Back to home page
The Quadratic Mutation
Four coils, four drivers, one pic chip - a programmable lightning display
The idea is to arrange four identical coils in a square. Primaries, secondaries, toploads are all made as equal as possible. Each coil has its own identical full bridge driver. The bridges are driven from port B of PIC micro computer chip. B0 & B1 drive bridge A, B2 & B3 drive bridge B etc. programming a sequence such as 01100000, 01100000, 10010000, 10010000, will drive coils A & B, 180 degrees out of phase while C and D will remain switched off. Causing an arc to form between A & B. Similar sequences are used to form arcs between any pair of coils. It is also possible to swap the sequences used hundreds of times a second so as to give arcs apparently simultaneously between all the coils. The photo above (a simple snap shot) shows arcs along all 4 sides of the square and the diagonals.
The PIC controller
The voltage contolled oscillator in a 74hc4046 is used as an external clock for the PIC. The control voltage is taken from a ten turn pot which is used for tuning. The PIC divides the clock signal by 32 to produce the 280kHz or so needed to drive the coils. Other loops swap the outputs at audio frequences producing notes and tunes.
Port A is used as a digital input and connected to six switches, port C is connected to a row of LED's to indicate where the program has got to.
The Power Stages
An almost complete full bridge - fairly standard design 4 power Fets, plus diodes, driver transformers, and TC4422epa driver chips. Added later was a 12V PSU for the drivers. I had originaly planned to use a single 12V supply for all four boards until I thought about ground currents etc. I use 2 transformers each with 2 12V, 10VA windings giving four separate 12V AC supplies which are rectified (4 of 1N4002) smoothed (2 of 470uF 25V) and regulated (7812) on each board.
There is a separate ammeter for each board.
The board in the very centre provides the DC for the PIC - there is a 7805 on the board.
Running
two across
and the other two sides
And all four sides.
It was quite a push to get the machine ready for running at the UK Teslathon. I did not have time to build a second DC supply its designed to run from 2 330V DC 10A supplies (rectified mains, smoothed). With a single supply I put extra turns on the primaries to limit the total current draw to about 10A. Doubling the power will improve the display, a great deal. Also the sequences were being switched too quickly - reducing arc intensity. It played the five note sequence from close encounters. Dropping it a couple of octaves will improve arc power. I had thought that I would need to carefully tune each coil. In practice they are so closely coupled, electrically through the common earth wire, electrostatically and magnetically that they worked together well and tuned over quite a wide range.
The future
Change the programming.
Build 2nd DC supply.
Remove at least 2 turns from the primaries.
Mount in a vertical frame
This should limit the magnetic interaction of adjoining coils, and should give a boost to the diagonal mode. But how will it affect tuning?