Electronic fuse circuit

Hello, i have found this electronic fuse circuit from here.
From what i read the circuit works like this.

The circuit is normally off until i press the button, it connects the gate of the thyristor to the source voltage. This will close the connection and current starts to flow from the Source (+12V) to the load through the Anode to Cathode pin of the SCR and through R1 and through the load to ground and this gets latched on. Since R1 is in series with the load and Q1 is not activated, the current flowing through the circuit is proportional to the voltage across R1 and the circuit works when there is too much current flowing through the load then the voltage across R1 will increase until it reaches .7v and then Q1 will start conducting, thus turning off the circuit.

Is this correct how i explained i correct? and i was wondering if this circuit works okay before i purchase the SCR ( because on the website at the bottom, the guy demonstrates how the circuit turns on when there is not enough latching current and not over-current protection).

Is it a foldback current limiter circuit that you are looking for?

Is it a foldback current limiter circuit that you are looking for?

Im not sure what the foldback part means, but yes i am looking for a current limiting circuit like what a normal fuse would do. Is this circuit and how i explained it correct?

Im not sure what the foldback part means,


The first power supply I built in 1979 had multiple foldback current limiting outputs using the LM723

I built it into a wood cigar box and used wood screws for the output terminals.
I tested it by running a metal bottle opener back and forth shorting the outputs.
It made sparks but the voltage returned as soon as the bottle opener was removed.

Sorry, that circuit is bogus... The animation shows that reducing the load current below the hold-on current of
the SCR will turn it off. Not exactly news.

The circuit doesn't act as a fuse. Once the current in the load is high enough to turn on the transistor the
current in R1 will effectively be constant at one Vbe drop. Thus the current in the SCR will be constant
(ignoring the small base current). So increasing the load current won't turn it off.

An SCR can never act as a fuse in fact. You can use it as a crowbar to force a real fuse to burn-out,
but that's an entirely different scenario. If you want an electronic fuse you need a switching device
that can turn off under load (not an SCR), and some way to trigger it and some sort of bistable circuit to
hold it off until reset. Think MOSFET + comparator + CMOS flip-flop (or more sensibly these days just go and
search for the IC that does it all for you).

The first power supply I built in 1979 had multiple foldback current limiting outputs using the LM723

The fact that it worked - reliably - suggests that you used the correct compensation capacitor value of 100 pF.

A nasty design blunder from an Australian electronics magazine and consequently from some other places put the capacitor value at 100 nF, slowing the response to the point that shorting the output promptly burnt out the current sensing transistor and other bad things followed. :astonished:

On the other hand, applying the foldback circuit to the design meant that the current sensing transistor was not directly connected across the current shunt so that it would probably survive even with the inappropriate compensation capacitor value. :roll_eyes:

You'll forgive me if I don't remember the cap value after 40 years ?

A few burnt-out 723s leads one to remember it well. And to post a few articles over the years.