TONEOHM 950 OPERATOR MANUAL
4-18
4.6 Track voltage measurement
The 20mV and 2mV ranges provide a method for tracing the flow of a DC current through a PCB track.
This provides an alternative to using the 200mA or 2A Track Current ranges, and is particularly applicable
where the tracks are very thick (i.e. very low resistance)
If the fault is static it is recommended that the Drive Source is connected between the faulty nodes to
supply the current. Its output of 550mV will not damage devices or turn on silicon junctions, making
tracing of the current flow easier. As the output is DC, decoupling capacitors do not affect operation.
Disconnect power from the board under test if the Drive Source is used.
For a dynamic fault it will be necessary to power the board. In this case the current flow that is traced will
be a combination of the "normal" load to devices connected to the supply and the excess current due to
the fault.
It is then possible to trace the current flow through the track and its branches by measuring the voltage
drop along the track due to the current flowing in it.
Remember that the displayed value and tone correspond to the voltage being measured along the track.
This voltage is proportional to the current flowing, but will also vary with the distance between the probes
and the resistance of the track (e.g. a current flowing through a thin track will develop a larger voltage
drop than the same current flowing through a thick track). When comparing the current flow through
tracks, the distance between the probes should be kept approximately the same and the method of
probing shown in Figure 4-4 used.
As the tone is proportional to the voltage measured, it is usually sufficient to follow the tone change rather
than check readings. If the measured voltage is negative, a warble is generated as a warning of reversed
polarity.
NOTE: Do not rely too much on voltage readings, or differences between readings, of less than 15µV.
Readings as small as these can be caused by thermoelectric voltages instead of PCB track currents. For
this reason the TONEOHM 950 does not produce a tone for readings below about 50µV.
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