Project Hub /Basic Analog & Power Electronics/Sequential, Timing & Pulse Circuits/Building Voltage Protection Circuit with Adjustable Delay

Intermediate

ANALOG / DISCRETE

< ₹500

3 – 6 Hours

Partial Kit

Building Voltage Protection Circuit with Adjustable Delay

A voltage protection circuit that cuts out power to appliances when input voltage is too high or too low, featuring an adjustable startup delay to protect compressors.

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.

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
MB102 Adjustable Breadboard Power Supply Module (3.3V & 5V) for Arduino & Raspberry Pi Rs. 139.00
400 Points Mini Solderless Prototype PCB Breadboard High Quality Rs. 89.00
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: Op-Amp IC (LM358 / LM741 / TL072)
Transistors (BC547 / 2N2222 / TIP / MOSFET)
Diodes (1N4007 / 1N4148 / Schottky / Zener)
Capacitors
Header Pins / Terminals / Connectors
LED Bulb / Lamp / AC Load

Browse all components

How It Works

The Voltage Protection Circuit with Adjustable Delay uses an analog approach to monitor a 15V unregulated power supply and protect a connected 230V AC appliance from dangerous voltage fluctuations. At the heart of the circuit is an LM324 quad op-amp (IC1), which compares the incoming voltage levels against preset thresholds established by 10-kilo-ohm presets (VR1 and VR2). A 6V Zener diode (ZD1) provides a stable reference voltage for these comparisons. When the input voltage falls within a safe operating window, the op-amp outputs a signal that drives a 2N3904 NPN transistor (T1). When transistor T1 conducts, it energizes the 12V SPDT relay (RL1), closing its contacts to deliver mains power to the load via CON3. To prevent short-cycling—which is particularly damaging to compressor motors—an adjustable startup delay mechanism is integrated using a 100uF 35V electrolytic capacitor (C1) and a 5-kilo-ohm preset (VR3). When power is first applied or restored after a fault, C1 charges through the timing network, holding off the relay activation until the predetermined delay period elapses. Visual feedback is provided by two 5mm LEDs: a green LED (LED1) indicates normal operation or power-on status, while a red LED (LED2) lights up during fault conditions such as over-voltage or under-voltage. Rectifier diodes (D1-D4) handle power conversion and protect the active components from reverse-voltage spikes and inductive kickback from the relay coil.

Why Build This

Safeguard sensitive motor-based laboratory equipment like small refrigeration units and cooling fans from harmful grid voltage sags and surges.

Gain practical engineering experience in analog threshold detection, timing circuits, and electromechanical relay control suitable for diploma and ITI curricula.

Real-World Application

Protecting refrigerators, air conditioners, and other compressor-based appliances from damaging high/low voltage fluctuations and short-cycle power surges.

Skills You'll Learn

Analog comparator circuit design

RC timing network calculation

electromechanical relay interfacing

transistor switching applications

power supply voltage monitoring

Safety Precautions

Exercise extreme caution when working with 230V AC mains power to avoid electrical shock and ensure proper insulation.

Technology Tags

OP-AMP

LM324

RELAY

VOLTAGE PROTECTION

AC LOAD

COMPARATOR

DISCRETE COMPONENTS

Frequently Asked Questions

What is the purpose of the adjustable delay in this circuit?

The delay prevents compressor-based appliances like refrigerators and air conditioners from turning back on immediately after a power outage, protecting the compressor motor from damage caused by starting against high head pressure.

Can I use this circuit directly with a 230V AC appliance without a relay?

No. The 12V SPDT relay acts as an isolated switch that allows the low-voltage control circuitry to safely open and close the high-voltage 230V AC mains path to the load.

How do I adjust the upper and lower voltage trip points?

You can calibrate the trip points by adjusting the 10-kilo-ohm presets (VR1 and VR2) while supplying variable test voltages to simulate over-voltage and under-voltage conditions.

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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