Author Topic: New coil  (Read 1115 times)

Offline Usernametaken

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New coil
« on: June 04, 2018, 10:21:55 AM »
This is what dining tables are for, right?
Going to be asking a LOT of questions in here so I'm hoping you guys are willing to help me out.

Offline oneKone

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Re: New coil
« Reply #1 on: June 04, 2018, 11:23:35 AM »
Welcome to the forum, love the username!

My girlfriend has gotten use to my projects never leaving the table! Looking forward to the build.

Offline Usernametaken

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Re: New coil
« Reply #2 on: June 04, 2018, 11:34:16 AM »
So here are the specs, so far. Anyone able to confirm that this is all relatively OK?

Secondary winding height is 400mm on a 90mm dia. former wound with 36AWG wire. With the topload should give ~110Khz frequency.
MMC is 3 strings of 3 942C20P14K-F giving 6KV @ .15uF.
The bridge is yet to be decided on but probably a CM200DU-24F brick.
Loneoceans UD2.7C driver.
Diy 555 interrupter for testing but will probably use a Onetesla interrupter in the finished coil.
Secondary will be using 1/4 inch copper pipe.

I run off 240V here.
Not looking for a ridiculously high powered coil, more a solid, reliable conversation piece to put in the loungeroom for people to look at.

Offline Mads Barnkob

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Re: New coil
« Reply #3 on: June 04, 2018, 02:45:12 PM »
Welcome to HVF!

It all sounds within good proportions and with some good overhead on the IGBT brick, how about RMS/peak current on the MMC?

You could properly get away with thinner than 1/4" copper tubing for the primary, but it will still be just fine.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline Usernametaken

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Re: New coil
« Reply #4 on: June 04, 2018, 02:50:58 PM »
Peak should be 432A and RMS 13.5 according to the MMC calculator.

Offline Mads Barnkob

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Re: New coil
« Reply #5 on: June 04, 2018, 03:17:21 PM »
Peak should be 432A and RMS 13.5 according to the MMC calculator.

Assuming you are running at 100 kHz with a 15uH primary inductance, you will only be able to run at a primary peak current of 400 A, on time 200 uS and 200 BPS. Then you are at the edge of the RMS current rating of that MMC.

So you can decide that a little heat dissipation is not going to matter due to short run times, run with lower BPS or just keep it at 400A OCD, but that is kind of in the low end of a CM200 brick. http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline Usernametaken

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Re: New coil
« Reply #6 on: June 04, 2018, 03:33:12 PM »
Actually, that was reading from the Kaizer MMC calculator, adding or removing caps doesn't seem to be affecting the RMS, let me look into it.

Oh... you wrote it...
« Last Edit: June 04, 2018, 03:56:54 PM by Mads Barnkob »

Offline Hydron

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Re: New coil
« Reply #7 on: June 04, 2018, 09:23:05 PM »
RMS current depends on the number of capacitors in parallel. If you are just changing the number in series then it will not change the RMS number. This rating can be a real pain to find in datasheets - often you need to read small graphs of RMS voltage vs frequency and try and work back to RMS current. For 942C caps it's known (specified?) though which makes it easy.

Peak current limit depends on two things:
1) The datasheet peak current rating of the capacitor (for "snubber" caps this is not usually the limiting factor). Either given directly (rare), or expressed in dV/dt in V/us. The latter can be easily converted to amps by multiplying by the capacitance in uF.
2) Limitations due to the avoiding exceeding the voltage rating of the capacitor. This depends on the reactance of the capacitor at the TC frequency - if you divide the voltage rating by the reactance you'll get a peak current rating _at that frequency_.

Mads's calculator should do this (and all other required calculations) for you given the correct inputs.

Offline Usernametaken

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Re: New coil
« Reply #8 on: June 05, 2018, 08:09:30 AM »
Cheers for the help, I'll sit down later and work out some calcs but as I'm not looking for a massively powered coil, I may keep this cap back and run at half mains input for a bit.
My calculations for the tank cap size were derived from JavaTC, I take it this is incorrect?

