May, 2019


www.us- tech.com


Page 43


Four-Wire Measurement with INGUN’s Kelvin Probe and Contact Series


By Matthias Zapatka, INGUN USA T


o obtain electrical impedance of a device under test, technicians and engineers often employ a


technique called “four-wire sensing,” “Kelvin measurement,” or the “four- probe method.” While it is possible to use four individual probes, by using


and high-voltage blade connectors (INGUN VKF series) or even round connector posts with safety pin (INGUN HKR series — R designates “round”). Lastly, one can also use four individual probes, such as the GKS-112 M probe series, which is a lower-cost alternative to other op- tions, or the GKS-503 M series, with a continuous plunger and thus very low resistance design. The INGUN probes provide a reliable and repeatable temporary


electrical interconnect for end-of-line testing. While there are several probes available by the manufactur- ers of four-wire test equipment, al- most none of these can be imple- mented in a semi or fully automated tester for end-of-line testing. Usually those probes consist of handheld nee- dles with banana plug connections for the tester. This is where the INGUN probes


shine, because their spring contacts allow them to be easily disconnected


after the test is done. The probes themselves do not


have retention force, as this is applied by the test fixture.


Recommended Probes and Contacts


INGUN offers coaxial type


probes in its DPS, HFS and HSS se- ries, where two probes are needed: DPS-215 M, DPS-465 M, HFS-010, HFS-110, and the HSS-624 M. Flat


Continued on next page


Four-wire sensing for voltage and current.


certain INGUN probes and contacts, this task may be completed by using only two coaxial probes instead. Four-wire measurement uses a


pair of voltage-sensing and current- carrying pins to make more accurate measurement than a two-wire meth - od. The effects of the wire length are effectively calibrated out of the meas- urement. This type of sensing is used in some ohmmeters, impedance ana- lyzers, wiring for strain gauges, and resistance thermometers. One typical application for


these contacts and probes is produc- tion line testing of individual battery cells, for example in the automotive industry. The weld joint in some of those cells (for example, the hugely popular 18650 cells) contributes to the overall performance of an assem- bled battery pack. A loose connection or defective


joint could increase the internal re- sistance. Quite often, those resist- ances are very small and thus hard to detect with conventional methods.


Kelvin Sensing Four-wire Kelvin connections


use a pair of current connections, which are known as force connec- tions. These, according to Ohm’s law, generate a voltage drop across the impedance that has to be measured. The pair of the sense connections for voltage is run adjacent to the force connection, so there is no voltage drop in the force leads. This is where the INGUN


probes come in and why they are so handy for these types of measure- ments. Both the outer conductor and the center conductor travel the same distance and have the same length (e.g., the INGUN DPS probe series). Such a probe can be placed eas-


ily on individual battery cells or other components, where four-wire measurements are needed. The outer conductor usually has a higher cur- rent (force) connection, whereas the center conductor is used for voltage (sense). Of course, one is not limited to battery cells, as there are also clip connector probes available that can mate with a variety of high-current


See at NEPCON China, Booth 1J20 and SMTconnect, Hall 4A Booth 120


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