Precision Measurement Instruments from TTi - Thurlby Thandar Instruments

The Control Box

The control box provides the choice of three modes of operation along with bandwidth filters, an offset control and overload indicator.

It also incorporates the calibrator needed for measurement on PCB tracks of differing widths.

PCB track current measurement

The unique feature of the I-prober 520 is its ability to observe and measure currents flowing in PCB tracks by acting as a "positional" current probe.

The magnitude of the signal is critically related to its position relative to the conductor which means that the probe tip must be positioned carefully. The size of the conductor (e.g. the width of a PCB track) also has a significant effect.

This means that the sensitivity of the I-prober has to be adjusted to match the track width when quantitative measurements are required. A calibrator within the control box enables sensitivity adjustment in conjunction with a calibration graph.

The measurement result will also include other field effects present at the tip of the probe and not just that coming from the current through the conductor. This may include DC effects from adjacent magnetised components and from the earths magnetic field, plus AC effects from transformers and other field radiating sources.

Current in adjacent tracks, or tracks on the opposite side of the PCB will also affect the measurement.

There are potential solutions to these problems. The unwanted DC can be nulled out by observing the measurement without power to the circuit, whilst AC interference can be attenuated using bandwidth filters. The I-prober control box includes a wide range DC offset control and switchable filters.

Nevertheless, the use of the I-prober 520 requires interpretation based upon a proper understanding of circuits and systems. It is a tool for the professional engineer and is not suitable for use by just anyone.

Closed-loop current measurement (in a wire)

Whereas the primary purpose of the I-prober 520 is as a positional current probe, the are many circumstances where current measurements can be made in the conventional way by enclosing the conductor.

To increase its overall usefulness, the I-prober 520 is supplied with a clip-on toroid assembly which converts it into a closed magnetic circuit probe for measuring current in a wire.

The toroid is open until the probe is attached, allowing insertion of the wire without disconnection.

The wide bandwidth, dynamic range and low noise of the probe are retained but higher accuracy, repeatability and unwanted field rejection are achieved.

With the control box mode switch is set to Wire, the probe and toroid assembly are correctly calibrated for an output of 1 volt/amp.

Measurement of electromagnetic field

The very small size of the field sensor within the I-prober 520 gives it some unique capabilities when used to measure magnetic fields.

The variation of field with position can be accurately determined enabling the precise source of fields to be located and their variation in space measured.

With the control box mode switch is set to Field the output voltage is rescaled to measure in Teslas or in amps per metre.

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