The Direct Answer: Safe Default TVS Part Numbers

If you are protecting a microcontroller GPIO, a communication bus, or a power rail and need a reliable transient voltage suppression diode without spending hours parsing datasheets, use the industry-standard Littelfuse or Vishay SMAJ (400W), SMBJ (600W), or 1.5KE (1500W) series. These are the workhorses of board-level surge protection.

Safe Defaults by Voltage Rail:
  • 3.3V Logic (ESP32/STM32): SMAJ3.3A (Unidirectional) or SMAJ3.3CA (Bidirectional)
  • 5V Logic / USB VBUS: SMAJ5.0A or SMAJ5.0CA
  • 12V Automotive / Industrial: SMAJ15A (Never use a 12V TVS on a 12V rail; load-dump spikes require the 15V standoff rating)
  • 24V Industrial PLC I/O: SMBJ24A or SMBJ24CA
  • Mains / High-Energy Snubber: 1.5KE200A (Through-hole, handles massive joule ratings)

TVS Diode Operation Regions and Datasheet Ratings

A transient voltage suppression diode operates as an open circuit during normal conditions and transitions into a low-impedance short when a voltage spike exceeds its breakdown threshold. Understanding the four distinct operating regions is critical to preventing nuisance tripping or catastrophic failure.

Operation Regions Table

Operating Region Voltage State Current Flow Device Behavior
Reverse Standoff V < V_RWM < 1 µA (Leakage) Invisible to the circuit; acts as an open circuit with minimal parasitic capacitance.
Avalanche Breakdown V_BR(min) to V_BR(max) 1 mA to 1 A The PN junction avalanches. The diode begins to conduct heavily and clamp the voltage.
Clamping (Saturation) V_C(max) I_PP (Peak Pulse) Maximum designed surge current flows. Voltage across the diode rises slightly due to bulk resistance.
Thermal Runaway Exceeds V_C limits > I_PP rating Silicon melts. The device fails, usually shorting permanently to protect downstream components.

The most misunderstood parameter is the 10/1000 µs pulse rating. Manufacturers rate I_PP (Peak Pulse Current) using a 10 µs rise time and 1000 µs decay time. If your application faces an 8/20 µs lightning-induced surge (common in outdoor telecom), the diode can actually handle roughly double the rated I_PP. Conversely, a long-duration square wave from a shorted power supply will destroy the TVS at a fraction of its I_PP rating due to thermal saturation.

Data-Dense Spec Sheet Comparison (SMAJ Series)

Part Number V_RWM (Standoff) V_BR (Breakdown @ 1mA) V_C (Max Clamping) I_PP (Peak Pulse) Typical Capacitance
SMAJ3.3A 3.3V 4.1V - 4.5V 7.1V @ 57.0A 57.0A ~3500 pF
SMAJ5.0A 5.0V 6.4V - 7.02V 9.2V @ 43.5A 43.5A ~2000 pF
SMAJ12A 12.0V 13.3V - 14.7V 19.9V @ 20.1A 20.1A ~800 pF
SMAJ24A 24.0V 26.7V - 29.5V 38.9V @ 10.3A 10.3A ~450 pF

Source data derived from standard Littelfuse SMAJ Series Datasheets.

Symbol, Pinout, and Biasing Rules

On a schematic, a unidirectional TVS diode looks like a standard rectifier diode but with a distinct kink or zig-zag in the cathode bar, indicating its avalanche capability. A bidirectional TVS diode is drawn as two opposing Zener diodes in series, or two standard diodes facing opposite directions.

Physical Pinout: In standard SMA, SMB, and SMC surface-mount packages, the device has two terminals. The Cathode is marked with a prominent band (often white, black, or silver) printed on the plastic body. The opposite end is the Anode.

Biasing Rule: A unidirectional TVS diode must be reverse-biased during normal operation. This means the Cathode (band) connects to the positive voltage rail, and the Anode connects to ground. If you forward-bias it like a standard rectifier, it will clamp at 0.7V, instantly shorting your power supply and destroying the diode.

Application Circuit: Protecting an ESP32 RS-485 Interface

RS-485 buses are notorious for picking up common-mode surges and ground-loop spikes in industrial environments. While the MAX485 transceiver has internal ESD protection, it cannot survive a 500V inductive kick from a nearby motor contactor. Here is a robust, tested protection circuit.

