LM393 plus relay overvoltage protection circuit
The LM393 plus relay overvoltage protection circuit is an active protection architecture that combines a comparator with mechanical switch control. It uses the comparator (LM393) to monitor whether the input voltage exceeds the set threshold. If overvoltage occurs, the relay is driven to cut off the power or switch the circuit, achieving protection. This method is commonly used in power input ports, battery management, and charging systems.
This overvoltage protection circuit is suitable for high-power DC circuits, supports automatic monitoring and cutoff, and reacts quickly in the µs range. The output is flexible, allowing relay activation, power cutoff, or path switching. Hysteresis can be designed to avoid repeated triggering near the threshold, and it can operate independently without a microcontroller.
Example schematic of LM393 plus relay overvoltage protection circuit
In the example above, when the input voltage is 12V DC, the circuit operates normally. When the input voltage exceeds 14V, the relay disconnects the circuit, requiring manual reset or restart to restore normal operation. The core of this circuit is the LM393 voltage comparator, which uses a 2.5V reference voltage. When the input voltage is below 14V, LM393’s 1IN+ is lower than the TL431’s 2.5V reference, so LM393’s 1OUT outputs a low level, keeping the thyristor Q3 off and the relay inactive. The main circuit remains connected, and LED3 lights up normally.
When the input voltage exceeds 14V, LM393’s 1IN+ exceeds the 2.5V reference, causing LM393’s 1OUT to output a high level. This turns on Q3, activating the relay and disconnecting the main circuit. LED3 turns off, and LED4 lights up to indicate an overvoltage fault. Due to Q3’s holding current being higher than its turn-off current, the relay remains latched even if the voltage returns to normal, requiring a reset button to restore operation.
When designing this circuit, ensure the reference voltage is accurate. Use a Zener diode (e.g., 5.1V) or a precision voltage reference (e.g., TL431) as the comparator’s reference, which must remain stable and unaffected by input voltage fluctuations. For voltage divider resistors, design them based on the input voltage and LM393’s input range (0–Vcc), and avoid excessively high resistor values to prevent input impedance from delaying the comparator’s response.
In terms of relay selection, the voltage rating and current capacity must match the load, and the driving current must be provided by an external driver (such as an NPN transistor) of the LM393. Regarding circuit hysteresis design, without hysteresis, fluctuations in the input voltage near the threshold can cause the relay to operate rapidly and repeatedly. Therefore, a positive feedback resistor network can be added to the design (feeding back a portion of the output voltage to the comparator). Additionally, to prevent false triggering, a small capacitor can be connected to the comparator’s input for decoupling, and an RC delay can be added to suppress misjudgments caused by instantaneous voltage pulses. Furthermore, when separating the relay from the load, if controlling high voltage or large current, it is recommended to use an optocoupler and MOSFET to drive the relay, avoiding interference from entering the logic circuit.
The LM393 plus relay architecture is simple, cost-effective, and allows precise threshold setting, making it suitable for 12V, 24V, and other low-to-medium voltage protection. It also facilitates hysteresis, delay, and multi-threshold designs.
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