Subject: Over-voltage protection for mobiles.

 Over-voltage Crowbar circuit
                      ----------------------------

The following circuit is better than the suggested idea of a high power
Zener (where would you get one, and how much would it cost?)
Mine uses a low power zener and a slave thyristor/SCR, which
are easier to come by (and cheaper if you actually have to pay hard cash!)
Befriend anyone in AC power engineering if you want a supply of SCRs.

+------!30-amp-fuse!---------------------------------------->to radio
12V                        !             !
supply.                    !           __+__
                           !            /~\ 20-volt zener
                           !           /---\ 1 watt rating.
                           !             !
                         __+__           !
                  SCR     /~\------------!
                100 volt /   \           /
                100amp  /-----\          \100 ohms
                           !             / 5 watt resistor
                           !             !
 -earth------------------------------------------------------->to radio

This will protect you if the voltage goes above 20 or so. It has the
merit that the SCR will 'latch' and always blow the fuse. If the supply
has spikes on it (alternator commutation whine, or induced voltages from
electronic ignition) then it may trigger falsely. A 10nF capacitor in parallel
with the 100 ohm resistor acts as excellent electronic Valium,
alternatively, stock up with 30-amp fuses.
Similarly, if the radio is one with defective RF filtering on the supply
leads, and RF gets into it, then the crowbar circuit will false trigger.
You can hang this circuit on the output of your 12-volt shack power source
too - it will save your rig should one of the series-pass regulator transistors
in the power unit decide that its going to go short-circuit.
It is cheaper to protect your rig than have to replace all the
semiconductors!