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Navrit Bal

Sat Nov 27 2010, 02:30PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Ah, thanks Artikbot. I am on 450V, so this looks like its very good, I'm glad I asked

I really like how this diode is called FRED (Fast Recovery Epitaxial Diode). What does 'Epitaxial' mean (just out of interest) ?

Artikbot

Sat Nov 27 2010, 02:44PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

I have no freakin' idea

I know that I use the 400V version of that one for my coils, and they perform like da bossez with a proper copper heatsink to help them not overheat with cemf spikes.

Navrit Bal

Sat Nov 27 2010, 03:55PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

I think I'll use some aluminium I have for a heatsink

Artikbot

Sat Nov 27 2010, 04:08PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Yeah, I used copper because I needed to do a custom sink. The alu ones I had didn't fit well under the coils together with the barrel, diodes, optical triggers, SCRs and all the shitz I have in such a small piece of space.

It will work. Not as good as copper, but good enough for sure.

Navrit Bal

Sat Nov 27 2010, 04:12PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Since I'm not really concerned with space, this should do fine

I've just been looking on how to measure ~1kA, all I've found is a Rogowski coil, does anyone have any suggestions? Could Rogowski coils be used for coilguns? I would have thought that the EMP from the main coil would disrupt the Rogowski coil.

Also, any suggestions on the silicon steel screwdriver bits as projectiles?

Navrit Bal

Thu Dec 02 2010, 09:25PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

I tested 1 SCR today, and it looks like its running fine.

Also, I have just ordered this sweet FRED diode (CEMF)

Since I have some spare 18SWG magnet wire, should I use for connecting everything together rather than not using it? Is it really necessary to connect capacitors with bus bars?

Saz43

Fri Dec 03 2010, 03:33AM

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

Navrit Bal wrote ...

I tested 1 SCR today, and it looks like its running fine.

Also, I have just ordered this sweet FRED diode (CEMF)

Since I have some spare 18SWG magnet wire, should I use for connecting everything together rather than not using it? Is it really necessary to connect capacitors with bus bars?

Compared to wire, bus bars tend to have lower resistance because of their increased cross-sectional area and you can get better contact with the capacitor screw terminals. The difference isn't really that noticeable though so if you don't have copper bars or sheet, don't worry about it. Also, I wouldn't use the magnet wire to connect things because 1. it's hard to bend that stuff when its low gauge and 2. the thin enamel insulation scratches easily and you might get a short.

Barry

Fri Dec 03 2010, 05:22AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Navrit Bal wrote ...

Since I have some spare 18SWG magnet wire, should I use for connecting everything together rather than not using it? Is it really necessary to connect capacitors with bus bars?

First let's compare copper wire to copper busbar, you might be surprised at the difference.

The resistance of copper is inversely proportional to its cross-sectional area. Lets take your 18 swg wire for example, with a diameter of 1.30 mm. Its cross-sectional area is pi r[sup]2[/sup] or 1.33 sq mm.

Now let's assume your busbar is made from a copper pipe (like mine) of thickness 3mm and width 25mm. Its cross-sectional area is 75 sq mm.

The ratio of resistance is therefore (75)/(1.33) = 56.4. So, in round numbers, your wire has about fifty times more resistance than my busbar.

Second, that sounds well and good and impressive and stuff, but it doesn't actually answer your question. Is a busbar necessary? How can you tell when the resistance is low enough?

Answer: The wire resistance is "low enough" when it's less than 10% of the circuit's ESR (total equivalent series resistance).

For example, if you think the capacitors in series have 330 mOhms, then a busbar and connection total resistance of 33 mOhms (or less) is as good as it gets. Investing in further reductions has no payoff.

By the way, I happen to think magnet wire is fine for the high-current path. Sure it scratches easily but you have lots of room to keep other things away. My biggest concern would be getting it very clean for good connections. It can be difficult to remove every trace of coating. Try using sandpaper (fine or medium grit) to polish the copper at connections.

Cheers, Barry
I have a life size tattoo of my face, on my face.

Navrit Bal

Fri Dec 03 2010, 07:06PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Barry, if I can get my hands on some copper pipe then I will exactly what you did, as it is probably the cheapest solution to getting bus bars, or at least nearly equivalent. Otherwise, this enamel coating is pretty hard to scratch off, I have to scrape quite hard with a knife to get anything off.

Since we have estimated 40 milliohms per capacitor and they will be in parallel, wouldn't their resistance be (1/R) = (1/40)*3 which comes out as 13 1/3 milliohms. If I can, should I try to connect everything with some form of bus bar? It will be a lot easier to get my hands on scrap aluminium than copper.

Barry

Sat Dec 04 2010, 05:18AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Navrit Bal wrote ...

This enamel coating is pretty hard to scratch off, I have to scrape quite hard with a knife to get anything off.

Since we have estimated 40 milliohms per capacitor and they will be in parallel, wouldn't their resistance be (1/R) = (1/40)*3 which comes out as 13 1/3 milliohms. If I can, should I try to connect everything with some form of bus bar? It will be a lot easier to get my hands on scrap aluminum than copper.

It's very hard to use a flat blade to clean enamel from magnet wire. It seems like it often damages the wire. But medium grit sandpaper to clean the magnet wire works pretty quickly and reliably.

Oh, your caps are in parallel? Excellent. This does wonders to reduce ESR, as you've correctly noted above. Since coilguns are sensitive to the last milliohms then it would be good to use a busbar.

Aluminum can work as well as copper. Note that Al has about 1.5x more resistance than Cu for the same cross-section. Google for "bulk resistivity aluminum" to find the values. So it takes about 50% more cross-section to achieve the same resistance. Either way, run the math, crank the formulas, design exactly the resistance you want, spend hours to get it perfect, then just go out and pick up whatever scraps you can afford anyway, lol. I just used copper pipes leftover from a plumbing project and pounded them flat.

Always polish the Al or Cu surface before making contacts. Both metals form a thin film of surface corrosion when exposed to air.

Cheers, Barry
Q: How many reindeer does it take to screw in a lightbulb? Q: Eight. One to screw in the lightbulb and seven to hold down poor Rudolph.

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