MIL-STD-1275E
3.6.3 Voltage surge.
A surge is a transient waveform having a duration greater than 1 ms and a specific wave shape, typically a rising/falling edge and a slow exponential decay for the falling edge. Surges result from the switching of reactive loads containing a significant level of stored energy or sudden disconnection of a constant load. Surges may also occur due to the application of high-demand loads.
3.6.3.1 Positive voltage surge.
A positive voltage surge is a positive-going transient, which exceeds the nominal supplied voltage. This may occur when a high current or inductive load is suddenly disconnected. The most common occurrence of a positive voltage surge, or "alternator load dump," occurs when the alternator is working to charge a partially or fully discharged set of batteries and the connection
to the battery positive terminal is suddenly disconnected. The alternator cannot immediately decrease its output to compensate for the sudden loss of load so the energy delivered during this settling period is distributed to the vehicle's electrical system. A positive surge (VPEAK) with a short rise time (tRISE) and long exponential decay is generated above the nominal battery voltage of the system (VNOM) and last for a given time (tWIDTH). Figure 4 shows an example of an alternator load dump waveform.
Volt
VPEAK
V90%
VNOM
VJ0%
t
RISE
tWIDTH
0
Time
Figure 4. Sample alternator load dump waveform.
3.6.3.2 Negative voltage surge.
This event is similar to an alternator load dump as specified in Section 3.6.3.1 except it represents the negative-going transient generated when a sudden load is placed on the alternator. The alternator cannot immediately change its output so the system voltage decreases until the alternator can compensate for the sudden increase in load.
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