The 24 V rail in an industrial cabinet and the 24 V rail in a truck look alike on a label and behave very differently under transients. The industrial rail carries switching spikes from contactors and motor drives; the truck rail adds load dump, cranking dips and the harsh pulses of commercial vehicles defined by ISO 7637-2 or ISO 16750-2. A TVS diode selected for one environment can fail in the other, because the voltage window, pulse energy and repetition rate are different. This guide walks the 24 V TVS selection from steady-state tolerance to the test standard, with the numbers that decide the part.
Which TVS voltage should a 24 V rail use?
A 24 V rail normally needs a TVS whose stand-off clears the highest continuous voltage the rail reaches, whose breakdown band sits above that level with margin, and whose clamping voltage stays below what the downstream circuit survives — on many nominal 24 V systems with 28 V-class charging that window lands near a 26-28 V stand-off, but the rail’s own specification sets the number.
The stand-off voltage must sit above the highest normal rail voltage — including tolerance, ripple and charging excursions — or the TVS conducts during normal operation and fails early. For a 24 V industrial rail with ±10 percent tolerance, that puts the stand-off near 27 V; automotive 24 V systems may need more headroom for jump-start and charging conditions, so the exact value follows the system’s own voltage specification. The breakdown band must then clear the normal window, and the clamping voltage at the peak pulse current must stay below the DC-DC converter or load’s absolute maximum. All three points — stand-off, breakdown and clamping — come from the same datasheet line, and all three must fit the system’s window.
Industrial and automotive 24 V transients differ in waveform and repetition
Industrial 24 V transients are dominated by switching surges and IEC-defined events, while automotive 24 V transients add load dump — a high-energy alternator disconnect pulse with defined voltage, source impedance and repetition — and cranking dips.
An industrial cabinet sees inductive-load switching, motor-drive disturbances and coupled surges; the relevant tests are typically IEC surge and EFT standards, and the TVS must survive the defined waveform a specified number of times. An automotive 24 V module faces the ISO 7637-2 or ISO 16750-2 pulse set, whose load-dump pulse can be large and must be survived several times with defined intervals. The difference matters because the TVS power rating is quoted against a specific waveform: comparing an industrial 10/1000 µs rating with an automotive load-dump rating as if they were the same test is how parts get undersized. TI’s 24 V automotive transient note documents the severity of that environment from the component-selection side.
Unidirectional TVS fits DC rails; bidirectional fits legitimate negative swings
Use a unidirectional TVS for a normal DC rail because it clamps positive overvoltage at a lower voltage for the same stand-off; switch to bidirectional only where the node must survive legitimate negative swings.
A DC rail is unidirectional in normal operation, so a unidirectional TVS gives the tightest positive clamp. A bidirectional part is needed when the protected node can swing negative — a signal line, an AC-coupled bus, or an input that must survive reversed polarity without the TVS clamping the negative excursion and conducting heavily. Automotive 24 V systems also include negative pulses in the standard test set, and the protection scheme must give those pulses a defined path: sometimes a bidirectional TVS, sometimes a series diode with a unidirectional TVS, sometimes a separate negative clamp. The direction decision is a circuit-level analysis of every polarity the node can see. Murata’s TVS direction FAQ explains the construction difference behind the choice.
How do you size the TVS for the surge waveform?
Size the TVS from the surge waveform’s peak current and energy, derated for pulse duration, repetition and ambient temperature — never from the steady-state rail current alone.
The TVS power rating is a pulse statement: 600 W or 1500 W means watts at a defined waveform such as 10/1000 µs, and the same part absorbs less energy in a longer or hotter event. The selection method is to convert the surge standard’s open-circuit voltage and source impedance into a peak current through the TVS, compare that current with the part’s rating at the actual pulse duration, and apply the derating curve for repetition and temperature. For automotive load dump, run the calculation for the full test sequence — several pulses with defined intervals — because a part that survives one pulse can fail the sequence. Vishay’s TVS application note and Microchip’s MicroNote 134 cover the waveform math from the component side.
Package thermal capacity decides survival under repeated 24 V surges
The package matters because repeated surges heat the die, and the package’s thermal capacity and board path decide whether the TVS survives the repetition rate or drifts into failure.
