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jaysun92

Mon Apr 12 2010, 07:43PM

Registered Member #2114 Joined: Fri May 08 2009, 11:58AM
Location: London, Ontario
Posts: 36

I have 4 100V 10000uF capacitors in series for a total of 400v 2500uF, would this SCR be able to handle switching them?
http://ca.mouser.com/Search/ProductDetail.aspx?R=S8040Rvirtualkey57610000virtualkey576-S8040R
Thanks for the help!

klugesmith

Mon Apr 12 2010, 08:35PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

jaysun92 wrote ...
I have 4 100V 10000uF capacitors in series for a total of 400v 2500uF, would this SCR be able to handle switching them?

Can't answer that without knowing the amplitude and duration of your current pulse. Then we can help you interpret the datasheet.

It would be a pretty sad design if actual current pulse does not depend strongly on the load, about which you have told us nothing. Railgun? Coilgun? Electrothermal gun? Disk launcher? Can Crusher? It's all about the circuit resistance ( how much less than 1 ohm? ) and inductance (how can we guess?)

For a coilgun, this simulator may help:

It gives you inductance and resistance values, which you can put into the RLC simulator at the same website
to answer the current pulse questions.

jaysun92

Mon Apr 12 2010, 11:01PM

Registered Member #2114 Joined: Fri May 08 2009, 11:58AM
Location: London, Ontario
Posts: 36

Sorry about that, It is for a Coilgun, the inductance is 1.409mH, the Resistance is 0.696 Ohms. the RCL simulator says a peak current of 302 amps that drops to 0 after 6.64ms. sorry i didnt post all the info before, i was kind of in a hurry.
Thanks for the help

Saz43

Tue Apr 13 2010, 04:01AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

All that's left for you to do is compare your peak coil current to the pulse current rating for the SCR, which can be found on it's data sheet.

SCR's have two current ratings- one for continuous current (that would be the advertised 40A in your case) and the pulse rating, which is the much higher max current that the SCR can pass for a very brief amount of time. Generally, the pulse rating is about 10 times the continuous current rating, but check the data sheet to be sure. Also mind the specified width of the pulse. If the pulse rating is specified for 3ms, and the pulse through your coil is 6ms, chances are you'll want to go with a lower peak current since it's going to last a lot longer and put more stress on your SCR.

Lastly, a 6ms pulse is a little long for a coilgun- this is due in part to your high inductance coil. Try fewer layers or something to get your inductance down a bit. Such a long pulse may cause suckback in your coil. Look it up in the HvWiki for more info.

Turkey9

Tue Apr 13 2010, 04:06AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Looks like it should be fine! It can handle 520A at 60hz and your pulse is about 154 hz. The voltage rating is high enough too so it should work. This is just a quick answer going off your simulation numbers, but there is quite a margin of error.

jaysun92

Tue Apr 13 2010, 12:10PM

Registered Member #2114 Joined: Fri May 08 2009, 11:58AM
Location: London, Ontario
Posts: 36

Thanks to both of your for your answers! Now i just need to order the parts and hopefully get it working soon.

klugesmith

Tue Apr 13 2010, 07:57PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I concur.
Note that the 520 A figure is for I_TSM, the tolerable single-cycle current at 60 Hz.

For pulse waveforms other than half-sine and decaying-exponential, one can check the limit for I-squared-T integral, which for this part is 1122 amps-squared-seconds. 300 amps for 6 ms (rectangular pulse) would be only 540 A^2s, so again there is plenty of margin.

Without coilgun experience, I would worry about voltage reversal. It's almost 20% with your component values. (note checkbox in RLC simulator). The SCR will turn off when the current reaches zero, leaving -20 V DC on each of your capacitors. (At best, assuming use of voltage balancing resistors and perfectly matched C values).

jaysun92

Wed Apr 14 2010, 12:37AM

Registered Member #2114 Joined: Fri May 08 2009, 11:58AM
Location: London, Ontario
Posts: 36

To stop the voltage reversal you just have to put a diode in antiparalel to the capacitors, so i am not too worried about that.

Turkey9

Wed Apr 14 2010, 02:00AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I think you'd be better off putting it in anti-parallel with the coil. That's where the voltage reversal builds up. This will protect your SCR as well as your caps.

jaysun92

Wed Apr 14 2010, 02:43AM

Registered Member #2114 Joined: Fri May 08 2009, 11:58AM
Location: London, Ontario
Posts: 36

Sorry about that, that is what i meant, here is a diagram i found for the firing circuit.
Coilgun Schematic

And thanks for the help!

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