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Primary Capacitor sizing

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uzzors2k

Sat Mar 03 2007, 09:53AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I'm planning on making a new SGTC, powered by an offline run flyback transformer, which can supply about 300W @ 20kV. The secondary resonant frequency should be around 571 kHz. All of the calculators (yes, I'm a noob.) I've found assume one is using a 50/60 Hz AC transformer. However, since this is not the case the capacitor size will only be limited by the flyback's power output. So how big can I go before the flyback can't charge the capacitor fast enough? I assume a larger primary capacitor means more power?

...

Sun Mar 04 2007, 03:11AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Well, you are designing a DC coil, not one that has to work at 50/60hz. What that means is that you can run at whatever break rate you want to

So, pick a break rate. In general, higher bps = fatter sparks, lower bps = longer sparks, however the DRSSTC people have shown there is a 'sweet spot' that gives longest sparks.

Then you take the # of bps, say 100, and find out how many joules you need in your tank cap. So, for 300w and 100bps, you need a 3j cap. (keep in mind that watts are joules/seconds). So then you decide what voltage you are going to set the spark gap at (10kv is probably about right) and then size the cap for that.

finally, you then use a calculator to find what size primary you need for the given cap size and secondary resonant frequency.

Have fun!

uzzors2k

Sun Mar 04 2007, 01:15PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Just the advice I was looking for, thanks!

uzzors2k

Fri Mar 09 2007, 06:02PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I'm ready to build the tesla coil this weekend, but I just need somw things checked first. I decided to use 58 plastic plates to make a 62nf primary cap, which with a 1.2ÂµH primary should resonate at 570kHz. What kind of primary shape should I use; pancake, helical or cone? If I use a pancake coil I'll only need 3 turns to hit 1.2ÂµH, sounds a bit low to me, what about coupling and such?

J. Aaron Holmes

Fri Mar 09 2007, 06:28PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

62nF! Wowzers. Maybe I'm wrong, but at 20kV, 60nF is like "pole pig territory". With a flyback, I'm thinking it's gonna take a long time to charge that 60nF, and you'll be left with a minescule breakrate unable to facilitate proper streamer growth (not to mention hideous peak currents associated with a 1.2uH primary--these can be hard on your caps!). I'd recommend trading a lot of that primary C for a lot more primary L.

EDIT: In fact, assuming 20kV is your RMS voltage and your peak is something like 28kV, and assuming your target power is 300W with a break rate of 100 (3 joules per "bang"), then I compute a required cap size of only about 7.5nF, which is 1/8 the size you're using! That would make 62nF good for over 2.4kW

Maybe not "pole pig territory", but real close!

EDIT #2 (sorry): Actually, RMS is probably a hard thing with your flyback, since it's not driven with a sinusoidal source. Perhaps somebody will hop on here and finish what I've begun here. But if 20kV were your peak and not 28kV, then your cap size would probably be about 15nF, which is still only a quarter of what you're using.

Regards,
Aaron, N7OE

uzzors2k

Fri Mar 09 2007, 08:20PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I can't quite remember the maximum output voltage, so I'm just going to set the spark gap for 10kV to be nice on the components. 20kV is just a number I based on striking distance while arcing, it may be a lot lower however. 10kV @ 60nf is within the power capability of the flyback (or so I hope). However, you mentioned large peak currents, would these destroy my capacitor? I do feel that 60nf and 1.2ÂµH is pretty unbalanced, but then again the secondary is pretty unbalanced too.

Any suggestions for primary coil type, this is my first proper Tesla coil so I'm still pretty green...

Dr. Drone

Sat Mar 10 2007, 02:16AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

Tom540

Sun Mar 11 2007, 04:39AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

Looks like someone shops at Trader Joe's

uzzors2k

Mon Mar 12 2007, 08:47PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

So clean Christop_eR, those primaries look really professional. I sure wish I had some acrylic, just wait 'til you see my ghetto coil.

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