I need to make a protection circuit for over voltages (protect from more than 5V, protect from 6V to 30V) and current(protect from more than 5A)

Could you please suggest components for following circuit or tell me the things I should consider when select SCR ?

Please suggest me ZD1, Q1, C1, C2, R1 values for above specifications.

Are there any IC with above spec which included crowbar circuit ?

enter image description here

  • 2
    \$\begingroup\$ Where did the circuit come from and why don't you trust the values specified on it? \$\endgroup\$ – Andy aka Aug 19 '19 at 11:00
  • \$\begingroup\$ @Andyaka The fuse rating i s a good start :-). \$\endgroup\$ – Russell McMahon Aug 19 '19 at 13:05

Is this an assignment?

ZD1 "triggers" Q1 when V_zd1 + Vgate_Q1_trigger equal the desired trigger voltage - here about 6V.
So Vzd1 needs to be about Vtrigger - Vgate_Q1_trigger

Q1 needs to be an SCR rated at usefully above Vin_max (here 30V) and able to smile at Ioverload (here 5A)

C1 is meant to deal with the odd voltage spike - OK as is - somewhat larger not harmful, probably.

C2 R1 form a time delay filter of sorts.
C2 charges via ZD1 which slows rise down a tad at very low overloads - with R1 keeping SCR gate low at low Zd1 currents.
C2 also holds gate high briefly after SCR triggers with R1 discharging it if the SCR gate current has not done so.
As the fuse will hopefully have blown long before then the values are not too critical.

You did not mention F1.
A 0.25A fuse "may be a little under-rated" for a 5A load.

It's not obvious that Sd1 is essential - IF the load is reactive (C or L) it may prevent (or modify) them affecting operation.

  • \$\begingroup\$ Of course the SCR will need to take the full short-circuit fault current of the power supply, which will be considerably more than 5A most likely, plus discharging any output capacitance - limited only by parasitic resistance, ESR and parasitic inductance. Some SCRs have relevant specs such as 300A surge capability. \$\endgroup\$ – Spehro Pefhany Aug 19 '19 at 11:42

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