Lab 1: DC Multimeter Measurements

Objective

The main goal of this experiment is for you to learn how to construct circuits on a breadboard and use a multimeter for measurements. Along the way, you should also come to understand the resistor color code, Ohm’s Law, and the distinction between series and parallel circuit elements.

Concepts: Voltage, Current, Resistance, and Ohm’s Law

DC (Direct Current) voltage and current do not vary with time. Voltage is an energy difference between two points. When a voltage source is included in a closed path with resistors, a circuit is completed, and current flows. Current is electron flow, or moving charge. When current flows across a resistor, which impedes current flow, voltage drop occurs across the resistor. This relationship between voltage (V), current (I), and resistance (R) is known as Ohm’s Law:

V = IR (1)Thus, if two of the above quantities are known, the third can be calculated.In the experiment, you will use a multimeter to measure resistance, voltage, and current. The experimental results will then be compared against Ohm’s Law.

Equipment and Components

This experiment requires the use of the Agilent E3630A Power Supply and Fluke Digital Multimeter (DMM), as well as a breadboard, 22 AWG wire, and resistors with nominal values of 1.5 k, 2.2 k, 3.3 k, and 4.7 k.

Operation of the Agilent E3630A Power Supply

To power your circuit, you‘ll use the Agilent E3630A, shown in Figure 1. You can use either the +6V, +20V or the -20V supplies by pressing the button on the button labeled Meter. The red lead goes to the selected supply (+6V, +20V,-20V), which is connected in the lower-right side labeled output. This terminal provides a voltage source to the circuit. The COM terminal is considered the 0 V (ground) reference voltages. Always begin by turning the knob for the selected supply to the

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counter-clockwise stop so that voltage begins at 0 V. These Knobs can be located in the upper-right side of the power supply under the label Voltage Adjust.

The power supply should always be turned off until a circuit is complete and ready for use. Once a circuit is complete, the supply can be turned on, and the voltage can be adjusted by turning the voltage knobs slowly clockwise until the desired voltage is reached.

Figure 1: Agilent E3630A front panel

Operation of the Fluke DMM

The multimeter you’ll be using in this lab is the Fluke 189 DMM, shown in Figure 2. A multimeter is a device used to take measurements of electrical quantities such as resistance, current, and voltage. DMM stands for Digital Multi Meter.

To use the DMM as a voltmeter to measure voltage, insert a red wire in the jack labeled V and a black wire in the jack labeled COM. Set the meter knob to the voltage setting. Voltmeters have very high resistance that typically exceeds 1 MΩ, so when making voltage measurements, be sure the voltmeter is connected in parallel with the circuit component(s) across which voltage is measured. A common mistake is to connect the voltmeter in series with the circuit components. This error would add a 1MΩ series resistance to the circuit and drastically change the circuit parameters.

For use as an ammeter to measure current, insert a red wire in the 10 A max fused input, and black wire in the jack labeled COM. Turn the knob to measure current, and select a current range (mA or (A) dependent on the expected current measured. For this experiment the 20m range is the most appropriate. Ammeters have very low resistance that typically is less then 0.5 Ω. A common mistake is to connect the ammeter in parallel with the circuit components. This error would effectively cause a short circuit, altering the circuit parameters, and possibly damaging the ammeter. When making current measurements, make certain the ammeter is connected in series with the circuit components through which current is measured. Never connect an ammeter directly across a power supply as it will cause a short circuit and will certainly damage the meter.

