madlabs.info

5B) Capacitors

Capacitors come in many forms. While there are many types of capacitors, only a few are really suited for TC work. The most common TC capacitors are: salt water caps, plate caps, rolled caps, doorknob caps, commercial pulse caps and MMCs (multiple miniature capacitors).

A) Saltwater (SW) caps. I listed these first because they are my personal favorite for beginners. Cheap and easy to make, and very durable and forgiving of mistakes. They are heavy and potentially messy however. A really good looking design, although I haven't made one, is the Geek Group's Bucket Cap .

People also complain about SW caps being "lossy", and it is true. In my personal experience it isn't that huge a difference though. I tested a coil of mine with SW caps and closely rated doorknob caps. I found that I did get about 10% more spark length, and the streamers gained in intensity. Doorknob caps themselves are known to be somewhat lossy, so I can't say what difference is to be found in comparison to commercial pulse caps. In a large system, this would be a considerable loss, but in a first coil not such a big deal. Very satisfactory results can be achieved using SW caps at low cost, and they are unlikely to go "poof" just as you are having fun. Click HERE for more information and pictures on SW caps.

B) Plate caps. These are constructed with multiple layers of conductors and insulators (usually called a "dielectric"). The conductors are typically tin foil or thin sheet metal stock, and the dielectric is often glass or some type of plastic. Thickness of the conductors and dielectric are important in determining the capacitance, and the dielectric qualities of the insulating layers is critical. Plate capacitors often must be submersed in oil to prevent flashover. My personal experiences with plate caps haven't been very good, and disposal of blown ones is a pain. I don't have any pictures of my past plate caps.

C) Rolled caps. These are generally constructed with strips of foil or very thin sheet metal, and strips of polyethylene plastic film or sheet. These are often immersed in oil like plate caps. I have never made one, but it seems as if they would be a pain to make. They have the same disposal issues as plate caps.

D) Doorknob caps. These are usually made out of ceramic, but some are plastic. These will work in TC service, but have a limited life and are known to be somewhat lossy. My earliest TC used doorknob caps, and one by one they blew. Overall not recommended, but if you see some at a good price, pick them up as they are great for other HV projects, like Marx banks. They are quite expensive new. Click HERE for pics.

E) Commercial pulse caps. These are caps that are made for HV pulse duty and are usually very low loss. However, not all are created alike, so investigate carefully. Look for the rated life span of the cap, usually in number of pulses it can withstand, and remember that in TC application it is going to see a lot of pulses. I have used a Maxwell commercial pulse cap a little and it has not failed, although it has gotten warm. It was expensive, almost US $100.00 used. And since I don't know its origin, I cant really say how long it will last. Many people swear by them though. To me, it seems that when they blow you are left crying. Click HERE for pics.

F) MMCs. These seem to be the way of the future. They are constructed using a number of smaller capacitors connected together to form a large capacitor of adequate voltage and capacitance for TC service. While many types of caps can be used for MMCs, the best are polypropylene, as they tend to be "self healing". This means that when over volted and punctured by internal arcing, the dielectric will seal itself, and the cap will retain it's rating. The word is they can be punctured like Swiss cheese and still survive. Another benefit is they are not likely to fail catastrophically and blow every cap in the MMC. This means they can be repaired cheaply, a big, big plus. I built mine using the Geek Group's MMC cap, made by Cornell Dublier. There is a list of know good and bad caps HERE.Click HERE for pics of my MMC and more information. While I believe in this cap system, I just built mine so I can't really say yet.

There are at least two makers of glass capacitors, Glassmike and Plastic Capacitors. These are doubtless fine for their intended use, but DO NOT USE THEM in TC service. I have had one crack and break apart when I picked it up, and stories abound of them exploding like a glass bomb. Click HERE for pictures of these dangerous little suckers!

Capacitors can retain their charges for really long periods of time, even years. They can also acquire charges from a phenomenon called "dielectric memory" or from their surroundings. This means that all caps should be "caged" when not in use by connecting a wire across the cap. It also means to ALWAYS ASSUME A CAP IS HOT. Capacitors in TC service should be equipped with bleeder resistors. These are connected across the cap, and will discharge the cap within a period of time determined by the value of the resistor.Don't count on bleeder resistors though, as they can fail like anything else. I have a "chicken stick" that I use to ensure discharge of the caps after every run.

Even well suited capacitors die in TC service, and many will only last fraction of a second. Take care in choosing and constructing your caps. It really bites when the caps blow just as you are having fun. I hate that! I really do recommend SW caps at first, then progress to an MMC.

Back to INDEX.