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According to Ohm's law, the electrical resistance of a component is equal to the voltage drop across the component divided by the current applied to it. In other words:
Where R = resistance, E=voltage and I=current
This is the law that is used in the four-wire, Kelvin-based test that Harris IRT Enterprises utilizes in their Industrial Resistance Testers.
The Kelvin-based test is connected in this way:
A known current is applied by a precision constant current source to the component being measured through the outer test leads (current carrying leads or "I"). The inner test leads (DVM leads) then measure the voltage drop across the component with a high-impedence Digital Volt Meter. Because the current is constant in the current leads, the connection resistance and the resistance of the leads will have no effect on the current passing through the component. The extremely high impedance of the Digital Volt Meter ensures that the voltage drop due to the DVM Leads and contact resistance will be insignificant.
Using Ohm's law, as described above, the resistance of the part is then calculated and displayed on a digital LED readout. This digital resistance measurement can also be made available via an optional RS-232 communications port (RS-232 communications standard on the Model 9250-05). How does the Ambient Temperature Compensation (TC OPTION) feature work?
With Ambient Temperature Compensation, a probe sends ambient temperature data to the instrument. This temperature information is used to modify the constant current that is being passed through the part under test. The resistance then registered is the resistance the item under test would have at the specified temperature. The compensation option can be calibrated for the measurement of either of two measurement coefficients or it can be operated without any compensation. The accuracy of the compensation is within 0.2% of the actual resistance between 0°C and 50°C.
Because the Ambient Temperature Compensation feature just measures the ambient (or room) temperature, the reading may not be completely accurate because the part may not be at ambient temperature. The Infrared Temperature Compensation feature utilizes an infrared temperature probe to measure the actual temperature of the part under test. The accuracy of the infrared temperature compensation is ±3% of the reading or ±5.4°F (±3°C), whichever is the greater.
The Remote Range Option is typically used in on-line testing when various resistance's must be tested. This option allows a PLC or other controller on the production line to set the range of the resistance tester. This option is usually used in conjunction with the BCD or RS-232 Option.
The Diode Test Option verifies the presence and proper polarity of the DC shunt diode installed on a solenoid to prevent high voltage transients during coil activation and de-activation. Because the maximum test voltage product by the Model 5012-05A is 0.200V, the shunt diode forward voltage drop is not exceeded and the diode does not conduct. This permits accurate measurement of the coil resistance up to 2000 Ohms, but does not make it possible for the resistance tester to detect the diode or its polarity.
Our newly introduced Diode Test Option effectively tests for the presence and the polarity of the shunt diode on a DC coil. The required test polarity (which terminal is the diode anode and which is the cathode) is selected by a jumper on the option board. This jumper can be front-panel mounted as necessary for production setup.
The resistance tester test circuit functions in the normal manner. When the resistance test is complete, an external contact closure from the test controller or PLC is used to initiate the diode polarity test. The polarity test automatically locks the resistance reading and limit status so that it may be read while the diode test is in progress.
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