Stefan's Tesla-Pages

My third solid state HV source

(output peak voltage 6.7kV)
again using a fly-back transformer

Table of contents:

Specs of the circuit

Photos

Circuit

This circuit was extracted from a really huge CO₂-LASER (1kW beam power continious duty!) where it was the ignition circuit:

Sorry for the german text in the schematic,
english version is coming soon after more detailled testing of the circuit!

• Output is AC. The secondary coil is floating. The limiting factor in voltage is the potential against the grounded ferrite core. Perhaps it would be a good idea to connect the low voltage end of the secondary coil to the earth lead of the power plug...
• I have no idea what the initial driving voltage was. The circuit starts to run at aproximately 14V. The input cap is rated 40V. Therefore I guess it originally was designed to run at 24V. I made a DC source for 25V. When drawing an ac, the power resistors R7-R10 get hot but not as much as to worry about. The Heatsink of the transistors gets warm.
• With the peak voltage rectifier I described here, I measured the peak output voltage to be 6.7kV (Vss=13.4kV, Veff=4.75kV assuming sinusoidal waveform).
• With no arc, the input current (measured at the fuse) is 0.7A. With an arc at the output, it increases up to 2.5A. Since it was not designed to deliver a continious arc, the 1A fuse makes sense and the 24V driving voltage seem to be ok.
• I for sure want the circuit to deliver considerable output power. The only parts which get hot at the moment are the BD438 and the four 0.82 Ohm power resistors at the drivers.
• Seems as if the purpose of the BD438 is to control the power into the circuit. I want to maximize the power, so I'll try to bridge the BD438 with a power-pot and see if I can increase the input current (while an arc is drawn) by decreasing its resistance, hopefully down to zero Ohms. If that works, I'll remove all connections of the feedback circuit (connection FB from the gate of the BUZ20 and the 3.9k resistor).
• Next step is to bridge the four 0.82 Ohm power resistors at the drivers with an aditional power-pot. I'll decrease the value until anything other than the power resistors will start to get hot.
• Hopefully, adding some capacitors will improve the circuits pulse capability (read: more output voltage). I plan to add 220uF to the 47uF in the input filter to adjust to the increased power level. A high performance pulse capacitor (FKP or MKP with approx. 1uF) will be added to get fast raising pulse edges.

Here the housing is closed:

And here with a paper clip installed to make a miniature Jacobs ladder:

And now with some power applied:

Here are two movie clips. the first one shows me starting the arc by sticking some kind of metal tool between the rails, the second one using a lighter (click the images):

Experiments, max. arcs etc.:

• I got 120 caps 4.7nF@30kV and plan to build a villard cascade from them using the ignition circuit above as the driver.
  Charging the all caps (564nF) up to 20kV, the total energy in them will be 113J (176J at 25kV). 
• some suitable diodes (suitable for the high frequency of the flyback circuit) will be either KYX28/15 (15kV/2mA) like here http://www.rapp-instruments.de/diverse/multiplier2/multiplier.htm or Diotecs DD1800 (18kV/20mA) http://diotec.com/pdf/dd300.pdf.
• In a villard cascade, all the elements have to be designed for 2 times the peak input voltage. You'll need 2 caps and 2 diodes per stage.
• Output power from the AC-supply (ignition circuit) is approx. 40W. At 60kV output of the cascade, that's roughly 0.6mA. Not enough for a big bad lifter...  
• I'll try to boost the ignition circuit to deliver 100W and add a cascade to get 50kVpeak voltage, that will result in 2mA for the lifter at 50kV.
• Alternatvely, a two paralelled OBITs with 5kVeff (90W power each) could be used to get the desired 50kV:
  I have 25pc. of diodes type H2612-22 (probably 10kV / 100mA, can't find any datasheet, probably just microwave oven diodes designed for 50Hz...) 
  With 7kVpeak from the xfmrs, two stacked diodes have to be used (total of 4 per stage), that's 6 stages for a total of 24 diodes.
  6 stages will result in (6+1)*7kV = 50kV, so the goal would also be reached. Still have to calculate the voltage drop when the lifter is connected (1mA/2mA/5mA).