Date: Mon, 28 Aug 1995 12:39:29 -0600 From: Richard Quick Subject: CAPACITOR INFO I just posted several times on the TCBOR rolled plastic capacitor. Since the detail of this particular unit was pretty well covered, I will focus on other homemade types; the flat stacked plate type capacitor, a little on the salt water cap, and a little on capacitor theory as it applies to Tesla coils. I have seen several types of homemade stacked plate capacitors. The two types differ as to the orientation of the plate stacks. Some are stacked vertically, others are stacked horizontally. Before I go into construction details I should cover some of the advantages of flat stacked plate caps for use in Tesla coils and other high voltage applications. Flat plate caps have very little inductance. Rolled caps contain two or more plates which are tightly rolled up. Rolled plates exhibit some properties of coils, they contain a certain degree of self-inductance. This limits the size of the rolled cap in Tesla applications. As plates grow in size, the self-inductance grows, and the caps exhibit self-resonance that will interfere destructively with the oscillation of the Tesla tank circuit. The rolled cap that I posted about previously is self resonate at about 7 megaHertz. Flat plate caps are better adapted for pulse applications. Rolled caps have to discharge a long plate. The further away the open end of the plate is from the high current terminal, the longer it takes for the cap to discharge. In essence this distance is also an extension of the tank circuit wiring, as the plate gets longer performance decreases. As the rolled cap gets larger, efficiency of pulsing drops off. Flat plate caps can be constructed to handle higher voltages. Rolled caps have efficiency limits in individual units as to the breakdown voltage. A single dielectric is used per plate. If dielectrics are made thicker, efficiency drops off, if made thinner efficiency increases, but they break down. Using standard materials, the rolled cap I posted about is at the edge of this design limit as well. Flat plate caps can be built for larger capacitance. The rolled cap, because of the design constraints listed above, won't give you much additional capacitance without increases in losses, problems with self-resonance, and lowering of the capacitor Q. The rolled cap that I posted is a good unit. I have built nearly 20 of these caps, and I use them a lot. But do not look to expand much on this design. It has passed through several improvements and I really think it is pushing the design limits in all of the important areas. Next we need to look at the flat plate cap, as there is much to be done yet, but first look at the dielectric. The best dielectric for homemade Tesla capacitors is low density polyethylene plastic. Whether you build rolled, stacked plate, or salt water caps you should look hard at this plastic before settling on anything else. It has an extraordinarily low RF dissipation factor for the cost. The actual "in use" dielectric constant on homemade caps using this plastic is right around 2. This is a little lower than the book value, but homemade applications of this dielectric rarely have the close plate bonding that are achieved commercially with clean room vacuum presses. This dielectric melts at 100 deg. C. But because of the very low dissipation factor the plastic is subject to very little in- ductive heating. There is little loss, therefore little heating. When using this plastic however, it is imperative to cover in mineral oil to distribute any heat that is formed, suppress corona and displace air. Plastic caps not covered in oil are almost guaranteed to fail in seconds. Plates, dielectric, and oil MUST BE CLEAN!... BTW The cheapest and most common plate material is aluminum. In the rolled cap, aluminum flashing is available precut in a perfect plate width, and there are other widths available. Flat plate caps can use flashing, but it is frequently more cost effective to use foil. Now that we have established a few basics, lets talk plate cap design. The first type of flat stacked plate requires the cap be pumped down to a pretty hard vacuum to remove air. This is the horizontal stacked plate capacitor. Typically these are built in a Tupperware type storage box. Plastic, plate, plastic, plate etc. are stacked one atop the other to build up the value. The breakdown voltage is directly related to the dielectric thick- ness used. 60 mil poly sheet is recommended and will have a breakdown voltage in the Tesla tank circuit between 11-15 kv rms input voltage in pulse discharge applications. This of course depends on the quality of material, and the cleanliness of the construction. Once the box is filled, and all parallel plate connections are made, high current busses are brought through the lid of the container and sealed airtight with hot glue. Then the lid is snapped on, and it too is sealed with a bead of hot glue around the edges. The next part is important: A single hole is made in the lid for the vacuum connection. A fitting is hot glued into the hole and a hose is attached to the vacuum pump. The cap is pumped down, then the hose is clamped off and disconnected with- out allowing air back into the cap. Submerge the hose in a bucket of clean mineral oil and release the clamp. This allows the oil to backfill the capacitor, and displaces the air that was removed. Once backfilled to normal pressure, I pump them down a second time, and repeat the procedure to make sure that all trapped air between the plates is removed. Air bubbles will form corona hot spots that will cause dielectric failure. The vertical stacked plate capacitor is much like the cap I just covered. But the vertical cap does not require pumpdown. A tank is used to hold the vertically stacked plates and dielectrics. The unit I examined was built in a glass fish tank that employed no metal in construction. Stiff foam padding was laid in the bottom of the tank, and wedged in around the sides of the vertical capacitor stack to cushion it and wedge it in place. The foam padding also reduced the mineral oil