Multimeters are an extremely useful tool for troubleshooting electrical problems.
They can be purchased in either analog or digital form and have a price range
to suit any budget. A multimeter is a voltmeter, ammeter and ohmmeter (along
with other features) combined into one instrument. It is often used when testing
solid state circuits because of its high input impedance (usually 10 megaohms
or more). A brief description of the multimeter main test functions follows:
- Voltmeter — the voltmeter is used to measure voltage at
any point in a circuit, or to measure the voltage drop across any part of
a circuit. Voltmeters usually have various scales and a selector switch to
allow the reading of different voltage ranges. The voltmeter has a positive
and a negative lead. To avoid damage to the meter, always connect the negative
lead to the negative (-) side of the circuit (to ground or nearest the ground
side of the circuit) and connect the positive lead to the positive (+) side
of the circuit (to the power source or the nearest power source). Note that
the negative voltmeter lead will always be black and that the positive voltmeter
will always be some color other than black (usually red).
- Ohmmeter — the ohmmeter is designed to read resistance (measured
in ohms) in a circuit or component. Most ohmmeters will have a selector switch
which permits the measurement of different ranges of resistance (usually the
selector switch allows the multiplication of the meter reading by 10, 100,
1,000 and 10,000). Some ohmmeters are "auto-ranging" which means the meter
itself will determine which scale to use. Since the meters are powered by
an internal battery, the ohmmeter can be used like a self-powered test light.
When the ohmmeter is connected, current from the ohmmeter flows through the
circuit or component being tested. Since the ohmmeter's internal resistance
and voltage are known values, the amount of current flow through the meter
depends on the resistance of the circuit or component being tested. The ohmmeter
can also be used to perform a continuity test for suspected open circuits.
In using the meter for making continuity checks, do not be concerned with
the actual resistance readings. Zero resistance, or any ohm reading, indicates
continuity in the circuit. Infinite resistance indicates an opening in the
circuit. A high resistance reading where there should be none indicates a
problem in the circuit. Checks for short circuits are made in the same manner
as checks for open circuits, except that the circuit must be isolated from
both power and normal ground. Infinite resistance indicates no continuity,
while zero resistance indicates a dead short.
WARNING
Never use an ohmmeter to check the resistance of a component or wire while there
is voltage applied to the circuit.
Ammeter — an ammeter measures the amount of current flowing
through a circuit in units called amperes or amps. At normal operating voltage,
most circuits have a characteristic amount of amperes, called "current draw''
which can be measured using an ammeter. By referring to a specified current
draw rating, then measuring the amperes and comparing the two values, one can
determine what is happening within the circuit to aid in diagnosis. An open
circuit, for example, will not allow any current to flow, so the ammeter reading
will be zero. A damaged component or circuit will have an increased current
draw, so the reading will be high. The ammeter is always connected in series
with the circuit being tested. All of the current that normally flows through
the circuit must also flow through the ammeter; if there is any other path for
the current to follow, the ammeter reading will not be accurate. The ammeter
itself has very little resistance to current flow and, therefore, will not affect
the circuit, but it will measure current draw only when the circuit is closed
and electricity is flowing. Excessive current draw can blow fuses and drain
the battery, while a reduced current draw can cause motors to run slowly, lights
to dim and other components to not operate properly.
