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Which cap is the best?

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LAN_403

Tesla Fan

Sun Oct 24 2010, 07:36PM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

Something interesting, Panasonic rates the ECW-H 1250V DC at 1000V AC , but Wima rates the FKP1 1250V DC at only 600V AC ...

Cornell Dublier 942C20P15K 2000V DC => 500V AC !

What should be the right rating used for calculating your MMC bank?
eg for a 10KV NST, will a series of 16 1250VDC caps be ok?
I know that the tesla coil current is AC, but what would be a resonable safety margin vs cap count (= price)?
Most of MMCs i seen use NST voltage ~ x2 but with DC voltage rating (eg for 10KV NST 10 x 2KV DC caps).

Herr Zapp

Sun Oct 24 2010, 08:27PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Tesla Fan -

Look carefully at the Panasonic datasheet showing the relationship between the cap's DC rating, and the peak AC voltage.

For the 1250 VDC ECW-H series, the maximum AC RMS voltage is 354 (or ~500V pk-pk). The 1000 V rating is for the special case of a CRT horizontal oscillator circuit.

It's common practive to use the cap's DC voltage rating as the "baseline" voltage, before multiplying by your "safety factor" multiplier (usually 1.5X - 2X your PEAK AC voltage).

If you are going to power your TC with a 10kV NST, the peak output voltage will be ~14kV, so design your MMC with this in mind.

In truth, there are a number of different types of specialized capacitors that could be used in a TC tank circuit, but the C-D caps are bulletproof and readily available as "new" parts. If you are a scrounger, and ready to take some risk with "used" parts, any of these caps could be made to work in a TC, if you can find the correct combination of capacitance, voltage, and current ratings:

1. Mica transmitting capacitors from high-power commercial or military radio transmitters;
2. Pulse capacitors from medical or industrial laser systems (Maxwell, General Atomics, NWL, High Voltage Components, etc);
3. Ceramic or mica capacitors from industrial indiction heating systems.

For your first TC, I'd strongly recommend an MMC made with the 942C caps.

Herr Zapp

Tesla Fan

Sun Oct 24 2010, 08:53PM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

Hmmmm...

Herr Zapp, if AC voltage is 354Vrms , then Vp-p (pk-pk or Vpp) is 1000V, Vp is 500V right?

eg europe mains is typ. 230Vrms which is 325Vp and 650Vp-p, right?

I assume "Peak Voltage" is Vp and "Peak-to-Peak Voltage" is Vp-p

So i suppose that a "10KV NST" is a 10KVrms and not peak. So for (safety factor=0 => will go boom) 10KVrms you would need 10000/354 = ~28 caps in series?

Herr Zapp

Mon Oct 25 2010, 04:13AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Tesla Fan -

Yes, your "10kV" NST output is RMS volts.

Terry Fritz performed some dielectric stress testing of the C-D 942C series caps a long time ago, running them at 1400 VAC, 60HZ (well above their 500 volt AC rating). The caps failed after about 100 hours due to dielectric breakdown.

See attachment for test data.

Herr Zapp
]cd942c20p15k-mmc-capacitor-lifetest.pdf[/file]

Tesla Fan

Mon Oct 25 2010, 08:55AM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

Thanks for the info Herr Zapp, ~3 x rated voltage is impressive!

Tesla Fan

Mon Oct 25 2010, 02:02PM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

For anyone who might be interested, BÃ¼rklin will ship to Greece (so i suppose to most European countries), they prefer "cash on delivery" method of payment and the total cost will be 22 EUR + value of goods + tax (for my case at least).

Thanks monokel

Tesla Fan

Mon Oct 25 2010, 02:23PM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

monokel,

I see that in your 450W SGTC you use a PIO cap

, it looks like a K75? How come, did you have that cap already? How did that work out? 70cm sparks from 450W seems more than fine to me!

monokel

Mon Oct 25 2010, 03:23PM

Registered Member #2981 Joined: Thu Jul 08 2010, 01:47PM
Location: Germany
Posts: 35

Tesla Fan wrote ...

For anyone who might be interested, BÃ¼rklin will ship to Greece (so i suppose to most European countries), they prefer "cash on delivery" method of payment and the total cost will be 22 EUR + value of goods + tax (for my case at least).

Are you sure that they deliver also to private persons ? On their website they write that they deliver only to concerns (On the other hand, I ordered more than 500 of these caps via an university at BÃ¼rklin

. And I also heard that they make exceptions sometimes).
I'm not sure, but I believe that the German "Mehrwertsteuer" (MwSt, some tax) of 19 % will also be added to the price when shipping to Greece. And maybe the Greek country will add some more tax.

Tesla Fan wrote ...
Thanks monokel

You are wellcome

Tesla Fan wrote ...

I see that in your 450W SGTC you use a PIO cap

, it looks like a K75?

Yes, it's a K75 !

Tesla Fan wrote ...
How come, did you have that cap already?

I bought it at ebay some time ago.

Tesla Fan wrote ...
How did that work out? 70cm sparks from 450W seems more than fine to me!

Thanks

Pinky's Brain

Mon Oct 25 2010, 05:52PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Sales tax only has to be paid in the country of origin, no extra excise taxes within the European Economic Area ... the EU isn't all bad.

Tesla Fan

Mon Oct 25 2010, 10:56PM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

monokel,

I 'm quite sure, send them an email sunday (**link**), stated that i am a private individual and do not represent a company, and asked if they would sell and send me caps (i stated that i would buy 50 or more, i doubt they would bother to send me eg 2 caps costing 5euros). Monday they replied. i quote:

"We also deliver to greece."
"We would like to handle this with C.O.D. (cash on delivery)."
"The charge herefor is 22 EUR + value of goods + tax, because you donÂ´t have a vat-number."

As a matter of fact, the person who replied was very kind and helpful, i felt bad replying him that i was not ready to buy the caps yet...

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