The circuit in this Design Idea detects short-circuit faults or excess-current conditions in line-operated equipment, such as a UPS (uninterruptible power supply), across an isolation boundary. If the load current rises higher than a predetermined level, the circuit generates a 5 V pulse to immediately shut down the power inverter. This compact circuit provides a reliable means for protecting power semiconductors and batteries against overcurrent conditions. It features an isolated output and a fast response. The ac mains self-power the sensing network. The circuit requires a 5 V power supply to the output-enable circuit on the other side of the isolation boundary. The circuit detects bipolar over-currents through the load, using shunt resistor RSHUNT’s resistance.
During the positive cycle of the ac source, components R1, IC1, R5, C1, Q1, D1, and C3 work to detect an overcurrent (Figure 1). If the load produces an overcurrent, the voltage drop at the shunt resistor generates a positive voltage on the base of Q1 that turns it on. Consequently, capacitor C1, which R5 initially discharges, receives charge from current across R1 and IC1’s photodiode. IC1’s photoresistor saturates, charging capacitor C5 and providing a 5 V output at VSENSE, indicating the overcurrent detection. D1 increases Q1’s base-voltage turn-on threshold to approximately 1.4 V. Capacitor C3 filters any noise or peak voltage that may accidentally turn on the transistor, thus enhancing the system’s detection reliability.
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| Figure 1. | This isolated circuit detects bipolar overloads in a UPS or another ac load. |
A circuit, comprising R2, IC2, R6, C2, Q2, D2, and C4, for fault detection during the negative cycle of the ac source works similarly. In this case, the direction of current flow through the shunt resistor generates a positive voltage on the base of Q2. D5 and C6 rectify the ac source, providing the isolated supply voltage to polarize the photodiodes. The inverter logic’s power supply supplies the 5 V source. When the load currents are within limits, the output at VSENSE extinguishes due to the discharge of C5 through R8.