The transient voltage suppression diode symbol on a schematic tells you two critical things: whether the component clamps voltage in one direction (unidirectional) or both (bidirectional), and which drafting standard the engineer used. Unlike resistors, TVS diodes do not use color bands; instead, they rely on alphanumeric package markings to specify their breakdown voltage and power rating. Below is the complete reference to identify schematic symbols, decode physical silkscreen markings, and select a direct replacement part.
TVS Diode Schematic Symbols: IEC vs. IEEE/ANSI Standards
Electronic component symbols are governed by IEC 60617 (International/Europe) and IEEE/ANSI 315 (North America). Note that the NEC (National Electrical Code) applies to mains wiring and conduit, not semiconductor schematic symbols. Read the table below to match the symbol on your blueprint to the physical component type.
| Component Type | IEEE/ANSI 315 Symbol (US) | IEC 60617 Symbol (Intl) | Physical Equivalent | Use Case |
|---|---|---|---|---|
| Unidirectional TVS | Diode symbol with a 'Z' bend on the cathode bar and a transient 'U' arrow | Standard Zener diode symbol (diode with bent cathode bar) | SMAJ5.0A, 1.5KE15A | DC power lines, logic level GPIO, automotive 12V |
| Bidirectional TVS | Two opposing Zener symbols in series, or a diode with a 'Z' bend on both ends | Two opposing Zener symbols sharing a cathode/cathode junction | SMAJ5.0CA, 1.5KE15CA | AC lines, RS-485 data buses, audio signals, relay coils |
| Standard Zener (For Contrast) | Diode with a bent cathode bar (no transient arrow) | Diode with a bent cathode bar | 1N4733A (5.1V Zener) | Voltage regulation, NOT for high-energy surge suppression |
| TVS Array (Multi-line) | Multiple TVS symbols grouped in a single dashed box | Multiple TVS symbols grouped in a single dashed box | PRTR5V0U2X, TPD4E05U06 | USB, HDMI, Ethernet ESD protection |
Decoding TVS Package Markings (The 'Color Code' Equivalent)
Because TVS diodes handle massive surge currents (up to 5000W in some packages), they are too small to print full part numbers on their bodies. Instead, manufacturers like Littelfuse, Vishay, and Bourns use abbreviated alphanumeric codes. Here is how to decode the silkscreen on common SMA, SMB, and SMC packages.
| Marking Code | Full Part Number | Working Voltage (Vrwm) | Direction & Tolerance | Power Rating |
|---|---|---|---|---|
| 5.0A | SMAJ5.0A | 5.0V DC | Unidirectional, 5% | 400W (SMA) |
| 15CA | SMBJ15CA | 15.0V DC/AC | Bidirectional, 5% | 600W (SMB) |
| 33C | SMCJ33C | 33.0V DC/AC | Bidirectional, 10% | 1500W (SMC) |
| 1.5KE18A | 1.5KE18A | 15.3V DC | Unidirectional, 5% | 1500W (Axial) |
| 5KP24CA | 5KP24CA | 24.0V DC/AC | Bidirectional, 5% | 5000W (Axial) |
Key Decoding Rules:
- The Number: Indicates the nominal breakdown or working voltage. For the 'SMAJ/SMBJ/SMCJ' series, the number is the Reverse Working Voltage ($V_{RWM}$). For the '1.5KE' series, the number is the Breakdown Voltage ($V_{BR}$).
- The 'C': Stands for Bidirectional. If there is no 'C', it is Unidirectional.
- The 'A': Stands for a tight 5% voltage tolerance. If absent, the tolerance is typically 10%.
Rows and Markings People Get Wrong
Misidentifying a TVS diode on the bench leads to immediate component failure when the circuit is energized. Watch out for these specific pitfalls:
2. The '1.5KE' vs 'SMAJ' Voltage Trap: A marking of '15' on an SMAJ part means 15V working voltage ($V_{RWM}$), and it won't clamp until ~16.7V. A marking of '15' on a 1.5KE part means 15V breakdown voltage ($V_{BR}$), meaning its working voltage is only 12.8V. If you swap them, the 1.5KE15 will clamp your 13V DC rail continuously and overheat.
