brian@ucsd.Edu (Brian Kantor) (11/28/89)
Back in the days when I was the engineer in a company that sold CB linears (ancient history!), we had a somewhat elaborate protection circuit to try to save the RF transistors from damage from the car's electrical system. Understand that there are spikes of well over 50 volts and more on the 12v power line while the starter is cranking, and if the voltage regulator should happen to short, you can see more than 25 volts or so as the battery boils dry. We were using RF power transistors that would cook solid at about 25 volts, so this was a problem. The solution was to bring the +12 power lead in through a fuseholder that was accessable to the user and had a 10 amp fuse in it. After that, there was a 25 amp (continuous, 150 amp peak) diode to ground, which would blow the external fuse if the dude hooked it up backwards or to a positive ground truck (which many 18-wheelers were in those days). The power then went through a ferrite bead for RF, through a second 15 amp internal slo-blo fuse, another ferrite bead, a 16-volt MOV to ground, an 18-volt 10 watt zener, a 27 ohm 2 watt resistor, and a choke - before it got to the power switch. After the power switch, there was another choke and bypass caps - a .01 and a 1000 uF. The chokes, MOVs, and zener seemed to flatten the spikes sufficiently; the resistor was to dampen any resonances, and the bypasses took out most of the crud. There didn't appear to be much RF on the DC line either. We regularly got amps back with the internal fuse fried; seems that a lot of people would replace the external fuse with a bigger one when it blew, instead of finding out the problem. The idea was to level out spikes, and to have the Zener self-destruct on overvoltages - it would short and blow out the fuses. I think if I were doing it now, 15 years later, I'd use a big SCR with a zener on the gate, or a DIAC. SCRs cost too much back then. Many big diesel trucks of the era (perhaps still) use 24 volts to crank the starter. Our amps seemed to survive that - the fuses blew, and the zener shorted out, but the transistors lived through it. At $75 a pair, that's what we were after. Zeners were cheap insurance. We got very few back that had fried transistors. - Brian