High and Low AC Voltage Cutoff with Delay and Alarm

Electrical appliances are vulnerable to voltage fluctuations, especially in regions where power stability is a concern. A High and Low AC Voltage Cutoff Circuit helps protect appliances by disconnecting them from the power supply when voltage goes beyond a safe operating range. In this blog, we’ll explore how such a circuit works, its components, applications, and how to build one with a delay mechanism and alarm feature.

Why Do We Need a Voltage Cutoff Circuit?

AC mains voltage ideally should be around 230V (or 110V in some countries). However, voltage surges and drops are common and can:

• Damage sensitive electronic equipment
• Reduce the lifespan of appliances
• Cause fire hazards or overheating

By using a cutoff circuit, we can automatically disconnect the appliance when voltage is too high or too low and reconnect only when it stabilizes.

Key Features of the Circuit

• High Voltage Protection
• Low Voltage Protection
• Time Delay on Reconnection (to prevent flickering or false triggering)
• Audible Alarm for fault indication
• Relay-Based Switching (for handling high current loads)

Block Diagram

pgsqlCopyEdit AC Input
    |
Voltage Divider (AC to DC Scaling)
    |
   Comparator (for high and low limit detection)
    |
Control Logic (includes delay timer)
    |
  Relay Driver
    |
  Relay --> Load (Appliance)
    |
  Buzzer (for alarm on fault)

Working Principle

1. Voltage Sensing: The AC input is scaled down using a step-down transformer or voltage divider and rectified to DC.
2. Reference Levels: Two comparators are used to compare the sensed voltage with preset high and low thresholds.
3. Cutoff Trigger: If the voltage exceeds either limit, the comparator sends a signal to deactivate the relay, cutting off power to the load.
4. Delay Timer: When voltage returns to normal, a delay timer (e.g., using 555 timer IC) waits for a few seconds before reactivating the relay.
5. Alarm System: An optional buzzer alerts the user during fault conditions (high or low voltage cutoff).

Components Required

Circuit Explanation

1. AC Voltage Sensing

• A transformer steps down 230V AC to a manageable level (like 12V AC).
• A bridge rectifier and capacitor filter convert it to DC.
• The voltage is further scaled and fed to the comparator inputs.

2. Voltage Comparison

• Comparators compare the sensed voltage with two reference voltages.
• If sensed voltage < LOW threshold → Low cutoff
• If sensed voltage > HIGH threshold → High cutoff
• If within range → System is normal

3. Delay Timer Circuit

• A 555 timer is configured in monostable mode.
• Once voltage returns to normal, the timer waits (e.g., 5 seconds) before turning the relay ON.

4. Relay Driver

• A transistor acts as a switch to activate the relay coil.
• Relay controls the power to the load (fan, fridge, etc.).

5. Alarm System

• A simple buzzer circuit triggers when voltage goes out of range.
• Can be disabled during the delay period to prevent nuisance beeping.

Applications

• Home Appliance Protection
• Refrigerators and Air Conditioners
• TV and Audio Systems
• Office Equipment
• Small Industrial Machines

Advantages

• Cost-effective protection solution
• Automatic operation without user intervention
• Prevents costly equipment damage
• Delay prevents false triggering due to transient changes

Upgrades & Smart Features

You can enhance this project by adding:

• Digital Display (LCD/7-Segment) for voltage monitoring
• Microcontroller-based logic for precise control (using Arduino or PIC)
• IoT Integration for remote alert and reset
• Battery backup for control circuit during blackouts

Conclusion

A High and Low AC Voltage Cutoff Circuit with Delay and Alarm is an essential protective device for modern electrical setups. Whether you build a basic analog version or an advanced digital one, it significantly enhances the safety and longevity of your appliances.