3. Zener vs. TVS Substitution: A 1N4740A (10V Zener) and a SMAJ10A look similar and have similar DC breakdown voltages. However, the Zener is rated for 1 Watt of continuous dissipation and will vaporize under a 1000W surge. Never substitute a standard Zener for a TVS in a transient protection role.
Safe Interpretation: Testing Faded or Unmarked TVS Diodes
Heat and flux residue often obliterate TVS silkscreen markings. Because you cannot rely on visual inspection, you must electrically characterize the component to find its $V_{BR}$ (Breakdown Voltage) before reusing it.
Bench Test Procedure for Unmarked TVS Diodes:
- Isolate: Desolder at least one leg of the TVS diode from the PCB. In-circuit testing will give false readings due to parallel IC impedance.
- Check for Shorts: Use a multimeter in diode mode. A unidirectional TVS should read ~0.4V to 0.6V in one direction and 'OL' in the other. A bidirectional TVS should read 'OL' in both directions. If it reads 0.00V (short) in either direction, the TVS has failed and absorbed a fatal surge.
- Find $V_{BR}$: Build a test circuit: A variable DC power supply in series with a 1kΩ current-limiting resistor and the TVS diode. Connect a multimeter across the TVS.
- Ramp Voltage: Slowly increase the power supply voltage. When the multimeter reads exactly 1mA of current flowing through the TVS, note the voltage across the diode. This is the official $V_{BR}$ at $I_{BR} = 1mA$.
- Determine Type: Reverse the diode and repeat. If it clamps at the same voltage, it is bidirectional. If it conducts at ~0.6V, it is unidirectional.
TVS Selection Decision Tree: Pick the Exact Part Number
Stop guessing. Use this decision matrix to lock in the exact TVS diode part number for your application. Follow the rows sequentially.
| Decision Step | Circuit Condition | Action / Parameter to Select |
|---|---|---|
| 1. Line Type | DC Power, Logic, Automotive | Select Unidirectional (No 'C' in part number) |
| 1. Line Type | AC Power, RS-485, Audio, Relay Coils | Select Bidirectional (Add 'C' to part number) |
| 2. Normal Voltage | Max continuous DC/AC RMS voltage | Select $V_{RWM}$ equal to or 10% higher than max line voltage |
| 3. Surge Energy | ESD / Logic level signals | Select 400W (SMA) or TVS Array |
| 3. Surge Energy | Inductive kickback, relays, motors | Select 600W (SMB) or 1500W (SMC / 1.5KE) |
| 3. Surge Energy | Lightning surges, main AC entry | Select 3000W (3KP) or 5000W (5KP) |
| 4. Clamping Limit | Max voltage the protected IC can survive | Verify $V_C$ (Clamping Voltage) in datasheet is below IC max rating |
Concrete Part Picks for Common Scenarios
Scenario A: Protecting a 12V DC Automotive ECU Input.
The line sees 12V nominal, up to 14.4V during alternator charging, and load-dump spikes. You need Unidirectional, $V_{RWM}$ > 14.4V, high surge rating.
Concrete Pick: Buy the SMAJ15A (15V working, 16.7V breakdown, 400W) or SMCJ15A if load-dump energy exceeds 400W.
Scenario B: Snubbing a 24V AC Industrial HVAC Contactor Coil.
The line is 24V AC (34V peak). Inductive kickback will generate massive reverse voltage. You need Bidirectional, $V_{RWM}$ > 34V peak.
Concrete Pick: Buy the 1.5KE36CA (30.8V working, 36V breakdown, 1500W bidirectional axial). Do not use a standard 1N4007 diode here; the AC cycle will destroy it.
For comprehensive datasheet parameters and cross-referencing, always verify your final selection against the Littelfuse TVS Diode Catalog or the IEC 60617 Symbol Database to ensure your schematic symbols match your physical BOM.
