resistance of a second 1kΩ resistor and add it in parallel with the first one. Note what happens to the brightness of the LED. Compute the equivalent resistance of the two parallel resistors and record this value. • Switch the DMM back to DC Volts function. Measure and record the voltage across the LED. Compute the current through the circuit. •
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Measure the resistance of a third 1kΩ resistor and add it in parallel to the other two. Again record the voltage across the LED. Don’t forget to switch back to DC Volts before trying to measure voltage!! Compute the equivalent resistance of the three resistors in parallel and record the value. Compute the current through the circuit. Now set up the DMM to measure current. The 0‐
20mA scale should be adequate. Insert the DMM into the circuit as shown in the lab introduction, and measure the current through the LED with all three resistors in parallel. Record the current for this arrangement. Is it the same as the current you computed? Remove the third resistor from the parallel Figure 1: Diode testing circuit arrangement and again measure and record the current. Compare to the computed current. Remove the second resistor from the parallel arrangement and again measure and record the current. Compare to the computed current. Measured
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Equiv. R Parallel
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Writeup • Write up this experiment as instructed in the Lab Report Guide. Plot diode current versus diode voltage, diode current versus series resistance and diode current versus 1/(series resistance). In each plot, the current is the dependent variable (vertical axis). Explain the results. Discuss any discrepancies you find between the measured and calculated values for the circuit current. Experiment 3: Photoresistor Equipment Breadboard, photoresistor, DMM, assorted fixed resistors, potentiometer, 5V power supply, wires. Procedure • Insert the photoresistor into the breadboard so it faces “up” toward the ceiling. • Measure and record the resistance of the photoresistor with no “shade” on it. • Cover the photoresistor completely and again measure and record its resistance. • Experiment with partially covering the photoresistor and determine how the resistance changes with different degrees of coverage. Project Design a circuit that uses a 5V power supply, one or more fixed resistors and a photoresistor (PR) to make a day/night detector. Depending on the range of your photoresistor you may or may not have the fixed resistors you need in your lab kit to construct your design. That’s OK, create the design and we’ll find you the resistors you need. Your circuit should be set up so that if it is “daylight” the voltage at the output of the circuit is less than 2 Volts, and if it is “night” the voltage is greater than 3.5 volts. To do this, you should have a photoresistor in which the ratio of Rdark to Rlight is at least 4:1, preferably more. If your PR is not that sensitive, try shining a flashlight onto it during the “daylight” phase and re‐measure its resistance to see if you get a 4:1 or better ratio. If not, speak to a T/A about getting a different PR. Document your design , including your calculations to choose a fixed resistor value, and build your circuit. Show the T/A your calculations and demonstrate your circuit to a T/A by showing the output on your DMM for the dark and daylight situations. Experiment 4: Potentiometers Equipment Potentiometer, DMM, 5V power supply, wires. Procedure Connect the power supply to your potentiometer as shown in the diagram at right. 1. Measure the voltage from ground to the center terminal of the potentiometer. Turn the shaft of the pot all the way counterclockwise and slowly turn it clockwise and watch as the voltage changes on the DMM. 2. Disconnect the power supply and set the potentiometer somewhere in the middle of its range. Measure the resistance between the “left” terminal and the wiper and between the “right” terminal and the wiper. Now change the position of the shaft and read again. Predict the voltage you will read at the wiper when you reconnect the power supply. Try it and verify your prediction. Writeup No writeup for this experiment.