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Questions regarding variacs & protection

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vasil

Sat Mar 25 2006, 06:14PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

It is still very close to 6.6 nF, so it will be a bit overvolted, but if you don't open too much the gap (in parallel with the transformer output), i think it will be ok.

infinitum

Sat Mar 25 2006, 07:57PM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

Ok, some more questions:

DeepFriedNeon says to use heavy wires to connect the spark gap, RF ground, MMC, etc. What rating should the wire I be using be?

I know that I should ground the high voltage side of stuff to the RF ground, but do I ground the 120V input to the NST through my house's ground?

Where can I get little metal balls suitable for use in a safety gap?

Thank you so much. I feel like I'm actually learning something

edit:

Almost forgot, could I use a light dimmer in place of a variac (I remember seeing this done, I just can't remember the website)? I would like to buy a Variac but I don't have fifty to a hundred more dollars to spend.

vasil

Sat Mar 25 2006, 08:20PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Use what you want, but thick wire (eg: wire used for the mains house frame). At least 4 mm^2 section. Keep all the wires insulated one from the others.

The last discussion on the tesla1 list suggest that it is the best to ground together the metalic case of the NST, the middle electrode of the safety gap and the bottom end of the secondary, to the same ground, to protect the mains frame from any RF from the coil. On 120 vac side use a filter, but do not connect the middle tap to the mains ground (you dont want RF there). Please use a strike ring around your primary, do not allow strikes to the primary turns. This can kill your NST instantly.

You dont need balls for the safety gap, only some screws with round head, or just common screws. At least some thick wires which go from each terminal to the case of the NST.

Dimmers and NSTs dont work together. Use a variac, or if not, run different setups of your spark gap for different voltages to charge your primary cap (between 0 and the maximum gap lenght)

infinitum

Sat Mar 25 2006, 09:12PM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

Where can I buy a filter for the 120V side? I haven't been able to find any cheap ones.

So any size wire should work for the tesla, it should just be thick and sufficiently insulated?

GimpyJoe

Sun Mar 26 2006, 01:11AM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

I got my filters for about $2 apiece at

but I don't know if they still have them. They are rated 2x20a, 120-240vac. As for wire, the best stuff is 6ga welding cable. It's super flexible and has thick insulation since it has to resist abrasion. However, if you can't get this, you can get away with regular 10-14awg house wire. I recommend running your wire through flexible plastic tubing for more insulation.

infinitum

Sun Mar 26 2006, 02:04AM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

Ok, since I can't get caps from RS (no international shipping), could someone tell me if these are any good?

According to my calcs I should need 1 row of 6 caps...is this correct?

17,000V peak / 3,000V = 6 caps (for 18,000V total)
.047 uF / 6 caps = .00783 uF (close enough to .0066)

Thanks for the replies so far, they have been very helpful.

edit:

Thank you for the link Funky, I found the filters.

infinitum

Sun Mar 26 2006, 08:49PM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

Ok, the link expired.

Digikey part#: 338-1185-ND
Family: Polypropylene Film
Vendor: Cornell Dubilier Electronics (CDE)
Capacitance: 0.047ÂµF
Voltage-Rated: 3000V
Tolerance: Â±10%
Series: 940C
Features: High Frequency (HF)
Maximum Temperature: 105Â°C
Lead Free Status: Lead Free
RoHS Status: RoHS Compliant
Other Names: 338-1185 or 940C30S47KF

Would these be suitable for use in my tesla?

Jrz126

Sun Mar 26 2006, 09:52PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

I'm still working on my first TC, so Im not an expert or anything. While I was researching the MMCs to use, I came across this list:

and it looks like the 940c series falls into the 'bad' catagory.
Where abouts are you located?

infinitum

Mon Mar 27 2006, 03:59AM

Registered Member #307 Joined: Sun Mar 12 2006, 06:41AM
Location:
Posts: 37

I'm in Towanda (Bradford county).

I'm going to try emailing CDE directly to see if they can sell me some. I can't find their 942C caps ANWHERE. :(

...

Mon Mar 27 2006, 04:15AM

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

If you really can't find a source I have 10 slightly used 942c20p15k's (2kv/.15ufd) that I could let go for $4/cap ($3 that I paid and what it cost to ship them to me) + shipping.

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