Printed Circuit Board Capacitor

2n2F PCB capacitor

Warning

When I first suggested the idea of making capacitors from fibreglass printed circuit board to other coilers they were a bit doubtful. I must be honest though, the idea was not an original one, I first came across it in a copy of the Scientific American in the late 60s or early 70s. In the article, the author was using exactly the same type of device for a nitrogen laser. the kind of duty was similar to that needed for Tesla coils. In other words a high voltage high discharge pulse capacitor. It seemed logical to try it out. To my mind there were a number of possible advantages over the various alternatives. First it would be a lot less work than making a rolled cap, less messy and heavy than a salt water cap, and as I had a pile of pcb board available free of charge, a lot cheaper than an MMC. Other advantages over some types of homemade devices were that it would be readily reproducible and predictable.

The Material

Standard 1/16" double sided fibreglass pcb is made from epoxy resin impregnated glass cloth. Copper foil is hot rolled onto the board using an adhesive under considerable pressure. This process ensures close bonding and excludes air.

Offcuts are commonly available on the surplus market, the largest I have come across being about 2' square. A good hunting ground is amateur radio rallies or hamfests. Often they have been discarded at the end of runs, or more commonly because of minor corrosion problems, although this is a problem for manufacturers it doesn't matter for our purposes because they can be easily cleaned up using a mild abrasive.

The biggest problem with fibreglass pcb is that it rapidly blunts tools!

Electrical Properties

Theory

In its simplest form a capacitor consists of two conductive plates separated by an insulator known as the dielectric. various types of capacitor are used in electrical and electronic circuits for a variety of purposes. Generally speaking in Tesla coil circuits we are primarily concerned with devices that store a lot of energy, at high voltage (in the kV range), and are able to deliver it as rapidly as possible. These factors mean that very large currents flow for very short timess This all means that such capacitors are very highly stressed, and that the majority of types in general use are of no value in this special case.

Imagine two metal plates, parallel to each other, and separated by an insulator. Now connect them to a dc supply such as a battery. Very rapidly electrons flow from the negative pole of the supply to the plate to which it is connected, and at the same time an equal number of electrons move from the other plate to the positive terminal of the supply. The electrons cannot move through the dielectric. We now have an excess of electrons on the negative plate, and a lack on the positive. Now if the supply is disconnected the situation remains the same. The capacitor holds a charge, it is storing energy. Charge is measured in Coulombs and energy in Joules. The amount of charge held on a capacitor's plates depends upon the applied voltage and the 'capacity' of the capacitor measured in Farads.

Q=CV

Where: Q = charge in coulombs, C = capacitance in Farads & V = potential difference in volts.

W=V2C/2

Where: W = energy in Joules, V = potential in volts and C = capacity in Farads.

A larger value capacitor will store more energy at a given voltage than a smaller one.

The value of a capacitor depends on three factors:

Mathematically the value of a capacitor may be calculated using the formula:

C=0.224KA/d

Where C = capacitance in picofarads (pF), K = dielectric constant of the material between the plates, A = area of one side of one plate in square inches & d = distance between the plates in inches.

Construction

The actual method of making the capacitor is simple. First of all you need to calculate the total area of board needed for your capacitor. using the formula and data above we find that the capacity of 1/16" fibreglass pcb is 16pF per square inch. In other words a capacitor with plates 10" x 10" will give a value of 1.6nF. A board itself needs to be an inch or more wider and longer than the finished plate size.

Cutting the board is easy if you use the right tools, but you must remember that fibreglass ruins most sharp tools quickly. Personally I use a ceramic tile cutter with a triangular pointed tungsten carbide tip. Use a steel straight edge as a guide, and score deeply using a series of cuts rather than one deep one. Score both sides od the board, then clamp it between the jaws of a vice or similar and snap the board off. It is easiest if you repeatedly bend the board back and forth, starting at one end of the cut.

A border 1/2" wide needs to be removed all round the copper on both sides of the board to prevent flash0ver. this means that the capacitor should be good at up to 30kV. The easiest way to remove this copper is to score it with a sharp kinfe and then peel it off. you could also etch the copper away using ferric chloride.

Prepared boards

 

It is quite possible that the size of board that you come out with following your calculations will be larger than is convenient. You can of course make a number of smaller boards and connectn them in parallel, however you must rmember that surfaces at the same potential will have to face each other.

Connecting the boards.

In order to reduce coronal discharge, de-burr the edges of the copper, rubbing them down with a piece of smooth metal is fine. You can then run a bead of hot-melt glue along the edges. As an alternative the case in which you place the finished plates can be filled with mineral oil (messy) or paraffin wax.

The capacitor needs to be finished off by mounting it in an insulated case. Varnished mdf, or a plastic food container are ideal. Various methods can be used to mount the boards, I have used hot mount glue, plastic extrusions and nylon blocks with equal success.