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Improving Cable Quality and Reliability.... Continued from page 50
Test Fixtures for 4-Wire Measurement Unlike a bench-top DMM which
has four test connectors (two for source and two for sense), today’s cable test equipment offers a multi- tude of programmable test connec-
Ohmmeter Lead Wire 1 Mating Connector RL1 Volt Meter V
+ –
RW I
Current Source
Resistance of Interest Ohmmeter Lead Wire 1 RL2 Lead Wire 2
Measured Resistance = V/I = RL1 + RW + RL2 ≈ RW if RL1 and RL2 are much less than RW
Volt 2-Wire resistance measurement.
tions, also referred to as “test points,” to which the UUT may be connected. Typical cable testers start with 128 test points and can be expanded upwards into the thousands of points. The advantages of 4-wire meas-
urement come at a cost. First, the test system requires twice the number of test points normally required of 2-wire resistance measurement, significantly increasing equipment cost. Second, test fixtures must utilize two wires for every pin on the mating connector, one for current source, and the other for voltage sense. Typical high-performance cable
test equipment not only provides 4- wire measurement, but also high
testing due to the number of wires employed, and the requirement that source and sense leads be joined at each pin in the mating connector. When high-density mating connec- tors must be used, there may not be enough space within the back shell of the mating piece to accommodate two wires if it was designed for only one. This then requires splicing a short single extension wire from each mat- ing pin to the wire pair, soldering all three together and insulating with heat-shrink tubing. Test fixtures are not mass-produced, but are custom- designed and assembled by hand. A typical 4-wire test fixture may cost from many hundreds to thousands of dollars.
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Sample Screen Report
4-Wire Kelvin to 1mΩ Available
128-Point HVX System Meter Lead Wire 3 VW V
+ –
IW
Current I Source
RW Mating Connector Conductor Being Measured Mating Connector Conductor Being Measured
voltage testing to check for dielectric breakdown and insulation resist- ance. With the cost of this equipment ranging from $25 to $50 per test point, depending on the total number of points ordered, the equipment cost can be significant, especially for larg- er assemblies. Construction of the test fixture itself contributes to the overall cost of
Mating Connector
Considerations for Building Before building 4-wire test fix-
tures, be sure that the test equip- ment to be used has sufficient test points for the fixture — two test points for every pin in the mating connectors. To determine the mini- mum test point requirement, add up all the pins on all the connectors of the cable or harness, including any ground or shell conductors, and dou- ble this number for the total required points. Also, be sure to develop a wiring standard; assign odd numbers and even numbers to the source and sense test points and keep the
December, 2015
test fixture is unimportant in 4-wire measurements since the lead wire does not figure into the entire resist- ance of the circuit. Thus 4-wire methodology may be especially advantageous when the UUT is located some distance from the test equipment. For example, if the UUT is in an environmental chamber apart from the test equipment, leads of considerable length can be fed through a sealed access port to obtain precision measurement re - motely. Note that for very long source wires with resistance exceed- ing 5W, ensure the voltage of the 4-
Lead Wire 4 Lead Wire 2 Measured Resistance = VW/IW = RW 4-Wire resistance measurement.
method the same for all fixtures. Because the source pin can
drive a current of 1A or more into a pin, 22-gauge or larger wire is suit- able here. The sense pin will carry almost no current at all, so its wire can be much thinner. This might be an advantage when trying to crimp two wires into a single pin of the mating connector. The length of the wire in the
wire system can rise sufficiently to drive the current through the wire. Several testers presently on the
market use 64-pin dual-row latch headers. For this type of test point interface, using Ampmodu connec- tors or their equivalent, which con- sists of a 64-pin socket body and gold plated crimp-and-poke pins, is rec- ommended. These connectors offer
Continued on next page
Demo Systems
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CAMI
® CAMI Research Inc.
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