The circuit shown in Figure 1 is important for protecting sensitive circuits from shorts or large voltage changes.
Figure 1 – Crowbar protection
The voltage drop across resistor R1 depends on the current. If the voltage drop rises to the trigger point of the SCR, the SCR turns on, putting the power supply in a short. This causes the fuse to blow, and the current flow is stopped.
The value of R1 depends on the current the designer specifies to blow the fuse.
The value is found by the following formula:
R = V / I
where
R = the resistance in ohms
V = SCR trigger voltage (typically between 0.8 and 1.2 V for the TIC106)
I = trigger current
Remember that the SCR must be able to tolerate the current necessary to blow the fuse.
