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Re: troubleshooting tesla coil, continued (fwd)
---------- Forwarded message ----------
Date: Fri, 21 Sep 2007 19:28:06 -0700
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: troubleshooting tesla coil, continued (fwd)
Hi Marko,
I don't see anything wrong with it. The dielectric constant is about 2.6
and volts/mil is around 500. Polyethylene (either hdpe or ldpe) has a
dielectric constant about 2.26, but the volts can vary considerably from
450 v/mil to 1200 v/mil. PP is actually a good choice and often used by
cap manufacturers for pulse applications. Polystyrene is also a good
choice. I've got some professional pulse caps which use polystyrene and
they've held up excellently.
Take note to what DC Cox has stated: Throw a safety margin into your
design. That's important especially for caps of this nature. Like he
says, MMC's are certainly easier, but sometimes it's fun to build your
own (I understand that fully).
Even with polymer insulation's, it's often a good idea to also use oil
to remove air from the gap. It does require some means of containment,
and that is usually the mechanical difficulty, but it does go a long way
into the longevity of the cap.
Take care,
Bart
Tesla list wrote:
>---------- 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.
>
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