Introduction to Instrumentation and Ohmic Circuit Elements

Phy101 Experiment

Introduction to Instrumentation and Ohmic Circuit Elements (revised for Phy101) Name: ______Date: ______

Lab Partners: ______

Introduction: In general, an electric circuit consists of a group of components such as batteries, lamps, switches, and motors that are connected together by conducting wires. Circuits containing semiconductor devices, such as transistors, integrated circuits, or thermionic devices, such as vacuum tubes, among their components are called electronic circuits. In a DC circuit, the direction of the electric currents in all of the components is constant in time.

Any points where a given circuit connects to devices that are not considered to be part of the circuit itself are called terminals. A single-loop circuit is one that consists of two or more components that are connected together in series, one after the other, to form a single closed path.

In this lab you will study single-loop DC circuits. This will help you develop an intuitive feel for the basic concepts and terminology used in electronics and gain hands-on experience with equipment that you will use all semester!

Some Definitions:

CURRENT (I) is the rate at which charge flows past a given point in a circuit. It is usually measured in amperes. One ampere (amp for short) is one coulomb of charge passing a given point in one second. The current, at any instant of time in a single loop circuit, must be the same everywhere along the circuit. Charge cannot accumulate at any point along the circuit path. This is the law of conservation of charge. In a direct-current (DC) circuit the current always flows in one direction and is usually constant. In an alternating current (AC) circuit the current oscillates, flowing in one direction and then reversing and flowing in the other direction, and then reversing again, etc.

POTENTIAL DIFFERENCE, or voltage, (ΔV) is the work (energy) required per unit charge for a charge to move from one point to another in an electric field. It is usually measured in volts. A potential difference of one volt between two points means that it takes one joule of work to move one coulomb of charge between the two points. In other words, when a charged particle is moved from point A to point B through a region of space containing an electric field, the electric field does work on the particle either giving energy to (positive work), or taking energy from (negative work), the particle. Potential difference is the energy gained or lost by that charged particle divided by the charge of the particle.

POWER (P) is the rate at which energy is transferred to or from a portion of the circuit. It is measured in Watts (W). The power delivered to, or taken from, a circuit component is equal to the product of the current through the component and the potential difference across the component. There are two possible situations that you might encounter in a circuit. The first is where the potential drops, or decreases, across the component. In this case, the point where current enters the component has a higher potential (higher voltage) than the point where the current leaves the component. This component draws, or uses, power from the circuit. Examples of this include resistors, motors, diodes, capacitors and inductors. The second case is where the potential rises, or increases, across the component. In this case the potential is lower (lower voltage) where current enters the component and higher where it leaves. This component gives

1 power to the circuit like a battery, generator, capacitor or inductor. Why are capacitors and inductors listed as components that both give and take power? You will find out in a future lab.

RESISTANCE (R) is a property of electrical devices. All devices have a resistance to current flow. This resistance is evident in the heat given off by a light bulb and the need for a fan in a computer. When you use a laptop computer for an extended period of time you may have noticed that it is quite hot on the bottom before you turn it off. The resistance is measured in ohms [Ω]. In this lab we will be looking at how resistance is related to current and voltage in a circuit. It is important to keep in mind that resistors always take power from the circuit.

Instruments:

NOTE: It is very important that you understand what these instruments do and how to use them properly.

The POWER SUPPLY that you will use is shown in figure 1. This supply is voltage regulated, meaning that when you set an output voltage and then attach it to a circuit that draws current from the supply, it will maintain the set output voltage. This is different from a simple battery where the output voltage of a battery goes down when you draw current from it. The power supply also has an adjustable current- limiting feature. Current limited operation of the power supply will be explored in detail in a later lab. For now, you should set the current limit control, the bottom knob, at its maximum value by turning it clockwise as far as it will go and leaving it there for the duration of this lab. The top control sets the regulated output voltage of the power supply.

The power supply has two digital meters, one to display the output voltage and the other to display the output current. You will find that these readings are not as accurate as what you can measure with the digital multimeter discussed below.

Figure 1: The Power Supply

The DIGITAL MULTIMETER (DMM) is a multifunction instrument that can be used as an ohmmeter to measure resistance, an ammeter to measure current or a voltmeter to measure voltage depending upon which of its functions is selected with the large rotary switch on its face.

2 Figures 2, 3, and 4 show three slightly different DMMs. When using a DMM you will always have one test lead, or wire, connected to the COM port and a second connected to one of the other three ports as discussed below.

Figure 2: The Fluke 77 Figure 3: The Fluke 83 Figure 3: The Fluke 87

Measuring Resistance: To measure the resistance of a resistor you would use the DMM as an ohmmeter by first setting the rotary switch to the  symbol and then connecting one wire to the port marked with and a second wire to the port marked COM. (These DMMs are self-adjusting in the sense that you do not need to make any adjustment to the meter if you intend to measure a very large resistance versus a very small resistance.) You would then attach one test lead to each end of the resistor with the resistor completely removed from the circuit. When the resistance measurement is displayed you will also see units such as M, k, or . Be sure to make note of the units along with the measured resistance. Always convert any measured values from M or k to before using data in calculations or plots.

Measuring Voltage: To measure the voltage (or potential difference) across a component you would use ~ ___ the DMM as a voltmeter by first setting the rotary switch to either the , ˙˙˙ , or  symbol and then V V mV connecting one wire to the port with the V symbol and the second to the port marked COM. Set the rotary ~ ___ switch to the symbol when measuring AC voltages or to the ˙˙˙ , or  , symbol when measuring DC V V mV voltages. For now we will be working with DC circuits.

