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Messages - MRMILSTAR

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1
Voltage Multipliers / Re: CW multiplier resistor string suggestions
« on: December 08, 2019, 05:28:09 PM »
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.

2
Voltage Multipliers / CW multiplier resistor string suggestions
« on: December 07, 2019, 11:32:49 PM »
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?

3
Electronic circuits / Re: Cost reduction in consumer electrics
« on: December 06, 2019, 08:35:29 PM »
Here is possibly the most dangerous consumer electronic item that I have seen that is available today. There is a Youtube video on the Electroboom channel that shows something very similar to what Dave is referring to except that it is available today on E-Bay and Amazon. Its basically a 240 volt heating element exposed to the water stream housed in a shower head. Apparently they are popular in less-developed countries as a way to get a hot shower. I wonder how many people have been electrocuted by this device?


4
Voltage Multipliers / Re: HV resistor in oil
« on: December 05, 2019, 05:52:11 AM »
David,

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

5
Voltage Multipliers / Re: HV resistor in oil
« on: December 04, 2019, 06:05:11 PM »
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.

6
Induction launchers, coil guns and rails guns / Re: Sense coil fabrication?
« on: December 04, 2019, 05:15:49 AM »
Nice work with the pancake spirals. Someday I may take on the task of a horizontal disc launcher which would make use of 2 spiral pancake coils facing each other.

I found this calculator useful for winding spiral coils. It computes the length of wire needed given various parameters. This calculator is actually intended for measuring the length of tape on a roll but it can also be used for coil winding.

http://www.giangrandi.org/soft/spiral/spiral.shtml

7
Voltage Multipliers / Re: HV resistor in oil
« on: December 04, 2019, 05:06:39 AM »
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.

8
Voltage Multipliers / Re: HV resistor in oil
« on: December 03, 2019, 05:07:28 AM »
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.

9
Voltage Multipliers / Re: HV resistor in oil
« on: December 02, 2019, 09:06:07 PM »
David,

What is your source for buying the Vishay resistors?

10
Voltage Multipliers / Re: HV resistor in oil
« on: December 01, 2019, 10:47:18 PM »
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

11
Voltage Multipliers / HV resistor in oil
« on: December 01, 2019, 08:51:31 PM »
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?

12
Voltage Multipliers / Re: Suggestions for CW HV feed-through
« on: November 30, 2019, 05:24:50 AM »
I really like this person's 20-stage CW generator. It has the type of construction that I am considering.

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13
Voltage Multipliers / Re: Suggestions for CW HV feed-through
« on: November 30, 2019, 05:13:13 AM »
I also like the circular contact idea.

I have seen clear PVC tubing for sale on E-Bay. I actually have some acrylic tubing. The problem is that my research seems to indicate that acrylic tubing is susceptible to attack by petroleum-based oils while PVC is not.  If anyone knows of some type of oil that will not degrade acrylic over time, I would like to hear about it. I am not really interested in vegetable oils though because they can turn rancid over time.

14
Voltage Multipliers / Re: Suggestions for CW HV feed-through
« on: November 30, 2019, 05:06:36 AM »
Steve,

I'd thought that "full-wave" applied to only voltage-doubling.   Is there a circuit for higher multiplication factors that's called "full-wave"?

At 1.7nF and 14 stages, parasitic capacitance will have some effect, but probably not excessive.  (I have 16 stages starting with only ~550pF.)

Another thought for your interconnect at the bottom:  How about using a pair of battery-contact springs on the bottom of the multiplier and two pieces of nickle or nickel-plated sheet metal at the bottom of the housing?  Alignment could have very loose tolerances that way.  I expect the weight of your multiplier is well more than twice the contact force of a typical AA battery holder.

Dave,

Attached is a comparison between the circuits for a half-wave and a full-wave CW multiplier. I won't be using the full-wave circuit because of the increased parts count. The full-wave circuit also requires a center-tapped transformer. I like the battery spring concept idea.

15
Voltage Multipliers / Re: Suggestions for CW HV feed-through
« on: November 29, 2019, 08:27:53 PM »
Banana plugs/jacks sounds like a good solution as long as you have the alignment issue solved.  The stamped metal "banana-peel" part typically produces sharp edges of contact, which likely have enough pressure to break through the oil film.  You could bend the banana-peel tabs a bit to create even more pronounced edge contact rather than contact along the surface of the stamped metal.  I'd suggest having the male plug parts on the multiplier in case you need to tweak the tab bending and/or find a part with stronger (higher insertion force) tabs.

BTW, have you ran any simulations on the multiplier including parasitic capacitance between the two capacitor columns?  I was surprised how much voltage loss came from capacitive coupling between columns.  (If you are making a high-power unit with relatively large column capacitors, stray capacitance may not be significant.  I was purposely using as small capacitors as possible for safety - using it as a Van de Graaff alternative.  Also added a string of 30 x 10meg 10kV resistors in series with the output.)

I haven't done any simulations yet. I am still in the conceptual and learning phase right now. I do have 28 TDK doorknob capacitors with 1.7 nF capacitance that I bought some time ago for a future project such as this. They are rated for 30 KV so they are physically rather large, probably about 2" in diameter by 0.5" thick. If I make a half-wave multiplier I could have up to 14 stages. I am planning on using an AC flyback transformer driven by a ZVS driver.

