TONEOHM 950 OPERATOR MANUAL
4-8
4.4 Track current measurement
The 200mA and 2A ranges are used to measure the current flowing in a PCB track without needing to cut
the track, lift components or break the circuit in any way.
• Either power the board from its own supply, or disconnect external power and
connect the TONEOHM 950's Drive Source between the shorted nodes. Use of the
Drive Source is recommended, as its output voltage is limited and its amplitude can
be adjusted using the Drive Source control.
• Use the Needle Probes to measure the current flowing in a track (see Fig 4-3). The
probes should be kept a few millimetres apart to ensure a valid reading. If the probes
are too close the UNCALIBRATED LED will light.
Note:
• When the probes are first connected to the track the UNCALIBRATED LED will
briefly light while the instrument auto-ranges.
• Unlike a conventional ammeter where errors are minimum at FSD the current
injection technique used on the 950 results in lower errors at LOWER readings.
100mA
75mA
25mA
Figure 4-3 Track Current Measurement
The 950 determines the current by measuring the voltage drop along the track, so for a measurement to
be meaningful do not probe a track at a branch (see Figure 4-4).
If the UNCALIBRATED LED lights, there is insufficient current and/or track resistance for the instrument
to make a valid measurement, although the reading will still be proportional to the current. Either increase
the distance between the probes or use Track Voltage.
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