HV resistor in oil

[MRMILSTAR]

My question concerns operating HV resistors in mineral oil. As an example, if I operate a HV resistor rated for 5 KV in mineral oil does that it extend its HV stand-off capability? If I assume that mineral oil has 4 times the breakdown voltage of air, does that imply that I can safely apply 20 KV to that resistor without flash-over?

[Mads Barnkob]

I would say that all resistors, wire wound, carbon or metal-oxide are rated by their internal voltage limit, the out distance is almost always way larger than their rating.

Oil submerging will however help against corona and possible flash-overs due to unbalanced voltage sharing, so there is advantages when pushing them to the limit, but it will never extend their voltage specification. They are cheap enough, just add some more

[MRMILSTAR]

One thing that I will need to make is a current-limiting resistor for the output of my proposed CW generator. Since the output will probably be in the range of 200 - 250 KV, it looks like I may be considering a water resistor in order to fit inside the top of a 4 inch tube. I would probably need about eight HV resistors each rated for 30 KV. That would take up a lot of space.

Does anyone know the voltage rating of those HV 100 mm red resistors that are offered on E-Bay? They are advertised as high voltage 10 watt resistors but they do not state the voltage rating. I have used them on some other HV projects but I really don't know their rating. I was mostly judging by the length. I was hoping that they were rated at 30 KV. Here is a typical listing.

https://www.ebay.com/itm/10W-500K-Ohm-High-Voltage-Bar-Glass-Glaze-Film-Resistor/311464048433?hash=item4884b43731:g:2uwAAOSwKjFdW~Sw

[davekni]

I use Vishay's VR68 series, such as VR68000004703JAC00 for 470K, or VR68000001005JAC00 for 10meg.  They are rated for 1W, 10kV, and much smaller than many HV parts.  Being interested in low current for human touch (standing hair on end etc.), I used a string of 32 10-meg parts on the output of my CW multiplier.  I use the 68meg ones for bleed/balancing resistors on my larger Marx generator.

[MRMILSTAR]

David,

What is your source for buying the Vishay resistors?

[davekni]

Steve,

For more specific electronic parts such as VR68 resistors, I purchase from electronic distributors.  There's a wonderful web site:
    http://www.oemstrade.com
Enter a part number, or the initial string, and it searches most distributors, displaying price@quantity (and quantity in stock). with links to the distributor's page for that specific part.  It's wonderful for finding the best price.  (For work, it's also helpful to see if a part is widely available.)

Arrow (arrow.com) often has one of the best prices, and has free shipping for >=$50 orders.  For a couple years they had free overnight shipping on all orders of any size.

DigiKey often has the highest price, but not always, occasionally the lowest.  They have the widest range of parts in stock and the best search tools (IMHO).  So, I often start at DigiKey, search for the parameters I want (ie. >=10kV 10meg resistor), look for the lowest price options on DigiKey, then enter those part numbers into Oemstrade.  (Sometimes I need to order a larger variety of parts or more unusual ones, so end up ordering from DigiKey or Newark or Mouser.)

Also, if exact value isn't critical, look on Oemstrade for several similar parts.  Sometimes one of the discount distributors such as chip1stop or Verical will have excess stock and dramatically-lower prices on specific parts.

I looked through the videos from backwoodsBrophil that you linked, and commented there with a question about source of his AC-output flyback transformers.  He's using old Sanyo FO241 transformers from Ebay.  I don't see any source for those at the moment.  Do you have AC-out HV transformer(s) for this project?  If so, where did you find them?  Or, are you frying the diodes in a DC-output flyback?

Saw your comment on backwoodsBrophil's video about more diodes than necessary.  My experience suggests this is necessary.  Series diode strings have a problem at the turn-off point.  Some diodes in a series string will have shorter Trr, so turn off first.  The first diode turning off now sees the entire reverse voltage.  It goes into avalanche breakdown, and must handle the remaining reverse recovery charge from the other diodes w/o frying.  My larger Marx generator is fed from a +-11kV home-made transformer and two-state multiplier to get 44kV.  Each diode needs to handle 22kV reverse and ~0.5A forward current.  Made strings of 1.5A 1kV 75ns Trr diodes.  (Also tried some 120ns diodes separately.)  Had frying issues until using 40+ diodes per string.  The diodes died slowly, increasing leakage current and lowering breakdown voltage from the avalanche current.  (They were avalanche-energy-rated diodes.  Initial try with diodes not rated for avalanche failed almost instantly.)  For my initial 30-diode strings, I kept testing each diode and replacing the failing ones.  When I didn't test often enough, the failures cascaded throughout the strings.  With enough diodes, the few fastest ones fry and there's still enough good diodes left.

BTW, my low-current CW multiplier uses pairs of 20kV 5mA diodes, even though the input voltage is ~20kVpp.

You may get as much or more voltage than backwoodsBrophil even with fewer stages.  If I heard his video correctly, he has 500pF caps.  With 20 stages, he's getting quite a bit of loss from stray capacitance.

