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OHMMETER RANGES | Up
Neets Module 03-Introduction to Circuit Protection, Control, and Measurement | Next
SHUNT OHMMETER |
1-35
Figure 1-33.—An ohmmeter with multiplication jacks.
Some ohmmeters are equipped with a selector switch for selecting the multiplication scale desired,
so only two test lead jacks are necessary. Other meters have a separate jack for each range, as shown in
figure 1-33. The range to be used in measuring any particular unknown resistance (Rx in figure 1-33)
depends on the approximate value of the unknown resistance. For instance, assume the ohmmeter in
figure 1-33 is calibrated in divisions from 0 to 1,000. If Rx is greater than 1,000 ohms, and the R x 1 range
is being used, the ohmmeter cannot measure it. This occurs because the combined series resistance of
resistor R ´ 1 and Rx is too great to allow sufficient battery current to flow to deflect the pointer away
from infinity (¥). (Infinity is a quantity larger than the largest quantity you can measure.) The test lead
would have to be plugged into the next range, R ´ 10. With this done, assume the pointer deflects to
indicate 375 ohms. This would indicate that Rx has 375 ohms ´ 10, or 3,750 ohms resistance. The change
of range caused the deflection because resistor R ´ 10 has about 1/10 the resistance of resistor R ´ 1.
Thus, selecting the smaller series resistance permitted a battery current of sufficient amount to cause a
useful pointer deflection. If the R ´ 100 range were used to measure the same 3,750-ohm resistor, the
pointer would deflect still further, to the 37.5-ohm position. This increased deflection would occur
because resistor R ´ 100 has about 1/10 the resistance of resistor R ´ 10.
The foregoing circuit arrangement allows the same amount of current to flow through the meter’s
moving coil whether the meter measures 10,000 ohms on the R ´ 10 scale, or 100,000 ohms on the
R ´ 100 scale.
It always takes the same amount of current to deflect the pointer to a certain position on the scale
(midscale position for example), regardless of the multiplication factor being used. Since the multiplier
resistors are of different values, it is necessary to ALWAYS "zero" adjust the meter for each
multiplication fact or selected.
You should select the multiplication factor (range) that will result in the pointer coming to rest as
near as possible to the midpoint of the scale. This enables you to read the resistance more accurately,
because the scale readings are more easily interpreted at or near midpoint.
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