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a b cFigure 2: Fluke Digital Multimeter shown in three different measurement configurations

Prelab (25 points) – Due at the beginning of lab1. In the discussion session, you were introduced to the color code by which the nominal values of resistors are represented. The nominal values of the resistors you will use in the lab are listed on the data sheet at the end of this document. Next to each value, write the color code. (10 points)

2. The Fluke DMM is shown in three different measurement configurations. For each setup a, b, and c, in Figure 2, determine what electrical quantity the meter is set to measure, and write them in the data sheet. Refer to the online user’s manual. (10 points)

3. You’ll be using a multimeter to measure voltage and current. Voltage through a circuit element such as a resistor is always measured in parallel with the circuit element, while current is always measured in series. Two circuit elements are in series with one another if there is no alternate path between them for current to flow. If two circuit elements are in series, the same current flows through them. If there is an alternate path between them, the elements are in parallel. Figures 1 and 2 show examples of series and parallel circuit elements.

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R 1

1 kV 1

5 V d c R 21 k

0

R 1

1 kV 1

5 V d c R 21 k

0

DMM

Figure 3: Series resistive circuit Figure 4: DMM in parallel with R2

In Figure 1, the voltage source, V1, is in series with the resistor R1, since there is no other path between them. Likewise, R1 is in series with R2. In Figure 2, the voltage source is still in series with R1. The DMM, is in parallel with R2, since current can be split between them. Resistor voltage is measured with the DMM in parallel with a resistor, and current is measured with it in series. Using Figures 1 and 2 as examples, draw the DMM in series with a resistor and voltage source on the data sheet. (5 points)

Part 1: Resistance MeasurementsResistor values vary slightly from their nominal value. Since voltage and current are related to resistance, it’s a good idea to measure the actual resistance value so you know exactly what you’re working with. Manufacturers specify the percent error from which the actual value of a resistor can deviate from the nominal value. This quantity is called the tolerance. Your first task is to measure the resistance of the four resistors from the prelab and determine whether they fit within the manufacturer’s specified tolerance.

1. A gold band represents 5% tolerance, a silver band represents 10% tolerance, and no band represents 20% tolerance. Write the tolerance for each resistor in your data sheet.

2. Measure the resistance and enter the values in the data sheet. The resistor should not touch your fingers or be part of a circuit when you measure it. The resistor leads should only touch the DMM terminals.

Calculate and enter the percent error for each resistor using the following formula:

Percent Error = ((Nominal value– Measured value) / Nominal value) x 100% (2)

On the data sheet, state whether each resistor meets the manufacturer’s tolerance specifications.

3. On the data sheet, give the reason why your fingers can’t touch the resistor when measuring it.

Part 2: Voltage and Current Measurements for Single-Resistor Circuit s

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Our first circuit will be a simple series circuit in which the source voltage, V s, the resistor voltage, VR1, and their corresponding current will be measured. The current will then be checked using Ohm’s Law. First assemble the circuit shown in Figure 5.

8 V d c R 14 . 7 k

0

Figure 5: Series circuit for measurements

Use the DMM to make the following measurements and write the values in the data sheet.

1. For the circuit in Figure 5, use the DMM to measure Vs and write this value on the data sheet. This value is the source voltage. Since the meter on the power supply is difficult to read accurately, it is a good idea to always measure the source voltage so that you know the circuit input. If the voltage value was not exactly 8 V, use the FINE knob and the DMM to get as close to 8 V as possible.

2. Reconfigure the meter to measure voltage instead of resistance. Measure the voltage across the resistor, as shown in Figure 6. The multimeter should be in parallel with the resistor for use as a voltmeter. This is because the voltmeter has a very high input resistance, and current tends toward the path of least resistance, and ideally we want all the current to pass through the resistor.

Figure 6: DMM in parallel to measure voltage Figure 7: DMM in series to measure current

3. Measure the current through the resistor as shown in Figure 7. Remember that the multimeter must be in series with the resistor to measure current, since ammeters (current meters) have low input resistance. We connect the DMM in series so that the current passing through the meter and the resistor are the same. Placing the DMM in parallel would short the circuit. This could blow a fuse in the meter, and you’d wait around while someone fixes it.

5

Lodestar 8102

+

Vs

-

4. Since the voltage source and resistor are the only circuit elements, all the voltage in the circuit is going across the one resistor. Given Ohm’s Law in equation (1), use your measured values of voltage and current to calculate the resistance.

