A low resistance that depends on how the probes sit

Tags: Measurement Magnetics

Symptom

The dc resistance of a winding with few turns, or of any low resistance, reads well above its calculated value (dc resistance of a winding), and the reading changes when the probes are pressed harder or moved.

Why

A two-wire ohmmeter passes its test current through its leads and reads the voltage across the same leads, so the reading is the part together with the leads and their contacts. The four-wire (Kelvin) method passes the current through one pair of leads and senses the voltage with another, so the leads’ resistance stays out of the reading [Keithley Instruments 2014]. A winding of a few turns can have a resistance well below that of the leads.

How to confirm

Touch the two probes together and read: that is the leads and their contacts alone. If it is not small against the part’s reading, much of the reading is leads. Then measure the part with four wires: the difference between the two readings is the leads and their contacts.

Fix

Measure with four wires: the current leads on the part’s terminals, and the sense leads on the same terminals, inside the current contacts, so that only the part lies between the sense points [Keithley Instruments 2014]. An impedance meter’s four-terminal connections keep the leads’ impedance out in the same way [Keysight Technologies, Impedance Measurement Handbook]. Note the winding’s temperature with the reading: copper’s resistance rises with it (resistivity at temperature).

References

• [Keithley Instruments 2014]: the four-wire method keeps the test leads’ resistance out of a low-resistance measurement.
• [Keysight Technologies, Impedance Measurement Handbook]: four-terminal connections keep the leads’ impedance out of an impedance measurement.
• Measuring magnetics: dc resistance and a bench exercise with two and four wires.