I have a general question about the key components of basic crowbar circuit below
I want to use a crowbar circuit for several DC to DC LM78xx linear regulators (various outputs, 7805 7809, etc.) to protect my devices against high voltage. At this point, I am more concerned about the general circuit behavior but I would highly appreciate if you could tell me what I need to pay attention when choosing components for my circuit.
Ok, Forget about components at the left side of the 7805. I know it is for AC to DC.
Please tell me what these components do during normal operation and during overvoltage condition? I want to know what happens during the normal operation while the zener is leaking current.
1. Fuse F1:
I have seen this fuse in various locations in various crowbar protection circuits. Is it important where to put it? I have seen it in this exact position, also after the output capacitor of LM7805 or even before the input capacitor of LM7805. Which place is more optimal?
What type of fuse is the best for these applications? I think Slow Blow fuses (like T1A) are better because I can choose them very close to my actual current consumption and it allows some transient without blowing.
2. Zener Diode D2:
I know how Zener Diodes work. I want to know how I should choose them to be optimal. Do you suggest TVS ? Can Zener in this circuit cause the output voltage of LM78xx raise or drop (I have seen this reported around the web)? Is it possible to use the Zener only configuration + Fuse for the crowbar circuit? Why not?
What are the most important points in selecting this zener diode?
What does the leakage current of the Zener do during the normal operation?
3. Resistors R1 & R2:
What do these resistors do exactly? Why would someone want to put R2 there? I have seen most of the circuits without R2. Is R1 by any chance for protection against gate floatation? If it is why some designs don't have this R2? What are the optimal values of these resistors? Should R1 > R2? or R1/R2 > some value?
4. Capacitor C4:
What does this C4 capacitor do? Should it be polarized? or is it better to be ceramic or film capacitors? How does choosing a high or low value of this C4 can affect the performance? Is it important for it to be low ESR or not?
I am not familiar with this device. I only know that it conducts when the current gets to it! How should I choose one that is optimal? Can you tell me several models so I can start with?
What are the main benefits of SCR against a thyristor?
6. Limitations, disadvantages, and drawbacks:
What are the limitations, disadvantages, and drawbacks of this circuit?
Relatively unrelated question about C3 > C2?
Is it normal to use C3 > C2? Is there any reason for this? Haven't seen this!!
Thank you very much.