5. Measure the voltage across the resistor again, but disconnect one end of the resistor while still measuring the voltage. One end of the multimeter should be connected to the end of the resistor that’s connected to the circuit, and the other end should be connected to the point on the circuit board where you disconnected the resistor. Enter the value in the data sheet. This value is known as the open-circuit voltage, Voc.

6. Now do the same thing with current. Measure the current, and without changing anything else in the circuit, disconnect one side of the resistor. Write the value of the open-circuit current in the data sheet.

What you should observe from steps 5 and 6 is that current needs a closed electrical path to flow. In this sense, it can be thought of as analogous to water flowing through a pipe. Voltage, however, can exist in an open circuit, since it is an energy difference between two points. In this sense, you can think of voltage as the pressure in a water pipe that exists when the faucet is turned off.

7. Compare your measured and calculated values of I and include your observations in your answer to question 4 on your data sheet.

Part 3: Voltage and Current Measurements for Four-Resistor Circuit .This last part of the lab is to gain get some practice taking voltage and current measurements, and experimentally determine resistance values using Ohm’s Law. First build the circuit shown in Figure 8.

Figure 8: Series circuit for measurements

1. As always, measure the source voltage and adjust it to be as close to the nominal value (in this case 10 V) as possible.

2. Answer question 5 on the data sheet. Refer to Question 3 of the Prelab on page 3 for hints.

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3. Measure the voltage across each resistor and write the values on the data sheet. To check if your voltage values are correct, use Kirchhoff’s Voltage Law (KVL). It states that the algebraic sum of the voltages around each loop of a circuit must equal zero. This will be the case if your measurements are right.

4. Measure the current through each resistor and write those values on the data sheet. You can check your measurements by Kirchhoff’s Current Law (KCL). It says that the current into a node (a point on a circuit where two or more circuit elements meet) is equal to current out of a node. Another way of putting it is that all the currents into a node (taking, say current out being positive and current in being negative – although you can also do it the opposite way, with current out being defined as negative and incoming current being positive – either way, you’ll get the same result) algebraically sum to zero. Kirchhoff’s Laws are based on conservation of energy, and we’ll be looking at them again in the next lab.

5. Use Ohm’s Law to calculate the resistance for each resistor and enter the values in the data sheet.

6. Lastly, calculate the percent error between your calculated resistance values and the nominal values.

When you’re finished, turn in the data sheets (pages 8 and 9) to your TA.

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Lab 1: DC Resistive Measurements Data SheetName_____________________________ Section________

Prelab (due at the beginning of lab). .

1. Next to each value, write the corresponding color code.

1.5 k _______________________________ 2.2 k _______________________________

3.3 k _______________________________ 4.7 k _______________________________

Figure 2a is set to measure _________________________

Figure 2b is set to measure _________________________

Figure 2c is set to measure _________________________

3. In the space below, draw the multimeter in series with a voltage source and resistor.

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Part 1: Resistance Measurements .1. Color codes, measurements, and percent error

Nominal Resistance

()Color Code

Nominal Tolerance

(%)

Measured Resistance

()Percent

Error (%)

Within Manufacturer’s

Spec (y/n)

1500

2200

3300

4700

2. Why can’t your fingers be touching the resistor when measuring it?

Part 2: Voltage and Current Measurements for Single-Resistor Circuit3. Voltage and current values

VS (V) VR1 (V) IR1 (mA) V/I () VOC (V) IOC (mA)

4. How does the value of V/I compare to the value you measured in Part 1? Give reasons for the similarities or differences.

Part 3: Voltage and Current Measurements for Four-Resistor Circuit

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5. Which two resistors should have the same current through them? Why?

6. For each resistor below, list the voltage, current, value of V/I, and percent error.

1.5 k 2.2 k 3.3 k 4.7 k

V (V)

I (mA)

V/I ()R PercentError (%)

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