Lab #1 Basic Measurements - University of Evansville



Lab #1 Basic Measurements

In this lab experiment we will be investigating some of the techniques to measure basic circuit properties such as resistance, voltage, and current. First, let us look at some theory about the quantities that we will be measuring.

Current: The flow of electrons. Quantitatively, current is the number of electrons flowing by a point of wire per second. The Amp is the unit of current and one Amp is equal to one Coulomb of charge per second.

Voltage: Voltage is a convenient way of saying “electrical potential.” A voltage may exist across any circuit element except a “perfect” wire. The unit of voltage is the Volt.

Resistance: A circuit characteristic that opposes the flow of current. The unit of resistance is the Ohm, symbolized by (.

A good analogy is to think of water flowing through a hose. The water flowing through the hose is the current, the water pressure is the voltage, and the resistance is equivalent to the nozzle setting on the end of the hose. When the nozzle is disconnected, lots of water (current) flows, but it comes out with little water pressure. If you try to put your finger over the end of the hose, the water squirts farther because you have increased the pressure (voltage) by increasing the resistance to the flow of water. Figure 1.1 shows the circuit symbol of a resistor with its voltage, current and resistance relationships.

Ohm’s Law

Ohms Law gives the relationship between voltage, V, the current, I, and the resistance, R, in a circuit element. Mathematically, Ohm’s Law is

(1.1)

Lab Equipment

The Multimeter

The multimeter is a basic circuit laboratory tool used to measure voltage, current, and resistance. This tool will become your best friend in the laboratory due to its endless versatility. There are two common types of multimeters, digital (DMM), and analog (AMM). Although we will be using a digital multimeter throughout this course, much of the following discussion applies to analog multimeters as well. The multimeter, or meter for short, is connected to the circuit under test via two leads (or probes). The leads usually have a male banana connection on one end, which plugs into the meter. The other end can have either a “grabber” type connection such as an alligator clip, or a bare metal tip much like the point of a pencil. The meter leads are normally color-coded, one red and one black. It is standard practice to connect the black lead to the negative (or reference) jack on the meter, and the red lead to the positive jack. While the meter is colorblind and does not care if you color-code your leads, it is good practice and will save your time and effort when using the meter. The value indicated by the DMM is determined by reading the digital value on the face of the meter.

Warning: Due to the display limitations of the LCD readout on the DMM, the symbol “M” means milli, or 10-3 when displaying Amps or Volts, and mega, or 106 when displaying Ohms.

The function buttons determine which variable the DMM will measure: volts, amperes or ohms. DC volts and amperes measurement functions, typically labeled VDC and ADC, are available on most DMMs. Some meters also provide a measurement function for time-varying signals, usually labeled VAC and AAC, which are collectively known as “alternating current” (AC) signals. The AC notation indicates that the signal changes periodically with time. The DC notation is used to indicate that the signal has a constant magnitude over time. This experiment uses a widely accepted vernacular to describe the three standard functions on AMMs and DMMs. Ohmmeter identifies an instrument used to measure resistance. Ammeter and voltmeter, as you might suspect, describe instruments used to measure current and voltage. These terms are often used in reference to a multimeter that is set to the specific function described by the term. The scale switch is used to set the sensitivity (or range) of the AMM. Typically, DMMs can measure voltage, current, and resistance on several different scales, with the capability of making measurements spanning three or more decades. In our case, the meter is auto ranging and will go to the scale best suited for the measurement by itself.

The Power Supply

The power supply is used to produce a variable DC output voltage between (20V. Figure 1.2 shows a schematic of the power supply.

The power supply as shown in Figure 1.2 is a triple output power supply. It is capable of producing an output of 0V to 6V, 0V to 20V or negative 20V to 0V. Which of these output ranges you use depends on how you connect the power supply to your circuit. There is a selector dial on the left side of the power supply (not shown in schematic) to select which of the three ranges will be displayed on the analog meters shown on the front of the power supply. It is important to note that you should not depend on those analog displays for important data. Use them only as a gauge and measure the output of the meter with the DMM to be more accurate. The dials on each of the lower corners of the power supply adjust the output voltage for their specific ranges. If you want to use the 6V scale of the supply, connect up your power between the +6V and common (com) connection points. If you desire to use the 20V scale, connect your output between the +20V and com connections. For the –20V scale, use the –20V and com connections. This is a floating power supply. That means that the output of the power supply is not directly connected to earth ground inside the power supply. That is the reason for the extra earth ground connection on the supply. If you wish for the output of the supply to be connected to earth ground, simply connect a wire between the earth ground connection point and the com point on the supply. You should note that the three scales operate independently of one another and you may use all three outputs simultaneously even though you will only be able to see one of them on the analog meters on the supply. For the sake of completeness, the tracking dial allows you to adjust the ratio between the +20 and –20 volt scales. You would use this if you needed to simultaneously have +12V and –18V supplies for instance.

Note: The output of the power supply is between the +6V, +20V, -20V and com connections and not the +6V, +20V, -20V and earth ground points!

