ENG 250 Lab #1



Lab #1 ENG 250-001 Name:Date:Introduction to DC CircuitsBasic Circuit Definitions and ConceptsThe concepts and definitions outlined here are extremely important for understanding how to construct and analyze electric and electronic circuits.In general, an electric circuit consists of a group of components such as batteries, lamps, switches and motors connected together, in some pattern, by conducting wires. Circuits that contain semiconductor devices such as transistors and diodes, or thermionic devices such as vacuum tubes, are called electronic circuits. A DC circuit is an electric or electronic circuit in which the electric current through every component is constant in time. All of the points where a given circuit connects to devices that are not considered to be part of the circuit itself are called terminals of the circuit.Current (we use the symbol I ) is the rate as which charge flows past a given point in a circuit. It is measured in units of Amperes. One Ampere (or Amp for short) is one Coulomb of charge passing a given point along the circuit in one second. I = Q / TA single loop circuit is one that consists of two or more components connected together in series (one after the other) to form a single closed path. The current in a single loop circuit will always have the same magnitude at all points around the loop, otherwise charge would build up or disappear somewhere which, by the law of conservation of charge, cannot happen.Multi-loop circuits contain nodes, points where the current can divide up and take alternate paths, or where currents can merge. Any circuit path between two nodes is called a branch. The sum of all branch currents arriving at a node must always equal the sum of all branch currents leaving the same node. This is called Kirchoff’s Node Law. Two or more circuit branches that connect together through the same pair of nodes are said to be connected in parallel.Potential difference, also known as voltage, (using the symbol V ) is the work required per unit of charge to move charge from one point in a circuit to another. It is measured in Volts. One Volt is one Joule of work per Coulomb of charge moved. When a potential difference is negative, it is often referred to as a potential drop. The sum of potential differences along any closed path within a DC circuit is always zero. This principle is known as Kirchoff’s Loop Law.Power (using the symbol P) is the rate at which energy is transferred to or from a portion of the circuit. It is usually measured in Watts or Joules per second. The amount of power delivered to or from a circuit component is equal to the product of the current through the component and the potential difference across it.Some components in a DC circuit will absorb energy and transform it to mechanical energy or to heat, or store it as chemical potential energy, while other components will supply energy to the circuit. Rechargeable batteries can perform either of these functions, depending upon whether they are being recharged or discharged at the time. When the potential is lower at the point where current exits from a component than at the point where it entered, power is being removed from the circuit by the component. But if the potential is higher at the point where the current exits, then that component is acting as a power source, delivering power to the circuit. Resistance (symbol = R) is a property of energy absorbing electrical devices that convert electrical energy into heat. Often the current, I, through the device is directly proportional to the potential drop, V, across it. The relation V = IR is known as Ohm's Law, and a device satisfying this linear relationship is called a resistor. Resistance is measured in Ohms. A one-Ohm resistor carries a current of one Amp if there is a potential drop of one Volt across its terminals. The Digital Multimeter The digital multimeter (DMM for short) is a multifunction instrument that can be used to measure Voltage, Current or Resistance, depending upon which function is selected and how it is connected to the circuit. A large rotary switch on the face of the DMM sets the range of the instrument as well as selecting the quantity to be measured. The decimal point in the DMM’s digital display will move automatically, as you change ranges, but you need to be aware of the units implied on each particular range of the instrument. Please Note: One of the test leads must be plugged into a different jack on the meter when it is used to measure Current, than the jack used when measuring Voltage or Resistance. One lead is always connected to the COM jack.To measure the potential difference between any two points in a circuit, first select an appropriate DC Voltage range, plug the test leads into the COM and V/? jacks, and then connect the test leads to the appropriate circuit points. There is also a switch near the top to select DC versus AC.To measure the current in a single loop circuit or in a particular branch of a multi-loop circuit, select an appropriate DC Current range, plug the test leads into the COM and A terminals, break the loop or circuit branch, and connect the DMM leads across the break, to complete the circuit. Note: To measure the current in a circuit branch, the DMM must always be connected in series with the components that make up that branch. To measure the resistance of a device, you must remove it from the circuit and connect it directly across the DMM test leads, plugged into the COM and V/? jacks. Switch the DMM to one of the resistance ranges. Attempting to use the DMM as an Ohmmeter (to measure the resistance of a component) while the component is connected in a circuit will generally lead to erroneous results, and it may permanently damage the meter as well. The Power Supplies There are several power supplies in your trainer kit. On the left side of the breadboards are jacks for the following power supplies: A variable DC supply from +1.25 V to 20 V; A variable DC supply from –1.25 V to –20 V; A fixed AC supply at 15 V and 30 V; A ground jack; and fixed +5 V, +12 V and –12 V jacks. In most labs you will be using the +5 V DC fixed