Sometimes, the need arises for a short-circuit tester that supplies a low current to the device under test (DUT) and also uses voltages lower than 100 mV to prevent conduction of semiconductors. The circuit in Figure 1 meets these requirements. R1 limits the current in the DUT to 0.9 mA. The voltage on the DUT can not exceed the value set by the ratio R2/(R1+R2). The NE5230 micropower op amp compares the voltage on RX (representing the DUT) with the voltage at the junction of R3 and R4.
|Figure 1.||This short-circuit detector uses little power, and provides low currents and voltages to avoid damage to the device
You can adjust the op amp’s supply current by trimming R5; in this circuit, the current is 0.1 mA. If the value of RX falls below 14 Ω, the output of the op amp switches low and the LED illuminates. The circuit derives its power from a 1.5 V battery. IC1 converts the battery voltage to 5 V.