___ The  setting is used to make more precise measurements of small voltages, but the maximum voltage mV that can be measured using this setting depends upon the specific meter. For both the FLUKE 83 and 87, the maximum voltage is 400 mV, while for the FLUKE 77 the maximum is 300 mV. If you do not know approximately what voltage to expect always start with the rotary switch set to the ˙V˙˙ setting. If you see ___ that your measured voltage is less than the maximum mentioned above, then switch to the  setting to mV obtain a more precise measurement. 3 To make an actual measurement you would connect one wire to each side of a component in your circuit without disconnecting the component from the circuit. We say that the voltmeter is connected in parallel with the component. This is VERY important so if this is not clear ask your instructor to demonstrate.

Measuring Current: The following discussion is very important. Be sure you understand this otherwise you could cause serious damage to the DMM.

To measure the current passing through a component you first need to set the rotary switch to the ˙A˙˙ symbol and connect one wire to the port with the A symbol and the second wire to the port marked COM. This sets up the DMM as an ammeter. Next, disconnect one end of the component from the circuit. Connect one wire of the ammeter to the disconnected end of the component and connect the other ammeter wire to the circuit where the component had been connected. Connected in this way we say that the ammeter is in series with the component. Remember, you must always break the circuit to measure current. (A negative sign in the ammeter display simply means the current is traveling into the COM port and out of the A port. Normally you would connect the ammeter such that current flows into the A port and out of the COM port, but often you may not know the direction of the current. Don’t worry though, this won’t hurt the ammeter.)

If you try to measure a current larger than 10 A, a fuse will blow in the DMM and it is possible that the DMM will be damaged. If you see that your measured current is small, you may obtain a more precise measurement by switching the second lead to the mA port, but be careful. The FLUKE 77 can handle currents only up to 300 mA while the FLUKE 83 and 87 can handle currents up to 400 mA on this more precise setting. So always measure current first with the second lead in the A port and then switch if the current is small enough depending on the meter being used.

Even more precision can be obtained using the FLUKE 83 and 87 by moving the rotary switch to the A symbol. The second lead must be in the mA A port and the current must be less than 400 mA.

Part I, Measuring Voltages: Set the DMM to measure voltage and connect the leads to the red (+, positive) and black (-, negative) terminals of the power supply. Be sure the current knob is fully clockwise. Adjust the voltage of the power supply to several values from fully counter-clockwise to fully clockwise. For small voltages (below 300 mV for the FLUKE 77 and below 400 mV for the 83 and 87) rotate the rotary switch to obtain a more precise voltage measurement. In table 1 record the measurements you have obtained from the voltmeter and also write down what is displayed on the power supply’s meter. Be sure you include several voltages less than 1.0 V and several above, up to the maximum voltage, and be sure to write down all decimal places displayed. Don’t forget units!

Before proceeding to Part II, answer the questions below table 1.

Part II: A simple circuit: Set up the circuit shown in Figures 1 and 2 below using your 24resistor. Be sure to have your instructor check the circuit before you begin!

4 Figure 1: Photo of Lab Set Up

Figure 2: Circuit Diagram

5 Adjust the power supply to several voltages below 4.00 V and several voltages above 4.00 V, but not higher than 7.0 V. For each voltage, record the current through the resistor and the voltage across the resistor in table 2. DO NOT leave the switch closed very long. Keep the switch closed only during the short time it takes to record your current and voltage.

Repeat the above procedure for a 100and a 220resistor.

Before proceeding to Part III, answer the questions after table 2.

Safety Check: When current is flowing in the circuit be careful not to touch the metal alligator clips or leads on the resistor. It is possible that you could get a good shock. When you need to make changes to the circuit like adding a resistor, or inserting an ammeter, turn the power supply off or include a switch in your circuit so the power supply can be safely disconnected temporarily.

Think Carefully! Be sure to follow the instructions outlined above in the discussion of the DMM. Remember that two steps are necessary:

1) Set up the DMM properly. If you know the current will be less that 300 mA, or 400 mA depending on the DMM, you can use the mA port on the meter and set the rotary switch to the mA setting. Otherwise be sure to measure current starting with the rotate switch set to the A setting and a wire in the A port.

2) Connect the DMM to the circuit properly.

Part III, Direct Measurement of Resistance: Remove the resistor from the circuit and set the DMM to measure resistance as described above. Record your measured resistance in Table 3 and answer the questions that follow.

Data Part I

DMM Voltage Power Supply Reading Voltage Reading

6 Table 1

1.1) What is the minimum voltage the power supply can deliver? Be sure to use the voltmeter on its most sensitive setting.

1.2) What is the maximum voltage delivered by the power supply?

Part II

2.1) Power Supply Current Through Voltage 24 Resistor

4.00 V

Table 2

7 Power Supply Current Through Voltage 100 Resistor

4.00 V

Table 3

Power Supply Current Through Voltage 220 Resistor

4.00 V

8 Table 4

2.2) Plot Current Vs. Voltage using Logger Pro for all three resistors. Use the curve fit function to find the function that is the best fit for the data (e.g. linear, quadratic, exponential). The currents you measured are in mA (10-3 A). Convert the current values from mA to units of A before inputting them into Logger Pro.

2.3) Write an equation that best describes the relationship between Current and Voltage.

2.4) A device is linear if the current through it is directly proportional to the voltage across it. Is the resistor a linear device? Explain.

2.5) If you have a linear curve fit fill in the table below.

Slope 1/slope

Table 5

Part III:

9 Resistance 1/slope Percent Difference Measured by DMM from Table 5

Table 6

3.1) Compare the resistance measured by your DMM to the data in table 5. What does the slope of I Vs. ΔV represent physically?

3.2) Write an equation (symbols only, no numbers) representing the relationship between current, voltage, and resistance.