Every automotive engineer has heard of load dump, and most have heard only one number: 87 V. Fewer can explain why the rail spikes that high, why the spike has an energy and not just a voltage, and what actually survives it. Load dump is not a single clean pulse; it is the consequence of the alternator being an inductive source that loses its load, and understanding that mechanism is what turns the 87 V figure from a memorized number into a design that holds. This article tells the alternator’s story, walks the test pulses that define the event, compares the suppression strategies, and closes with the diode and rectifier margin that decides whether the 12 V input stage survives.
The Alternator’s Revenge: Why Disconnecting a Battery Spikes to 87 V
The name “load dump” describes what happens when the battery — the alternator’s biggest load — is suddenly removed from the circuit while the engine is running. The classic trigger is a battery-terminal disconnect at speed, but the same event follows when a heavy load is switched off or a connector is dislodged. The mechanism is the key.
An alternator is fundamentally an inductive machine. It drives current through its field and armature windings, and the battery normally absorbs the alternator’s output. When the battery terminal is disconnected, that load vanishes abruptly, and the energy stored in the alternator’s inductance has nowhere to go but to push the rail voltage up. The inductive energy releases as a voltage spike whose peak is set by the alternator’s internal impedance, its speed, and the field current at the moment of disconnect — which is why the result is not a modest overshoot but a punch into the tens of volts.
The number that survives in memory is the 87 V peak of the ISO 16750 load-dump test. It is a useful anchor, but it is only one point on a waveform: the spike rises fast, holds for a duration set by the alternator’s time constant, and carries an energy that can persist for tens to hundreds of milliseconds. That duration is what makes load dump an energy event, not a fast electrostatic pulse — a distinction that decides which protection component can absorb it. The load-dump TVS article and the automotive MOSFET design-in guide develop the waveform and the survival requirement in detail.
Test Pulses: ISO 16750 Load-Dump Shape and Energy
Because load dump is a real, measurable event, the industry has standardized it, and the standard’s waveforms tell the designer what to test against. The two relevant families are ISO 16750 and ISO 7637, and the load-dump pulse has a defined shape and energy.
The ISO 16750 load-dump test models the worst-case alternator event. It specifies a pulse that rises to the peak voltage, holds for a duration tied to the alternator’s time constant, and then decays — and it is characterized not only by the peak voltage but by the energy the pulse carries. The ISO 7637 family adds the broader set of automotive transients, with different pulse classes for inductive spikes, coupling, and other events, and the load dump sits among them. Reading the standard correctly means checking the peak, the duration, and the energy, because a component that survives the peak but not the energy is still a failure.
The energy is the part that separates a TVS from a fast clamp. A TVS is rated not just by its clamping voltage but by its pulse-power or pulse-energy capability — how much energy it can absorb in a single event — and the load-dump pulse must stay inside that envelope. The surge and ESD testing guide walks the ISO 7637 waveforms, and the TVS selection guide shows how to read the energy rating against the pulse shape. The standard is the contract: it defines the event, and the component’s ratings are the promise to survive it.
Suppression Strategies: TVS vs Clamp vs System Response
With the event defined, the design question is how to suppress it, and there are three strategies with different trade-offs.
The first is a TVS across the rail. The TVS is the fastest and most common answer: it conducts when the voltage exceeds its clamp level, absorbing the pulse energy and holding the rail to a survivable voltage. Its strength is speed and energy handling; its limitation is that it must be sized for the specific pulse energy and that it may need to be paired with other elements. The second is a linear or active clamp, which regulates the rail rather than simply absorbing the spike — used where the downstream needs a tight, controlled voltage rather than just a clamped one. The third is the system response: the alternator’s own regulation, a deliberate clamp inside the alternator, or a load that can absorb the energy, which reduces the spike at the source.
