Introduction to Circuit Analysis



Introduction to Circuit Analysis

Due: At recitation the week of April 9-13.

Objectives

At the end of this lab, you will be able to:

• check continuity with a multimeter

• describe breadboard internal connections

• create circuits using breadboard connections

• measure dc voltage, current and resistance using a multimeter

• identify resistors by value

• calculate resistance from dc voltage and current measurements

• calculate power consumption from dc voltage and current measurements

• validate voltage divider calculations by experiment

• validate current divider calculations by experiment

• plot voltage-current relationships for non-linear devices

The circuits covered in this laboratory are the beginning basics of an area of electrical engineering called Electronic Circuits, Devices and Transducers at UW. You can find a summary of this area of specialization within electrical engineering in your supplemental notes for the EE 215 course. The notes are part of your reading assignment. Later labs will touch on some of the other areas of specialization.

Materials and Supplies

Here is a list of what you need for this lab, where to get it and about how much it will cost:

|What |Where |How Much |

|Multimeter |Electronics Store |$30-50 |

|Breadboard |EE stores or Electr. Str |$7.10 |

|Jumper Wire Kit |EE stores or Electr. Str |$6.00 |

|Parts Kit |EE stores only |$12.90 |

|9V Battery |Any store |$2.50 |

Groups may choose to purchase one multimeter and move it from person to person to take measurements, although I suspect you may find that it is more convenient for each individual to get their own meter. Groups may also try to share jumper wire kits, although there is no guarantee there is enough wire in one for all group members to do each circuit in every lab. Every individual must purchase their own breadboard, parts kit and battery.

Multimeter: You need a multimeter that can measure ac and dc voltage, dc current and resistance. Ones that measure ac current, frequency and/or capacitance can be useful in the long run, but are not required for the course. A good multimeter costs $40-50, and you can spend more if you want to. You might be able to scrape by under $20 with the piece of junk special. Before you take the low road, think about this: a good multimeter will be with you for years (I still have mine from 1969 - rc) and is a useful part of a house, apartment or automotive toolkit, as well as essential for any electronics work.

Local electronics stores:

• Radio Shack: 4223 University Way NE (206) 632-4720 or University Village Mall (206) 523-0534

• Active Electronics: 13107 Northup Way, Bellevue (425) 881-8191, has a 5% student discount

You can also find multimeters at Sears! Or from the Web (allow enough time for delivery!):

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Breadboard: Typically a piece of white plastic with lots of tiny little holes in it. You stick wires and component leads into the holes to make circuits. Some of the holes are already electrically connected with each other. The holes are 0.1 inch apart, which is the standard spacing for leads on integrated circuit dual in-line packages. You will test the connections in this lab.

You can purchase breadboards at EE stores. Yes, breadboards from other sources are just fine, though from what I've seen EE stores is decent value for money. Breadboards are used in other electronics courses in the department.

Jumper Wire Kit: This kit contains assorted lengths of pre-stripped wire. (Stripping means removing the insulation from an end of a wire.) Working with pre-cut and pre-stripped wire is much, much easier than cutting and stripping your own wire. Purchase from EE stores. (EE stores has some free breadboard wire in uncut, unstripped form. To make use of this wire you will need the needle nose pliers, diagonal cutters and wire strippers mentioned below as optional stuff. The jumper wire kit is undoubtedly more convenient and cheaper than buying the tools.)

Parts Kit: This kit contains components for all the labs in the course. Because it is course-specific, it can only be purchased at EE stores. You can probably find the components from the kit elsewhere, but, like the wires only more so, having all the parts you need in one place, and having them be mostly the right parts, is a real convenience.

9V Battery: This is the standard 9V transistor radio battery with the snaps on the top, as sold in drugstores, convenience stores, grocery stores, etc, etc, etc.

Optional Stuff: If you get a multimeter that does not have clips at the end of at least one set of leads (sometimes these are an option), you might want to invest in a pair of wires with alligator clips on either end. This can make taking measurements much more convenient, and they're cheap. Get the small ones.

If neatness in circuit construction is important to you, you can buy a pair of needle-nosed pliers to make neat, precise bends without annoying your fingers, a pair of diagonal cutters to snip leads and wires to length, and a wire stripper to remove the insulation from the newly cut wire ends. Combination tools tend to work less well than separate tools, especially the cheap ones! But needlenose-cutter combinations are common. Note that making the circuit neat will take extra time (although it can improve clarity of understanding) and you will be graded on the function of your circuits, not on their appearance.

