OHM METERS
Test Instruments & Measuring Devices
Equation (14-10) is the mathematical representation for determining full-scale deflection meter
current.
(14-10)
Im
V
Ro
When the unknown resistance Rx is connected across the ohm meter terminals, the current is
measured by calculating the total series resistance and applying Equation (14-10). Equation
(14-11) is the mathematical representation of this concept.
(14-11)
I
V
Ro
Rx
An easy way to determine ohm meter deflection is by use of a deflection factor (D). Deflection
factor is the ratio of circuit current to meter current. Equation (14-12) is the mathematical
representation of the deflection factor.
(14-12)
D
I
Im
V
Ro
Rx
V
Ro
Ro
Ro
Rx
The current through the circuit can be determined by solving for I. Equation (14-13) is the
mathematical representation of this relationship.
(14-13)
I
DIm
To solve for Rx using Equations (14-10) through (14-13), the relationship between deflection
factor and the meter resistance to the unknown resistance can be shown. Equation (14-14) is the
mathematical representation of this relationship.
(14-14)
Rx
1
D
D
Ro
If half-scale deflection occurs, then Rx = Ro, so that the value of Ro is marked at mid-scale on
the ohm meter face.
ES-14
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