SCR as fuse

[Sigmoid]

The fact that SCRs "stay on" as long as there's current through them got me thinking...

Fuses are kinda slow, expensive, gotta change them after they've blown, etc... So I'm wondering if an SCR could be used as circuit protection instead! My idea: connect an SCR and a resistor in series, and connect this up to the positive and negative rails... When it gets gate voltage, it just shorts the power rail down to ground, poof!
Gate could be driven from an op-amp hooked up somewhere buffering some voltage level or current flow converted to voltage, with the output through a Zener to define the blow voltage limit...

What I'm wondering about is the reliability of this setup... Is it a good idea for low-voltage systems?

[c4757p]

You just described either the first or second most common use for an SCR. But what exactly do you think is going to go "poof"? The resistor? In that case you just made it into a fuse, why not use a proper one?

[Jeff1946]

google crowbar

[TerminalJack505]

Such a circuit might protect the load and, if you're lucky, blow a mains breaker.  If you're not so lucky then your house burns down.

With the exception of the missing fuse, it sounds like you are talking about a crowbar circuit.  They're used to protect a load from (typically) over-voltage conditions.  They do this by shorting the power rail (with an SCR, typically) and blowing an on-board fuse.

[Sigmoid]

Yea that sounds reasonable... I guess it'll help you get away with using a slower, bigger fuse that costs a lot less.

[c4757p]

Fuses aren't expensive for the level of protection and certainty they give you, and if you design the device and spec the fuse correctly, it will never blow. Just use a fuse.