True measurement of current requires the
circuit to be broken and a current measurement device inserted
(e.g. a shunt that converts current to voltage). However, breaking the
circuit is impractical in many circumstances and, in the case of
PCB tracks, may be impossible.
Closed magnetic circuit current measurement
DC capable current probes do not measure current, they measure
field density. Current flowing through a conductor creates an H
field which is directly proportional to the current.
If a conductor is surrounded by a closed magnetic circuit of
high Mu material the whole of the field is ?captured? by the
magnetic circuit and the field density can be scaled to
Conventional current probes achieve this by concentrating the
field into a gap within a loop of high Mu material. The field is
then measured by a field sensor inserted into the gap, often a
Hall effect device.
Alternatively ac current can be measured by transformer action
whereby the loop of magnetic material creates a one turn primary
from the conductor that is enclosed. Hybrid devices typically use a field
sensor for dc and low frequencies plus a transformer for higher
Normally the probe provides a method of mechanically splitting
the magnetic circuit to enable the conductor to be inserted. The
position of the conductor within the loop has relatively little
effect upon the measurement.
Note: Illustration shows high performance current probes
from Tektronix inc.
PCB track current measurement
Measuring current in a PCB track presents particular
difficulties because it normally not possible either to break
the track or to enclose it within a magnetic circuit. Typically
engineers have to guess at the current flowing in a track from
voltage measurements made in other parts of the circuit.
As electronic design moves towards ever higher densities,
development omits the ?bread board? stage and goes straight to
PCB design. The inability to observe and measure currents in a
circuit under development can pose a serious problem for