Is it possible to DC bias a Tesla coil?

[bobfrancis1980]

I want to get a voltage source that oscillates between 500kV and 0V, preferably high frequency.

At first I thought of using a Marx generator but their pulse rate is relatively low and they are difficult to insulate.

Then I was thinking I might be able to put a Tesla coil which is high wattage and Cockcroft-Walton generator in series but I don't think the CW's capacitors could take the 250kV from the Tesla coil and they are designed to output DC. And I don't think the Tesla coil receiving a DC voltage pulse from the capacitor into the primary would alter the output of the Tesla coil from being AC.

Then I was thinking I might be able to put a Tesla coil which is high wattage and Van de Graff generator in series but Van de Graff's are low wattage relatively speaking.

Lastly I thought about creating a 30-60hz (ideally higher) Cockcroft-Walton alone but try to design it with a very high ripple voltage.

Do you guys have any insights onto how I might go about generating the high voltage (and ideally high frequency) as mentioned above?

[davekni]

At first I thought of using a Marx generator but their pulse rate is relatively low and they are difficult to insulate.

My 440kV Marx generator repeats at 120Hz.  Don't know if that is enough for your needs.  What is this feeding?

Difficult but probably possible option (with enough money and experimenting):  250kV TC with 250kV CM multiplier feeding bottom of secondary.  Requires insulating TC secondary bottom from primary (and from everything else).

Another difficult but perhaps possible option:  250kV TC feeding a 250kV capacitor with >500kV diode to ground.
BTW, I have a bit of experience with the difficulty of making a 500kV diode.  I had wanted to rectify output of my Marx generator in order to charge a capacitor.  Made a series string of 1710 BGY23M diodes (1kV 1A 75ns avalanche-rated).  Enough diodes to allow for over half to fail.  Diode string wound (spiral wrapped) around the outside of a 1.2m long 110mm diameter pipe.  I thought that would be long enough to allow operation in air at 440kV.  Lots of arcing between turns of the diode string.  Probably due to wave front delay down string and varying reverse recovery times.  Fried diodes fairly quickly.  I have ideas on improvements, but not anything I plan to try again.  Took a very long time to solder 1710 SMD diodes into a series string.

[MechatEng2023]

A HF HV diode and capacitor makes a simple voltage doubler.

[bobfrancis1980]

Thank you for your response.

Oh wow, I did not realize a mark generator could fire at that rate. Tesla coils typically burst around 60-120 times a second so I would say your marx generator might be the simpler solution for me.

What was the voltage rating of your charging capacitors?
What was the pF rating of your charging capacitors?
What were the voltage, wattage, and ohm rating of your charging resistors?
What was the input voltage into the marx? Was it with a flyback?
What did you use to trigger the first spark gap? I have a SF6 switch I got off ebay years ago but I think it is limited to about 30 pulses a second.
Do you use compressed air through the spark gaps? If so, do you know the PSI?
Does the amperage from the flyback make a difference? To increase the charging rate would putting two or more flybacks in parallel increase the charging speed?

Thanks

[klugesmith]

Dave wrote:
>> Made a series string of 1710 BGY23M diodes (1kV 1A 75ns avalanche-rated).

Has anyone tried getting fast HV diodes in MELF type package?
Could put more than 100 in series without solder, inside a rigid insulating tube with a spring at the end for contact force.
Or reflow-solder them into sticks of 10 or so.

These guys are in stock at Digikey.    SUF4007 is 1000V 1A 75ns.

https://diotec.com/request/datasheet/suf4001.pdf

[davekni]

Oh wow, I did not realize a mark generator could fire at that rate. Tesla coils typically burst around 60-120 times a second so I would say your marx generator might be the simpler solution for me.

TC would provide much higher frequency within each burst in case that helps with whatever your application is.  With enough input power and cooling, TC could run continuously.

I just made a quick retrospective post of my Marx generator:
    https://highvoltageforum.net/index.php?topic=2755.msg19787#msg19787
Hope this answers your questions.  If not, please ask again.

Has anyone tried getting fast HV diodes in MELF type package?
Could put more than 100 in series without solder, inside a rigid insulating tube with a spring at the end for contact force.

Nice idea.  BGY23M would work with that technique too.

These guys are in stock at Digikey.    SUF4007 is 1000V 1A 75ns.

I don't see any avalanche energy specification for these diodes.  I've tried similar non-avalanche-rated diodes in series strings.  They fail very quickly due to mismatches in reverse recovery times.  Even avalanche rated diodes fail given the high (although short) avalanche current carried by the fastest diodes in a string.  The CW multiplier feeding my Marx generator uses 44-diode strings for each diode.  Even with the avalanche rating, several burn out within each string.  If half or more turn out, string fails.  Several strings failed that way.  Finally have four strings that seem reliable.  Brought spares for demos, however.