Given that the parts count of a full-wave circuit is higher than a half-wave circuit, is there any significant advantage of using a full-wave circuit over that of a half-wave circuit? I don't really care about voltage ripple since I am only using this to make long sparks.

16
Voltage Multipliers / Re: Suggestions for CW HV feed-through
« on: November 29, 2019, 05:01:08 AM »
I posted this on 4hv too:
PCB edge connector, with some of the central pins removed, or just a couple of banana sockets sealed into the bottom. Either would allow you to simply unplug your multiplier.

Do you think that the banana plug would make a good electrical connection being immersed in oil?

I am also thinking about using clear PVC tubing for aesthetic reasons. This would also allow me to see if the banana plug male and female ends were in alignment.

17
Voltage Multipliers / Suggestions for CW HV feed-through
« on: November 28, 2019, 06:05:46 PM »
My next project is going to be a large (300 - 400 KV) Cockroft-Walton generator. I want to house it in a 24" x 4" PVC tube. The CW generator will be operated vertically. The entire unit will be immersed in oil. My conundrum is serviceability. I am not going to assume that I will never need to repair this, therefore I will need to be able to remove the electronics from the 4" tube. I am envisioning all of the components mounted on a plastic strip or smaller PVC tube that fits inside the 4" main tube. That would enable me to withdraw the entire assembly from the 4" tube for repair if needed.

My biggest unknown right now is how to get the two HV power input leads through the bottom of the tube without leaking oil and to be able to remove the electronics assembly. I obviously won't be able to reach down into the bottom of the tube to disconnect the two HV power input leads. The best idea that I have come up with is to be able to slide the outer 4" tube off of the base. The base would probably be some kind of PVC end-cap. To make that joint oil-tight I thought about machining a groove into the 4" tube so that an O-ring could be installed between the tube and the end-cap.

Any ideas?

18
Induction launchers, coil guns and rails guns / Re: Sense coil fabrication?
« on: November 28, 2019, 05:04:49 PM »
Looks good kluge. That technique works well with lighter gauge wire and of coarse will work well for a test coil.

This is the first coil-winding method that I tried with the 9 AWG solid wire and I didn't like the result. I even used thick plywood backing plates. I used my lathe to turn the form very slowly so that I could pull on the heavy wire tightly to keep the kinks out.  Even with the 0.5 inch plywood backing plates the wire would manage to occasionally slip under the previous winding because I was pulling so tightly to enable the wire to bend. The other problem with thick wire is that first turn has to be very accurate because all subsequent turns will try and conform to it. The wire is thick though and that makes it is very difficult to get an accurate bend in a very tight radius as is required on that first turn.

I haven't quite given up on this technique though. I have 6 feet of 9 AWG magnet wire left over from my initial failed attempt that I will use again. I think this time though I will not use my lathe so that I can have fine hand-control over the wire. I will place the jig on a stool and then I will walk my body around the jig as I wind. This worked fairly well on my second attempt, the result of which is the coil that you saw posted earlier. The only difference being that I used a cone as a winding jig on my first attempt and then compressed the resulting spring-like coil down into the G10 mount.

19
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« on: November 28, 2019, 05:40:21 AM »
I have a Marx generator which is powered by an average-size flyback transformer driven by a ZVS driver. I use a 24 volt DC supply to power the ZVS driver. The flyback and ZVS driver have no problem with the 24 volts. Perhaps this will allay your concerns about the 24 volt power supply.

20
Induction launchers, coil guns and rails guns / Re: Sense coil fabrication?
« on: November 28, 2019, 05:21:30 AM »
I've considered most of those methods but I haven't decided on the best method. If your going to use a HV capacitor discharge (~ 5000 volts) such as I use, hook-up wire may not have adequate insulation. For lower voltage with more capacitance it may be OK. Double-build magnet wire will probably be needed for HV. You can also get more turns with magnet wire.

My preferred method would involve what I have on-hand in my lab. The good thing about a spiral is that once you ACCURATELY start and secure the initial turn, the remaining part of the spiral can just be wound around each successive turn assuming that you want the turns to touch. The hardest part is keeping the turns laying flat and in place while you are winding the coil. That first accurate turn could be made by following a traced line on a board which would be the bottom surface of the winding jig. That line could be defined by small nails to supply a winding form for that first turn.

I envision making a cross out of a stiff material such as G10, steel, or aluminum that will press down on the coil as it being wound. The cross will be the upper part of the winding jig. A bolt through the center of the cross could be used to apply the proper amount of force. After the coil has been wound and is still in the winding jig there are 2 options.

Option 1 would entail tacking the coil in place, while being held securely by the cross, with a few select spots of 2-part epoxy. After the epoxy sets, the cross can be removed and the entire coil can then lifted off as a complete unit. It can then be dropped into a machined hole in a sheet of G10 and immersed in 2-part epoxy

Option 2 would be similar to option 1 except that the finished coil would not be epoxy-tacked in place. The cross would be removed and the entire coil would be removed. Since no tacking was done, it will spring outward to some extent larger than the designed diameter like a clock spring. After the recessed hole has been machined into the G10 holder, the coil can then be tightened up and wound inside of the G10 coil holder.

Having experienced the "pleasure" of trying to wind these coils with 9 AWG wire I am tending to prefer option 1 for my next attempt.

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