Sensing short circuits in equipment that performs under water is especially critical, but Fig. 20-4`s wet-mat
e connector design also suits other remote short-circuit sensing applications. Because of the limits imposed by the battery and voltage levels, the circuit uses the data line to sense short circuits. The differential voltage that develops across the 422- resistor in the data line drives a low-bandwidth op amp, which amplifies and filters the differential signal. The resistor values produce a gain of 3. The op amp"s output controls the voltage regulator"s shutdown pin. To operate correctly, the circuit must have a continuous stream of digital data. Under normal conditions, and using high-speed CMOS logic, the data source sinks less than 10 . This normal operation generates about -3 mV across the sense resistor. The op amp"s output will be slightly negative, producing a solid on signal to the voltage-regulator chip. When a short occurs, the resistor and op amp together produce an average of 2.4 Vdc. This voltage provides a solid off to the voltage-regulator chip. The peak signal-line current is about 12 mA (5-V data divided by 422 ), which HCMOS logic can handle. The addition of the resistor and op amp only changes the rise time to about 40 ns and doesn"t cause any problems with the 2.5-MHz data rate. When the short is no longer present, the voltage regulator chip turns on again. You can use the same circuit with any TTL on/off-type voltage-regulator IC.