Measurements Of Current In A Wire - TTI i-Prober 520 Manual De Instrucciones

Application Notes for Magnetic Field Measurements
Because of the very small dimensions of the probe tip, the probe is capable of investigating the variation
of magnetic fields over very localised areas. It can investigate the fields around inductors and the gaps in
their cores, which can sometimes cause unexpected cross-talk into an electronic circuit. It can also show
the fields radiating through any slots and holes in an equipment case, which are often the source of EMC
compatibility problems. The probe has a bandwidth which covers most of the waveforms in switched
mode power supplies.
The low noise of the system makes it capable of measuring fields much smaller than the earth's field,
which at high scope sensitivity settings can take the trace a long way off screen. The TRACE POSITION
adjustment on the control box has much more range that the typical Y-shift control of a scope. In many
instances, AC coupling of the scope can be used, which will minimise the inconvenience if the low
frequency distortion of the waveform is acceptable.

Measurements of current in a wire

A toroid attachment is provided to allow measurement of the current flowing in a wire or cable. This
attachment contains a magnetic core which concentrates the field from around the cable onto the sensor
of the current probe. It mechanically locks onto the nose of the probe to hold the correct relationship
between the gap in the toroid and the sensor in the probe tip.
Attaching the Toroid to the Cable
Before attaching the toroid assembly to the cable to be measured, first ensure that
there is no risk of electric shock to the operator by either making sure that the cable is
adequately insulated for the voltage it is carrying, or by disconnecting it from its supply.
Pass the cable through the open end of the jaws of the toroid housing, then align the probe so that the
large pips on the nose are aligned with the gaps in the housing and push them together in a straight line.
The jaws of the attachment are forced apart as the nose of the probe passes through until they lock (with
an audible click) into the locating slots. Confirm that the two parts are securely locked together by gently
rotating and pushing the probe. For calibrated results arrange the cable to be at the back of the hole in the
toroid, away from the sensing tip of the probe.
Select the Wire position on the MODE switch on the Control Box. This gives a calibrated sensitivity of 1
Volt per Amp. Set the scope Y-axis sensitivity as required and select a suitable BANDWIDTH setting.
Note: The 1 V/A calibration only applies to a matched set of probe and toroid. The toroid
attachments are not interchangeable between probes. Check that the serial numbers of probe and
toroid match to ensure that the pairing being used is calibrated.
Operating Notes for Using the Toroid Attachment
The magnetic circuit of the toroid reduces the sensitivity to external magnetic fields (including the earth's
field) by a factor of about five, so the measurement is much less affected by positioning of the probe;
nevertheless, for best measurement consistency, arrange for the probe to rest in a fixed position away
from strong local fields.
Additional sensitivity can be obtained by winding multiple turns of the wire around the toroid. The
resulting increase in insertion inductance will impair the frequency response slightly, and might affect
some high frequency circuits, but otherwise the scaling factor is multiplied by the number of turns.
When measuring DC, the measurement can be affected by small hysteresis and remanence effects in the
magnetic material of the toroid. For best accuracy, first apply the current in the required direction to pre-
bias the magnetic circuit, then remove the current and adjust the TRACE POSITION control to set the
zero point on the scope; then reapply the current to measure it. This zero point will not change much as
the current varies providing it remains in the same direction. If, however, the polarity of the current is
reversed, the zero should be reset. When measuring alternating current this effect is negligible, but note
that when the current is switched off the zero point may exhibit some offset.
The polarity marks on the probe barrel are the reference for indicating the direction of the current: the
output to the measurement instrument will be positive when the current flows from the side with the
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