How to Select and Source TVS Diodes for 24V Industrial Control Systems

Industrial 24V DC systems are rugged, but the surge environment they sit in is punishing. Inductive kickback from solenoids and contactors, coupled transients from adjacent motor drives, and direct lightning-induced surges on long sensor cables all land on the same rail. IEC 61000-4-5 defines the test pulse: a 1.2/50 µs open-circuit voltage and 8/20 µs short-circuit current combination wave. Selecting the right TVS diode comes down to matching three parameters — standoff voltage, clamping voltage, and peak pulse power — against your installation class.

Step 1: Determine the Standoff Voltage (V_RWM)

For a nominal 24V DC rail, the TVS standoff voltage (reverse working voltage) must sit above the maximum DC operating voltage, tolerance included. A 24V industrial supply typically runs at 24V ±20%, which gives a maximum of 28.8V. Add margin for ripple (typically 1Vp-p on switch-mode supplies) and you land on a minimum V_RWM of 30–33V. We've seen 28V parts end up on 24V rails because they were the cheap option in stock — they don't survive long once the contactors start switching.

Common choices:

Vishay SMAJ30A — V_RWM 30V, unidirectional, 400W peak pulse power (10/1000 µs)
Littelfuse 1.5SMC33A — V_RWM 33V, unidirectional, 1500W peak pulse power
ST Microelectronics SM6T33A — V_RWM 33V, 600W peak pulse power

For 24V PLC I/O modules, the SM6T33A is a common fit — adequate surge margin for internal rail protection without committing to a larger DO-214AB footprint.

Step 2: Verify the Clamping Voltage (V_CL)

The clamping voltage is the maximum voltage the protected circuit will see during a surge event. For a 24V system with a 33V V_RWM device, the clamping voltage at rated peak pulse current typically sits between 53V and 70V.

Why this matters: downstream components — DC/DC converters (e.g., Traco Power TSR 1-2450), relays, and sensor power supplies — have absolute maximum input ratings. If your TVS clamps at 70V and your downstream regulator's abs max is 60V, the regulator fails first. The fix is either a lower-clamping TVS (which means higher capacitance — problematic on signal lines) or a higher-voltage-rated converter. We see this mismatch on returned boards often enough that it's the first thing we check.

Rule of thumb: Choose a TVS whose V_CL(max) is at least 10% below the lowest abs max rating of any powered IC on that rail.

Step 3: Calculate Required Peak Pulse Power (P_PPM)

IEC 61000-4-5 installation classes define the surge energy:

Class	Test Level (open circuit)	Generator Impedance	Typical P_PPM Need
1	0.5 kV	2 Ω	400W
2	1.0 kV	2 Ω	600W
3	2.0 kV	2 Ω	1500W
4	4.0 kV	2 Ω	3000W+

Using class 2 as an example: a 1 kV surge through 2 Ω source impedance delivers 500A peak into a short circuit. A TVS with 33V V_RWM clamping at 53V at the rated current must handle 53V × (1000V − 53V) / 2Ω ≈ 25 kW for a moment. The peak pulse power rating is specified for a 10/1000 µs waveform, not the 8/20 µs of the combination wave, so you need to derate.

Derating factor: For an 8/20 µs waveform, a TVS rated at 10/1000 µs can handle roughly 3–5× its rated P_PPM. An SM6T33A rated at 600W P_PPM (10/1000 µs) handles approximately 1800–3000W for an 8/20 µs pulse — adequate for Class 2 indoor installations with moderate wiring length.

For outdoor sensor lines (Class 3+), step up to a 1500W device like the Littelfuse 1.5SMC33A. The 7–8 mm creepage distance on a DO-214AB package also gives better clearance for high-altitude or humid environments.

The Bidirectional Question

For 24V AC-powered industrial equipment (which sees both positive and negative half-cycles), use bidirectional TVS diodes. For 24V DC rails, a single unidirectional device is sufficient and provides about 0.3V lower clamping voltage than a bidirectional pair in series.

If you're sourcing TVS diodes for an upcoming control panel design, check current TVS diode inventory and alternatives or upload your component list for cross-reference against available stock.

One more practical note: always check the IV curve at the actual peak pulse current your installation will see — datasheet clamping voltages are often specified at a single test current, but the clamping voltage increases linearly with surge current above that point. If you're borderline, oversize the package by one step (e.g., from SMAJ to SMBJ) rather than changing the V_RWM value.

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