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Old 21st May 2018, 11:06 am   #1
mark_in_manc
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Default EHT insulation

Hi folks

I am working on a rather unusual project for a Polish friend - a 30-yr old pair of USSR night vision binoculars PNW57E. They used to attach to a padded tank helmet (sadly missing ), binos at the front, PSU at the back.

They work, but the connector which joins the PSU to the wires going to the optics is missing. I'm going to make one, but - it has to deal with about 19kV without tracking, and I'm a newbie in this area. I've got that figure from the internet - I was originally told they produced rather less, and indeed into my AVO8 it can manage about 1200v - the meter loads the inverter power supply audibly and squashes the output.

Circuit diagram for the power supply is attached below for interest.

The leads to which I need to attach a connector have an earthed outer screen, and the core is insulated with what looks like shredded polythene bags! This insulation works OK in the lead itself, but where it emerges it can make sparks jump up to maybe 10mm.

So the question - what can I experiment with in the plug I am making, to stop the core arcing to the connector body? Heat shrink? Plasticine? Silicon sealant? The connector will be based on a DIN plug with a screw collar since this thread fits that provided around the PSU socket - and I may start with a tubular Tufnol 'prong' about 2" long with a brass tip to make the connection.

cheers
Mark


(Safety - I know that getting bitten by something is not a great way of proving how non-lethal it is, but anyway, we've been there and it's rather like a hit from a vehicle HT lead )
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Old 21st May 2018, 11:27 am   #2
Al (astral highway)
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Default Re: EHT insulation

Quote:
Originally Posted by mark_in_manc View Post
...it has to deal with about 19kV without tracking, and I'm a newbie in this area. I've got that figure from the internet - I was originally told they produced rather less, and indeed into my AVO8 it can manage about 1200v - the meter loads the inverter power supply audibly and squashes the output.
Hey Mark,

Looks like an interesting project! Only I don't understand these figures. If the output is 19kV then it can't be 1200V reading from an AVO8. That would need a big old current to be flowing., completely inconsistent with the input impedance.

From what you're describing, if an arc is jumping 1cm, that's more in line with your 19kV than 1200V, so I'll ignore the AVO figure as spurious.

I'd try military spec silicone or plain old two part epoxy, for the isolation you need.

Good luck with it. I'm kind of relieved the helmet part isn't still around - it feels spooky to have an inverter up near the old brain tank!
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Old 21st May 2018, 11:31 am   #3
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Default Re: EHT insulation

When I was sorting out the EHT side of a Barco colour monitor, the only (common) material I could find that did not break down was PTFE. I found a model engineering supplier who sold such rod. I would certanly try to use that for the insulator in the 'plug''

Be warned it is not an easy material to work. It does not cut or machine 'cleanly'.
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Old 21st May 2018, 11:48 am   #4
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Default Re: EHT insulation

The uniform-field DC breakdown strength of air is 10kV/cm or so. Sharp tips on the metalwork will give some field enhancement which allows breakdown over longer distances with lower voltages. A 19kV supply should manage a 1cm spark comfortably.

As far as a connector design is concerned, distance is your friend. For a reasonably compact design that distance will have to be tracking distance along a surface. When I used to fettle temporary HV connectors at work we would commonly use the metal part of a banana plug. We'd turn the hex section down to a circle and then round off the sharp edges with a file, again to avoid field enhancement. Sharp edges can also start corona discharge into the air. This is bad a) because the current can load your supply, b) because it emits UV which ionises the air and lowers the breakdown threshold and c) because the ozone it makes is chemically corrosive to surrounding materials and is also toxic. The plug, and a turned down socket, each with the cable inner attached, were pushed into a short length of plastic tube from opposite ends and the coaxial ground braid was run over the outside of the tube and the whole lot held together with a jubilee clip on the outside. With a 6" length of tube and some care to smooth off all sharp edges (solder blobs are good) we could propagate single 300kV, microsecond pulses through a coupler like that. DC is a lot tougher though and I wouldn't have trusted it much above 40kV.

Silicone sealant is a good material for HV as are most plastics - polythene, polypropylene, PVC, teflon, PET (as long as it's pinhole-free) and even some composites such as GRP. Not carbon fibre though. Your shredded polythene bags might be layered teflon. Plasticene can be useful as a temporary cover for sharp points but at HV you really have to treat it as a conductor, not an insulator. Heatshrink might work but I'm not sure about the hot-melt glue inside the adhesive-lined stuff.

If you want to make measurements then a chain of resistors (I've used as many as 50 x few megohms soldered in series) is a good way of getting the voltage down to a manageable level and reducing the loading on the supply. For safety's sake it's wise to put at least one resistor in parallel with the meter just in case anything goes open circuit in the latter. You can, of course, buy specialist HV resistors from the big retailers which will give you tens or hundreds of megohms at many kilovolts.

Have fun and stay safe.

Cheers,

GJ
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Old 21st May 2018, 12:02 pm   #5
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Default Re: EHT insulation

That's very interesting. I have some PTFE rod and some Tufnol, so I'll have a go with both. I've also read up making an EHT probe on here with a bunch of high value resistors as a divider, so I'll see if I can measure the voltage properly. And yes - potting the inside with Araldite sounds like a way forward, though I'd better get it right first time!

