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Low resistance measurement considerations
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by Ivan S. Veg
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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 |
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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.
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| 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. |
