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SCR Over Voltage Protection

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Suraj143

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I need to blow the fuse if the output voltage of the regulator exceeds 13.1 V.

The problem is this is a sensitive gate SCR.The gate trigger current is something between 200uA range. I cannot find any other SCR.

Can somebody fine tune my circuit.
 

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It seems unusual to boost an SCR with a transistor.

Reducing the value of R3 should make it less sensitive, try 1k.
 
So what's the problem? Are you saying the fuse is blowing when it shouldn't? Is the SCR triggering on just the zener's leakage current?
 
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My problem is I'm not sure will the gate will damage by the gate current?

Shall I add a 1K resister to the base?

My supply is 24V.So when the 12V regulator damaged the hole 24V comes out.I want to stop that 24V.
 
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If the circuit is built properly, a 7812 will be 10 times more reliable than that overvoltage circuit.

Are you having reliability problems with 7812 regulators?
 
Hi

I'm building a 24V to 12V converter with 5A current.

The thing is I'm using a PNP high current pass transistor with this regulator & I haven't mention in this drawing.First I used a LM317 instead of 7812.LM317 will limit current when temperature rises.So I had to apply a huge heat sink.Also I need another two resistors for voltage selecting.

I more prefer use a 7812 instead of a LM317.

The two main things I'm concerning is

*When the output short circuit detected it must take some action - Blow the fuse
*When the output transistor shorts (burned) it must take some action - Blow the fuse

Blowing the fuse is my own idea.If you have any other ideas please tell me.You can give me some ideas on those two issues!!!
 
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Yes, nothing new - and no reason for the transistor to be there.

The Ferguson 3000 series colour TV, which was the worlds first domestic product to use a switch-mode PSU, included a crowbar circuit - using a 2N4443 thyristor and a 72V zener - way back in 1972 or so.
 
Hi

I'm building a 24V to 12V converter with 5A current.

The thing is I'm using a PNP high current pass transistor with this regulator & I haven't mention in this drawing.
Details, details. Obviously the details you were concealing makes my answer wrong.

You should then notice that the TIP42's collector current is beta limited at these levels. The beta falls off rapidly as the collector current passes 6A. So the fuse will not blow instantly. Meanwhile the transistor is well outside its safe operating area, trying to absorb a transient of 240 or more watts. Typically, the failure mode for transistors so abused is C-E short circuit. But this can be good; see below.

The easiest solution is to replace the entire regulator, never just the fuse. There should be no circumstance where the crowbar will trigger alone without another fatal circuit fault. If you allow the fuse alone to be replaced, you are giving the user permission to operate equipment which is known to be failed.
 
Why not just get rid of T2 and use the SCR?

Using a transistor to buffer an SCR is a silly idea because SCRs are normally more robust than transistors.
 
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