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Electronic Design

Improved short-circuit detector

A previously submitted Idea For Design by Jim Wood (“ ‘Beeper’ Finds Short Circuits,” Oct. 24, 1996 “BEST OF ISSUE” SPECIAL SUPPLEMENT TO ELECTRONIC DESIGN, p. 18) impressed me because of the circuit’s simplicity to realize a useful gadget. It suffers, however, because of the power required: ±12 V @ approximately 60 mA—hardly a small portable gadget! This is used to develop a ±100-mA current audio square wave. With the availability of audio ICs designed for two-cell battery operation, it seemed that a better solution could be found. National Semiconductor’s LM4861 fills that requirement.

The circuit designed differs in the oscillator and its frequency control (see the figure). The normal Vo2 (−1) amp has capacitive positive C feedback rather than a series R shunt C on the negative input. Variation of the Vo1 amp’s output changes the bias of the oscillator Vo2 since it’s connected internally. The pitch polarity is reversed —frequency decreases as the short circuit is approached. This could be avoided, but the virtue of reducing the speaker volume when the leads are open would be lost. It is relatively quiet when not being used.

A two-cell supply, together with the load resistor R4 reduction to 10 Ω, creates a ±130-mA square wave to the short circuit. Reasonable speaker level is set by reducing R6 to approximately 47 Ω. The maximum current drain is approximately 80 mA. The capacitor C2 eliminates some circuit instability.

Typically, with the center-tapped battery, a double-pole On-Off switch is required. By using the shutdown mode of the LM4861, the unit can be turned Off (0.6 µA residual) by a single-pole logic switch. All of the notes in Jim Wood’s article apply. Note the differential input requirement and the recommendation of piercing phono-tip probes.

The circuit can easily be built on a 0.7-in2 printed circuit board. Using AAA cells, it can occupy the space in the back of a 2-in. speaker, resulting in a very compact tester that can locate board shorts.

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