DIY 100KV 4-30NF VARIABLE HIGH VOLTAGE Air/Film CAPACITOR Project - Plans
This article will guide you to construct custom made high voltage variable capacitors with in range of nanofarad. The current capacitors are 4-30nf range! Usually those Capacitors are very rare and expensive but many high voltage researchers need such equipment.
Those two Capacitors was builded for my needs and they worked perfectly at 100KV. Their limit in voltage is unknown yet, but I believe they can handle 150KV without internal arcing. So the following instructions are for two capacitors.
Details of Plates
For capacitor plates, we will use lamination from a big old transformer. Get the largest possible mains transformer that you will find in your scrap yard basement, but make sure you don't need it first!
Carefully unwind all copper wires and keep them for future projects.
Take of all the "E" lamination carefully not to bend them... See how many straight they are and divide them by 4,,. the four sides, 2 sides for every cap. If you need to work with voltages above 120KV I suggest to submerge the final construction in capacitor oil. That off course means that you have to seal internally the boxes, etc. The size of plates/lamination will affect the capacitance.
Typical old mains transformer
"E" and "I" lamination of a typical transformer
The size of "E" lamination that has being used, was taken from a 10KVA 220V-220V isolation transformer. The exact size is shown in the following image.
"E" Lamination size
Hint:(you can use the "I" lamination and make smaller size cap).
In all mains transformers, the central iron core is constructed from thin silicon steel lamination which are electrically insulated from each other, by a very thin coating of insulating varnish. So the area where the holes are, we need to scrab off that coating so the washer will contact the lamination. Use your Bench Drill with a Wire Brush Drill Bit and can clean off the insulating coating easy and fast. Do that to all holes of lamination, at both sides.
Typical Wire Cup Brush Drill Bit
Ready lamination for use as capacitor plate
Just rub off the insulation in a wide circle (more than the washer' s diameter) above every hole and every side of every "E" lamination. So where the screw and washer go, will make a good contact.
For separating lamination we will use washers. The washers that has been used was 18mm outer diameter, 8mm hole diameter, 1.5mm width. They are very common size and you can find them in all DIY stores.
Hint: Calculate the washer height, you may want to place two washers to much the space, or more film layers to increase the voltage. You decide.
There are 45 plates on each side. 90 lamination for every capacitor. Now once you have ready the lamination with clean holes, and the washers by side, proceed to make the dielectric..
Find the cheapest A4 transparent film, usually its the films for photocopier machines.
They are 100 microns every film. (Count how many you need for your project)
There are 4 layers of film between every pair of opposite plates.
Cut the A4 films in sizes to fit your lamination.
Find the cheapest A4 transparent film, usually its the films for photocopier machines. They are 100 microns every film. (Count how many you need for your project) There are 4 layers of film between every pair of opposite plates. Cut the A4 films in sizes to fit your lamination.
A4 transparent film
If you Place 1 washer between two plates of the same side. The gap between lamination will be 1,5mm = 1500 microns. The width of opposite plate is 0,5mm = 500 microns. So we get 1000 microns free space to place the dielectric. So 400+400 microns of dielectric gives 200 microns air so the sides are comfortable to move.
Remember that when a big charge is present the opposite plates attract each other and the friction will make heavier the dial, so just make sure the cap sides are no very tight and difficult to move and you are good to go.
Now as you have all the films packed aligned and tight stable on your bench drill Put a lamination on top to mark the two holes of the lamination on the first film. Mark also the perimeter of the lamination leaving 2cm more space at the edges.
Take the lamination off and take a ruler and the cutter and cut all 4 sides of films. Pack them again all together as a block and with the drill for wood 20mm (18mm is the washer plus 2mm space to be comfortable the washer in the hole), Drill all the way down, all the 740 films, at both holes. (Films should not insulate the plates from their washers).
Transparent film cutted and drilled, ready for layering.
To make your life easier... All washers have 1 side with curved ends and one completely straight side, its due to the cutting press. So remember to place the washers with the straight side down as you build, so the film will not go under the washer when you place it or move it, or when you place the next layers on top, etc.
First capacitor layering ready
You will need also 8 screw bars and nuts, (for 2 caps) in approximate size.
So now you are ready to build the first one.
Position the 4 screw bars (two pairs) vertically on the table. The two pairs of screws need to be in a half distance, meaning that the plates will be at half way.
Be carefull not to separate the two sides. Dont move them until you finish all layers. Start from left side..
left side: 2 films, 1 plate, washers, 2 films ... then
right side: 2 films, 1 plate, washers, 2 films ... then
left side: 2 films, 1 plate, washers, 2 films ... then
right side: 2 films, 1 plate, washers, 2 films ... until you place all layers.
Film, washer and lamination layering
The gap between plates have mixed, 400 mikrons polypropylene films and 600 mikrons air gap. Use 4 angles at the screws endings to be fixed at the moving parts of the box. Then put the nuts on, and its ready to be fitted in its box.
Your cap is ready. You can connect on your capacitor meter, then pull or push the one side to test its capacitance.
Its up to you what kind of boxing you will make, or mechanism to puss-pull the two sides.. A long screw and rail at bottom is used to push or pull in horizontal plane the one side, with the other side fixed. Its up to you, what materials you have and how you want it.
Railing and boxing the capacitors
There are no caps in market like these. Actually 30nf for variable is probably unique. Most of them they value few hundreds of pF, because you suppose to use more caps in parallel to be inside the value you want. But there are some high voltage projects that need wide range of capacitance, especially those in energy research field.
Capacitor Dial ready
Above view of inside structure
Ready to install terminals on top of the Box
Connect two wires at each side of capacitor and drive them on top with separate screws on boxing. Make sure the distance of the terminals is the highest. Be very carefull not to forget the capacitor charged, It may kill you instantly. Remember to discharge them immediately after every use, to be safe. The dial screw is well insulated from the plates and and a plastic tyre from a toy car is being used as a dial, to avoid electrostatic shocks.
Finished HV Variable Capacitors
The following video shows the range of the finished capacitor.