8 _ _ _ _ _ _ _ _ _ _ SlARJECH
CURRENT TRACING
Another instrument very useful in troubleshooting digital
circu its is the current tracer. The model 547 A Current Tracer,
manufactured by Hewlett-Packard, has the same general
appearance as a logic probe, but it is used for measuring
current without the necessity of breaking into the circuit. The
current tracer tip is placed against the foil conductor on the
circuit board, as shown in Figure 10. An indicator illumination
level indicates the relative value of the current in the circuit.
The HP 547A Current Tracer operates over a range of 1
milliampere to 1 ampere. A sensitivity control, used in con-
junction with the indicator lamp, indicates the relative values
of currents from one point to the next.
CIRCUrT BOARD
FIGURE 10
HE:AVY CURRE:NT
PATH
A
FIGURE II
/-
...... "'--'---,GROUM)
F"AULT
B
FIGURE II
-
The current tracer is used in a manner similar to that of the
logic probe. However, it can be used to trace along a circuit
board foil to identify the exact nature of a fault. As an example,
the circuit in Figure 11A shows a ground fault at the input to
IC 8 . A conventional logic probe would indicate a zero logic
level at the interconnection of the gate inputs. However,
unless the integrated circuits were individually tested or
connections unsoldered, the faulty circuit would be very
difficult to locate. By using the current tracer, you can go
along the circuit board foil to each of the gates and readily
determine that the heavy current path is to the shorted gate
on IC 8 .
The current tracer is very useful when employed in conjunction
with a device such as the logic pulser. The logic pulser
provides the stimulus signal at the various test points and the
current tracer is used to determine the incorrect current path.
This technique can be used when one circuit point is shorted
to another. A fault such as this would be verydifficultto isolate
with the logic probe alone.
Shorts from Vee to ground are also difficult to find on a large
circuit board if you do not have the proper test equipment.
Again, the current tracer can be used in conjunction with the
logic pulser to find either the faulty area on the circuit board or
a defective component. First the power supply lead is discon-
nected from the power supply. Then a logic pulser is used to
apply a pulse to the Vee bus, as shown in Figure 11 8. Then, by
using the current tracer to look for the heavy current path on
the various Vee bus leads, the circuit board fault can be readily
located.
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