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Topics - MRMILSTAR

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1
Voltage Multipliers / Different styles of HV electrodes
« on: July 14, 2020, 09:34:39 PM »
On large Marx generators, CW multipliers, and other HV equipment I often see large shielding electrodes  composed of many smaller bowl-shaped elements. I assume that this construction technique is used because its simpler to build and is a close approximation to an ideal shape such as a toroid or sphere.

Other times I see the shielding electrode made from tubular sections which, in the Tesla coil world, we usually call a skeleton electrode which again is a close approximation to an ideal shape.

And then other times I see an actual spun aluminum electrode with the ideal shape.

Does anyone know why one technique is used instead of another? I assume that its just simplicity of construction and cost for the large electrodes. The attached image shows examples of these.

2
Voltage Multipliers / E-Bay Marx generator
« on: June 27, 2020, 09:36:57 PM »
I just saw this very interesting Marx generator on E-Bay. No specs unfortunately. I wonder if its actually a CW multiplier. Does anyone know anything about it?

https://www.ebay.com/itm/MARX-IMPULSE-GENERATOR-LIGHTNING-INDOORS-MADE-BY-VARIAN-SEMI-CONDUCTOR-FREE-SHIP/184340134249?hash=item2aeb873969:g:qD8AAOSwsvhe8-iR

3
Capacitor Banks / Crimping using pulsed power
« on: May 09, 2020, 10:02:16 PM »
I was playing around with my pulsed power machine a few days ago and decided to try something different. I took a 2 inch long piece of 0.875 inch diameter copper pipe and did a shrinking experiment. I placed the piece inside a 12-turn 12 AWG work coil and hit it with 7000 Joules. The result was an almost-perfect hourglass pinch. The work coil wasn't wound tightly against the pipe so I could have done the same thing with less energy with a tighter coil (better coupling) but I already had a few of these spare work coils made for quarter shrinking laying around.

This technique is actually used in industry where one piece has to be crimped on to another piece. Supposedly this method produces a better more uniform crimp than can be accomplished mechanically.

4
Spark Gap Tesla Coils (SGTC) / Effect of UV radiation on white PVC
« on: May 02, 2020, 05:41:09 PM »
About a week ago I was inspecting the base of my SGTC (6.5 KVA) and saw this brown spot on the bottom of the secondary coil form. This spot is about 4" x 4" and is in the direct line of site of my RSG electrode gaps. Its about 12" away from the spark gap. The UV radiation from the spark gap is so intense that it has turned this spot on the PVC tube brown. Total exposure time to UV was probably about one hour.

My fix was to use aluminum tape to cover that spot and protect it from further UV radiation damage. The pictures show before and after the patch.

The lesson learned is to remember that white PVC is very susceptible to UV radiation damage.

5
Spark Gap Tesla Coils (SGTC) / G10 and GP03 UV susceptibility
« on: April 29, 2020, 03:54:07 PM »
There isn't a "materials" section on this forum so I posted my question here since it is related to spark gap Tesla coils.

I have a large Tesla coil (6.5 KVA) which uses a RSG (rotary spark gap). The insulating parts of the RSG are made of several different materials. The base is made from a sheet of GP03 which is frequently used on outdoor HV transformers. Many of the support structures, including the RSG rotor,  are made of G10. Does any one know what the susceptibility of G10 and GP03 are to UV light as emitted from a spark gap? These materials are about 6" away from the spark gap.

I have been using this RSG for several years now and it has held up well with no problems. While recently inspecting the underside of the base of my TC I noticed that a 4" x 4" section of the PVC secondary coil tube was yellowed because it is in the direct line of sight of the spark gap which is about 12" away. I do know that PVC is highly susceptible to UV light so I plan on shielding that spot with a little bit of aluminum tape. However, this episode got me to wondering about the integrity of the RSG parts which are made of G10 and GP03 which are also in the direct line of sight of the spark gap and are much closer. I can see no apparent discoloration.

Do I need to add some sort of UV shielding to those parts? There's not much that I can do about the G10 rotor since it spins at 3600 RPM and must maintain excellent rigidity and also remain an excellent insulator against 14.4 KV RMS. I also can't use the aluminum tape on the support structures for UV shielding as I did on the PVC tube because they must insulate against 14.4 KV RMS. I do happen to have a lot of teflon tape which is claimed to be UV-resistant so I could possibly use that if needed.

6
Maybe most members are already aware of Chinese counterfeit USB flash memory sticks. If not, here's a warning. I see a lot of memory sticks on E-Bay and Amazon that purport to have an extremely large amount of memory that costs almost nothing. For example I saw a 2 TB USB memory stick on FB today for about $25! It even offered buy one get one free!

Everyone should be aware that almost any memory stick can have its internal software hacked to make it appear to be almost any size. For example a 8 GB stick can be hacked to appear to have 2 TB. Windows Explorer will indicate 2 TB but that isn't what you have. Your only indication of a counterfeit will be when you try and write more data to the stick than it actually has. Your data will then usually begin to over-write itself. Many people don't realize for a long time that they are fake until that day when they finally try to exceed its actual memory capacity.

