This experiment introduces the use of a digital multimeter for resistance
measurement. It is also designed to increase understanding of resistance.
Resistance is theopposition to current flow between two points. This value
has only magnitude, there is no direction or polarity associated with
resistance—it is always a positive value.
Although resistors are created with color bands that indicate their values, the
total resistance of an object can be more accurately measured with an
Ohmmeter. In this experiment, we will use a Digital Multimeter (DMM) as an
Ohmmeter to measure the resistance of resistors. We will also examine a few ways
that measurements can be done improperly to produce incorrect results.
Before you begin, you should:
Be able to identify fixed resistors by their color bands.
After you're done, you should:
Be able to use a digital multimeter to measure resistance.
Inventory:
Qty
Description
Typical Image
Schematic Symbol
Breadboard Image
1
Digital multimeter (DMM)
1
Any resistor
1
A different resistor
2
Analog Discovery™ pins
Procedures
Step 1: Measure the Resistance of an Individual Resistor
A. Create the circuit
Place the two resistors on the breadboard so that none of their
terminals are connected to one another.
Once on the breadboard, place the mode selector on your DMM to the
Ω symbol. Ensure that your leads are plugged into the
V-Ω and COM ports.
Place one probe on each side of a resistor to obtain its resistance
value.
B. Measure Resistances
Measure and record the individual resistance values of the resistors.
Use the color codes on the resistors to determine an expected
resistance value. Does the expected value agree with the measured value?
Step 2: Measure Resistance of a Resistor in a Circuit
A. Create the circuit
Connect the resistors to one another as shown.
Connect the AWG lead (W1), to the first resistor.
Connect the second resistor to the ground ()
terminal.
Do NOT turn on the AWG!
B. Re-measure Resistances
Re-measure the individual resistance values of the resistors.
The values you have now should be different than those you first
measured. What caused the resistance measurement to change?
Because these resistors are now in a circuit, the Ohmmeter
measures how much resistance there is to current flow through
EVERY available path. This includes the path through
the Analog Discovery and the other resistor.
The moral of this experiment: always remove components from circuits
before measuring their resistance values.
Test Your Knowledge!
We can use our Ohmmeter not only to tell the value of resistance, but we
can also check continuity. (To see if current can flow between two points)
Place several Analog Discovery pins in the same numbered row on
your breadboard and measure the resistance between them. All pins on
the same side of the channel (the divider through the middle) should
read 0 Ohms. This means that pins in the same row are all
electrically connected.
Place several Analog Discovery pins in the same lettered column on
your breadboard and measure the resistance between them. They should
all read OL, or infinity. This means that pins in different
rows, even if in the same column, are not electrically
connected.
Everything has a resistance. Grab some components out of your parts kit
and measure the resistance between their leads.
It is important to realize that although everything has some
resistance, the resistance value is not always constant. Transistors
and diodes, for example, change their resistance depending on the
voltage applied to them. This is how they received the name
semi-conductor.
There is a component in your kit that has only 2 leads and changes
its resistance as it warms. Can you find it? (Hint: Temperature
effects are called thermal effects. Check the parts kit list
on the Digilent web site for a part with something like
“thermal” in its name. Beware: there are similar
components listed on the web site that have more than two
leads!)
There is a component in your kit that has only 2 leads and changes
its resistance as it is exposed to light. Can you find it? (Hint:
Light is caused by photons. Check the parts kit list on the
Digilent web site for a part with something like “photon”
or “photo” in its name.)