Electronic Electrical Devices  Electronic Electrical Components
Simple Ohm Meter Review
The ohm meter is an instrument used to determine resistance, such as that created by a resistor. A
simple ohm meter
consists of a battery, a meter
movement calibrated in ohms, and
a variable resistor. Ohm meters are connected to a
component which is removed from
the circuit as illustrated below. The reason for removing the
component is that measurement of
current through the component
determines the resistance. If the
component remains in the circuit,
and a parallel path exists in the
circuit, the current will flow in the path of least resistance and give an erroneous reading.
R_{o}, shown below, is an adjustable resistor whose purpose is to zero the ohm meter and correct for
battery aging. It is also a currentlimiting resistor which includes the meter resistance Rm.
Zeroing the ohm meter is accomplished by shorting the ohm meter terminals ab and adjusting
R_{o} to give fullscale deflection.
The equation below is the mathematical representation for determining fullscale deflection meter
current
I_{m} = V / R_{o}
When the unknown resistance R_{x} is connected across the ohm meter terminals, the current is
measured by calculating the total series resistance and applying the equation above. The equation
below is the mathematical representation of this concept.
I = V / ( R_{o} + R_{x} )
