|What Are Transients?
These are faults which cause
the voltage of the power supply to go outside normal limits for a period
of time. Many transients are capable of causing immediate equipment
failures. But, most of the time they cause minor damage to semiconductors,
degrading their performance. This damage is cumulative and eventually reaches
a point where sudden and complete failure of the component results.
Because of the subtlety of the fault process, equipment failures
are often incorrectly blamed on other 'perceived' causes. Equipment is
repaired, the 'perceived' cause of the problem is fixed, but failures
continue. The only cure is to keep transients out of equipment!
Destructive 'LOAD DUMP'
transients occur when a battery is disconnected from the charging system
during moderate or high charging rates. Load dump transients typically
reach peak voltages of 60 to 125 volts in 12 volt systems with relatively
slow rise times. Their duration usually exceeds several hundred ms and
can extend out to 1 second or more depending on the characteristics of
the charging system.
Load Dump transients also
occur when heavy loads are switched off although their magnitude and duration
will be lower. These transients are capable of destroying semiconductors
on the first 'fault event'.
These are high energy, high
voltage NEGATIVE transients. These typically occur if an ignition switch
is turned off while current is flowing in an inductive load such as an
electric motor or alternator field coil. Therefore, it can occur several
times per day.
A negative voltage transient
appears on the supply rail on the same order of magnitude as a field dump
transient, i.e. -60 to -125V. These transients tend to oscillate between
negative and positive values, decaying slowly.
Because typical integrated
circuits are highly susceptible to negative voltages, this is a potentially
devastating (but all too common) fault.
|Failed 12 Volt Regulator
|Booster Start (12V systems)
|Inductive Load Switching
||-300V to +80V
||up to 320 ms
|Alternator Field Decay (each
||-125 to -60V
||up to 200 ms
|Ignition Pulse (battery
||up to 75V
||90 ms typ.@ 500 Hz
|Mutual Coupling in wire
||up to 200V