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Now let's consider ohmmeters.
A simple ohmmeter can be pictured as a circuit containing an
ammeter, a resistor, a power supply and a pair of leads.
When an ohmmeter tests the device, that device completes the circuit.
Ohmmeters send a current into the component or circuit under test.
The amount of current that flows through the component is
determined by the component's resistance.
The current is indicated on the Ammeter, but the meter's markings
are in Ohms rather than Amps.
Most often ohmmeters are contained as part of a multimeter
called a volt-ohm-milliammeter, or VOM for short.
We'll talk more about multimeters later in this lesson.
Portable vom's have one or more batteries inside to provide the current for the ohmmeter.
Electronic multimeters have a power supply which provides
the current that is fed out by the ohmmeter.
Some meters have their Ohm scale reversed. Infinity is on the left
and zero 0 is on the right.
When the leads of the ohmmeter are not connected to a circuit or device
the ohmmeter circuit is open and no current flows, thus the pointer
is all the way to the left and is marked with the infinity symbol
which means there is infinite resistance. In other words
resistance is so high it is uncountable.
When the leads are connected directly together the pointer moves all the way to the right
which is marked zero (0) on the scale, which means there is no resistance whatsoever
that is a perfect connection allows maximum current flow.
All ohmmeters have a potentiometer that allows the meter to be
properly calibrated as the meter's voltage changes.
This is used to compensate as the battery ages and weakens.
On some meters this control is a "zero adjust."
When the leads are connected together the pointer should read zero (0).
On electronic meters one potentiometer adjusts zero
and another adjusts ohms infinity.
While many ohmmeters have the reverse scale we've discussed
a different type of ohmmeter circuit is used in electronic meters
it has zero ohms on the left and infinity on the right.
Ohmmeter scales are almost always the top scale on a meter
and are always non-linear. The scale markings near zero are widely spaced.
This is the scale area where the most accurate readings are taken.
The space between scale markings near infinity is narrow
showing the infinite progression of numbers. Readings here should be avoided.
The range switch on an ohmmeter selects different resistors to
allow greater or lesser current flow depending on the interference
presented by the components being measured.
Ranges on an ohmmeter are multiplier. Each is indicated as
R X 1, R X 10, R X100 and so forth. Measurements are taken
by first noting the reading indicated by the pointer on the ohm scale.
The number is then multiplied by the range setting.
For example, if this ohmmeter is set to R X 10, and it measures
a resistance of 5 on the meter scale, that 5 needs to be multiplied by 10.
The resistance we have measured, then, equals 50 ohms.
Here are some techniques to be used for proper and
accurate resistance measurements.
The zero adjust and/or ohms adjust setting should be checked
each time an ohmmeter is used. The test leads are connected to
each other, and zero (0) ohms should be indicated.
With the leads separated infinity should be indicated.
An ohmmeter should never be connected to a component or circuit
that is powered, since an ohmmeter supplies current into the component
or circuit that is being measured. If the circuit or component under
test is not disconnected or isolated from power the meter could be
damaged and incorrect measurements will result.
Also care should be taken that no other components are connected
to the component under test. Additional components connected to
the component or circuit under test can interfere with the proper measurement.
Finally measurements should always be made on a range that
allows pointer indication in the more expanded section of the scale.
Measurements made on the compressed side of the scale are less accurate.