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I'd like to use the below circuit to protect my device from overvoltage.

schematic

simulate this circuit – Schematic created using CircuitLab

D1 is rated to 15V. SCR1 is supposed to be ON Semi MCR100-8G with maximum current of 800 mA. F1 supposed to be rated at 0.5A in order to blow below maximum rating of SCR1. So my question is, can I place fuse to F1 position? All schematics I've seen had fuse in place of F2. I want to omit it, and use only F1 instead. Also, should I add some resistor between D1 anode and SCR1 gate? If yes, how to calculate its value?

Thanks to @rdtsc who noticed a very silly mistake of mine. Obviously the only possible place of fuse is F2. So now the question is:
Provided that SCR1 has maximum current rating of 0.8A, and maximum surge current of 10A, can I use F2 with 3A rating?.

Zhenek
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    If F1 blows, what will prevent current from entering the load? – rdtsc Aug 13 '19 at 13:24
  • @rdtsc that was extemely stupid of me. Obviously nothing. So the only option is to use SCR1 with higher current rating? – Zhenek Aug 13 '19 at 13:29
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    While a crowbar and fuse is great for extra protection, a fuse alone is probably enough for this circuit. Unless the fuse is not acting fast enough, then just remove F1 (take the SCR cathode to ground) and the SCR should absorb anything else. Except a really big pulse, then it might become a LER, a light-emitting rectifier. – rdtsc Aug 13 '19 at 20:13

1 Answers1

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Your circuit with F1 overlooks how a crowbar circuit is designed to operate--the SCR is intended to conduct a very large current, forcibly blowing the supply fuse and isolating your circuit from the overvoltage.

The idea of a crowbar circuit presumably takes its name from the act of throwing a metal crowbar onto a live circuit causing protection elements to blow, either accidentally or to prevent a more severe incident -- electrified trains often carry third-rail crowbars to short the supply in an emergency when time is of the essence.

If F1 is present and blows, then your crowbar circuit does nothing--F2 does not blow and the overvoltage is applied to the load. Instead, you should have F2 (only) and ensure that SCR1 can handle the full current needed to blow the fuse. According to its datasheet, SCR1 is intended to handle up to 0.415 A2s of I2t.

I don't know what fuse you are using, so you will need to consult your fuse's specs to determine what I2t is needed for it to blow. For example, this fuse will blow in 0.004 seconds at 10A, for an I2t of 0.4 A2s, which is quite close to the absolute maximum rating. You may want to consider a faster-blowing fuse than my example, a lower current, or a larger SCR.

nanofarad
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