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Re: troubleshooting tesla coil, continued (fwd)
---------- Forwarded message ----------
Date: Fri, 21 Sep 2007 11:54:49 -0400
From: Marko Ruban <Marko@xxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: troubleshooting tesla coil, continued (fwd)
Will this work? Pretty cheap.
http://acetal-rod-sheet.com/polypropylene-copolymer-sheet.htm
A couple of 48x48 sheets cut to size (0.063") thickness should be enough
to get needed cap value (13nF recommended by javaTC).
Can grocery store aluminum foil be used as cap plates? Last time I used
10mil copper sheets, and that looks a bit heavy.
Tesla list wrote:
---------- Forwarded message ----------
Date: Thu, 20 Sep 2007 19:29:31 -0700
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: troubleshooting tesla coil, continued (fwd)
Hi Marko,
Try some PP (Polypropylene). I'm not sure if places like Home Depot or
ACE Hardware sells it in rolls, but certainly worth a shot (caster oil
is always an option also). The soot you found is certainly a problem.
The gap as you are finding is important. If gap is heating up, you'll
find when you start running the sparks will be their best, but as a
little time goes by they will diminish as the heat in the gap lowers the
arc voltage (which is why they diminish). Glad to see you working on this.
Take care,
Bart
Tesla list wrote:
---------- 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.