Parts for This Lab

Separate and identify the following from your parts kit:

• 30 ( 1/4 Watt, 5% tolerance resistor (Rings: Orange Black Black Gold)

• Mystery resistor (Rings: Brown Black Brown Gold)

• 1 k( 1/4 Watt, 5% tolerance resistor (Rings: Brown Black Red Gold)

• 1.5( 1/4 Watt, 5% tolerance resistor (Rings: Brown Green Red Gold)

• Diode (1N4148)

• 1000 ( potentiometer (1/4 Watt)

• Tweaker

• 9V battery connector

Resistors are the small light brown tubular things with wires (leads) sticking out of each end and four colored rings on the body. The rings are closer to one end than the other, and the ring nearest the end is the first ring. To find out what the rings mean, look in Appendix E of Dorf and Svoboda (your text) or search for "resistor color code" on the web. (Suddenly the mystery resistor is not so mysterious after all…)

The diode is shaped sort of like a resistor, only smaller. It has a tubular glass body with orange insides, and one end of the glass has a black band on it. The number (1N4148) is a standard part number. Diodes with different numbers will have different characteristics.

The 1000 ( potentiometer (called a "pot" in casual conversation) is the oblong blue thing with a silver shaft with a slot in it sticking out of one of the long ends. A potentiometer is a variable resistor, and you vary its resistance by turning the shaft. In this case, it takes 15 turns to go from maximum to minimum.

The tweaker is the white plastic thing shaped like a pencil only smaller, with one end flattened into a blade that just fits the slot in the potentiometer shaft. Any idea what it's used for? (Any small screwdriver used for adjusting components like the potentiometer is called a tweaker by electrical engineers and technicians.)

The 9V battery connector is the black plastic cap with two snaps mounted on it and a couple of wires coming out of its side. The snaps fit with the snaps on the top of the 9V battery. The red wire is the positive end.

Laboratory Procedures, Measurements and Questions

Record your data and the answers to questions on a separate sheet (or sheets) of paper and hand it in at recitation section when the lab is due. You will also have to bring your breadboard with designated circuits on it to your recitation section the week the lab is due.

Procedure 1

Use your multimeter to identify the connections between the holes in your breadboard.

Sketch enough of the holes on your breadboard to illustrate its connectivity. If you want, you can copy the illustration on page 2. (Do NOT sketch the entire breadboard! Just enough so the pattern of connections is clear.) Set your multimeter on resistance. To find out if any two holes are connected, measure the resistance between them with the multimeter. This is called a continuity check.

a. (2 points) What will the resistance be between connected holes?

b. (2 points) What will the resistance be between unconnected holes?

c. (2 points) Does it make a difference which probe goes in which hole?

d. (14 points) Draw the connections you find into your sketch of the breadboard holes.

Some hints on measuring resistance:

Don't try to measure resistance in energized circuits (ones with the power on). You won't get an accurate value and you could damage your multimeter or the circuit.

Your multimeter probes probably don't fit into the breadboard holes. Stick the stripped end of a wire into each hole, and touch the other stripped ends of the wires with the multimeter probes. If you have clips at the end of your multimeter leads, or you bought those optional alligator clips, you can clip on to the ends of the wires and move the wires from hole to hole. Resistor leads also work for this purpose, but make sure you are not measuring the resistor resistance as well as the breadboard resistance!

Because the multimeter uses a low voltage to measure resistance, you can safely use your fingers to press the wires to the multimeter probes to be sure you have a good contact. If you do, though, you will put your body in parallel with the resistance you are measuring. This can be important for certain values of resistance, those near your body resistance. It's usually not a problem for continuity checks.

Switch the multimeter to off or to the voltage setting when you are not actively measuring resistance. This minimizes battery use in the multimeter and is also safer. (Electrical safety will be covered more thoroughly in later labs. This lab is safe.)

Procedure 2

Build a circuit similar to the one that the multimeter uses to measure resistance, and measure a mystery resistor. Construct the following circuit on your breadboard:

a. (5 points) Draw the circuit schematic diagram using standard symbols for resistors and batteries.

b. (5 points) Measure the voltage v21 and the current i1 using your multimeter. Note the values.

c. (5 points) Calculate the value of the mystery resistance R2. Take R2 out of the circuit and measure its resistance with your multimeter. Write down the calculated value, the measured value, and the value according to the color code. Are they within the resistor accuracy tolerance?

d. (5 points) Use the value for R2 you think is most correct and use the voltage divider method to calculate the value of v21. List at least two reasons this is different from the value you measured in part b. (If by some chance it is not different, list two reasons it should be!)

