Measuring Current and Voltage
The multimeter shown below can be used to measure voltage, current and resistance.
Figure 1 shows how to configure the meter to measure voltage. Typically the meter is
easier to use for voltage measurements. Voltage measurements place the meter in
parallel or across the component of interest. This often seems to be a more natural
configuration to the novice.
Insert the black probe in com and the red
probe in V/Ω. Voltage difference is
measured between the two probe ends.
V
20
10A A
Red probe
com V/Ω
Black probe
Figure1: Configuration for Measuring Voltage
Voltage is measured by inserting a probe into a location in the circuit. Be sure that the
metal of the probe is in contact with the metal of the component. A voltage measurement
requires a starting and ending location. The location of the black probe is the start and
the red probe location is the end. The voltage displayed is the difference between these
two locations. One can easily be confused with voltage but height has the identical
features. All height measurements intrinsically involve a start and an end. The start is
often obvious and therefore not stated [elevation of Harrisonburg above sea level, my
height from feet to head, building from ground to roof.] Similarly voltage measurements
have starting an ending points. If you say a battery is 1.5 V you mean from the negative
side to the positive side.
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Current measurements require that the meter be included in series. The meter must
become part of the current’s path. A problem, however, is not getting the correct
measurement but rather destroying the meter. If a current meter is placed across a power
supply the resulting current can often destroy the meter. Meters are designed with fuses
to protect the inner circuitry from improper use but the fuses do not always work and
replacing fuses can be cumbersome. PLEASE acquaint yourself with the proper use
guidelines contained in this handout before using the meter to measure current.
Always consult your instructor.
MEASURING CURRENT
Because there is a possibility of meter damage we expect all students to follow the
following steps.
1.
Configure your meter so that it can measure currents up to 10 amps, Figure 2.
a. Place the red probe in the 10A plug.
b. Place the black probe in the COM plug.
c. Set the dial to 10A.
d. Energize the circuit (Power on).
Because the devices in the lab do not typically produce currents greater than 10 amps it
will be difficult to expose the meter to currents beyond the maximum range for this setup.
If your meter is measuring a large current you may have improperly connected the meter.
Check with your instructor before proceeding.
V
Red probe
Insert the black probe in com and the red
probe in 10Α. Put the meter in series with
the component.
A
10A
10A A
com V/Ω
Black probe
Figure 2: SAFE Configuration for Measuring Current
If your meter is measuring zero or a small value (less than 0.200 on the 10A scale), then
it is safe to set the meter on a more sensitive scale. You should not rewire the circuit
only reconfigure the meter as listed below. Please consult your instructor if there are
any questions as to the proper use of the multimeter.
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1.
Configure the meter to read mA and therefore more accurately determine the current.
a. De-nergize the circuit (Power off).
b. Place the red probe in the A plug.
c. Place the black probe in the COM plug.
d. Set the dial to 200mA.
e. Energize the circuit (Power on).
Debugging Circuits [some hints]
•
Check the 5V posts on the protoboard.
o A very small measured voltage indicates a short circuit
o The voltage should be in the range of 4.9 to 5.2
•
The best place to check the volt inputs is by touching the actual pins on the IC
with the voltage probe.
•
Check to see that all ICs have a 5V power and 0V ground.
•
A banana to banana cable can be used to connect the voltmeter COM to the 0V or
ground post on the protoboard.
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