Introduction
The purpose of this lab is to introduce the electrical circuit and how to use electrical equipment to test electrical circuits.
Equipment
Power supply, digital multimeter, function generator and oscilloscope, resistors and wires.
Background
In order to study electrical phenomena, we have to be able to make measurements such as the current and the potential. These things are not directly observable. Therefore, it is necessary that tools be used to detect and to quantify them. In this lab, you will be introduced to the tools for generating and detecting electrical phenomena. You will also be introduced to the electrical circuit and electronic components.
The Electrical Circuit
Electrical phenomena can be most easily produced in an electrical circuit. An electrical circuit is a loop through which charges can flow directed. Here is a representation of a circuit that you will build first.
power supply wire wire resistor
A power supply is a device that acts like a battery that gives energy to the charges in the circuit. A resistor is just a generic device or appliance that converts the energy in the charges to other forms. The wires are guides for the charges to go where they are supposed to.
The Digital Multimeter (DMM)
The digital multimeter is a device that can measure multiple electronic properties of electrical components and circuits. We will use three of these: resistance, voltage, and current.
Resistance
Resistance is a measure of how hard it is for electricity to pass through a certain object. For the resistance measurement, the DMM acts as an ohmmeter. It measures the resistance between the
“ Ω ” end and the “COM” end of the meter. The DMM knob must be set to the “ Ω ” (ohmmeter) function. As a check, connect the two wires of the DMM directly to each other. You should have a reading of 0.0 Ω . Note the prefixes on the display like milli (m) and kilo (k).
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Have one person in you group pick a random resistor from the plastic box, don’t tell anyone what it is. Have the others the group connect the two ends of the resistor to the two wires of the DMM.
The resistor must be by itself (not in a circuit). Record the resistance with unit.
The color rings of the resistor represents the resistance. They are read from left to right oriented with the gold or silver ring on the right. The colors have the following interpretations.
black brown red orange yellow green blue
0 1 2 3 4 5 6 violet
7 gray
8 white
9 gold
5% silver
10%
The way to read the bands is to place them in the following rubric. The first two digits are the significant digits. The third digit is the exponent. The last is a percentage for the tolerance.
digit of 1st band digit of 2nd band x 10 digit of 3rd band ± % of 4th band
For example, a resistor with the bands “red black yellow silver” represents the value
2 0 x 10
4 ± 10%
20
× 10 4 Ω ± 10% or 18 × 10 4 Ω to 22 × 10 4 Ω
Questions: What is the labeled resistance of the resistor you just tested? How far off is your measured value from the labeled value? Is it within the tolerance?
A resistor out of tolerance is usually damaged. If too much current goes through the resistor, it is said to be “fried” and its resistance is no longer as advertised and predictable.
Repeat the above exercise with another random resistor. Measure its actual resistance using the
DMM and compare it to the advertised value.
Build the Circuit
Use the 1,000 Ω resistor to build the above circuit. Set the voltage to 5 V and turn off the power supply until you have completed your circuit. The colored red and black plastic parts are identical.
Electrical energy is being used by the circuit even though you see nothing going on.
–
5 V
+
1,000 Ω
Question: Where is the energy going?
Replace the resistor with a lightbulb and observe the light as you increase and decrease the voltage of the power supply. Don’t adjust the voltage above 10 volts as more light means more heat and the filament will melt at a high enough temperature.
Turn the voltage knob down to about 0.5 V.
Question: Is there any light from the bulb? Why is there no light that you can see when the voltage is NOT zero?
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Voltage
The voltage represents the driving force applied to the charges; that is, how hard the charges are being pushed. Use the above resistor circuit with the 1,000 Ω resistor.
–
5 V
+
1,000 Ω
Set the voltage of the power supply to about 5 V (we will measure this accurately later). Positive charges flow from the “+” side to the “–” side of the power supply through the circuit. This is called the “conventional current.” The real current is due to electrons.
For the voltage measurement, the DMM knob must be set to the V function. In the diagram, the
DMM is represented by as a voltmeter, V. The DMM measures the voltage difference, ∆ V, between the “V” (red) end and the “COM” (black) end of the meter. The reading is defined as follows.
reading = Δ V = V
V
− V
COM
Voltage is also like a potential energy, it is measured relative to a zero that is arbitrary.
