CW multiplier resistor string suggestions

[MRMILSTAR]

I need some suggestions on the HV current-limiting resistor string for my CW multiplier. Briefly, here is my proposed design.

* 14-stage design based on 28 TDK 1.7 nF doorknob capacitors rated at 30 KV
* Each diode string consists of three 20 KV 30 ma diodes in series
* To be powered by 1 or 2 flyback transformers in parallel driven by a ZVS circuit
* Current-limiting via HV resistor string, resistance TBD
* Everything will be under oil with the exception of the HV supply

My plan is to supply no more than 15 KV peak to the first CW stage since I have 30 KV capacitors. This should give me a theoretical output of 210 KV (15000 * 14). In order to protect the diodes I plan on having a HV resistor string attached to the final stage of the CW multiplier.

The current-limiting HV resistor string is what my question concerns. The most readily available, reasonably-priced, and compact resistor appears to be a Vishay VR68  with the following specs:

* Resistance: 470K ohms
* Voltage rating: 10 KV
* Power Rating: 1 watt

If I use 250 KV as the target resistor rating, this will require 25 resistors in series for a total resistance of 11.75M ohms. This should limit the current to more than 21 ma. I would actually like to use about half of this resistance value but the only way obvious to do that would be to place resistors in parallel which would double the number from 25 to 50 resistors. I would rather not do that for space reasons.

Does anyone have any suggestions concerning the HV resistor string?

[plasma]

1.7nF doorknob caps, will have a high discharge rate,
I had 10 1kv 1A diodes and 200nF at 50hz Z, the diodes should handle the current from the Mot, but when the sparkgap fired they all arc over, I added 30kohm ceramic and carbon film who h stopped it.
The CW boosted 2.4kv to 8kV, with two stages, the last stage had 5 1uF in series.

[davekni]

Steve,

The VR68 series goes down to 100K.  Arrow happens to have 150k in stock, VR68000001503JAC00.  Digikey has all the way down to 100k, but at a bit higher price.  Or, to you already have a set of 470k parts?

[MRMILSTAR]

Steve,

The VR68 series goes down to 100K.  Arrow happens to have 150k in stock, VR68000001503JAC00.  Digikey has all the way down to 100k, but at a bit higher price.  Or, to you already have a set of 470k parts?

Thanks Dave! I tried using "oemstrade.com" that you suggested as the search tool. It does appear to be a very good parts search tool. I didn't know the exact part number I was searching for so I tried a generic "VR6800000". The only thing that came up as available in small quantities was the 470K part. When I entered the exact part number that you supplied a longer list of suppliers appeared. The moral of the story seems to be "know the exact part number".

I haven't bought any parts yet other than the capacitors which I already had because I got a great deal on them about a year ago. I generally don't buy any parts for projects until I have a complete design in hand otherwise I often end up wasting money because of mid-project design changes.

[davekni]

That's the value of searching on Digikey first, searching by parameter, then looking at oemstrade for better prices.  OEMs trade works reasonably well with partial part-number strings IF there are reasonably few matches.  Adding a single extra digit to your search makes it reasonable, "VR68000001".

BTW, the VR68 series has the value encoded in the four digits after the long string of 0's, three digits followed by a power-of-ten digit.  So, the VR68000001503JAC00 part is 150E3 or 150k.