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Tesla coils => Spark Gap Tesla Coils (SGTC) => Topic started by: MarcusFunt on June 09, 2022, 08:44:49 PM

Title: Cheaper Tesla coil capacitors
Post by: MarcusFunt on June 09, 2022, 08:44:49 PM
I am thinking of making a dual mot sgtc, and therefore i need capacitors.

My coil will be running at around 320khz.
I ran some calculations on the max voltage of the mots and doubled the value to 12000V.
I am aiming for slower arcs of around 150 bms. So my max capacitance becomes 95nf.


All decent tesla coil capacitors i found are REALLY expensive, so my idea was:

https://www.amazon.de/-/en/gp/product/B08NVLCGBS/ref=ox_sc_act_title_2?smid=A3TAA9JQ6SVWB5&psc=1 (induction cooker capacitors) induction cookers KILL capacitors, and so do tesla coils. (i would get a capacitor rated 12kv 33nf)

The safe route: https://dk.rs-online.com/web/p/products/7441461/ (Fancy CDE cap)(12kv but only 12nf)

Experimental: https://dk.rs-online.com/web/p/filmkondensatorer/1051060 (REALLY CHEAP) i could build a large mmc that is WAY in excess of the others

Also: https://dk.rs-online.com/web/p/filmkondensatorer/7123905(12kv 15nf)


Let me know if you know any decent capacitor that fit in my budget of around 20£(Has to be from within the eu because of import taxes)

I heard somewhere that you look at the dc voltage of the tesla coil. is that true?
Title: Re: Cheaper Tesla coil capacitors
Post by: davekni on June 10, 2022, 05:16:48 AM
Quote
I am thinking of making a dual mot sgtc, and therefore i need capacitors.
MOTs are particularly dangerous:  Enough voltage to arc through clothing with current capability several times lethal threshold.  NSTs are much safer, generally limited to 60mA or less, so much less likely to be lethal if you make a single mistake.  I have a dual-MOT SRSGTC, but rarely set it up because I'm too afraid of making a mistake.

Quote
I am aiming for slower arcs of around 150 bms. So my max capacitance becomes 95nf.
That is rather low power for two MOTs.  6kV on 95nF is 1.71J, or 257 watts at 150 BPS.  Two MOTs are capable of 2kW (or more for intermittent use).  My coil runs at 120 BPS with ~700nF and resonant charging to 9kV.  Of course, if you have two MOTs in hand, that's cheaper than purchasing anything safer for 257 watts.

Your first link to 0.33uF 1200Vdc induction cooker capacitors looks like roughly the same parts I use in my DRSSTC and have ran some destructive testing to 2100Vpk (4200Vpp) at 80kHz 1% duty cycle.  Your third link labeled "VERY CHEAP" would require 4 times as many parts for the same final Vdc rating and capacitance, since they are only 630Vdc rated.  Would likely work fine, though I have no experience with these.

Have fun, and experiment safely!
Title: Re: Cheaper Tesla coil capacitors
Post by: MarcusFunt on June 10, 2022, 09:41:02 AM
Quote
I am thinking of making a dual mot sgtc, and therefore i need capacitors.
MOTs are particularly dangerous:  Enough voltage to arc through clothing with current capability several times lethal threshold.  NSTs are much safer, generally limited to 60mA or less, so much less likely to be lethal if you make a single mistake.  I have a dual-MOT SRSGTC, but rarely set it up because I'm too afraid of making a mistake.

Quote
I am aiming for slower arcs of around 150 bms. So my max capacitance becomes 95nf.
That is rather low power for two MOTs.  6kV on 95nF is 1.71J, or 257 watts at 150 BPS.  Two MOTs are capable of 2kW (or more for intermittent use).  My coil runs at 120 BPS with ~700nF and resonant charging to 9kV.  Of course, if you have two MOTs in hand, that's cheaper than purchasing anything safer for 257 watts.

Your first link to 0.33uF 1200Vdc induction cooker capacitors looks like roughly the same parts I use in my DRSSTC and have ran some destructive testing to 2100Vpk (4200Vpp) at 80kHz 1% duty cycle.  Your third link labeled "VERY CHEAP" would require 4 times as many parts for the same final Vdc rating and capacitance, since they are only 630Vdc rated.  Would likely work fine, though I have no experience with these.

Have fun, and experiment safely!

I only felt safe enough to build it because a am doing it with two friends.

I think of using the cheap caps because I can distribute the load across 80( or so) caps.
Title: Re: Cheaper Tesla coil capacitors
Post by: Mads Barnkob on July 05, 2022, 08:23:07 PM
Let me know if you know any decent capacitor that fit in my budget of around 20£(Has to be from within the eu because of import taxes)

I heard somewhere that you look at the dc voltage of the tesla coil. is that true?

20£ is a small budget for a MMC and you will properly have to find a good bargain on ebay to make it for that small amount. Another route is to make it smaller than needed, going low on voltage and current rating will however shorten the life time of the MMC considerably.

Using DC voltage ratings instead of AC voltage ratings is a "trade off". You can design it cheaper and the short pulse operation of a Tesla coil can to some degree justify this, as the heat losses have more time to dissipate. But there is no doubt, that designing with DC voltage ratings, you again shorten the life time of the MMC.

The capacitors you look at are maybe designed for 5-10 years full spec operation, so if you can get 100 hours of full abuse on them, well 100 hours run time of a Tesla coil is a really long time and most amateur built coils will rarely see 100 hours in their life time :)
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