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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.
>>
>>
>>