What is load dump and how does it damage vehicle electronics?
Load dump is the battery suddenly dropping out of the circuit while the alternator is running under load; the magnetic energy of the alternator turns into a high voltage pulse on the supply line lasting hundreds of milliseconds. On centrally suppressed alternators the residual voltage is typically limited to 35 V in a 12 V system and 58 V in a 24 V system. The protective device must be selected above these levels.
The sequence of events is this: while the alternator is producing high current, the battery connection breaks (a loose terminal, a battery cable removed while running, a corroded end). The system, which uses the battery as a buffer, loses that buffer and the energy stored in the alternator's magnetic field is dumped onto the supply line. The resulting pulse is far longer than an ordinary switching transient; its energy is high and it easily damages electronics.
Modern alternators have central suppression; the peak of the pulse is clipped. The residual voltage is typically limited to a level of 35 V in a 12 V system and 58 V in a 24 V system. These two numbers are the starting point of the protection design.
A critical design rule: the breakdown voltage of the TVS device used at the input must be selected above these levels. In a 12 V system, a TVS that breaks down below 35 V pulls the entire load dump energy of the alternator into itself and is destroyed. In other words, selecting the protective device with a "the lower the better" mindset becomes the cause of the failure.
Measures to take on the installation side:
- Check the battery terminals and chassis connections regularly; the most common cause of load dump is a loose or corroded battery connection.
- Never disconnect the battery cable while the engine is running.
- Connect the supply of electronic devices so that they benefit from the buffering effect of the battery.
Full coverage of the topic: Installation, redundancy and field continuity