HP HP 547A free download

File name HP574A current tracer.pdf

© Copr. 1949-1998 Hewlett-Packard Co. Current Tracer: A New Way to Find LowImpedance Logic-Circuit Faults By tracing current pulses, this sensitive probe helps locate so l d e r b r i d g e s , s hort ed c onduc t ors in ca b le s, sh o rts in voltage distribution networks, shorted 1C inputs and dead or stuck outputs, stuck wired-AND circuits, and stuck data buses. by John F. Beckwith FOR TROUBLESHOOTING DIGITAL systems, instruments such as logic probes, logic pulsers, logic comparators, state analyzers, and computerbased board testers enable an operator to localize a system malfunction down to the faulty node, that is, down to a collection of 1C terminals and the network of printed circuit traces and/or wires that electrically tie the terminals together. However, after the faulty node is identified the operator still lacks the ultimate information needed to make the repair, namely, ex actly what part of the node has failed? Is the driving 1C dead, or has one of the driven ICs developed a shorted input? Has the interconnecting network shorted to another node, and if so, precisely where, or has the interconnecting network developed an open circuit? Except when the fault is due to an open circuit, vol tage-sensing instruments provide no further infor mation, simply because all points of the faulty node are constrained by the interconnecting network to be at the same voltage. At this stage, techniques such as cutting traces or lifting ICs are usually employed until the defective element is identified. This ap proach is time-consuming, tedious, and often damag ing to the printed circuit board and ICs. Although voltage provides no additional informa tion, there is a quantity whose variation about the node provides the information needed to pinpoint the faulty element. This quantity is current. To date, little use has been made of the information provided by the nodal current distribution simply because of the difficulty of measuring current flow. Traditional methods, such as cutting a trace and inserting an ammeter, or encircling the trace with a magnetic path, are clearly very awkward to use on printed cir cuit boards. The question thus arises whether there is a more convenient means to determine current flow in logic circuits. The new Hewlett-Packard Model 547A Current Tracer (Fig. I) was developed to meet this need. The current tracer is a self-contained, hand-held probe containing a single easily-viewed display lamp Printed m U S A whose intensity indicates the relative magnitude of current steps occurring in the vicinity of the current tracer's tip. The reference level for the display can be varied from one milliampere to one ampere by means of a conveniently placed sensitivity control. The 547A responds to the current changes normally preCover: A new troubleimt SnoВ°t'n9 team finds many вўвў*^ low-impedance faults that elude voltage-sensing in struments. Model 547 A Cur rent Tracer (right) reveals the presence and relative size of current step