Component List and Values

  • U1: MAX485ESA RS-485 Transceiver
  • D1 (TVS): SMBJ6.5CA (Bidirectional, 600W, 6.5V Standoff)
  • R1, R2: 10 Ω, 1/4W series termination resistors
  • R3: 120 Ω, 1/2W bus termination resistor
  • U2: ESP32 DevKit V1 (Microcontroller)

Wiring and Pin Mapping

  1. Bus Connection: Connect the RS-485 A and B incoming bus lines to the circuit.
  2. TVS Placement: Solder the SMBJ6.5CA directly across the A and B lines, before the series resistors. Because RS-485 is differential and swings above and below ground, a bidirectional TVS is mandatory.
  3. Series Resistors: Route the A line through R1 (10 Ω) to MAX485 Pin 6 (Driver Output A). Route the B line through R2 (10 Ω) to MAX485 Pin 7 (Driver Output B). These resistors limit the surge current into the transceiver's internal clamp diodes during a massive spike.
  4. Termination: Place R3 (120 Ω) across MAX485 Pins 6 and 7 to match the characteristic impedance of the twisted-pair cable, preventing signal reflections.
  5. Logic Side: Connect MAX485 Pin 4 (DI) to ESP32 GPIO 17 (TX), and Pin 1 (RO) to ESP32 GPIO 16 (RX).

Why SMBJ6.5CA and not 5.0V? RS-485 drivers output differential voltages up to 5V, but high-speed edges can ring and overshoot. A 5.0V TVS will clip these valid signals, corrupting your data packets. The 6.5V standoff ignores normal ringing while still clamping dangerous surges well below the MAX485's absolute maximum rating. For deeper design rules, refer to Vishay's TVS application guidelines.

Failure Modes and Multimeter Testing Procedures

Unlike standard signal diodes that fail open when overloaded, a TVS diode is designed to fail short. When subjected to a surge exceeding its I_PP rating, the silicon junction melts and fuses into a solid short circuit. This is a deliberate sacrificial mechanism: the shorted TVS pulls the rail to ground, which blows the upstream fuse or trips the power supply's overcurrent protection, saving the expensive microcontroller downstream.

Testing with a Digital Multimeter (DMM)

Testing a TVS diode requires understanding your multimeter's limitations. A standard Fluke or Brymen DMM in "Diode Test" mode outputs roughly 2.5V to 3.0V open-circuit.

  1. Testing Unidirectional (e.g., SMAJ5.0A): Place the red probe on the Anode and black on the Cathode. The meter should read a standard forward voltage drop (typically 0.4V to 0.7V). Reverse the probes (red on Cathode). The meter should read "OL" (Over Limit) because the 5.0V breakdown voltage is higher than the meter's 3V test voltage.
  2. Testing Bidirectional (e.g., SMAJ5.0CA): Place probes in either direction. The meter will read "OL" both ways, because both internal junctions are reverse-biased relative to the meter's low test voltage.
  3. The "False Dead" Trap: If a bidirectional TVS reads "OL" both ways, it might be good, or it might be blown open (a rare failure caused by extreme, instantaneous vaporization of the die).
Bench Verification for V_BR: To definitively prove a TVS diode is functional and measure its exact breakdown voltage, use a bench power supply. Set the supply's current limit to exactly 1.0 mA. Slowly ramp up the voltage from 0V while monitoring the voltage across the diode with your DMM. The voltage will climb, then suddenly stop rising and hold steady. That steady voltage is your V_BR. If it matches the datasheet table (e.g., 6.4V to 7.02V for an SMAJ5.0A), the part is healthy.

Unidirectional vs. Bidirectional Selection Matrix

Choosing between unidirectional (suffix 'A') and bidirectional (suffix 'CA') comes down to the signal topology and the consequences of a reversed power connection.

Criteria Unidirectional (A) Bidirectional (CA)
Ideal Application DC power rails, single-ended GPIO, I2C/SPI lines referenced to ground. AC lines, RS-485, CAN bus, audio signals, hot-swap connectors.
Response Time Slightly faster (< 1.0 ps theoretical junction response). Marginally slower due to dual-junction capacitance, but identical in real-world PCB parasitics.
Reverse Polarity Protection Provides reverse polarity protection (conducts at 0.7V if power is flipped). No reverse polarity protection; will allow negative voltage to pass if used on a DC rail.
Parasitic Capacitance Lower capacitance (better for high-speed single-ended data). Higher capacitance (can degrade edges on >10 MHz single-ended buses).
Cost & Availability Cheaper, higher volume, wider selection of low-voltage standoff ratings. Slightly more expensive, essential for differential pairs.

The Golden Rule: If the signal or power rail never swings below 0V (ground), use a unidirectional TVS. It offers lower capacitance for high-speed data and acts as a bonus reverse-polarity crowbar. If the signal swings positive and negative relative to ground, or if the connector can be plugged in upside down (like a USB-C or bare terminal block), use a bidirectional TVS to prevent the protection diode from shorting out valid negative signal swings.