EELE 250: Multimeter Notes
In the lab we will use a digital multimeter (DMM) to measure resistance, voltage, and current in
our circuits. There are pushbuttons on the meter to indicate which type of measurement is being
made, and other pushbuttons to select the range (scale) for the measured values.
Be sure to note that the input connections on the meter are different for measuring current than
for measuring voltage or resistance. Always make sure you have the proper input connections
and pushbutton settings BEFORE attaching the meter to the circuit!
If you do not know the expected value of the measurement, first be sure that it does not exceed
the maximum rating of the DMM. If it is within the limits, start with the range pushbutton set for
a large number. If the value displayed by the meter is small compared to the range setting, push
the appropriate smaller range setting to optimize the display. In other words, choose the smallest
range setting that will accommodate the meter reading. This gives the most precision (number of
digits) in the meter reading.
Voltage Measurement
(1) Voltage is the potential difference between two nodes in a circuit.
(2) Voltage is measured in the unit “volts” (V), which is “joules per coloumb”: how much
energy (joules) is required to move some charge (coloumb) between the two nodes.
(3) A voltmeter indicates the voltage between the two nodes. The meter has a very high
input resistance so that essentially no current flows through the meter itself. The
voltmeter is connected by touching the meter probes to the two nodes of interest. See the
figure below: measuring voltage between node 2 and node 3.
(4) Voltage has a polarity (positive or negative). In order to help avoid making careless
polarity errors, we will connect a red probe wire to the positive terminal on the voltmeter
and a black probe wire to the negative terminal on the voltmeter. The color of the wire
doesn’t affect the wire’s behavior, of course, but simply helps us keep things straight
when hooking up our circuits.
(5) When the probes are connected to the circuit, the meter will indicate a positive value if
the red probe is at a higher voltage (potential) than the black probe, and a negative value
if the red probe is at a lower voltage than the black probe.
+
5V DC
-
R
2
R
3
R
1
volt
meter