I regret to say it but the only reliable place to buy a USB flash memory stick is a brick-and mortar store. If you buy one on-line you are almost guaranteed to get a counterfeit if it is a very low price and high capacity. Brand names are no guarantee on-line because they are counterfeited as much as anything else.

Any memory sticks advertised to be larger than 256 GB are guaranteed to be counterfeit. If you check any of the major manufacturers such as Sandisk, Kingston, Sony, etc. you will see that none of them makes anything larger than 256 GB.

I bought one of these several years ago and learned the hard way. At the time I bought what was supposedly a 256 GB stick but it was actually 8 GB. The problem vexed me for a while. I even tried re-formatting the drive but I eventually figured out it was fake. The crook wouldn't even refund my money. Been there, done that.

7
Transformer (Iron Core) / MOT efficiency improvement idea
« on: April 16, 2020, 11:20:50 PM »
Like many others I use MOTs for various HV projects because they are usually free, readily available, and do the job even though they are very inefficient. I recently had an idea to make better use of MOTs to improve efficiency. Tell me what you think.

The problem with MOTs is that they are very cheaply designed and barely have enough iron and copper to operate properly. They seem to operate right on the edge of saturation and get very hot even with no load. I have measured a few and they will typically draw at least 400 watts with no load. The problem to be solved is how to get them to operate in the linear region and far away from saturation without major modifications and preferably no modifications.

The idea I have is to combine two very similar, ideally identical, MOTs. The primary coils would be connected in series and the secondary coils would also be connected in series. Proper phasing of the coils would have to be observed. With this arrangement each MOT is only being powered by 60 volts instead of 120 volts. This should be a much better match for the available iron and copper. The output voltage would be the same as that of a single MOT but the arrangement should waste much less power and run much cooler although with twice the weight. Effectively these two combined MOTs should act the same as a much larger transformer with the proper amount of iron and copper.

I haven't tried this yet. Any thoughts?

8
Electronic Circuits / Motor-run capacitor RMS current
« on: April 16, 2020, 06:03:22 PM »
I use several 100 uF, 400 volt motor-run capacitors for PFC on my SGTC. I have tried to find the RMS current capacity of these capacitors with no success. Does anyone know what the typical RMS current capacity of a typical 100 uF, 400 volt motor-run capacitor is?

9
Light, Lasers and Optics / 95 watt blue laser array
« on: March 14, 2020, 07:25:00 PM »
I am very tempted to build a hand-held laser using this blue laser array. The output is 95 watts at about 30% efficiency! Since I would probably blind myself or someone else, its probably not a good idea. Very cool though. The laser diode array is $465 on E-Bay. Have a look at this styropro video where he built such a hand-held laser. It will ignite almost anything with no focusing.


10
Electronic Circuits / Unusually long-lived electrolytic capacitor
« on: February 26, 2020, 05:43:34 AM »
I have a project that I built in 1969 that still operates normally. It is a small vacuum tube tesla coil (VTTC) but that is not important. One of the components is a 10 uF electrolytic capacitor rated for 450 volts which sees a maximum voltage of 350 volts. The capacitor is used to smooth the output of a full wave bridge rectifier for driving the plate of a 6L6 tube. The VTTC is used very infrequently.

My question concerns the lifetime of this capacitor. This capacitor is still functioning after 50 years! I measured its capacitance and it is within tolerance. From my reading I was under the impression that this capacitor should have dried out and died many years ago. Is it unusual to have an electrolytic capacitor last this long? Should I replace it or should I follow the "if it ain't broke don't fix it" rule?

11
Radio Frequency / 500 KW AM radio transmitter
« on: February 12, 2020, 05:05:53 PM »
I found this very interesting video describing the only 500 KW AM radio transmitter to ever operate in the US. It was built in the 1930s and is now decommissioned. Everything about it is enormous. Huge vacuum tubes, huge inductors, huge capacitors, huge transmission lines. Huge motor-generators were used to convert AC to DC for the vacuum tube filament power to prevent 60 Hz hum on the audio. The vacuum tubes were water cooled and had their own outdoor cooling pond.

This is radio engineering on a massive scale which is not done anymore.You can skip the first 10 minutes if you want to get right to the good stuff. Have a look.


12
Vacuum Tube Tesla Coils (VTTC) / 833C versus 833A vacuum tube
« on: February 09, 2020, 09:16:13 PM »
My VTTC currently uses an Amperex 833A tube. As most of you know, you have to be careful of the power level when running in CW mode (non-interrupter). When in CW mode you have to pay careful attention to the plate color. If it gets too hot it will melt. Because of this I can't operate my VTTC at full power in CW mode, only in interrupter (staccato) mode.

