Can't get my head around this

[Mr Paul]

Hi All

I am trying to make a very simple but fast fuse circuit using transistors, but to be honest I know very little about transistor switch circuits.

The circuit is 5volts and the transistor that is acting as a fuse needs to make open circuit within 40ms of it recieving the 5volts. I think the best way is to use a potentiometer to get the timing right and use a NPN Tranny. The circuit that I have tried to replicate from uses a 30ma fuse (or less) which is marked (A) and I have tried polyswitches but they are just not fast enough (also some of them increase in resistance which is of no use to me).
So if anyone could provide me with advise I would be really gratefull as this project is doing my head in.

Regards

Paul

[SamRodgers]

The circuit that I have tried to replicate from uses a 30ma fuse (or less) which is marked (A)

What does this fuse look like. I don't believe that a 30mA fuse is feasible. There wouldn't be enough power to melt the wire.