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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
Field Decay Transients
Other Transient Sources
 
 

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 Transients

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.

 

Other Transient Sources
 
TRANSIENT: Typical Voltage: Length:
Failed 12 Volt Regulator +18V Continuous
Booster Start (12V systems) +/- 24V 1-5 min.
Load Dump 60-125V 5-400+ ms.
Inductive Load Switching -300V to +80V up to 320 ms
Alternator Field Decay (each engine turn-off) -125 to -60V up to 200 ms
Ignition Pulse (battery disconnected) up to 75V 90 ms typ.@ 500 Hz
Mutual Coupling in wire harness up to 200V 1 ms

 

 

 

         
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