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SK065K SCR/Coilgun

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wpk5008

Tue Dec 28 2010, 10:22PM

Registered Member #3546 Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18

This SCR is rated at 1000V, 41A steady, and 950A pulse. This sounds great for a coil gun design, but it is a three pin through hole mounting type. It just doesn't seem as robust at the hockey puck or bolt type SCRs that your usually see with those ratings. The price is fantastic though, but I'm just concerned for my own safety when discharging under a kJ through it. Not to mention I don't want to spend 6 dollars to vaporize it and anything around it.

Here is a link to additional specs... http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=US&WT.z_homepage_link=hp_go_button&KeyWords=SK065K&x=0&y=0

On a similar note I am planning on building a coil gun, I have already purchased come caps, 4 450V 2400uF electrolytic caps to be exact. I am hopefully going to get a .25 inner diameter .375 outer diameter carbon fiber tube, once the company gets back to me. I just sent them an email asking if they sand the inside as well as the outside of their smooth carbon fiber tubes. If they say yes that the inside is sanded then I have already ran barry's simulators for my single stage coil, using only three capacitors charged to 400V.

The parameters from barry's simulations are as follows 400V, 170mohm, 7.75mF, 257uH. This gives me a 1238A forward surge, that ends after 5.00ms and reaches <-320A negative emf surge. Sorry I am new and not familiar with how to post the pictures yet. The coil is as follows 16AWG, 10mmID, 29mmOD, 40mmL, 210 turns, 7 layers, for 170mohm, and 257uH.

I am just not sure right now how to chose my SCR from this data. Reading the datasheet it typically lists the peak forward current surge, which would be the initial current, however reading some of the posts on this forum I am under the impression that perhaps it is the emf (negative spike on barry's simulator) that the peak forward current surge needs to be rated to. I hope it is the latter because that would reduce my costs greatly.

Thank you for any help.

Saz43

Wed Dec 29 2010, 06:42PM

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

I think it's worth a try. If you're worried about explosions, you could always charge your capacitors to a lower voltage and steadily increase the energy of your discharges.

wpk5008

Wed Dec 29 2010, 09:56PM

Registered Member #3546 Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18

Yeah worth a shot I guess. Next time I put an order in I will have to get one or two of those SCRs.

I just thought of something last night/this morning(what do you call 5am?). Carbon is conductive to some extent, so a carbon fiber tube would generate eddy currents too? While the eddy currents of a metal would be of concern, do you think carbon would generate enough eddy currents to effect the performance of a coilgun?

Saz43

Thu Dec 30 2010, 08:38PM

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

Probably not- carbon has a very high resistivity compared to most metals. Carbon is usually the material inside a resistor to impede current.

Sir Fixalot

Thu Dec 30 2010, 09:37PM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
Location:
Posts: 22

Hmm...I agree-I wouldn't want to use a little SCR Like that for a coilgun Like you're building. I would look on ebay, as you can find some cheap scr's for <$100. Here, try this, it's probably exactly the sort of thing you were looking for, and its not too expensive either:

It's the one on the bottom-left of the page.

Sir Fixalot

Thu Dec 30 2010, 09:38PM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
Location:
Posts: 22

Oh, I almost forgot-this one too:

Arcstarter

Fri Dec 31 2010, 02:30AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I have been using these very SCRs for some of my cap bank stuff. They are great. I paralleled 5 of them and with 1.5v on the gate they have withstood 100's of dead shorts with a 1950uf 800v 624 joule cap bank. Never had a failure.

wpk5008

Fri Dec 31 2010, 07:52AM

Registered Member #3546 Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18

I got emails back from two companies today and a physics professor. Both companies said they make their tubing on steel cylinders and in order to remove the tubes it has to be free of imperfections. The one company offers an extruded version for $8 and the other offers a fabric version for $30. I think I will go with the $8 one just in case carbon does induce strong enough eddy current to effect efficiency. As far as the physics professor... he was about as useless as his class was, he just told me one of his students built a coilgun recently and he would forward my eddy current email to him. Maybe this student will be a good local help for me though?

As far as the SCRs go, I was unaware that you could wire them in parallel and that they would equally share the load. I might have to give that a try at first to see if I can save some money. I actually think it was one of your posts Arcstarter, that made me aware of that particular SCR, and it just looked to good to be true.

I have one more question though. I'm rather new to dealing with high voltage. For my charging circuit I was thinking of building a boost converter. But I do not have the materials right now to build and test one. So I was wondering how dangerous it is to charge caps with a voltage doubler or tripler straight from the wall? Should I run the wall through a ballast first to limit the current? But I am curious if this would drop the voltage too much to make my voltage multiplier useful because all the ballasts I can think of that I could find around my house are resistive...

Sulaiman

Fri Dec 31 2010, 03:34PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Put a filament lamp (e.g. 100W) in series with the ac line
and it will be safer than a fully charged capacitor bank,
rectifier/doubler/tripler/transformer/ whatever load.

Saz43

Fri Dec 31 2010, 06:57PM

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

wpk5008 wrote ...

I have one more question though. I'm rather new to dealing with high voltage. For my charging circuit I was thinking of building a boost converter. But I do not have the materials right now to build and test one. So I was wondering how dangerous it is to charge caps with a voltage doubler or tripler straight from the wall? Should I run the wall through a ballast first to limit the current? But I am curious if this would drop the voltage too much to make my voltage multiplier useful because all the ballasts I can think of that I could find around my house are resistive...

Uzzor's boost converter costs like $7 if you buy your parts from Mouser

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