This overvoltage detector can be connected to any dc power source (Fig. 1). Its purpose is to provide a visual indication of when the voltage exceeds a preset value, which may range from around 3 V to several hundred volts. The circuit also implements latching overvoltage detection, allowing it to capture transient overvoltage spikes as narrow as 30 µs.
Under "normal" voltage conditions, the whole circuit draws less than 25 µA, making it ideal for monitoring battery-powered systems. But despite its frugal diet, the detector is a fairly tough character, able to withstand continuous mains overloads without damage.
IC1, an LTC1541, is a micropower comparator, op amp, and voltage reference (1.20 V nominal) with a maximum quiescent current of just 13 µA. The op amp and the Q1/Q2 Darlington pair form a precision current sink. With R5 = 1.1 MΩ and R6 = 100k, the op amp puts a nominal 100 mV across R11. By doing so, it sinks 1 mA through the LED (D7). If the LED is a low-current type, such as the HLMP-D155, 1 mA is sufficient to achieve adequate brightness.
With no overvoltage condition present, IC1's comparator output is low. It clamps Q1's base to around 0.6 V via D6, which holds the current sink off. R1 and R2 determine the trip voltage, VT (the value of VIN at which the circuit indicates an overvoltage).
When the voltage (VR2) across R2 exceeds the 1.20-V reference, the comparator trips and its output goes high. D6 becomes reverse-biased, enabling the current sink to illuminate the LED. This provides immediate visual indication of the overvoltage event. At the same time, D5 becomes forward-biased, pulling the comparator's noninverting input toward VDD. The circuit is now latched, and the comparator output remains high even if VR2 subsequently falls below 1.20 V. By either disconnecting VIN or momentarily closing switch SW1, the circuit can be reset.
Under normal conditions, when D6 clamps Q1's base, the op-amp output rises toward VDD, desperately trying to bias Q1 on. Consequently, a substantial value is needed for R10 to minimize the op amp's output current (which adds to IC1's quiescent current). This means that only a few microamperes are available at Q1's base, dictating the use of the Darlington pair. The ZTX458 devices specified for Q1 and Q2 have an hfe high enough to furnish the LED with 1 mA when Q1's base current is as low as 1.5 µA. Since they're high-voltage types, rated to 400 V, they can withstand the 350-V peak voltage of the 240-VRMS mains supply.
If a MOSFET were substituted in place of the Darlington pair, its voltage and power ratings would have to cope with the worst-case anticipated overload. In addition, the MOSFET's gate-threshold voltage would need to be low enough to allow operation at lesser values of VIN.
D1 to D4 and R7 to R9 supply IC1 with overload protection. Diode D3 blocks any excessive negative inputs. The 1N4005 has a 600-V reverse voltage rating, which is more than adequate for a 240-VRMS overload. Diode D2 protects the comparator input against high positive voltages.
For precision voltage monitoring, R1 must be connected "upstream" of D3 as shown. Therefore, D1 is required to protect the comparator input against excessive negative voltages. Selecting a large value for R3 provides ample current limiting for D1 and D2. It also makes it necessary for the comparator to source negligible current through D5, permitting a relatively small value to be selected for R2 if required. Resistor R4 offers additional current limiting for the comparator input. Plus, it prevents the comparator output from being dragged low via D5 when SW1 is closed to reset the circuit.
Because the comparator's input bias current is very low (1 nA maximum), the voltage dropped across R3 and R4 is negligible. Zener diode D4 clamps IC1's positive supply to a safe value. Since the LTC1541's maximum working voltage is 12.6 V, using an 11-V zener is an appropriate selection.
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