If you want a safe, automatic, and professional AC Changeover for your inverter system, this project is the perfect solution. Using a custom PCB, relay, diodes, and a small transformer, this circuit automatically switches your load between AC mains and Inverter AC within milliseconds — without any manual interaction.

Step 1: Components Required

Main Components

1. Relay: NT90HCE12CB0 (12V DPDT / 30A recommended for 1000W)
2. Transformer: 220V → 12V (1A / 1.5A recommended)
3. U2: 7805 Voltage Regulator
4. Diodes:
5. D1, D2 → 1N5822 (High-efficiency rectification)
6. D4 → 1N4007 (Relay flyback protection)
7. Capacitors:
8. U1 → 220µF
9. U3 → 220µF
10. Resistor: R3 → 1kΩ
11. Indicator LED: LED11
12. Connectors: Screw terminals for AC & Transformer

Additional Tools

1. Soldering iron
2. Screwdriver
3. Multimeter
4. Hot glue / enclosure

⭐ Project Highlights

1. ✔ Automatic AC Main → Inverter switching
2. ✔ Handles up to 1000W load
3. ✔ Works with any inverter (IPS, Solar, Online UPS)
4. ✔ Compact transformer-based isolated DC supply
5. ✔ DPDT relay ensures clean AC routing
6. ✔ PCB-based design = No messy wiring
7. ✔ Fully safe, reliable, and easy to build

Main Components

1. Relay: NT90HCE12CB0 (12V DPDT / 30A recommended for 1000W)
2. Transformer: 220V → 12V (1A / 1.5A recommended)
3. U2: 7805 Voltage Regulator
4. Diodes:
5. D1, D2 → 1N5822 (High-efficiency rectification)
6. D4 → 1N4007 (Relay flyback protection)
7. Capacitors:
8. U1 → 220µF
9. U3 → 220µF
10. Resistor: R3 → 1kΩ
11. Indicator LED: LED11
12. Connectors: Screw terminals for AC & Transformer

Additional Tools

1. Soldering iron
2. Screwdriver
3. Multimeter
4. Hot glue / enclosure

1. Transformer Module

1. Converts 220V AC → 12V AC
2. 1N5822 diodes convert to DC
3. 7805 stabilizes output to 5V DC
4. LED indicates system is powered

2. Relay Logic

1. When AC mains present → Relay energizes → Load receives mains
2. When AC fails → Relay de-energizes → Load shifts to inverter

3. AC Routing

The relay’s DPDT structure allows full AC line switching, ensuring:

1. No back-feeding
2. No sparking
3. No short circuits
4. Safe, isolated switching

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🔷 Top Layer PCB

Your image shows:

1. Relay centered
2. Transformer input at bottom-left
3. AC Mains & Inverter input at left
4. Load output at right
5. LED indicator at front

🔶 Top + Bottom Routing

1. Red tracks = AC & relay high-power lines
2. Blue tracks = DC control lines
3. Adequate spacing is used between AC and low-voltage areas
4. Grounding is kept clean for 7805 stability

🔩 Schematic Diagram

Includes:

1. Power supply filtering
2. Relay switching
3. Protection diodes
4. Terminal connections

Perfectly matched with the PCB.

This section is expanded for clarity and written EXACTLY in "Instructables step style".

5.1 – Connect the Transformer to PCB (TR Primary)

Take your 12V transformer.

Terminal labeling:

1. Transformer Secondary Output → TR(Primary)
2. Pin-1 → Transformer wire-1
3. Pin-2 → Transformer wire-2

This powers your DC supply section.

5.2 – Connect AC Mains Input to the PCB (AC(M))

Use a 2-pin screw terminal for AC Mains.

Connect:

1. AC Line (L) → AC(M) terminal pin-1
2. AC Neutral (N) → AC(M) terminal pin-2

This is your utility/main power input.

5.3 – Connect the Inverter Output (AC(NV))

This is the inverter’s 220V AC output.

Connect:

1. Inverter Line → AC(NV) terminal pin-1
2. Inverter Neutral → AC(NV) terminal pin-2

⚠ Make sure inverter and mains share a common neutral (standard household wiring).

5.4 – Connect the Load Output (LOAD1)

This is the final output that powers your appliances.

Connect:

1. Load Line → LOAD1 pin-1
2. Load Neutral → LOAD1 pin-2

This output will automatically switch between:

1. AC mains (when available)
2. Inverter AC (when mains fails)

5.5 – Verify Relay Orientation

Ensure the relay seating matches PCB marking:

Pins:

1. 4 & 6 → Load path switching
2. 1 & 2 → Coil (powered by 5V)
3. 5 → Common switching node

The diode D4 (1N4007) protects the coil.

5.6 – Check Voltage Regulator Section (7805)

Before applying AC:

1. Power the transformer
2. Measure DC output of rectifier (should be 12–15V DC)
3. Measure 7805 output (should be 5.0V DC)
4. LED should turn ON

If LED does NOT glow → check polarity, regulator orientation, and diode connections.

5.7 – Relay Switching Test

Now test the relay:

When AC mains ON:

1. Relay should click ON
2. Load gets Mains AC

When AC mains OFF:

1. Relay should release
2. Load gets Inverter AC

Switching delay is less than 50ms.

5.8 – Final Installation

1. Place PCB in insulated box
2. Tighten all terminals
3. Label each connector
4. Add cable protection bushings

System is ready for long-term use.

Test 1 – Transformer DC Section

1. Check 5V
2. Check LED
3. No overheating

Test 2 – AC Mains Switching

1. Plug mains power
2. Load should be powered from Mains

Test 3 – AC Fail Condition

1. Turn off mains
2. Relay instantly switches
3. Load now powered by Inverter

Test 4 – Backfeed Safety

Ensure inverter does not send power back to mains.

DPDT relay routing prevents this 100%.

For long-term safe operation:

1. Use heat-resistant plastic enclosure
2. Add ventilation slots
3. Avoid mounting near inductive loads
4. Keep away from moisture
5. Add 5A/10A fuse on Line path

Working with AC (220V) is dangerous.

Follow these:

1. Always disconnect power while wiring
2. Never touch AC tracks on the PCB
3. Install the system inside an insulated enclosure
4. Keep proper grounding
5. Use proper terminal blocks

Congratulations! You have successfully built a professional-grade Automatic AC Changeover System capable of handling 500W to 1000W inverter power.

This design is:

1. Safe
2. Compact
3. Reliable
4. PCB-based
5. Perfect for solar, IPS, UPS, or home inverter systems

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