Duncan Instruments Canada Ltd
Duncan Instruments Canada Ltd.
Electronic Measurement Solutions since 1968
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Low resistance measurement considerations

by Ivan S. Veg

There are many electrical devices, such as relays, high current contactors, motors and transformers, where resistance measurements are less than 1.0 Ohm. The issues associated with this type of low resistance measurement are listed below:
Lead resistance

A DC resistance measurement is based upon: 1) a supply of current and 2) the measurement of the voltage drop. Using Ohm's law this voltage drop is expressed in terms of resistance: R=V/I.

Test leads of a multimeter typically have a resistance of about 0.2 Ohms. When measuring resistances of less than two Ohms, this could produce a large error of 10%. The simple solution in this instance, would be to subtract the residual resistance of the test leads but with less than ideal connections, this method is at best, limited. Most manufacturers of dedicated milli or micro ohmmeters use the 4-wire Kelvin test lead method to arrive at error-free results.

In a 4-wire Kelvin test lead, such as our STERLING Instruments KTLS2M, the supply points for the current are placed outside the voltage leads and an accurate measurement of resistance is possible at the contact of the voltage leads. Pix 4 wire hookup.

Lead length
While in general the length of the wire used in the leads is not an issue for the voltage leads, it is a concern for the current leads. At low milli-ohm and micro-ohm the supplied current maybe ten or more amperes. The current leads have their own resistance and produce a voltage drop. This in addition of the voltage drop across the DUT (device under test) may exceed the compliance voltage capability of the meter. The solution for extra long leads is to increase the size of the conductor thus lowering the resistance and the voltage drop on the current leads.
Temperature
Most metals used in electrical devices have a temperature coefficient (in the case of copper 0.4% per degree C) so a 10 degree Celsius change in temperature will cause an 4% difference in the reading. The meter itself is also subject to temperature change. Typical calibrations are at room temperature and the specifications may apply over a fairly narrow temperature band.
Inductance
Measuring the resistance of motors, transformers and other inductive loads presents another challenge. Before stable measurement can be obtained the inductor has to be charged up. Depending on the amount of voltage and current available from the meter this can take minutes for large devices. Furthermore the meter can not be disconnected from the inductor until this charge is dissipated ( typically the same amount of time as for charging). If the circuit under test is opened too early a dangerous flash may occur possibly injuring the operator and/or damaging the meter.
Connections
Dissimilar metals produce s small voltage that can cause inaccuracies and in many cases special gold-plated clips or prods are used to connect to the DUT(device under test). Oxidized copper is also a poor conductor and has to be cleaned in order to obtain an accurate measurement.
Electrical Noise
The low level of voltage drop makes this measurement susceptible to electrical noise and if encountered shielded leads may have to prevent interference with measurements.

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