Tesla Coil Mark IV

As my son is now old enough and "sparked" too, we started a coil project together:

The Parts

HV Source

As HV source we want to use a classical NST (Klinger). The ratings on the label state 6 kV, 75 mA for the secondary side and 220 V, 2.5 A for the primary side.
After removing the magnetic shunt from the transformer core I did some measurements of the actual values. The free wheeling voltage is about 7.6 kV. With a load of 549 Ω the voltage breaks down to about 7 kV, whereas the current is 270 mA.
At first I couldn't quite believe this. So I measured the primary current under load and got 8.2 A which gives 1.9 kVA. This result makes the 7 kV @ 270 mA plausible and should be quite enough for the planned setup.

HF Filter + Safety Gap

To protect the HV source from surges and overvoltage I designed a combined HF filter and safety gap. The gap is adjusted to fire at approx. 10 kV. Additionally the filter is bridged by 2x7 VDRs rated 625 VAC (Epcos S20K625). The filter itself consists of 2x7 caps rated 4700 pF @ 2000 V balanced by 2x7 resistors rated 10 MΩ @ 1 W (Vishay VR68) and 2 resistors rated 549 Ω @ 50 W. The filter and the safety gap are symmetrical to ground.
You can find the schematics here.

Spark Gap

For the spark gap the remainder of the drain pipe was reused. It consists of 6 iron bolts of 8 mm diameter and 8 cm length. 4 of them have stainless steel cap nuts. To improve quenching the pipe is fitted on a fan.

MMC Capacitor

The main capacitor consists of 6 pulse caps from a dismantled laser setup. Each cap is rated 50 nF @ 10 kV. As balancing resistors 22 MΩ @ 1 W types are used (Vishay VR68). The total capacitance is 75 nF.
To protect the MMC against voltage surges the array is bridged by 6 surge arresters (Okaya RA-362M-V7; 3600 V / 3500 A).
You can find the schematics here.


Made of copper cable for car-hifi power supply (25mm²); 9 turns at 45°


The secondary coil is wound on a polypropylene drain pipe:
  • copper wire (0.63 mm; 22 AWG)
  • diam.: 11 cm / 4.33"
  • length wound: 55 cm / 21.65"
  • turns: 870
  • 15 Ω / 12 mH
The top load is a toroid made of flexible aluminium pipe (inner diameter: 21 cm; outer diameter: 43 cm; pipe diameter: 11 cm).

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