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Tolerance and Measurement of Resistance at Low Values

Writer:Microhm Page View:Date:2019-08-22
The ability to measure low values to tight tolerances is a concern to both the manufacturer and the user. Microhm Electronics' low ohm resistors with tight tolerance covering shunt resistosrs, metal foil resistors, metal film resistors and more. The problem is compounded in the cases of high current shunts where the self-heating will cause the “in-service” resistance value to be different than that obtained with low current level measuring equipment. Therefore, the measurement conditions must be defined at the time of specification preparation, i.e., resistance value as determined by specified current and measured IR drop following a period  of stabilization.

Measurement equipment is available from a number of sources with varying stated accuracies. Digital multimeters with from 5½ to 8½ digits may have a 1 Ω full-scale range. If the stated accuracy is sufficient, these devices are suitable for direct reading of resistance down to 0.001 Ω when equipped with Kelvin connections. If indirect readings, calculated from current and voltage readings, are acceptable, then the digital multimeter can be switched from the ohms to the voltage function, and with a constant-current power supply across the known and unknown resistors in series.
IR drops across the potential leads can be measured and compared. This method permits measurement at rated current which is not available with the “ohms” function of multimeters and/or most low ohm bridges. Thus, measurement with the higher rated current assures correct results at anticipated operating conditions.

One useful variation of this scheme is to read many resistors at one time by passing the constant current through all resistors in series and switching the voltage probes from one resistor to the next. Large quantities are thus measured quickly. It should be noted that manufacturers, including Microhm Electronics will generally prefer a resistance specified at room temperature and not have to deal with the stabilization time necessary for rated current readings. If rated current readings are required, a test charge may be imposed.

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