Electronic Design

Printed wiring board short-circuit pin-pointer

The quality of printed wiring boards and other electronic components seems to be improving all the time. As a result, it’s less common these days to see circuit boards with shorts between traces due either to manufacturing defects in boards themselves or to failed components. Nevertheless, especially with handassembled prototypes, it still happens on occasion. When it does, the search for errant particles of copper, specks of solder, or shorted components can be a frustrating ordeal involving cutting/repair of traces and unnecessary removal of good parts.

The battery-powered short-circuit locator shown uses the small but measurable resistance of circuitboard traces themselves, in combination with a low-voltage excitation current, to directly sniff out bad connections (see the figure). It thereby greatly short-cuts (short circuits?) the diagnostic process. The ±1-µV typical input offset voltage and high gain (1,000,000:1) of chopper-stabilized, rail-to-rail op-amp A1 makes it possible to detect voltage developed across trace resistances on the order of microhms.

The “short search” begins by connecting test clips P1 and P2 to the shorted traces. This applies a test current of approximately 300 mA. Simultaneously, Q2 is turned on and applies power to the rest of the circuit. The test probes P3 and P4 are then touched to point a centimeter or so apart on one of the shorted traces (point A on PWB trace 1, for example). In the case of point A, because it lies between P1 and the short, current flowing through the copper lying between P3 and P4 will generate a positive voltage (P3 > P4) on the order of tens of microvolts to millivolts. This will unbalance A1 and cause its output to slew until LED D1 glows.

If point B on trace 2 is subsequently probed, however, no differential voltage will be observed because no current is flowing through this portion of the PWB. So neither LED glows, correctly indicating that B lies outside the conducting path from P1 to P2. As the search continues, we find C is “below” the short and the reversed polarity will light D2. Point D has no current and produces no glow. Subsequently, though, there’s no current at E, and reversed polarity at F. After that observation, a few further probe points will usually zero in precisely on the point of the problem, making definitive repair (relatively) easy.

Fabrication of the locator circuit is straightforward. High-efficiency LEDs should be used for D1 and D2. Then the 3- to 4-mA maximum output from A1 will make for a satisfactory indication, and the relatively high voltage drop of this type LED will prevent any noticeable zero-voltage glow. Note that no ON/OFF switch is necessary because the circuit turns itself off automatically when P1/P2 are disconnected. Battery life is adequate, yielding several hours of continuous operation from a pair of AA cells. This usually transdifferlates into months to years of typical, intermittent service. After all, circuit board quality is improving.

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