Electric-crowbars

  
To avoid grief when using 12-V power supplies with mobile transceivers, especially when there is a short -circuit failure of the series pass transistor, crowbar circuits provide protection by clamping the power line and blowing the fuse within microseconds of an overvoltage condition. It is a good idea to incorporate the crowbar directly into the transceiver.
Electric-crowbars - schematic

The main difference between the two circuits is that less complex circuit B depends on component tolerances for the exact trigger level, while the circuit A includes a unijunction trigger to permit precise setting of the operating point.




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