TONEOHM 950 OPERATOR MANUAL
4-10
Excessive Vcc Loads
A common problem is a power supply rail being held low by a faulty device drawing excessive current.
This is not a low resistance short (less than 200 mΩ) or the rail would read 0V and the shorts location
procedure (Section 4.2) could be used.
The fault may be caused by a component going low resistance, producing a permanent low resistance
between Vcc and ground. This is an example of a static fault, which remains when power is removed from
the board under test.
Another common component failure is an IC that draws excessive current, causing the Vcc rail to be held
low when the board is powered. When the board is un-powered, the Vcc to ground resistance is normal.
This is an example of a dynamic fault.
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.
As the tone frequency is proportional to the voltage measured, it is usually sufficient to follow the tone
change rather than check readings. If the measured current is negative the tone is turned off and the
PROBES REVERSED LED is lit.
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