Protoboard

The protoboard or breadboard as they are commonly called is a tool used for constructing and testing a circuit. Don’t let the size of the protoboard fool you. Even small protoboards can accommodate surprising large and complex circuits. Skill at using a protoboard is an item found in every practicing engineer’s toolbox. A protoboard is shown in Figure 1.3.

The protoboard will either be your best friend or worst enemy. It is a friend because it allows you to easily and neatly fabricate complex circuits. It can be an enemy if you forget how the protoboard is connected. It can also be an enemy because protoboards are infamous for having bad connections. The connections on a protoboard are easy to remember. See Figure 1.4 for a protoboard with the internal connections shown with a thick black line. The three banana type connections on the top of the protoboard are for the convenience of connecting power or other inputs to the protoboard. They are not connected to anything else on the board unless you make the connection yourself with a wire. The strips along both vertical sides of the protoboard are common. This means that the points in the first column on the left as shown in the figure are connected internally. Therefore any wire inserted in column 1 will share a common node with any other wire inserted in column 1 no matter which row you select. The same is true of column 2. However, column 1 and 2 are not connected. The same connections apply to the 2 columns on the right side of the protoboard. These columns or strips are usually used as power or ground busses when building circuits. The two large groups of points in the middle of the protoboard are where you would place the majority of your circuit components. Each horizontal row of 5 points is common to itself, but not connected to any other group of 5 points. This means that the two groups of five points in any row are not connected across the divide that runs vertically through the center of the protoboard. Also, the each horizontal group of 5 points is not connected to the 2 vertical columns that run vertically on the sides of the protoboard.

Measuring Voltage, Current, and Resistance

Voltage is measured in parallel. That means you connect up the DMM in parallel with the voltage you would like to measure. Figure 1.5 shows this.

In Figure 1.5, the DMM is connected to measure the voltage across the resistor R3. This is labeled as VAB in the figure. When two subscripts are used to identify a voltage, the first subscript is at the more positive potential. So VAB is the voltage at point A relative to point B. If a voltage has only one subscript, it is the voltage at that point relative to ground. When measuring voltage with a DMM, be sure that the probes are connected between the voltage measurement terminals and the proper setting for either AC or DC measurement has been selected. The voltage terminals on any meter will usually be labeled. In the case of our DMM, this is labeled as a 300V maximum input between the upper two inputs. This means the upper two inputs are used to measure voltage and we can measure any voltage up to 300V.

Current is measured by placing the meter in series with the current being measured. This is shown in Figure 1.6.

Figure 1.6 shows the DMM connected to measure the current flowing through the resistor R3. This is labeled as IAB. Current labels are always from the tip to the tail as shown in the Figure. When measuring current, you have to actually break a connection in the circuit, and insert the meter in series where the break was made. Changing circuit connections like this should always be done when the power is turned off. On our DMM, current is measured between the bottom two connections and we can measure up to 10A. The voltage and current measurement connections on the DMM share a common point. This is the ground connection for voltage measurement and the tail of the arrow for current measurement. When measuring current, be sure that the current button has been selected for either AC or DC current measurement.

Resistance is measured similar to voltage and through the same terminals on the meter. It is measured in parallel. Resistance cannot be measured with a DMM when power is connected to the circuit. So to measure equivalent resistance between two points, turn off the power supply and replace it with a short circuit. This is shown in Figure 1.7.

In Figure 1.7, the DMM is connected in parallel with resistor R3. Since there are other resistors connected to the same points (A and B), the DMM will measure the total resistance between the points, A and B. If it is desired to measure only the resistance of R3, then R3 must be removed or at least separated from the circuit. This is shown in Figure 1.8.

The resistance being measured in Figure 1.8 is the resistance of R3 since R1 and R2 have been removed from the circuit. When measuring resistance, be sure the resistance button has been pressed on the DMM. Also note that resistance does not have a polarity associated with it so RAB = RBA.

Things to Remember when Measuring Voltage, Resistance, and Current

1. Voltage is measured in parallel.

2. Current is measured in series.

3. Resistance is measured in parallel.

4. Resistance is measured with the power off.

5. Select the correct function on the DMM (Voltage, Current, or Resistance).

6. Be aware if you measuring an elements resistance or a circuits equivalent resistance.

Instructional Objectives

1. Measure the resistance of a resistor using the DMM.

2. Measure DC voltage using the DMM.

3. Measure DC current with the DMM.

4. Determine the resistance of a resistor by reading the color bands.

5. Determine the resistance of a resistor using Ohm’s law.

Procedure

1. We would like to measure the resistance of several resistors. Why bother, since they are color coded to their resistance values, right? Well yes, but not very accurately. Remember that the last band tells us the tolerance of the resistor. So a 10k( resistor with a tolerance of 5% could have a value anywhere from 9500( to 10500(. If you happen to pick one resistor that is on the low end and one on the high end, you could have two resistors that are marked the same but whose values differ by 10%. Get 5 resistors from your TA and record their labeled resistance value and tolerance values. Check the validity of your Ohmmeter and probes using the “2 wire (” setting on the DMM. It should read about an Ohm. Measure the resistance of each of the resistors to the nearest Ohm with the DMM. Record your data in Table 1.1. Make sure that each lab partner has an opportunity to measure this important electrical quantity.

