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Re: troubleshooting tesla coil, continued (fwd)



---------- Forwarded message ----------
Date: Thu, 20 Sep 2007 21:42:27 -0400
From: Marko Ruban <Marko@xxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: troubleshooting tesla coil, continued (fwd)

I've upgraded my design with 1/4" copper tubing primary and a toroid 
topload (3"x12") made from aluminum air duct.  I can now pull about 2" 
sparks off the topload.  I'll work on new spark gap next, current 
(temporary) one seems to stop working properly after a minute or so.

I also decided to give polyethylene a try for capacitor, but it didn't 
work.  Burned a hole right away (3 layers of 4mil), think I'll need more 
or better quality PE.  When I was putting the Mylar cap back together, I 
noticed soot on the margins from arcing.  Looks like I'll need more than 
an inch margin for my next cap.


Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Sun, 16 Sep 2007 22:06:17 -0700
> From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: troubleshooting tesla coil, continued (fwd)
>
> Hi Marko,
>
> Yes, now that you identified 4 pairs of copper sheets and the 1" 
> clearance using Mylar, 7nF is just about right! And yes, Mylar has a 
> dielectric constant value of 3.2 and breakdown at 7,500 V/mil. But there 
> is a problem here that you have identified in this particular post. The 
> problem is Mylar (aren't you sick of people bringing up bad news?). 
> Sorry, it's just that it sticks out like a sore thumb. You accounted for 
> the volts/mil and dielectric constant, but you didn't account for how 
> dielectrics behave with alternating currents such as RF (Tesla Coil 
> frequencies).
>
> Dielectrics have what is termed a "dissipation factor" identifying 
> losses (always in the form of heat) when the dielectric is operated in 
> an alternating current. The higher the frequency, the more losses that 
> are generated. Some dielectrics have a high dissipation factor and 
> others a low dissipation factor. What is desired for high frequency is a 
> low dissipation factor. Mylar unfortunately has a high dissipation 
> factor and makes them unsuitable for Tesla Coil tank caps. It's always 
> best to go with a known dielectric which has a low dissipation factor 
> such as polypropylene, polystyrene, polyethylene, etc.. just as the cap 
> manufacturers themselves go with. Even with our MMC's, we account for 
> the dissipation factor (tangent loss) as specified by the cap manufacturers.
>
> There are a lot of things you'll be working on, but I just wanted to 
> point out that Mylar is not a great choice for tank caps (even if the 
> volts/mil is high and dielectric is in the medium range). Your cap being 
> a plate cap would likely do better than a rolled mylar cap simply due to 
> the thermal dissipation of the plates, so you may want to give it a go 
> anyway and see how it reacts. Of course, if breakdown occurs (even a pin 
> hole punch through), that will end the caps use. I personally used oil 
> in my plate cap, mainly for the self healing ability.
>
> If you are ever curious as to what constitutes a good dielectric or not, 
> copper vs. steel in the primary, etc.., just do a search at 
> www.pupman.com of the list archives. Cap dielectrics are one of those 
> items where there is a lot of information available.
>
> Take care,
> Bart
>
>
> Tesla list wrote:
>
>   
>> ---------- Forwarded message ----------
>> Date: Sun, 16 Sep 2007 16:43:56 -0400
>> From: Marko Ruban <Marko@xxxxxxxx>
>> To: Tesla list <tesla@xxxxxxxxxx>
>> Subject: Re: troubleshooting tesla coil, continued (fwd)
>>
>> As I mentioned a bit earlier, I made an error in units, my cap was an
>> estimated 7nF (not pF).  I finally got the multimeter, and that value was
>> confirmed since initial post.
>>
>> Capacitor is constructed with 4 pairs of 10mil copper sheets (6" x 8")
>> separated by 10mil Mylar sheets with 1" margin on the sides.  I picked
>> Mylar because it has a high dielectric value and high breakdown voltage.
>> 10mil sheet is rated at 70kV.
>>
>>
>> Tesla list wrote:
>>
>> ---------- Forwarded message ----------
>> Date: Fri, 14 Sep 2007 18:17:24 -0700
>> From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
>> To: Tesla list <tesla@xxxxxxxxxx>
>> Subject: Re: troubleshooting tesla coil, continued (fwd)
>>
>> Hi Marko,
>>
>> Your inserting 0.007uF into Javatc so the frequency's do match as they 
>> should at that capacitance. However, your "saying" 7pF and that is way 
>> off from both Javatc and what the plate cap really is. Judging by your 
>> plate cap configuration, it is about 0.7nF (not 7nF) if your using glass 
>> as an insulator. If air is your insulator, then it's down to 0.1nF.
>>
>> So you are way out of tune. The fix is the cap. For a plate cap using 
>> glass with about 0.2" spacing, you would need a plate surface area of 
>> 900 square inches (30" x 30").
>>
>>
>> If your going to use a plate cap, you might want to consider Castor oil 
>> which has a high dielectric of 5. About 15 plates (8" x 11") with a 0.2" 
>> spacing between plates would yield near 7nF (which is 0.007uF). Plate 
>> caps are fun to play with, but when you start building to a capacitance 
>> such as this, it can get bulky and messy. I of course recommend an MMC 
>> for performance.
>>
>> Take care,
>> Bart
>>
>> Tesla list wrote:
>>
>>  
>>
>> ---------- Forwarded message ----------
>> Date: Thu, 13 Sep 2007 19:19:45 -0400
>> From: Marko Ruban <Marko@xxxxxxxx>
>> To: Tesla list <tesla@xxxxxxxxxx>
>> Subject: troubleshooting tesla coil, continued
>>
>> Hello guys, I'm back from a long summer vacation.  As per previous 
>> suggestions I have acquired an old NST transformer (12kV, 60Hz, 60mA 
>> output).  And the spark gap does run now with capacitor/primary in 
>> circuit.  However, there's no visible effect on the topload, still.  
>> Could it be that badly un-tuned?
>>
>> Please see the photo of my simple setup and make any suggestions...
>> http://marko.dppl.com/TC_setup.jpg
>>
>> The flat rectangular thing in front, is the capacitor, which should be 
>> about 7pF.  Can't make a more specific measurement since my new 
>> multimeter is still in the mail.  Spark gap is on top of the NST, and 
>> consists of two screws facing each other (about 0.2" adjustable 
>> spacing).  Wire running in the back to the right is the ground.
>>
>> Also, here's a dump of JavaTC for my coil...
>> http://marko.dppl.com/coil.txt
>>
>> Thanks in advance for any assistance.
>>
>>
>>