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INTRODUCTION
The MRM-100 is a portable micro-Ohmmeter used to test circuit breaker
contact resistance, or anywhere low resistance measurements at currents up to
just over 100 Amps are required. The MRM-100 uses a low voltage, high current
power supply to run a user-defined test current of between 10 Amps to 105
Amps to any resistive load that requires no more than 2.5 Volts across it to test.
The MRM-100 does detect if the test is able to reach desired current, and allows
for immediately stopping the test.
THEORY OF OPERATION
The MRM-100 uses a two-voltmeter method where each voltage reading
implements a four-wire Kelvin connection. One voltmeter monitors the voltage
across a known, highly stable shunt that passes the test current through it, while
the other voltmeter monitors the voltage across the unit under test. A test
current is then applied (see ANSI C37.09 and IEC 62771-100 for test current
requirements). This current is ramped up, held for a period of time and then
ramped down. While the current is relatively constant, the voltages across the
known resistance and unit under test are amplified and filtered. Then they are
sampled several times by analog to digital converters to reduce any noise, and
processed by a microcontroller. The microcontroller calculates the ratio between
the test current and sensed voltage to determine the resistance. The micro-
Ohmmeter is designed to test resistances of 1 micro-Ohm to 240 milliohms.