Until I get bored and end up building a better MMC anyway.
« Last Edit: June 05, 2018, 08:19:37 AM by Usernametaken »

Offline Teravolt

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Re: New coil
« Reply #9 on: June 05, 2018, 08:53:23 PM »
Hi Usernametaken, when you are looking for a capacitor for MMC you will be looking for a cap that is a snubber cap or pulse rated. the factors you should look for is low ESR, ESL, and low dissipation factor, and a high dV/dt . A good material to look for is Polypropylene capacitors

http://www.farnell.com/datasheets/1747413.pdf?_ga=2.50515118.602371384.1528224431-465729543.1521044120&_gac=1.45237136.1528224431.EAIaIQobChMIxte4yJe92wIVCNlkCh3rCgJbEAAYAiAAEgKc2PD_BwE

Wima fkp1 series

https://www.mouser.com/Search/Refine.aspx?Keyword=fkp+1

Offline Hydron

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Re: New coil
« Reply #10 on: June 05, 2018, 11:33:39 PM »
The other thing to note is that for a DRSSTC the expensive 942C caps that everyone swore by for spark-gap coils are not so necessary - a DRSSTC is less demanding on peak pulse capability. This means you have many more options, including the Wima FKP series mentioned. I'd also look at EPCOS - they have good value caps which are a good match to DRSSTC (especially QCW) work.

For my smaller QCW coil I have bought new caps from Farnell (element14 in Aus/NZ) as it was quite inexpensive, but for my larger DRSSTC I ebayed some used Aerovox 2uF/1kV snubber caps (the MMC needed a LOT in series and only 2 in parallel, but it worked well). You'll probably have better luck with ebay in the US (and to a lesser extent Europe) than e.g. Aus/NZ, but while the shipping basically doubled the cost of the caps I bought from the US and got shipped to NZ it was still much cheaper than buying a lot of bigger caps new.



Offline Hydron

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Re: New coil
« Reply #13 on: June 06, 2018, 10:16:57 AM »
I found these which look promising otherwise I'll go for the FKPs if I can find bulk at a decent price.

https://www.ebay.com.au/itm/4-x-1-2KV-2UF-Pulse-High-Frenquency-Snubber-Capacitor/362320230706?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3Dde24a80363c5469e925eec1f00bbf9b7%26pid%3D100675%26rk%3D2%26rkt%3D15%26sd%3D272431785177%26itm%3D362320230706&_trksid=p2481888.c100675.m4236&_trkparms=pageci%3Ad85c0cc0-693e-11e8-bf17-74dbd180f9ae%7Cparentrq%3Ad345588a1630aa47cd879a6afff8f282%7Ciid%3A1

For a smaller coil I would look for something with a bit less capacitance - you'll need quite a few in series to get the capacitance down to a usable number, especially if you want more than one string in parallel for increased RMS/peak current handling. I did use 2uF capacitors myself, but for a bigger coil and in total about 40 of them (2 strings of ~20 in series for 200nF/20kV), and they had much better pulse and RMS ratings than the mystery Chinese ones. Mine were Aerovox RBPS 2uF/1kV I got in a lot of 50 for 100USD (plus at least that again for shipping).

Offline TDAF

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Re: New coil
« Reply #14 on: June 06, 2018, 10:18:41 AM »

Offline Hydron

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Re: New coil
« Reply #15 on: June 06, 2018, 11:28:56 AM »
You might want a bit more than 10nF per cap, but those EPCOS parts are definitely worth looking at - I am using a different cap from the same series in my QCW coil. Good ratings, and they are also rated for intermittent, low-duty-cycle use beyond their normal voltage rating, which is perfect for tesla-coil use (see the end of the datasheet).

Offline TDAF

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Re: New coil
« Reply #16 on: June 06, 2018, 12:00:12 PM »
What do you think of the WIMA caps??

Offline Hydron

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Re: New coil
« Reply #17 on: June 06, 2018, 12:03:20 PM »
What do you think of the WIMA caps??
At 630VDC and 10nF you'll need an awful lot of them to get both decent capacitance and voltage rating for a MMC. WIMA MKP/FKP parts are definitely worth looking at though, the FKP should perform well in DRSSTC use, and the cheaper MKP might be OK too (though I'm not sure if people have tested them in this application).

Offline Usernametaken

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Re: New coil
« Reply #18 on: June 06, 2018, 12:49:10 PM »
JavaTC tells me that a .15uF capacitor is pretty close to what I need for resonance.
To save a bit of $$$ I may for now run at half mains voltage in the hope of keeping things robust and then in the future go to full mains with a new capacitor bank.

Offline TDAF

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Re: New coil
« Reply #19 on: June 06, 2018, 02:10:28 PM »
JavaTC tells me that a .15uF capacitor is pretty close to what I need for resonance.
To save a bit of $$$ I may for now run at half mains voltage in the hope of keeping things robust and then in the future go to full mains with a new capacitor bank.

I don't think there's much of a saving to be made with running at half mains instead
But the voltage increase is sure worth it
So, I suggest you go with full mains

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Re: New coil
« Reply #19 on: June 06, 2018, 02:10:28 PM »

 


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