Protection Circuits: Diodes, Fuses, and TVS Devices

Protection circuits make a product survive mistakes and real-world electrical stress. Reverse polarity, overcurrent, inductive kick, ESD, and surge events are common enough that they should be designed for at the connector, not discovered after field failures.

Learning Objectives

By the end of this lesson, you should be able to choose basic reverse-polarity protection, size a fuse conceptually, explain TVS clamping, protect inductive loads, and avoid common protection placement mistakes.

Protection Starts at the Boundary

External connectors see cable inductance, user mistakes, electrostatic discharge, and supply faults. Protection should be close to the connector so fault energy is diverted before it enters sensitive circuitry.

flowchart LR CONN[External connector] --> FUSE[Fuse or PTC] FUSE --> REV[Reverse protection] REV --> TVS[TVS clamp] TVS --> LOAD[Protected circuit] TVS --> GND[Low impedance return]

Reverse-Polarity Protection

A series diode is the simplest protection. Correct polarity conducts; reverse polarity blocks.

The cost is voltage drop and heat:

$$
P_D=V_F I_LOAD
$$

For a silicon diode with VF = 0.7 V at 1 A:

$$
P_D=0.7V\times1A=0.7W
$$

A Schottky diode reduces voltage drop, and a MOSFET ideal-diode circuit can reduce loss further in higher-current products.

Overcurrent Protection

A fuse opens when current remains above its rating long enough. A resettable PTC increases resistance when hot and resets after cooling, but it does not open as cleanly as a fuse.

Selection checks include normal operating current, inrush current, maximum fault current, voltage rating, time-current curve, ambient derating, and downstream trace and connector current limits.

TVS Diodes

A transient voltage suppressor is normally off. During a fast surge, it avalanches and clamps voltage to a safer level.

Important datasheet values:

VRWM: reverse working voltage, the maximum normal operating voltage.
VBR: breakdown voltage where conduction begins.
VC: clamping voltage at a specified surge current.
peak pulse power and waveform rating.
capacitance, important on high-speed data lines.

A TVS must have a short, low-inductance path to the return or chassis. Long thin traces make the clamp much less effective.

Inductive Loads and Flyback

Relays, solenoids, and motors store magnetic energy:

$$
E=\frac{1}{2}LI^2
$$

When current is interrupted, the inductor tries to keep current flowing:

$$
V=L\frac{di}{dt}
$$

A flyback diode across a DC coil gives the current a safe path. A TVS or diode-plus-Zener clamp can release the coil faster when turn-off speed matters.

Data-Line Protection

UART, USB, RS-485, CAN, and sensor inputs need protection matched to their speed and voltage. Use low-capacitance TVS arrays for fast buses, series resistors where allowed, common-mode chokes when EMI requires them, and solid return paths.

Worked Example: 24 V Sensor Input

A 24 V industrial input feeds a controller drawing 200 mA.

Choose a fuse or PTC above normal current but below connector and trace limits.
Use reverse-polarity protection rated for the supply and current.
Choose a TVS with VRWM above the maximum normal supply and ensure VC stays below downstream absolute maximum ratings after any series impedance.
Place protection at the connector with a short return to chassis or power ground.

Practical Checks

Confirm absolute maximum voltage of the protected IC pins.
Check power dissipation in series diodes and MOSFETs.
Check fuse clearing against the available source current.
Put the TVS before narrow traces that need protection.
Verify creepage, clearance, and safety rules for high-energy circuits.

Common Mistakes

Selecting a TVS by nominal voltage only and ignoring clamping voltage.
Placing the TVS far from the connector.
Using a fuse rating lower than inrush current, causing nuisance trips.
Forgetting flyback protection on coils.
Assuming ESD protection is unnecessary on rarely used connectors.

Summary

Protection circuits are deliberate failure-control paths. Diodes block reverse polarity, fuses and PTCs limit overcurrent damage, TVS devices clamp transients, and flyback clamps absorb inductive energy. Placement and ratings matter as much as the component names.

Mind Map

mindmap root((Protection Circuits)) Core concept Divert fault energy Limit current Clamp voltage Applications Power input Relay coils Data ports Sensor connectors Formulas Diode loss equals Vf times I Inductor energy equals one half L I squared Inductive voltage equals L di dt Fuse I squared t Design rules Protect at connector Short TVS return Rate fuse for source Check clamp voltage Practical checks Inrush current Trace current TVS heating Coil turn off Common mistakes TVS too far away Fuse too large No flyback path Ignored capacitance