Construction hints:

Bend the resistor leads at right angles near the resistor body to make a U-shape with a flat bottom, then insert the leads directly into breadboard holes. Make sure they go in to the spring clips in the holes. Sometimes the springs don't want to let the leads in. With a little experience you will be able to tell when you have the leads in, and when you don't. If a hole is particularly stubborn about accepting leads, consider using a different one.

You can do the same thing with wires, or just curve them into place. If you really like being neat, you can use a pair of needle-nosed pliers to make neat, precise bends without annoying your fingers, a pair of diagonal cutters to snip the leads and wires to length, and a wire stripper to remove the insulation from the newly cut wire ends. Neatness can improve your clarity of understanding of the circuit. However, you will be graded on the function of your circuits, not their appearance.

Your breadboard has vertical rows of connected holes that run the length of the breadboard, often with red and blue stripes marking them. They are usually used for the positive and negative terminals of the battery or other power supply voltage.

Measurement hints:

The red lead is the positive probe.

Use a meter scale with a maximum value higher than the quantity you are measuring. If you do not have a good idea of what the quantity will be, start at the highest scale and work down until you have a good measurement. Some meters have autoscaling, which is a convenient feature that does this automatically.

At this low voltage it is safe to hold energized wires against the multimeter probes. (This is not always true, and you should never come in contact with 120V AC from the wall socket.)

You may need to change the lead connections to the multimeter as well as the function switch settings when measuring current. This varies from meter to meter. Read the instruction manual for your meter!

Minimize the amount of time the multimeter is on the current setting while measuring current, and always return it to the off position, or the voltage position, as soon as you are done with the measurement. The multimeter is a short circuit in the current position, and short circuits are more likely to damage equipment and the meter if the probes inadvertently contact something in the circuit. Even with this simple circuit, you could damage the meter if you short circuited the battery through it. Leaving the meter on off or voltage is a good habit to develop.

If you do try to measure voltage with the multimeter set on current, and your meter stops working for measuring current, you have probably blown a fuse in the meter. Read the manual, go get another fuse, and resolve not to measure voltage with the meter on the current setting.

Disconnect the battery (pulling out one of the battery wires works quite well) when you are done making measurements. Otherwise the battery will run down, and you'll need a new one for the next lab.

Procedure 3

Build a current divider by connecting the 1 k( and 1.5 k( resistors in parallel across the 9V battery in series with R2 from Procedure 2:

a. (5 points) Measure i1, i2 and i3, and battery voltage vb. Copy the circuit and label the location of vb. Be sure you include polarity. Hint: If the value of i1 is not close to the sum of i2 and i3, something is probably wrong with your measurement.

b. (5 points) Calculate the expected values of i2 and i3 using the current divider method. Are the measured values within tolerance of the calculated ones?

c. (5 points) Calculate the power dissipated in each of the three resistors using current and the nominal resistance values (the ones from the resistor color code).

d. (5 points) Calculate the power supplied by the battery using the voltage and current measurements from part a. What should the relationship be between this value and the values calculated in part c?

Leave this circuit on your breadboard and bring it with you to the recitation section the week the lab is due.

Procedure 4

Characteristic curve of a non-linear component (the diode).

Construct the circuit on the next page using the 1000 ( potentiometer and the diode. Connect the diode so the anode (orange end) corresponds to the base of the arrowhead and the cathode (end with black band) corresponds to the tip of the arrowhead. Note that the potentiometer has three leads. They correspond to the three black dots in the circuit. The middle lead corresponds to the variable arrow. One of the leads is not connected to anything in the circuit.

a. (4 points) Remove the potentiometer from the circuit. Set your multimeter to resistance and measure the resistance of the potentiometer between the two leads that were connected. Use the tweaker to turn the shaft of the potentiometer until the resistance between these two leads is 150 (. If the tweaker is not working well, try a fingernail, or an eyeglass screwdriver. (This is a case where alligator clips are very useful.) Put the potentiometer back in the circuit. Measure the voltage across the potentiometer, and the voltage across the diode. Calculate the current through the potentiometer using Ohm's Law. What is the current through the diode?

b. (12 points) Repeat part a for resistance values of 300 (, 600 ( and 1000 (.

c. (4 points) Plot the voltage across the diode on the x-axis and the current through the diode on the y-axis of a graph. Plot the diode resistance (y-axis) versus voltage (x-axis) on a separate graph. Is the diode a linear device? Explain why or why not in terms of your graphs. If you decide it is not linear, what does the graph of current versus voltage (I-V characteristic) look like? (e.g. exponential, square, square root, etc.)

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