TTI i-Prober 520 Manual De Instrucciones página 9

mark to the side with the
only to assist in consistent attachment; reversing the toroid does not reverse the polarity.
Removing the Toroid
Before touching the toroid, ensure that there are no dangerous voltages present.
To remove the toroid, hold both the toroid attachment and the probe body and gently twist through about
30º while pulling the two apart. The twisting action uses the lugs on the nose of the probe to lever the
arms of the toroid housing apart, which makes removal much easier than a straight pull.
Measurement of Current in a PCB Track
Before touching the probe onto tracks carrying high voltages, always check the condition of the
tip insulation. The probe tip is double-insulated; underneath the black, high melting-point, 'wear-tip'
moulding is a second light coloured moulding containing the sensor itself. If the light coloured inner
moulding can be seen then the probe is no longer safe for high voltage use. Careful inspection is needed,
as dirt or ragged wear might make the colour difference hard to detect. To maximise the lifetime of the tip,
avoid rubbing it across rough surfaces.
Measurements on apparatus with dangerous voltages exposed should only be made by
engineers with sufficient training and experience to recognise the hazards involved. Always
take care to keep the hands away from high voltages.
The probe measures the current in a PCB track by measuring the magnetic field around the track caused
by the current. The probe must be held centred over the track, with the long dimension of the probe tip
across the track and the probe body held as close to perpendicular to the track as possible. When the
output is positive on the measurement instrument then the current is flowing in the direction
+
indicated by the
probe will also be influenced by all external magnetic fields at the point of measurement. There are many
causes of such fields – the earth's magnetic field (which may be modified by steel metalwork), leakage
fields from inductors or transformers, magnetised components (such as screws) and even magnetic
materials in the electrodes or terminations of electronic components. As a result, it is quite difficult to
make accurate quantitative measurements, but there are many applications where a qualitative
measurement gives all the information required.
Qualitative Measurements
Major areas of application for this probe are in switched mode power supplies, power amplifiers or other
circuits where high currents flow. The probe is optimised for waveform fidelity, and the fact that its
response extends down to DC allows exact wave shapes to be viewed. Its very localised nature allows
detailed investigation of the exact paths these currents take.
It can, for example, be used to investigate the effectiveness of reservoir or decoupling capacitors – a
pulsed current should flow between the switching device or rectifier and the capacitor, and only a DC
component should flow from the other side of the junction. Residual switching signals, caused by
improper layout or inadequate components, can be easily seen.
The probe can be particularly useful in investigating the current flow in power and ground planes. It is
often found that if a ground plane is split and then re-joins, perhaps around a group of components, then
an unexpected circulating current can be induced around the resulting loop. It can also be used to show
radiation and cross coupling between circuits, and to check for proper cancellation in circuits which are
supposed to be balanced.
Another application is in finding short circuits on a PCB, where the current can be followed from its source
to the point where it is diverted through the fault. Comparing the signal with the current switched on and
off can distinguish between the fault current and external magnetic fields.
Currents can be measured not only in PCB tracks but also in component leads, including the terminals of
integrated circuits. Take care to avoid very hot components, such as wire wound resistors, which can
exceed the temperature rating of the probe tip.
–
mark. There are also polarity marks on the toroid attachment, but these are
–
and marks on the probe barrel. Because this is not a magnetically closed circuit the
+ –
to
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