| Preventing Failures: |
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| Common Electrical System Faults: |
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| 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!
TYPES: Load
Dump Transients
Load Dump Transients 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'. Field Decay TransientsThese 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.
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