Multi-Channel and Multi-Point Resistance Scanner

A four-wire meter answers one question at a time. When the thing being tested has sixteen pins, forty windings or sixty-four thermistors, the question is asked sixteen, forty or sixty-four times, and the cost of asking it is the labour, not the instrument.

Product overview

These are bench scanners. Each one contains a single four-terminal resistance measuring circuit and a switching matrix that connects it to each channel in turn, so a fixture can be wired once and scanned in a sequence.

The group spans a wide range of channel counts and two levels of measurement performance. The common families run 4 to 32 channels over 10 µΩ to 200 kΩ at 0.05%, with a 64-channel model on a larger chassis. The wide families reach 1 µΩ to 2 MΩ at 0.02% and add thermal EMF cancellation and a low-voltage mode. The high-speed family pushes 1 A through the part and reads 30 channels a second. One family is offered from 3 to 100 channels to order.

All of them take a fixture on screw terminals, four per channel, and all of them grade each channel against its own limits.

How the multi-channel resistance scanner works

Each channel has four terminals: HD and LD carry the test current, HS and LS sense the voltage. Inside the instrument, a matrix of relays connects the selected channel’s four lines to the single measuring circuit. The instrument closes one channel, waits for the line to settle, measures, compares the result with that channel’s limits, stores the result, and moves to the next.

The settling wait is the part that decides real throughput. A fixture with long unshielded wiring presents capacitance and picks up noise, and the reading needs a moment after the relay closes before it is valid. That is why the families expose a scan delay and a trigger delay rather than quoting a scan time: the same instrument scans a short fixture faster than a long one.

Per-channel ranging matters when the points are not alike. A harness may have a 2 mΩ pin and a 200 Ω pin in the same connector, and a single range would either clip the small one or lose resolution on the large one. The families that select a range per channel handle that; the families with one range for the whole scan are cheaper and suit fixtures where every point is similar.

How to select the right multi-channel resistance scanner

Count the points first, then add the points that will be added next year. Channel count is the one specification that cannot be changed afterwards without a new chassis, and the step from 32 to 64 changes the body size and the screen.

Then look at the spread of resistance across the points. If every point is a similar winding or contact, one range is enough and the common families will do. If the fixture mixes milliohm contacts with hundreds of ohms, per-channel ranging or a wide-range family is the requirement.

Then the accuracy and the current. The 0.05% families cover sorting; the 0.02% family is for a drawing that calls for it. The 1 A family is the one to specify when the lowest point is below 100 µΩ and the part is heavy enough to take the current.

Then the fixture interface. Screw terminals suit a harness built in-house; a connector suits a fixture that is swapped between products. The terminal numbering has to match the channel numbering in the setup, or the results will be assigned to the wrong point, which is the most common way a scanner produces a wrong pass.

Then the output. A scanner that only displays results is a bench instrument; one wired to a handler or PLC is part of a line. Where the fixture will grow past one chassis, cascading over MODBUS is the cheaper route than a larger instrument.

Measurement limitations and selection considerations

The channels are scanned, not sampled together. Two points that must be compared at the same instant cannot be, and a reading that changes during the scan will be captured at different moments on different channels. For a batch of static parts that is irrelevant; for a part that is warming up under load it is not.

Every channel carries its own wiring, and that wiring is part of the measurement. A long channel will read differently from a short one on the same part, because of lead resistance in the drive path and thermal EMF in the sense path. The four-terminal connection removes the first, but not the second, and a fixture that mixes wire types will show a channel-to-channel offset that looks like a product defect.

Thermal EMF is the limiting error at the low end. A relay contact and a copper terminal form a thermocouple, and at 1 µΩ with 500 mA the effect is small but at low current it is not. The families with thermal EMF cancellation or a low-voltage mode are the ones to choose when the bottom of the range is the working point.

Contact resistance at the fixture, not at the instrument, sets the practical floor. A screw terminal that is not tight, or a probe that has lost its spring, produces a reading that drifts between scans and is easy to mistake for a marginal part.

The switching hardware has a life, and the material does not state a contact rating for it. A fixture that scans continuously at 30 readings per second accumulates a large number of operations, so the relay or switch rating should be confirmed for the duty cycle before the instrument is specified into a 24-hour line.