Simple Over/Under Voltage Protection Circuit Using LM393

[Sekhohub.online]

Voltage fluctuations can damage sensitive electronic appliances. Whether you’re powering a DC fan, LED strip, battery charger, or controller, it’s essential to protect your system from over-voltage and under-voltage conditions.

In this project, we will build a simple over/under voltage protection circuit using the LM393 comparator IC, BC547 and BC557 transistors, and a 12V relay.

This circuit automatically cuts off power to the load when the voltage goes below or above safe limits.

Materials for the Project

1x LM393 IC
1x BC547 TRANSISTOR
1x BC557 TRANSISTOR
2x 5K VARIABLE RESISTORS
4x 1N4007 DIODES
1x 3V ZENER DIODE
3x LEDs (RED, GREEN, AND YELLOW )
5x 1K RESISTORS
3x 4.7K RESISTORS
2x 10K RESISTORS
1x 12V RELAY
2x 2-PIN TERMINAL BLOCK
1x 3-PIN TERMINAL BLOCK
VERO BOARD
JUMPER WIRES

Circuit Overview

The LM393 is a dual comparator IC that compares two input voltages and changes its output accordingly.

We’ll use:

Comparator 1 to detect over-voltage
Comparator 2 to detect under-voltage

Each comparator is connected to a voltage divider and reference zener diode, and its output drives a transistor that controls the relay.

⚙️ How It Works

The input voltage is divided using resistors and compared with Zener reference voltages.
If the voltage goes below the under-voltage threshold, the comparator output goes high → the transistor turns off → the relay turns off → the load is disconnected.
If the voltage goes above the over-voltage threshold, the comparator output also goes high → the transistor turns off → the relay turns off.
Only when voltage is within a safe range, the output of both comparators is low, the transistor conducts, the relay is ON, and the load is connected.

️ Threshold Adjustment

You can set:

Under-voltage cut-off point using the first potentiometer
Over-voltage cut-off point using the second potentiometer

Typical settings for a 12V system:

Under-voltage cut-off: ~10.5V
Over-voltage cut-off: ~14.5V

Testing Steps

Power the circuit with a variable DC supply.
Slowly increase the voltage from 0V to 15V and observe when the relay clicks ON and OFF.
Tune the potentiometers to set your exact cut-off values.
Add a 12V LED or bulb as a load to verify switching.

Why Use LM393?

Low power consumption
Built-in dual comparator
Fast switching time
Open-collector output (perfect for transistor control)

Applications

✅ Battery over/under-voltage protection
✅ Solar charge controllers
✅ DC power supplies
✅ Industrial DC equipment safety
✅ Smart home power systems

⚠️ Safety Notes

Always use a diode (1N4007) across the relay coil to suppress back EMF.
Use the correct relay rating for your load (DC or AC).
For higher current loads, use a relay driver with a heatsink.

Video Tutorial

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Download Circuit Diagram

✅ Conclusion

With just an LM393, a couple of transistors, and a relay, you can build a compact and efficient voltage protection circuit for any 12V system. It’s low-cost, easy to assemble, and highly reliable.

For more DIY electronics circuits and tutorials, keep visiting SekhoHub.online—Pakistan’s growing platform for electronics learners and makers.