Warning: When measuring resistance, avoid touching your skin to both probe tips at the same time because the resistance of your body will affect the reading.

|Labeled Resistance |Measured Resistance |Labeled Tolerance |

|(() |(() |(%) |

| | | |

| | | |

| | | |

| | | |

| | | |

Table 1.1: Measured and Labeled Resistance Data.

2. It is good engineering practice to turn all power supply voltage adjustments all the way down when finished working with them to avoid accidentally damaging equipment. Set the function switch on the DMM to DC Voltage.

3. Turn on the power supply and turn the output all the way up. Observing proper polarity, measure the maximum output of the supply using the +20V outputs. Turn the output all the way down and measure the minimum output from the +20V output. Record your data in Table 1.2.

4. Repeat step 3 for the +6V and –20V outputs.

|Output Setting |Maximum Output Voltage |Minimum Output Voltage |

|+20V | | |

|+6V | | |

|-20V | | |

Table 1.2: Output Voltages of the DC Power Supply.

5. With the power supply off, connect the common from your power supply to the black post of the protoboard. Connect the hot (+20V positive side of the power supply) to the red post. Using wire with the ends appropriately stripped, make a connection from each post to a different vertical column on the sides of the protoboard. This will serve as the hot and ground bus. Insert your resistor into the middle area of your protoboard being sure that both ends of your resistor are not connected to the same group of five points. With a jumper wire, connect one end of the resistor to the positive bus and one end to the negative bus. This can be done by inserting the jumper wire into the same group of five points on the protoboard that one end of the resistor is in, and the other end into the column of the positive or negative bus.

Warning: If the current needle on your power supply pegs, immediately turn off your power supply and correct the problem, most likely a short to ground.

6. Set the DMM for DC voltage measurement. Turn on the power supply and adjust the power supply voltage to be 10V. Record the actual value of the voltage across the resistor to the nearest hundredth of a volt. Turn off the power supply and remove the jumper wire that connects the ground end of the resistor to the negative bus. Turn on the power supply and measure the voltage across the resistor.

Vresistor (original jumper position) ____________________

Vresistor (final jumper position) ____________________

7. Construct the circuit shown in Figure 1.9 on your breadboard using one of your resistors.

8. Measure the value of Vs using the voltage settings on your DMM. Then configure the DMM to measure current (push Amps DC setting and use the two bottom inputs for current). Measure the current in your resistor. Turn off the power, replace the resistor with another one, turn on the power supply and again measure the current. Do this for each of your five resistors and record the data in Table 1.3. Make sure each of the lab partners has an opportunity to change the circuit and measure current.

Vs ____________________

|Labeled Resistance |Measured Current |

|(() |(mA) |

| | |

| | |

| | |

| | |

| | |

Table 1.3: Measured Current Data.

9. In step 8 you determined the resistance by an indirect measurement since you measured the voltage across and the current through the resistor. Using Ohm’s Law and the values of voltage and current you measured in step 8, calculate the resistance of each of your resistor. If the resistance is more than 5% different from the nominal value, you may have measured the current incorrectly in step 8. If this is the case, take this opportunity to redo step 8.

10. Obtain a mystery resistor from the TA. Using the protoboard, construct a circuit that will allow you to measure the voltage across the resistor and the current through it. Record these values.

VmysteryR ____________________ ImysteryR ____________________

11. As verification, measure the resistance of the mystery resistor directly with the DMM.

RmysteryR ____________________

Post Lab Questions

1. Explain why there is still a resistance reading on the DMM even when the two leads are shorted together.

2. In this lab, you got different resistance values depending on the method of measurement. Explain the possible origins of any error in these resistance values.

3. What range of resistance values would you expect to measure if a resistor had bands of brown-black-green-gold?

4. Calculate the resistance of each resistor using the data you measured in step 8. Using the resistance values you measured in step 1 as the accepted or true value of resistance, calculate the percent error for your indirect resistance measurements.

5. Calculate the percent difference in the two methods you used to determine the value of the mystery resistor.

6. Explain what happened in step 6 when you changed the position of the jumper wire.

Name: ____________________ Section: ____________________

Pre-Lab #2: Voltage and Current Division

1. Calculate the values of V1, V2 and V3 for the circuit shown in Figure 2.0a.

2. Calculate the values of I1, I2, and I3 for the circuit shown in Figure 2.0b.

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Figure 1.1: Resistor Circuit Symbol.

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Figure 1.2: Schematic of the DC Power Supply.

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Figure 1.3: Protoboard Schematic.

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Figure 1.4: Protoboard Connections.

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Figure 1.5: Measuring Voltage with the DMM.

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Figure 1.6: Measuring Current with the DMM.

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Figure 1.7: Measuring Equivalent Resistance with the DMM.

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Figure 1.8: Measuring the Resistance of a Resistor.

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Figure 1.9: Circuit to Measure Current.

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Figure 2.0a: Series Circuit for Problem 1.

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Figure 2.0b: Series Parallel Circuit for Problem 2.

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