On November 2016 Radio Electronics Kit it has published a project for the construction of a protection circuit called Crowbar. The files of the circuit diagram, the PCB, the assembly diagram and parts list, in PDF format, can be downloaded from the following link:
i6dvx 
17-10-2016 
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It 'was developed a second version of the Crowbar where the reference zener is the integrated circuit TL431. The motivation for this is due to the fact that the low-voltage zener, as may be the BZX55C 3V3, have a Breakdown voltage not very stable to vary the current. The variant guarantees a better stability of the reference voltage. The graph shows the measurements of Breakdown voltage as a function of the supply voltage, for both the TL431 for both BZX55C zener 3V3. The values shown in the graph refer to the averages of 5 samples of the two devices.
The files of the circuit diagram, the PCB, the assembly diagram and the parts list in PDF format, the second version of Crowbar, can be downloaded from the following link:
The most important thing is the appropriate choice of fuse-coupled SCR. E 'you need to compare the I2t parameter, which is the energy necessary to the merger of the device. The value of this parameter of the SCR diode, noted from the data sheet, must be far superior to the same fuse parameter. Otherwise there is a risk that the SCR diode bases before, or at best, together with the fuse.