The trade is between cost, complexity, and how much of the energy the downstream must tolerate. A TVS absorbs the energy locally and is cheap and simple; an active clamp gives tighter control but adds circuitry; a system-level answer reduces the event but depends on the whole vehicle. Most 12 V modules use a TVS as the practical default, sized against the ISO 16750 pulse, and the choice of which strategy depends on the sensitivity of the load. The TVS protection article and the co-design of rectifier and TVS compare the approaches on a real 12 V input.
The trade between the three strategies is worth making explicit, because each answer has a different cost and a different failure mode. A TVS is the cheapest and most direct: it conducts at its clamp voltage, absorbs the pulse energy, and holds the rail to a level the downstream can tolerate. Its failure mode is energy-limited — a TVS sized too small for the pulse absorbs what it cannot hold and fails short or open, which is why the energy check is not optional. An active or linear clamp holds a tighter voltage, which protects sensitive electronics, but it adds a controller, a pass element, and their own heat and cost, and it can be slower to engage than a passive TVS. The system-level answer — reducing the spike at the alternator with internal regulation or a dedicated clamp — removes the energy before it reaches the module, but it is a vehicle-level decision that a module designer cannot make alone.
The practical rule is to match the strategy to the load’s sensitivity and the event’s energy. A rugged load like a relay or a motor drive can tolerate a higher clamped level, so a modest TVS may be enough; a sensitive load like a microcontroller rail needs either a TVS clamped low enough or an active regulator behind it. The module-level default is a TVS sized against the ISO 16750 energy, and the choice of whether to add active clamping follows from how tight the downstream rail must be. The comparison is the subject of the TVS protection guide and the 12 V co-design article, and the decision table below summarizes it.
The Rectifier and Diode Margin in the 12 V Input Stage
The TVS takes the energy, but it does not do so alone. Behind it sits the diode and rectifier margin that the whole 12 V input stage depends on, and that margin is a separate decision from the TVS sizing.
The rectifier’s job in the load-dump context is to hold reverse and forward voltage with margin while the TVS clamps. The reverse-voltage rating must clear the clamped level with the standard derating margin — the rule that turns a working voltage into a VRRM choice — and the surge rating must cover the current that flows during the event. The forward drop and the heat it produces matter because the rail carries real current, and a diode that is electrically right but thermally marginal fails in the hot engine bay. The rectifier voltage ratings method gives the margin rule, and the automotive rectifier mission-profile article connects the rating to the thermal duty.
The margin is what separates a part that survives one test from one that survives a decade. A rectifier sized exactly to the test peak has no room for lot variation, temperature, or repeated events; a part with the standard derating margin has the budget that the real vehicle consumes. The 12 V input stage is a chain — TVS absorbs, diode blocks and rectifies with margin, filter smooths — and the load dump tests the whole chain, not just the TVS. The automotive ECU power design article walks this chain as a complete input stage.
Verifying Survival: What Your Design Must Show
The last step is proving the design survived, and verification is where the engineering evidence is recorded. A load-dump design is not verified by a datasheet read; it is verified by showing the waveform and the component margins that survived it.
The verification checklist has four parts. First, confirm the TVS was sized against the actual pulse energy, not just the peak voltage — read the ISO 16750 energy against the TVS’s pulse capability. Second, confirm the rectifier and downstream parts hold their reverse-voltage and surge margins with the derating applied. Third, run the test at the worst-case temperature, because the engine-bay heat changes both the clamping and the derating. Fourth, record the before-and-after waveform and the junction or case temperature, so the survival claim is evidence rather than an assumption. The surge and ESD testing guide covers the test setup and waveform, and the field reliability checklist ties the test evidence to the long-term reliability case.
The story of load dump is the story of the alternator’s stored energy and the chain that absorbs it. The 87 V number is the headline, but the design is won in the energy, the suppression strategy, the diode margin, and the verification that proves the chain held. A designer who understands why the alternator punches 87 V — and who sizes the TVS, the rectifier margin, and the test against the energy, not just the peak — builds a 12 V input that survives the event it was named for. The automotive rectifier category supplies the parts sized for the duty, and the co-design article closes the chain.