required to cover the stack. If foam padding is used it is important that "sponge" type padding NOT be used. This padding can release latent air bubbles into the capacitor plates. Use a quality "sealed-cell" padding. The reason these caps do not require pumpdown is that eventually the oil will displace the air trapped in the unit. A break in period of low voltage operation assists the removal of trapped air, as the pulsing of the cap vibrates the plates and agitates the air bubbles. The disadvantage of the unit I examined was the glass fish tank. I have seen plastic waste cans that could be cut down for use as a tank in this construction. Higher Qs, higher voltage, and additional capacitance in stacked plate capacitors can be easily obtained. The trick is to use thinner dielectric. The dielectric strength of polyethylene is given as 1000 volts per mil, but this is not the case in Tesla coils. The standard breakdown voltages of a dielectric are calculated using a static DC voltage. When you run AC across the dielectric, the breakdown voltage must be divided by two. Then you must figure that the peak voltage from a AC sine wave is higher than the rms voltage most people go by. You meter won't see it, but your dielectric will. Then you have resonate rise in the Tesla tank circuit. To give you an idea of resonate rise in a tank, think about the tidal forces that can be created with timed pushes in a bathtub. It don't take much energy to push water over the side. The same principal operates in the tank circuit in a coil, especially with a synchronous gap system. The current pulsing back and forth from capacitor plate to capacitor plate causes a voltage rise that appears on the dielectric in the capacitors. The standard 60 mil poly is supposed to hold up to 60,000 volts per the book. I have blown holes through 60 mil poly with a 12 kv neon sign xfrmr in a Tesla tank circuit and my gap wide open. My pinky finger fit inside the hole. One of the neatest homemade stack plate caps I have seen was built by Bill Richards of T.C.B.O.R., the cost was pretty low, the materials came from his laundry room, the grocery store, and the drugstore. The only thing required was 56 hours of time in arranging the plates according to Bill. But he did end up with .03 uf 15 kv pulse capacitor in a five gallon bucket. It was quite a performer on his coil at 3600 watts! He shopped around for one gallon ziplock freezer bags with a 3 mil thickness. With a sharp scissors he cut the ziplocks off of the tops of the bags. Then he cut aluminum foil squares that fit inside the bag leaving a 1/2" of space around all four sides of the plate. So the plate had dielectric borders 1/2" on all sides. When two bags were stacked on top of one another, there were two layers of dielectric, for a total of 6 mils. Being practical, Bill figured correctly that the stacked bags would hold up to at least 1000 volts rms input in the Tesla tank. He built up stacks that had a value of about .45 uf each, with each stack rated at 1000 volts. Then he wired stacks in series. By squeezing fifteen stacks vertically into a bucket, and covering the whole thing in about three gallons of mineral oil, he got the required capacitance at the required voltage. Since the electrical forces are so well distributed among hundreds of dielectrics, he had plenty of breakdown safety margin. He gave the unit a couple of days to rest after construction, topping it up with oil as required, and gave her the works at 15 kv on a big coil. The heavy buss wiring never even got warm, and even though it bubbled out enough air to displace a few more pints of oil, it did not break down. It turns out that this is a homemade version of commercial pulse discharging capacitors. Stacked capacitor sections of very high value are placed in series until the proper voltage requirement is met. The cap has a very high Q because all of the plates are very close together, with a minimum of connections and bussing required. They deliver a very sharp pulse discharge. Bill's cap was pretty cramped in the bucket. Because of the square shape of the bags, a rectangular tank would have made things easier to fit and wire. But he ran his buss bars through the side of the bucket (sealed with hot glue) and by snapping on the lid, he could pick it up by the handle and move it around with ease. The novice coiler should think about the capacitor requirements and experiment some before beginning large scale homemade caps. Shop for materials; frequently a wholesaler can be found where bulk products (like mineral oil in 5 gallon pails) can be purchased for a fraction of the retail cost. But just because you don't have some big bang pulse caps on line does not mean that you should wait to begin firing a small coil. Nearly every beginner gets hir feet wet in salt water capacitors. Tesla used salt water tanks in Colorado Springs. A tribute to the genius of the man was his ability to develop his huge peak powers using low Q saltwater/glass caps. I do not recommend glass as a dielectric for coiling work. The dielectric constant is much better than plastic, but the RF dissipation factor is so great that they can rupture from dielectric heating (even in salt water the trapped water under the bottles does not circulate) and they always give a spindly, violet colored spark. Polyethylene again is the material of choice, and bottles and buckets can be assembled in a couple of hours that will fire small stuff. I mentioned he before that I have a friend who is firing 5 kVA coils, and still using banks of salt water caps to keep his investment down. As with any homemade capacitor, the salt water must be covered in oil to suppress surface corona. But the quality of oil need not be high, and the capacitors need not be exceptionally clean. A saturated solution of rock salt is all that is needed for the plates. I think I have accomplished what I intended to say on this subject. As always, I am happy to respond on any unclear areas, the need for additional information, or to note corrections. Richard Quick .. If all else fails... Throw another megavolt across it!