[MRMILSTAR]

David,

Thanks for your detailed reply. I have several AC flybacks that I can use. I got them at the Mid-Ohio Teslafest this year. Here is a link for a reasonably-priced new one from E-Bay:

https://www.ebay.com/itm/F0239-New-Replacement-Tv-Flyback-Transformer/110953941243

I'm sure you already know that you can find NOS AC flybacks on E-Bay. The only problem with them is that they are so old that frequently the insulation is cracked. They may be repairable by melting new wax over them.

I have found some 20 KV 30 ma diodes on E-Bay. I plan on using them in strings of 3.

I suggested that "backwoodsBrophil" should join this forum so that he can get some useful comments instead of the usual juvenile comments that you get on Youtube.

[johnf]

We buy the VR68 rersistors at work by the box full 1000 pieces per box we got ours through Arrow components but I see mouser and digikey have them as well as element14 and radiospares if you want the more expensive way to buy

[davekni]

Steve,

Thank you for the EBay link!  At the picture looks to be in very nice shape.

Is there a list of AC flyback part numbers?  I haven't found anything showing that F0239 is AC-output, nor any way to search EBay (or other places) for AC flyback transformers in general.

This question isn't critical.  At the moment I have only one HV AC transformer plan (DIY plasma globe), which I have by frying diodes in an old DC flyback.

[MRMILSTAR]

Steve,

Thank you for the EBay link!  At the picture looks to be in very nice shape.

Is there a list of AC flyback part numbers?  I haven't found anything showing that F0239 is AC-output, nor any way to search EBay (or other places) for AC flyback transformers in general.

This question isn't critical.  At the moment I have only one HV AC transformer plan (DIY plasma globe), which I have by frying diodes in an old DC flyback.

Someone on the Facebook high voltage forum has bought several from that E-Bay source and states that they are AC and work great. If you want an alternative source and don't mind being gouged ($79), there is Information Unlimited.

[MRMILSTAR]

The current plan for my proposed 14-stage CW multiplier for the HV current-limiting resistance is to to use 30 of the Vishay VR68 resistors in series. The specs for the VR68 are:

* 470K ohm resistance
* 10 KV maximum voltage
* 1 watt power dissipation

This will yield a resistance of 14.1M ohms. Assuming an output voltage of 250 KV, this should limit the current to 18 ma. Since I plan on using 30 ma diodes this should be enough to protect the diodes. Everything will be under mineral oil.

I had two ideas for connecting the 30 resistors and making them fit into the top of the 4" PVC main column. The first idea involved connecting them in one long string and then placing the string inside a 36" length of vinyl tubing. I would then coil the string along the inside circumference of the top of the 4" PVC column. I decided against this because I'm not sure if I could ever get all of the air bubbles out of the vinyl tubing.

The second idea is to break the string up into 6 sub-strings. Each sub-string would be placed inside a 6" length of 1/2" PVC pipe. The 6 individual pipes would then be placed in a circle around the top of the main 4" PVC tube. Each tube would be staggered vertically by about an inch with respect to the adjacent tube. It would look sort of like a circular Pan's pipe. The stagger is needed to provide sufficient  voltage standoff at the ends of the individual pipes. All 6 of the individual strings would be connected.

[davekni]

Steve,

You could use well less resistance if you want to make sharper sparks.  The 30mA diodes are likely rated well higher for peak current, probably over 100mA.  Any sudden discharge event will draw current for only a short pulse.  Even that short pulse sends only half of the discharge current through diodes.  Half will go through each capacitor chain.  The one chain connected to ground and your resistor string will not contribute to diode current during a discharge event.

For limiting average diode current, the capability of the ZVS/flyback is likely sufficient.  To reach 30mA average into even a shorted load, the flyback needs to output +-60mA, with each diode conducting for half-cycles.  That would be +-6A flyback input current if the turns-ratio is 100:1 (not including the resonant current).  With a 2meg output resistor string, the current would share some among the first several diodes, so could handle more than +-60mA.  It would require simulation for a more precise analysis, but I suspect 2meg total would be enough.

Of course, 15meg would be more conservative if that's your bent.  It would also lower the dV/dt of the caps during discharges - don't know what your caps can handle.

For mounting, how about making the single long string without covering, then wind it around a smaller internal piece of PVC pipe, perhaps an extension of something used to mount the caps and diodes?  Or make a zigzag up one side of the internal PVC.  Either way, there are many options for keeping the string in place.  Drill holes in the pipe and have excess lead stubs from the resistors stick into the holes.  Or, make small saw-cuts along one side of the pipe and wrap string or fishing line around the pipe and resistors,  with the string in the saw cuts.  The string could wrap up and back down and tie together.  Or, use epoxy on the resistor leads or on string instead of saw cuts.  Or, cut small scraps of PVC (say semicircles of pipe) and use PVC glue to attach them between turns of resistors...

[MRMILSTAR]

David,

Thanks for your suggestions. You have given me some good ideas.