supply. On the top of the kit are a series of knobs that control what is called a function generator. With these knobs you can create a varying voltage supply with variable frequency and shape. The function generator will be discussed in a later lab.The Breadboard Permanent circuits are usually constructed by soldering the components to a fiberglass circuit board that is constructed with metal “traces” that interconnect all of the components, completing the circuit. These permanent circuits are inexpensive to construct, easy to test and reliable, but difficult to modify or experiment with. For experimentation with new or modified circuits, the best strategy is to build the circuit with a “breadboard” and jumper wires. This is an ideal way to make temporary but fairly reliable electrical connections and keep things spaced out to avoid accidental short circuits (connections where you don't want them). The layout of connection points on the breadboard is designed for dual-in-line package integrated circuits, which you will be using in a few weeks, but it's useful in working with discrete component circuits as well. The narrower strips on the breadboard are called bus strips; they have two long lines of connection points and all the points in one line are connected together under the board. Typically you'll connect the +5 Volt output of the power supply to one bus line and the power supply common (ground) output to another bus line; then wires plugged into the bus lines at any point can conveniently connect power to your circuit. The wider breadboard strips with a groove down the middle are wired quite differently: Groups of 5 connection points running perpendicular to the length of the board on each side of the center groove are connected together. This allows an integrated circuit to be plugged in straddling the groove with four available connection points to each pin of the integrated circuit. ( See a copy of layout at the end of your lab.)Resistors Carbon-film resistors obey Ohm's law quite accurately and come in a broad range of resistance values: from 10 Ohms to 22 Mega ohms. (“Meg” means million). They carry a color-code to make it easy to identify their resistance value. The color of the first color band (closest to one end) represents the first decimal digit of the resistance value. The next band is the second decimal digit. The third band represents the number of zeroes that must be added behind the two digits to represent the resistance as an integer.0:Black5:Green1:Brown6:Blue2:Red7:Violet3:Orange8:Gray4:Yellow9:WhiteThere is also a fourth color band that gives you the tolerance of the resistor. The tolerance tells you how close the color code resistance is to the actual resistance. The following table shows the colors of the fourth band and the corresponding tolerances:Brown1 %Gold5 %Silver10 %None20 %Experimental Procedures 1. Using a DMM check the output voltages available from your power supply and record them. (Notice that you may connect the DMM leads to the power supply either way and that the DMM gives you a minus sign on its display when the potential is lower at the V/? terminal than at the COM terminal. Use the DC Volt range that gives you the greatest number of significant figures.)2. Select three different resistors from the front of the class, record their color codes from lowest to highest using the above table, and record the nominal value. Measure the resistance of each resistor with the DMM's ohmmeter function and compare the measured values with the nominal (approximate) color-code values. Adjust the range to get the largest number of significant figures. Color CodeNominal Value Measured Valuea.________________________________________________________________________b. ________________________________________________________________________c. ________________________________________________________________________3. Construct a series circuit using the 1.5 Volt power supply and tow of your lowest resistor. Calculate the current in this circuit. Draw a schematic of your circuit with the polarity and insert an ammeter to show how you connect your meter in the circuit.I cal = Measure the current using your DMM as an ammeter (start on the 200 milliamp DC Amp scale). Change the range until you get the largest number of significant figures. Record your value.I Meas =4. Repeat part 3 using all three resistors in series. Draw a schematic with the polarity, calculate and measure voltage drop across each resistor. Insert the ammeter at different points in the circuit loop to confirm that the current is the same everywhere around a single-loop circuit. V1 cal =V2 cal =V3 cal =I meas = 5. Construct a circuit with two highest value resistors in parallel. Predict and then measure the current in each branch of the circuit. Draw a schematic of your circuit and insert an ammeter to show how you connect your meter in the circuit. Is Kirchoff’s node law obeyed? Can you think of a reason that it might not seem to be obeyed exactly? (Hint: Use one DMM to measure the voltage drop across the terminals of the other DMM.) I1 meas = I2 meas = 6. Add the third resistor in series with that parallel pair. Draw a schematic of your circuit and insert an ammeter to show how you connect your meter in the circuit. Predict and then measure the voltage drop across each resistor in this circuit. Do the results make sense? I1 meas = I2 meas = I3 meas =7. Using Multisim, build each of the circuits and verify your data for each of the steps (3-6.).Measured ValueSimulated ValueStep 3.I Meas =__________________________Step 4. V1 Meas =_______________________________ V2 Meas =_______________________________V 3Meas =_______________________________I Meas =_______________________________Step 5. I1 meas =_______________________________I2 meas =________________________________V Meas =________________________________Step 6. I1 meas =_______________________________I2 meas =________________________________I3 meas =________________________________ Did you find your measured and simulated value to be different? Explain why. ................
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