Powering electronic equipment from a vehicle’s electrical system can be a risky business, especially if the equipment wasn’t originally designed for in-car use.

Large positive and negative voltage transients occur regularly during "normal" operation of vehicle electrical systems.

Opened-out view of the supply, immediately before final assembly. The hardest part is probably drilling the holes for the LEDs and cutting the slot for the switch - these must be done very accurately.

The alternator is undoubtedly the main culprit. Load dump transients, which occur when heavy loads are switched off, can cause the alternator’s output to swing to as much as 100V for several milliseconds.

This effect is caused by the inability of the alternator to respond instantaneously to load changes. The response time of an alternator is bound by the forces of mechanical inertia and the long time constant of the excitation winding.

Other nasties, called field decay transients, occur when the ignition switch is turned off while current is flowing in inductive loads (windscreen wipers, alternator field coil, etc.) These are negative in direction, with a similar energy to the positive swing of load dump transients.

Switching spikes from inductive loads like windscreen wipers and power windows generate even higher voltages, as much as 200V positive and negative.

These transients have much lower energy in comparison to load dumps and field decay transients.