Conventionally, this zero is the “–” end of the power supply. Connect the “COM” end of the DMM to the “–” end of the power supply to mark that end as the zero of the measurement.
1.
Measure and record the voltage difference between the outputs of the power supply.
COM
V
V
–
5 V
+
R
1
Question: Which side of the power supply has a higher voltage, the “+” end or the “–” end? Use your reading and the reading definition above.
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2.
Measure and record the voltage between the ends of the wire connecting the power supply’s
“+” end and the resistor. Make sure that the red wire lead the black wire as you ago around the circuit.
– +
5 V
V
R
1
3.
Measure and record the voltage difference between the two ends of the resistor.
– +
5 V
R
1
V
4.
Measure and record the voltage difference between the resistor and the “–” endow the power supply.
– +
5 V
V
R
1
Questions: What are the voltage changes along the two wires? What is the sum of all of the voltages around the entire loop?
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Add a second 500 Ω resistor to the circuit after the first resistor as shown. Measure the voltages across the two resistors
– + – +
5 V
500 Ω 1,000 Ω 500 Ω
5 V
1,000 Ω
V
1
V
2
Questions: Which resistor reduces the voltage more and by what factor does it reduce the voltage? What is the total reduction in the voltage due to both resistors?
Question: What is the total reduction in the voltage due to the wires?
Question: If the two resistors were R
1
= 400 Ω and R
2
= 100 Ω and the power supply is 5 V, what is the voltage reduction (voltage drop) across each resistor?
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Current
The current is the amount of charges that flows through some section of a circuit. The current flows like water in a pipe. Unless there is a leak, the amount of current is the same everywhere in a loop no matter what it passes through.
In order to measure current, the DMM must be placed inside the circuit. The diagram below is a representation of how this is done. The DMM is represented, when used to measure current, as an ammeter, A.
–
5 V
+ mA
A
COM
1,000 Ω
Preset the power supply to 5 V and use the 1,000 Ω resistor. Turn off the power supply for safety.
Break the circuit at the junction where the wire connects to the resistor. Connect that wire to the
“mA” side of the DMM instead. Add a black wire to the “COM” side of the DMM and connect it to the resistor (where the red wire use to go). Set the function knob of the DMM to largest current scale, “400 mA”. Now turn on the power supply. Record the reading of the DMM? If the value display looks like zero, use a smaller scale.
How do you know if the reading is correct?
First, the sign of the current indicates the direction of the conventional current. A positive sign means the current is flowing from the mA side to the COM side. In the circuit above, the current flows from the + side of the power supply out through the circuit back to the – side of the power supply. This means your current reading should be positive.
Second, for resistors anyway, there is a relationship between the current and the voltage.
R =
Δ V
I
This is called Ohm’s law. Calculate the resistance of the resistor this way. Compare and verify that it is the resistance of the resistor within tolerance.
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Varying Voltage
The power supply is what is called a DC (direct current) source. The current is constant over time so the voltage is also constant over time. A time-varying power supply, like that coming out of the wall, is called an AC (alternating current) source. Rebuild the original circuit, but replace the power supply with the function generator. The function generator is an AC source.
Furthermore, the DMM can not keep up with the changing values. If the change is fast, which is almost always true, we have to be able to measure many points at the same time. This is what the oscilloscope does. It is a voltmeter that records lots of points very fast and displays them all at the same time. Connect it to across the resistor just like you would a voltmeter.
function generator
1,000 Ω osc
Turn on both instruments. Set the function generator to about 1,000 Hz sine wave, and turn the amplitude knob about a half turn clockwise from zero. Click on the “Autorange” button on the oscilloscope. From the oscilloscope display, identify the vertical scale, measure the peak-to-peak voltage of the output, then identify the horizontal scale and measure the period (the time taken by one complete sine wave) of the output as well.
Sketch out a sine wave of the oscilloscope output with axes and scale.
Questions: What is the frequency as read from the function generator? What is the frequency as measured by the oscilloscope? The frequency is the number of cycles of the sine wave every second.
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