I have been considering replacing the 833A with a 833C. My understanding is that the graphite plate of the 833C will allow me to operate it at a higher power level in CW mode. I have never used a 833C. My understanding is that the graphite plate never changes color regardless of power level. My concern is that I will no longer have that visual feedback to know if I am over-powering the plate. I wouldn't want to accidentally destroy an expensive 833C.

Does anyone have any experience using a 833C in this application? Do I need to worry about the plate temperature?

13
Capacitor Banks / Exploded energy discharge capacitor
« on: January 04, 2020, 11:52:40 PM »
Someone working at Laurence Livermore National Laboratory posted this picture a few months ago on FB. This is what the guts of a large energy discharge capacitor looks like. I believe it was 300 uF rated at 24 KV. The source states that it was used in laser fusion experiments and was pushed a little too hard.  :) This capacitor is made by ICAR. From the outside it looks similar to one that I have except mine is an Aerovox.

I must say that the inside of this looks nothing like I expected. It appears to be comprised of a bank of smaller capacitors immersed in oil. I was expecting a stack of alternating parallel sheets of foil and dielectric all immersed in oil.

Does anyone know if all of these large energy discharge capacitors are constructed in this manner?

14
I am offering a large Maxwell energy discharge capacitor. The approximate weight is 165 pounds. The item is only available for pickup. I will not ship. I am asking $150. Here is a link to the local Craigslist ad for more information.

https://cedarrapids.craigslist.org/ele/d/cedar-rapids-energy-discharge-capacitor/7047627361.html

I am located in Cedar Rapids, Iowa.

15
I see these little HV modules all over E-Bay for practically nothing. Does anyone know what circuit topology is used inside these things? Is it some sort of CW multiplier? The voltage ratings listed are nonsense. I have seen numbers ranging from 20 KV to 1 MV! I suspect it is close to 20 KV. Are they useful for anything? The comments that I have read indicate that they are very fragile. They are so cheap I am considering buying a few just to experiment with them.

Here is a typical listing:

https://www.ebay.com/itm/DC-3-6-6V-400KV-400000V-Boost-Step-up-Power-Module-High-voltage-Generator-Newest/223799936315?hash=item341b84053b:g:mtcAAOSws65d-Hqx

16
Voltage Multipliers / Are CW multiplier diode resistors needed?
« on: December 17, 2019, 09:52:40 PM »
Does anyone think that I need current-limiting resistors on the charging diode strings? My design specs are:

* 14 stages
* 1.7 nF doorknob capacitors rated for 30 KV
* Each charging diode string consists of three 20 KV diodes rated for 30 ma each
* 4.5 Mohm resistor string feeding the top load for HV current limiting
* Power supply consisting of 2 identical AC flyback transformers in parallel
* Flyback transformers driven by ZVS driver (Royer oscillator)
* Target peak HV power supply voltage of 15 KV
* Everything is in oil

I would like to avoid using current-limiting resistors on the diode strings because space is tight.

17
Voltage Multipliers / Mineral oil breakdown voltage
« on: December 14, 2019, 05:00:39 PM »
I plan on using food-grade mineral oil for my CW multiplier project as insulator. For purposes of parts separation I am assuming that the breakdown voltage of mineral oil is 10 KV/mm. Does this seem about right?

18
I saw this Aerovox energy discharge capacitor on E-Bay. What I find unusual about it is that it appears to have a preferred polarity. I seriously doubt that its electrolytic given its voltage rating so why does it have a preferred polarity? This seems to be contradictory to energy discharge applications which typically generate a damped sinusoid which is obviously AC. I couldn't find a data sheet for this series. Is anyone familiar with this capacitor technology?

https://www.ebay.com/itm/High-Voltage-Energy-Discharge-Capacitors-Aerovox-196uF-2200VDC/163973233399?hash=item262d90faf7:g:KbIAAOSwGrBd5wr9

19
I have been using Windows Media Center (WMC) with Windows 7 for almost 10 years. I host this on a home theater PC (HTPC) which I built. I use this in combination with a Ceton 4-channel cable tuner card and a cable card which I rent ($2 per month) inside the HTPC to record and watch encrypted and unencrypted cable TV programs. I also use the HTPC for all of my media needs including watching movies of any format and listening to music. Even though Microsoft stopped supporting WMC with the release of Windows 10, it still works great with Windows 7 which is pretty amazing considering that it hasn't been updated or supported in over 5 years.

Microsoft has now announced that both Windows 7 and the electronic program guide (EPG) needed for WMC will no longer be supported starting in January 2020. I don't need Windows 7 updates or support. I can just keep on using it with no problem. The problem is the EPG. Without an EPG WMC is useless for recording or watching TV programs. I need to find a replacement for the EPG.

I have been reading about the EPG123 guide as a replacement for the discontinued Microsoft EPG. It requires a yearly subscription of $25 which is reasonable. Does anyone have any experience with EPG123? Are there any other replacement EPGs that anyone knows about?

Please don't suggest that I just buy or rent a TIVO. I want to keep my HTPC as my sole media player/recorder.

20
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?

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