L a n d T r a n s p o r t A p p l i c a t i o n s
Mobile applications abound in the wireless environment. Unfortunately the moment you connect electronics to the vehicle electrical system, you begin a spiral of problems that will effect the operation, reliability and longevity of the equipment.
Computers, Communications, Navigation and Entertainment systems MUST be isolated from the vehicles electrical system. In the past, installers have recommended that a separate battery be installed to power this equipment. Unfortunately, that doesn't guarantee reliable operation since the second battery is usually charged by the vehicle's alternator. 'Isolation' diodes do not prevent electrical faults from reaching the electronics either.
Each time the engine is started, high voltage spikes are generated by the starter solenoid and starter motor. Contact bounce aggravates the problem by creating a series of positive and negative spikes that can reach several hundred volts. In 24 volt systems, spikes can reach 500 volts peak-to-peak!
For example, the following chart is extracted from MIL-STD-1275B and shows the 'normal' loci, or limits, found in a properly operated and maintained system:
Each time the engine is switched OFF, a 'Field Decay Transient' occurs as the electro-magnetic field collapses in the alternator. These are high energy, high voltage NEGATIVE transients. They also occur if an ignition switch is turned off while current is flowing in an windshield wiper, or power window motor. Negative voltage transients of -60 to -125V are generated. See the section on TRANSIENTS for more details
UNDERVOLTAGE or 'Brownout':
Perhaps the easiest to understand fault is UNDERVOLTAGE. Each time the starter is engaged, battery voltage can be expected to dip to as low as 6 volts. This causes havoc with embedded computer systems and other electronic gear.
All of the problems noted on the sidebar are to be found in vehicular power systems. There is little difference between the modest passenger car, a diesel powered bus, an ambulance, or military APC. ALL suffer from these problems.
The bottom line is thus: if you MUST HAVE RELIABILITY, you MUST keep electrical system faults out of electronic equipment. That was the reason for STO-P's development nearly 10 years ago. Today's dependence on mobile communications and navigation makes it even more important!