Need help in choosing right SCR

[clicco89]

Hi, I start this thread saying that this is my first post on this forum and that I'm not english so I could do some grammar mistakes.
I'm working on a coil-gun, here's the circuit:

The problem is that I am not able to choose the right thyrsitor, I've tried with a BT151-650R, but it died.
I've measured the resistance through the fire circuit, and I got .2-.3 Ohms, then I calculated the max current I could get at 48V and I got 240 Amps.
Can you suggest me a cheap SCR which can meet circuit's specs?

 

[FvM]

Your circuit will damage the SCR by massively exceeding the reverse gate voltage rating. Need a series diode to block negative gate voltage.

 

[clicco89]

Your circuit will damage the SCR by massively exceeding the reverse gate voltage rating. Need a series diode to block negative gate voltage.

Sorry I'm new in electronics world, what do you mean by "reverse gate voltage"?

 

[FvM]

Reverse gate voltage means Vgate-cathode < 0 which happens in your circuit. Gate is held at 9V, cathode voltage rises to 48 V, thus Vgk = - 39V.

 

[clicco89]

Reverse gate voltage means Vgate-cathode < 0 which happens in your circuit. Gate is held at 9V, cathode voltage rises to 48 V, thus Vgk = - 39V.

So, if I've understood, I could use the same 48v as a source for the gate and remove the 9v battery. Am I right?

 

[clicco89]

That wasn't my suggestion but should work, too, with useful increased gate resistor. My suggestion was to install a series diode that cut's negative gate voltage.

As for the SCR current rating, we should have a rough estimation of coil inductance to estimate I²t. Without any inductance, you get I²t > 200 A²s, exceeding BT151 rating of 50 A²s by a multiple. A SCR in the 25 to 35 A class would be able to stand the discharge pulse even without series inductance.

So you think this should work? https://docs-emea.rs-online.com/webdocs/0db2/0900766b80db29b7.pdf
And how did you calculate that I^2t value?

 

[FvM]

I could use the same 48v as a source for the gate and remove the 9v battery.

That wasn't my suggestion but should work, too, with useful increased gate resistor. My suggestion was to install a series diode that cut's negative gate voltage.

As for the SCR current rating, we should have a rough estimation of coil inductance to estimate I²t. Without any inductance, you get I²t > 200 A²s, exceeding BT151 rating of 50 A²s by a multiple. A SCR in the 25 to 35 A class would be able to stand the discharge pulse even without series inductance.

- - - Updated - - -

0.5 C V²/R

That's an upper limit. Real I²t is a bit lower due to coil inductance, if current flow stops at the current zero crossing.

 

[clicco89]

My suggestion was to install a series diode that cut's negative gate voltage.

So I would have to place a diode between battery + and gate's cathode?