Project Hub /Basic Analog & Power Electronics/Power Supplies & Voltage Regulators/Circuit To Protect Appliances From High/Low Voltages

Intermediate

ANALOG / DISCRETE

₹500 – ₹1,500

3 – 6 Hours

Partial Kit

Circuit To Protect Appliances From High/Low Voltages

A DIY circuit designed to protect home appliances like refrigerators and air-conditioners by automatically disconnecting them when mains voltage goes out of the safe 190V-230V range.

Originally published on ElectronicsForU

View original tutorial

Parts You'll Need

This project is sourced from another site, not written by us — this list is our best match of their components against what we stock, not a verified bill of materials. Always check the original blog (linked above) for the actual parts list before buying.

LM2596 Step Down DC-DC Buck Converter Rs. 99.00
10pcs 5mm RED Diffused Light Emitting Diodes - LEDs Set Rs. 39.00
10pcs - 220 ohm 1/4 W Resistor Set Rs. 39.00
5V 1 Channel Relay Module Rs. 45.00
400 Points Mini Solderless Prototype PCB Breadboard High Quality Rs. 89.00
MB102 Adjustable Breadboard Power Supply Module (3.3V & 5V) for Arduino & Raspberry Pi Rs. 139.00
Jumper Wire 10 Pin Male To Male 20cm Rs. 45.00
Jumper Wire 10 Pin Male To Female 20cm Rs. 45.00

Selected: Rs. 0.00

This list includes a generic resistor set as a placeholder — it is not matched to the exact value or part number this circuit needs. The correct one is specified on the original source page linked above; check it before buying.

We don't carry yet: Transistors (BC547 / 2N2222 / TIP / MOSFET)
Diodes (1N4007 / 1N4148 / Schottky / Zener)
GSM / GPRS SIM800L
Custom PCB / PCB Fab
Enclosure / Box / Case

Browse all components

How It Works

This appliance protection circuit functions by continuously monitoring the incoming 230V AC mains voltage to ensure it stays within a safe operating range of 190V to 230V. The heart of the power supply and sensing mechanism is the step-down transformer, which converts the high-voltage AC mains down to a safer, lower secondary voltage (12V-0-12V). The 1N4007 rectifier diodes (D1-D3) and filter capacitors (C1 and C2) then convert this alternating current into a stable DC voltage to power the discrete control circuitry. The core sensing stage utilizes 5.6V Zener diodes (ZD1 and ZD2) along with 4.7k potentiometers (VR1 and VR2) configured as adjustable voltage dividers. These components are tuned to detect voltage sags below 190V or surges above 230V. When an unsafe voltage condition is detected, the bias parameters shift, causing the 2N2222 transistors (T1 and T2) to switch states. Transistor T1 and T2 act as electronic switches that control the coil of the 12V single-changeover relay (RL1). Under normal voltage conditions, the relay energizes to keep the appliance connected to the mains. If the voltage drifts out of bounds, the transistor configuration cuts current to the relay coil, causing the switch to trip and physically disconnect the appliance. Additionally, a 5mm LED (LED1) serves as a visual indicator to show the operational status of the circuit.

Why Build This

Design and implement an analog voltage-window comparator circuit using discrete semiconductor components like BJTs and Zener diodes.

Safeguard sensitive undergraduate laboratory equipment or high-value home appliances from transient grid fluctuations and unstable supply voltages.

Gain practical experience in mains-referenced analog system design, power supply rectification, and relay-based electromechanical load switching.

Real-World Application

Protecting refrigerators, air-conditioners, television sets, and submersible pumps from unstable electrical grid conditions.

Skills You'll Learn

AC-to-DC power supply design

analog voltage threshold sensing

discrete BJT transistor switching

Zener diode regulation

electromechanical relay integration

Safety Precautions

Exercise extreme caution as this circuit deals with live 230V AC mains electricity. Ensure proper insulation and enclose the PCB in a secure plastic housing before testing.

Technology Tags

TRANSISTORS

ZENER DIODES

RELAYS

STEP-DOWN TRANSFORMER

AC MAINS

PCB DESIGN

Frequently Asked Questions

Why is a step-down transformer necessary in this circuit?

The transformer safely reduces the dangerous 230V AC mains voltage down to a manageable low-voltage AC level (12V-0-12V) that can be safely rectified, filtered, and processed by the low-voltage analog control components.

What is the purpose of the 4.7k potentiometers VR1 and VR2?

The potentiometers act as adjustable voltage dividers that allow you to precisely calibrate the upper and lower trip thresholds (230V and 190V) for the sensing network.

How does the relay protect the connected appliance?

The 12V single-changeover relay acts as an automatic physical switch; when mains voltage goes out of bounds, the control circuit cuts power to the relay coil, breaking the circuit path to the appliance.

Gallery

This page is part of Kitkraft's Project Hub — a curated collection of electronics & robotics projects gathered from the maker community to help students and hobbyists discover and learn to build. Kitkraft does not claim authorship of the original project, and makes no representation as to the accuracy, completeness or safety of third-party source material. The summary and "How It Works" explanation above are AI-generated, rewritten with reference to the original tutorial — not a reproduction of it. Full credit and a direct link to the original source are provided above. If you're the original creator and have a concern about how your work is represented here, please contact us and we'll address it promptly.

Kitkraft Project Hub — curated from the maker community, credited at the source.

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