Download Multimeter and Resistance Lab 1 Materials Identifying Resistors and

Multimeter and Resistance Lab
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Name____________________
Materials
Choose four resistors at random from between 470 ohms and 50K. In addition, choose two of the
same value. You should have 6 resistors in total.
Wireless breadboard
2.1 mm power jack to fit wall adapter (in your kit box)
6-volt power adapter (wall adapter)
Read Lady Adaʼs tutorial on using your multimeter. (the link is on our wiki)
Read the post about measuring current on our wiki.
Identifying Resistors and Measuring Them with a Multimeter
Using a resistance color code chart, identify four of the resistors and record their values. Then
measure the resistors with your meter and record the measured values. Think about the reasons why
the measured values might not match the labeled values perfectly (remember the tolerance band?).
Color Bands
Color Chart Value
Measured Value
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Hook up some power and measure voltage
Construct the circuit diagram below by inserting your power jack (it is in your lab kit box) into the
breadboard and then connecting the resistor to the other terminal.
The back terminal of the jack is positive. The terminal on the bottom center of the jack is negative.
Measure the voltage across the resistor and record it.
Measuring current
When your meter is set to the current range it has no internal resistance. It is a short circuit.
Remember it is important to always have some resistance in series with your meter. To protect the
current ranges, your meter has a small fuse. If you touch your probes to a voltage source (such as the
power jack terminals), you will blow the fuse. This can be replaced easily but is some fuss.
If you do blow the fuse in your meter, it is easy to replace. Just remove two screws on the back and
remove the back cover. We will try to have spares on hand in class.
Never put the probes of a meter set to the current setting across the terminals of a power supply.
To measure current, you need to break the circuit somewhere and inserted the meter probes. Always
start with the current setting on its highest range (200mA) and then work your way down through the
ranges for the most accuracy, and to protect your meter.
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Figure 1. Measuring the current by breaking the circuit
and inserting the meter probes to complete the circuit.
Measuring and calculating current
Now we would like for you to use your multimeter to find the actual voltage, resistance and current in
the circuit. Record the measured voltage, resistance and current in the chart below. Repeat the
measurements with three resistors.
Then use Ohmʼs Law to calculate the expected current and compare to the measured values.
To calculate the current with Ohmʼs Law, use the formula:
E = IR (Ohmʼs Law)
E = voltage in volts (V)
I = current in amps (A)
R = resistance in ohms (Ω)
Example:
9 volts through a 1.5KΩ resistor
9 / 1500 = 0.006 Amps or 6 mA
(1 milliamp = .001 amp 1 amp = 1000 mA)
Measured Voltage
Measured Resistance
Measured Current
Predicted Value for
Current w Ohmʼs Law
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Resistors in Series
Use your breadboard and connect two resistors in series. The schematic diagram for the two resistors
would look like the image below. Do not connect either resistor to power jack though.
Measure the resistance of each resistor separately and the total resistance from end to end. Repeat
this with three different resistor pairs. While you are doing this verify the color bands on the resistors to
get some practice with the color code.
R1
R2
Not surprisingly the math for two (or more) resistors in series is:
Rtotal = R1 + R2
Total R
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Resistors in Series
Measuring Voltage in a Series Circuit
Now hook up the series resistors to the power jack.
Resistors in a series circuit form something called a voltage divider. The resistors divide the voltage
(provided by the power supply) into partial voltages proportional to the resistor values. For example, if
R1 and R2 are both equal to 1KΩ, the voltage across each resistor would be 2.5 volts.
With your meter on the 20V range of the voltage setting, measure the voltage across each resistor, by
putting your meter probes on either side of the resistor body.
R1 value
R2 value
Voltage across R1
Voltage across R2
Total Voltage across
both Rʼs
Measuring Current in a Series Circuit
Current in a series circuit is the same throughout the circuit (In an example with pipes: no
matter how large or small the pipe, if they are all connected in series, the water flow is the
same at any point in the pipe).
Now measure the current in the circuit by breaking the circuit at point A-B and attaching your probes.
Do this again at point C-D to prove that the current is always the same in a series circuit, no matter
where you break the circuit.
R1
value
R2
value
Voltage across R1
Voltage across
R2
Current at
point A-B
Current at
point C-D
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Resistors in Parallel
Measure the resistances of each of the two resistors separately. Then measure the total resistance
when hooked up in parallel as shown below. Repeat with three different pairs. For one pair use
resistors of the same value. For the other two pairs use resistors of different values.
Figure 2. resistors in parallel
Measure the values of the resistors. Measure the resistors in parallel to get Total R.
R1
R2
Total R
Draw some conclusions about how total resistance relates to the component resistance values. The
math for two resistors in parallel is slightly more complex than for resistors in series.
The equation is below:
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After measuring the values of the two resistors, construct the circuit below and measure the current.
Remember to be careful when using your meter in the current range. The green circles are suggested
places to break the circuits to insert the meter when measuring current.
To measure the current through R1 you will have to disconnect one end of the resistor and then
complete the circuit with your meter. The green circles are indications of places to break the circuit and
measure the current. Repeat this for R1 and R2.
Remember to reconnect both resistors before measuring the total current at points A-B.
Figure 3. Two resistors in parallel share the current, but you must
isolate each one to measure the current through it.
R1 value
R2 value
Current through
R1 (points C-D)
Current through R2
(points E-F)
Total Current at Jack
(points A-B)
Write out some speculations about what is going on in terms of current flow.
Voltage across
Rʼs