The Capacitor

back to homepage

OLTC Offline Tesla Coil

The starting point for this project was acquiring some igbt  bricks CM400HA-12H 400 Amp 600V from eBay.  I then started to play with numbers in the Roffesoft calculator - thanks Derek.  I really liked the idea of a secondary about the height of the Eiffel tower with a zeppelin topload, A copper tube primary that you could drive through and a one Farad tank capacitor, that resonates at 2Hz and takes out small countries with a single strike :)

I decided this was too ambitious for a first OTLC – maybe the next one. I decided on more modest proportions.

The Triodal

A different way to build a top load - one sphere would give 15pF - my guess for the whole assembly is about 35-40pF

Further images and recipe here

The igbt’s


2 of CM400HA-12H bricks, 600V 400A, 800A peak, each driving three of the loops, interweaved.

The secondary

60.5cm winding length, 16cm dia wound with 0.315mm wire apart from the top 12cm which is 0.2mm, average is 0.3mm (based on number of turns).  The thinnest wire is at the top where the current is less. It is a way of packing a bit more induction into a coil – but I did it because I ran out of 0.315.  I worked out the number of turns and the wire averages at 0.3mm for the purposes of calculations.

The hi tech coil winder – I control the drill speed with a variac
The Capacitor

I pinched the idea of mounting the primary cap on a double sided pcb and mounting the caps in alternating directions so to cancel the induction currents. I discovered that I could remove a neat circle of copper and leave most of the glass fibre by using the drill bits intended for woodworking - which cut at the sides rather than the centre of the holes. Drill the hole with the 1mm drill, then use 4mm and then 6mm wood cutting drills.Managed to do all the holes without cutting through the pcb. 

The capacitors are 0.47uF @ 1KV polypropylene, RS components via eBay.

5uF is mounted at the moment - but it may need as much as twice that

The primary is 6 loops of 10mm copper pipe, (26cm dia) spaced 2mm apart, all the loops are run in parallel for a single turn primary.  It is then wired up using bus bars and heavy copper pipe

The primary circuit was wired using copper bus bar and thick (7 of 1mm) copper wire. Care was taken to make this as compact as possible to minimise induction.  It was very time consuming – it would have been simpler to use Steve Conner’s trick of using coaxial cable for the wiring up.
Dereks program can be found on his site Derek's program reckons that this will go at 78kHz with a 20pF top load and a 6.4uF primary cap, peak primary current is 1592A at 500V but I'll probably stick at 400V,

1300A and push up the break rate, or increase the top load.

The unexpurgated wisdom of the list I would have struggled to design this without the original inspiration of Terry Fitz and the input from many on the list especially Steve Connor who also did the control electronics for me, and Derek’s input – particularly his oltc calculator.
The top load is two ikea 36cm diameter bowls, I cut a wooden disc to the diameter - I thoiught I would have to use screws but it jamed the two halves together nicely.
First Light - Saturday 15th July.

Steve Connor, Richie Burnett, and Duncan Cadd came up to Burntisland to play.  Steve gavw me a lot of help to get the oltc up and running. We had to bring the tank cap up to 10uF to get it to tune.  It drew about 4A at 300v into the charging choke. The variac had to be kept below 90% to avoid firing the overvoltage crowba. (its very effective) it only blew two 5A fuses!

18 inch arc - about what I expected, and a noise like a demented banshee....

I do have plans to put up the spark length.

back to homepage