🛡️ Protection Circuits (Diodes, Fuses, TVS)
Electronics are fragile. A wrong connection, a power surge, or an accidental mistake can destroy a circuit instantly.
Protection circuits are your first line of defense. They stop dangerous conditions before they reach sensitive components.
💡 Spend ₹5 on protection to save a ₹2000 board.
❓ Why Protection Matters
Without protection, circuits are vulnerable to:
- 🔁 Reverse battery connections
- ⚡ Voltage spikes (lightning, inductive switching)
- 🔥 Overcurrent due to shorts or wiring mistakes
- 🧍♂️ Static electricity (ESD)
- 🌩️ Power surges
Protection components are cheap, passive, and proven.
🔌 Diodes – Reverse Voltage Protection
A diode is a one-way valve for current.
Reverse Polarity Protection
Place a diode in series with the power input.
- Correct polarity → diode conducts → circuit works
- Reverse polarity → diode blocks → circuit is safe
Example:
Battery connected backward:
📌 Cost: a few cents
📌 Protection level: saves the entire circuit
Good designers assume someone will connect the battery wrong.
🔥 Fuses – Overcurrent Protection
A fuse is a sacrificial safety device.
- Normal current → fuse stays intact
- Excess current → fuse melts → circuit opens
Instead of your PCB burning, the fuse dies.
How it Works:
- Normal: current flows
- Short circuit:
Fuse Rating Rule
Choose a fuse slightly higher than normal current.
Example:
- Normal current =
- Fuse rating =
❌ Too small → nuisance blowing
❌ Too large → no protection
⚡ TVS Diodes – Voltage Spike Protection
TVS (Transient Voltage Suppressor) diodes clamp sudden voltage spikes.
What They Do:
- Normal voltage → OFF
- Spike exceeds rating → ON (very fast)
- Voltage clamped to safe level
Example:
- Spike from relay coil =
- Circuit max rating =
- TVS clamps spike to ≈
🟢 Much faster than regular diodes
🟢 Designed to absorb high surge energy
🔗 Common Protection Combinations
🔋 Power Input Protection
- Fuse → overcurrent protection
- Reverse diode → polarity protection
- TVS diode → surge protection
🔁 Motors & Relay Coils
- Freewheeling (flyback) diode across coil
- Optional TVS diode for severe spikes
When coil turns OFF:
Diode safely dissipates this energy.
📡 Data & Communication Lines (USB, UART, RS485)
- TVS diodes on signal lines
- Series resistors for current limiting
🧍♂️ ESD (Static Discharge) Protection
- TVS diodes on exposed pins
- Series resistors
- Solid ground plane to shunt energy
🧪 Real-World Example
You control a relay using an Arduino.
- Relay coil is inductive
- Switching generates high-voltage spikes
❌ No diode → Arduino dies
✅ Diode across coil → system runs for years
One diode saves an entire microcontroller.
👶 Why Beginners Must Care
Protection is often skipped — until something fails.
Experienced engineers:
- Always add protection
- Assume misuse
- Design for the real world
A circuit that works only on your desk is not a real product.
❌ Common Mistakes
- Forgetting reverse polarity protection
- No diode across relays or motors
- Using underrated fuses
- Ignoring ESD on connectors
- Assuming “it won’t happen”
It will happen.
✅ Basic Protection Checklist
✔ Power input:
- Reverse diode
- Fuse
- Input capacitor
✔ Inductive loads:
- Flyback diode
✔ External connectors:
- TVS diodes
- Series resistors
✔ High-voltage switching:
- Clamp sensitive nodes
💰 Cost vs Benefit
| Item | Cost |
|---|---|
| Protection parts | ₹5–₹20 |
| Repair & debugging | Hours + frustration |
| Field failure | Reputation damage |
Protection is cheap insurance.
🏁 The Bottom Line
Protection circuits are not optional — they are essential.
- 🔁 Diodes stop reverse voltage
- 🔥 Fuses stop overcurrent
- ⚡ TVS diodes stop voltage spikes
Add protection by default, not as an afterthought.
Your circuits will:
- Last longer
- Survive real-world abuse
- Save time, money, and sanity