Load dump


Load dump

= Automotive Load Dump =

Load dump in vehicles refers to what happens to the supply voltage when a load is removed. The voltage regulation has a time constant and if a load is removed rapidly it can take a few ms for the voltage to stabilise. The vehicle battery acts to smooth out these pulses and keep the voltage more constant.

Overview

If the vehicle battery becomes disconnected whilst the engine is running there is not much control of the voltage. Most alternators do have a limiting diode but the voltage cut off is often quite high. ECUs on vehicles are tested to withstand these voltages but still damage can result.

The windings of an alternator have a large inductance. If the alternator is generating a large current, it will need to generate a large voltage to overcome the inductance as well as the battery voltage. The alternator can produce as much as 80 volts, with 66 volts drop across the windings, leaving 14 V DC on the terminals.

The open-circuit voltage generated by the alternator depends on the magnetic field produced by the rotor, as well as the speed of rotation. The voltage regulator turns the field voltage on and off to keep the output voltage constant. The field current does not change very quickly because of the large inductance of the field winding, and a freewheel diode is fitted to the field winding so that the field current decays over a second or so instead of being stopped quickly.

Because of this, turning the field voltage on and off only causes slow variations in the magnetic field. If the output voltage rises, the voltage regulator can only let the field current decay of its own.

If the current load is reduced to zero very quickly, the alternator's output voltage becomes its open circuit voltage, which then decays as the field current decays. If the battery is fitted, it can take charging current surges from the alternator which are far smaller than the discharge current surges that the starter motor takes. If the battery takes the current surges, the alternator current does not fall to zero quickly, and no voltage surge occurs.

Battery missing

If the battery is missing, there is nothing that can absorb the current and the full open-circuit voltage appears on the terminals if there is a rapid change of load. The action of disconnecting the battery can cause the change of load, so it is particularly likely to cause damage.

Load dump can be as high as 120 V and take as long as 400 ms to decay, but it depends on the magnitude of the load change and the characteristics of what loads remain.


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