Axial packages such as DO-201 and surface-mount packages such as SMC (DO-214AB) have different thermal mass and different paths to the board. A single large surge may be fine in either; a machine that generates a surge every few seconds, or an automotive module that sees repeated load-dump events, accumulates heat that the package must move. The derating curve shows how the peak pulse power falls as pulse width or repetition rises, and the ambient temperature shifts the whole curve. If the application’s duty is repetitive, check the derated rating at the expected rate and temperature before choosing the package class.
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An undersized TVS fails shorted and turns the fuse into the protection
An undersized TVS fails shorted during the surge it was meant to clamp, and the short then draws fault current until the fuse, the input protection or the board itself interrupts it.
The failure sequence is the reason the TVS and the fuse are designed together. If the TVS cannot absorb the pulse energy, it degrades first — leakage rises or the breakdown shifts — and then shorts on a later event. The shorted TVS protects the load by clamping, but it creates a sustained fault that the fuse must clear before the board traces or the input stage overheat. Field failures that look like “TVS burned” are often coordination failures: the TVS was undersized, the fuse was too slow, or the layout added inductance that pushed the clamp voltage above the load’s limit. Altium’s TVS layout guide covers the placement side of that coordination.
How do you verify the selection on the bench?
Verify with the actual test standard: capture the clamped voltage at the load during the surge, check the TVS temperature after the full pulse sequence, and inspect the part for drift afterward.
Run the product’s surge or load-dump test at the specified voltage, source impedance and repetition, with the load connected and the TVS in its final layout. Measure the voltage the load actually sees — if it exceeds the load’s maximum, the clamp is too loose or the layout inductance is too high. Record the TVS case temperature after the sequence and compare it with the derating curve, then re-measure the TVS’s breakdown voltage to catch drift. A part that survives the single pulse and fails the sequence, or survives electrically and drifts thermally, was sized on the wrong waveform or the wrong repetition rate.
| Check | Industrial 24 V | Automotive 24 V |
|---|---|---|
| Steady-state tolerance | ±10% typical | Wider, includes charging excursions |
| Dominant transient | IEC surge / EFT | ISO 7637-2 / 16750-2 incl. load dump |
| Pulse energy | Defined by standard waveform | Defined by alternator/load-dump parameters |
| Repetition | Usually limited | Multiple pulses in the test sequence |
Frequently asked questions
Can I use a 30 V stand-off TVS on a 24 V rail?
Only if the normal rail never reaches 30 V. The stand-off must sit above the worst normal operating voltage, including tolerance, ripple and charging excursions, or the TVS conducts continuously and fails. A 30 V stand-off is a common choice for a 24 V nominal rail with modest tolerance; verify the actual system window before selecting the class.
Why did my 24 V TVS fail during a jump-start test?
Because a jump-start can push the rail well above 24 V, and if the TVS’s stand-off sits inside that excursion, the TVS conducts during the jump-start event and absorbs energy it was not sized for. Check the system’s maximum auxiliary voltage and select the stand-off above it, then size the TVS for the transient that follows.
Is a higher power TVS always safer?
Not always. A higher power class usually has a different clamp window, capacitance or package, and the larger part may clamp at a higher voltage or fit worse in the layout. Choose the class that fits the waveform, then verify the clamp level and the thermal path; “bigger” only helps if the other parameters still fit the circuit.
Do I need a TVS on both the input and the DC-DC output?
Possibly. The input TVS protects against line transients; the output may need its own protection if the load can back-drive the rail or if the converter’s output must survive load faults. Protect each rail by its own transient sources — do not assume one TVS covers the whole board.
What the 24 V TVS selection comes down to
The selection comes down to four verified points: stand-off above the normal rail, breakdown clear of the tolerance band, clamping below the load’s limit, and pulse energy plus repetition inside the package’s derating — checked against the standard the product must pass.
Identify the environment first (industrial or automotive), pull the surge standard’s waveform and repetition, then run the window and energy checks on the datasheet. The TVS that survives the full test sequence at temperature is the one that protects the design; the one chosen from a stand-off number alone is the one that fails in the field.
The TVS category groups the axial and surface-mount options with their datasheets. Send the rail tolerance, the surge standard and the load’s absolute maximum to the engineering team, and the recommended stand-off and power class follow from the window calculation this article describes.