Test Instruments & Measuring Devices
OHM METERS
Example 1:
An ohm meter has a meter movement with a 100 µA full-scale deflection. The
open circuit voltage at terminals ab is 24 V. The ohm meter is zeroed and then
an unknown resistance Rx is measured, which produces quarter-scale deflection.
Find Rx.
Solution:
First find Ro.
Ro
V
Im
24
1 x 106
Ro
2.4 x 105 W or 240KW
Then solve for Rx:
Rx
1
D
D
Ro
1
1
4
1
4
(240)
(3)(240)
Rx
720 KW
Therefore, quarter scale deflection of this ohm meter face would read 720 KW.
Example 2:
An ohm meter with Ro = 30 W, and full scale current Im = 300 µA. Find I with:
1) 0 W, 2) 5 W, 3) 10 W, 4) 15 W, and 5) 1 MW resistors across the meter terminal.
Solution:
First, the deflection factor for each resistor must be found.
D
Ro
Ro
Rx
Rev. 0
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