modding a linear supply for 20A current limit?

[SArepairman]

I was thinking

PSU -> Relay -> thermal fuse -> manganin shunt -> output

The manganin shunt would have a instrumentation amplifier connected to it, the output of the instrumentation amplifier would have a reasonable LC filter, there would be a comparator and VREF/turn pot that would shunt the current to ground through a low RSon mosfet and open a normally closed relay, the mosfet would act as a "protector" and the relay would make sure the mosfet does not burn up. There could also be a thermal fuse that runs by the mosfet to open circuit the thing, just incase the relay fails.

Is this a good protection scheme for an old supply? This way I would not have to modify the feedback loop of the original supply. It seems unreasonable to attempt a constant current source @ 20A, and I am not quite sure how the supply I can buy at a bargin is built. It also seems like its safer then using a control loop, as it cannot really oscillate out of control.

[T3sl4co1l]

A relay isn't much faster than a fuse, and a fuse is only there to protect the wiring...

What's the circuit actually capable of (peak current, energy)?  How long can it sustain a short before it explodes?

Don't forget the flip-flop, either.  As described, relay opens = current goes to zero = ahah, it's safe to turn on again = f00f.

Using a suitably sized MOSFET for the switching allows active current regulation and quick turnoff, but you need a suitable circuit to do it.

Tim

[SArepairman]

well the idea was that the mosfet is turned on by the comparator, and it shunts the circuit to ground (so full 20A goes through a low Ron mosfet), and at the same time, a relay to break the circuit is triggered.

The fuse is connected to the mosfet thermally, incase the relay does not open the circuit, the mosfet will get hot and open circuit.

The idea is that the mosfet only works for as long as the relay needs to open, so it should not need a heat sink I think.

It seems more reliable to use a pass relay and a "grounding" mosfet then a pass mosfet. Oh yea, the comparator output would latch, and the mosfet/relay would be driven by slightly more beefy transistors then in a regular comparator. The relay is normally closed.

This is just a mod to attach to the side of a old school CV 30lb power supply to make it a bit more safe/lab worthy.