Functional Operation
7
5. Functional Operation
This section describes the functional operation and features of the RT3-100P and RT3-100L
micro-ohmmeters.
5.1. Operating Principle
The RT3-100 operates on a four-wire measurement principle in which a current of known
value is driven through the load under test, thereby developing a voltage across that load. The
Potential leads then couple the developed voltage back to the unit, where the sensed potential
is compared to the known current to determine the resistance of the load. The meter displays
the result in microhms or milliohms, depending on the range switch setting.
Figure 3. RT3-100P Block Diagram
As the block diagram shows, with the control power switch on, the RT3-100P is in a reduced
power consumption state. Power for the RT3-100L is derived from a 115VAC source.
5.2. Measurement Accuracy
In practice, total resistance in the test current path varies with the changing load resistance,
and the test current can deviate from its nominal value. However, one of the advantages of
the four-wire ratio measurement method is accuracy over a wide range of test current
variations.
In a properly adjusted system, the test current power supply delivers a nominal 100 amps to
the load under test. Output from the supply is enabled only while performing a test. In
addition to regulating load current, accuracy is further assured by the use of a ratio-type panel
meter, which senses actual load current across an internal shunt. If current regulation
changes, the meter senses the percent change and compensates scaling of input voltage.
Accuracy is constant for load current variations from 85 to 135 amps. This means the current
regulator and Error LED calibrations are not critical and can usually be done quickly. The
recommended calibration settings are:
Load Current............. 100A
±
3A
Error Monitor ............. 90A
±
2A