How this electronic fuse circuit works?

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electronic fuse question

I intend to use this electronic fuse

**broken link removed**

but I'm dount that the Q3 is wrong connected. If it is connected as above, U(R2) = U(BE). When U(BE) = 0.6, it would conduct but the current will flow from C to E (or from 0V to +V --> I can not understand this circuit if Q3 is connected as above.

But if we change it to PNP transistor, it would be wrong because U(R2) = U(BE) > 0. The transistor Q3 PNP will never conduct.

Can someone explain how this circuit work and if it is wrong, how to correct it to be right?
 

Re: electronic fuse question

As far as I see everything should be OK.

The circuit should work like this: normally the darlington Q1/Q2 should be fully switched on by R1 there will be a voltage drop of about 1.5V.

When the current flowing through R2 generates a voltage high enough for Q3 to switch on it will connect R1 to the voltage after R2 which is in this case about 1.5 + 0.7 = 2.2 V lower then the input voltage V+ and Q1/Q2 will stop being switched on full and will limit the flowing current.

best regards
 

Re: electronic fuse question

This circuit is current limiter, not fuse. Limit current will be 0.65V/R2 approximatelly.
 

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