Edit to add - In fact I'll follow GJ (x-posted) and try silicon sealant first, in case I need to get it apart...
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Old 21st May 2018, 12:36 pm   #6
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Default Re: EHT insulation

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Originally Posted by mark_in_manc View Post
That's very interesting. I have some PTFE rod and some Tufnol, so I'll have a go with both. I've also read up making an EHT probe on here with a bunch of high value resistors as a divider, so I'll see if I can measure the voltage properly. And yes - potting the inside with Araldite .
I would not pot with Araldite. Many years ago I had an associate that tried this for voltage multipliers used in insect exterminators, the resin fails. It tends to break down at high voltages. A much better choice is polyester resin, or professional high voltage withstand potting rubber compound. Also PTFE is better than Tufnol for high voltages. Tufnol can carbonize.

Don't re-invent the wheel for an EHT probe, you can easily get a 25kV rated EHT probe for DC measurements for standard TV work quite cheaply. These contain a large high voltage withstand long resistor. If you try to emulate this with many series lower voltage resistors, it will likely fail, despite what you have read up on this; I can assure you it is a really bad idea. I have been through this process many times developing high voltage withstand probes for laboratory investigations up to 50kV. It is much better to have a specially made high voltage withstand/rated single resistor. For example resistors like the Ohmite 10W rated 100M ohm 1%, part MOX94021006FVE. I think either E14 or RS has them. Over 30kV its best to put them in oil.

When it comes to measuring high voltages, the problem gets very tricky when it is high voltage complex AC waveforms that you need to measure, not in this case, you only need DC.

For example a typical TV 25kV rated EHT probe has a very very poor HF response because of the large resistance and distributed capacitances, if you try to measure a square wave with it you will find it integrates it into a near sine wave. So these sorts of probes are only any chop for high voltage DC measurements. But I think that is all you need.

At least your circuit is a very simple Royer oscillator driven by a linear negative voltage regulator and has a simple multiplier on its output, so it should be easy to work with and fairly robust.
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Old 22nd May 2018, 5:53 am   #7
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Default Re: EHT insulation

I've used melted milk bottle for EHT insulation Marc, though it's fiddly to work with. As for measuring high voltages I have a proper EHT probe you can borrow or a homebuilt one you can have.

Andy.
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Old 22nd May 2018, 6:25 am   #8
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Default Re: EHT insulation

If the outer is the diameter of a DIN locking ring, then the inner to outer distance isn't enough for 19kV in sea-level air, so the connector has to have an arrangement of interlocking pieces of insulation to put the air path length up. Look at the PTFE dielectric pieces inside the mating sections of male and female 50 Ohm BNC connectors for an example (75 Ohm ones are different) You'll need a deeper interlock to get enough creepage distance.

PTFE is good because it tends to stay clean, and the build-up of muck over time spoils EHT connectors.

If you have access to a lathe, then PTFE rod would be a good start.



There are EHT versions of Lemo 1w connectors and electojumble have them, but the prices are, um, discouraging.

David
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Old 22nd May 2018, 9:23 am   #9
mark_in_manc
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Default Re: EHT insulation

Quote:
Originally Posted by Radio Wrangler View Post
...so the connector has to have an arrangement of interlocking pieces of insulation to put the air path length up...

There are EHT versions of Lemo 1w connectors and electojumble have them, but the prices are, um, discouraging.
Yes, my insulation problem is all about how to deal with the inner wires as they enter a 2" long, tubular insulating prong, and how long to make them before soldering them to a brass tip which I think I'll screw into the end. Sounds like 'as long as poss, and until then hold the weird tattered insulation around them together with PTFE tape and silicon sealant'.

Expensive connectors - I just fixed an odd 'mil-spec' 6 pin thing on a large American load cell at work which was mostly sheared off, because we were quoted about £80 all-in (supply, not fit!) to buy a replacement. I could make them one at a time from brass and nylon stock for less than that, once I got into the rhythm!

Andy - I'd like to play with your spare home-brew probe - let me know about costs etc.

Thanks all

Mark
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Old 22nd May 2018, 12:25 pm   #10
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Default Re: EHT insulation

Insulation breakdown in Araldite could be due to the presence of air bubbles at which internal discharge can occur.At Plessey it was the practice to put potting compounds like Araldite in a vacuum chamber before use to get rid of air bubbles created in the mixing process for applications where high reliability was vital.
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Old 22nd May 2018, 1:09 pm   #11
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Default Re: EHT insulation

I'm sure that is correct. On the other hand polyester resin tends to set crystal clear with the absence of bubbles in most cases and doesn't require a vacuum setup to help get rid of the myriad of fine bubbles that are often seen in Araldite after its mixed.
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Old 23rd May 2018, 6:55 am   #12
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Leave it with me Mark, I'll dig it out and will drop you a PM, Andy.
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Old 23rd May 2018, 9:36 am   #13
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Default Re: EHT insulation

Could be worth looking at the characteristics of the inner polythene core-to-braid dielectric/insulation of coaxial cable: the thicker varieties (used for amateur-band transmitting antenna feeds) can handle quite a few Kilovolts.

Also, what about the old-style copper-cored car spark-plug cable? That will take 25Kv without issues.
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Old 26th May 2018, 6:55 am   #14
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Default Re: EHT insulation

What about that powder you get in heating elements? Not sure if it can handle EHT, it'd need some research. If would be easy to pour inside a plug.

Andy.
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Old 26th May 2018, 3:59 pm   #15
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Default Re: EHT insulation

That powder is Hygroscopic, which means it readily absorbs moisture from the air, a similar compound is used in mineral insulated cables (MICC) or also known colloquially as "Pyro"
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Old 26th May 2018, 6:13 pm   #16
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Ah, no good then. nice to see you back R2B, havn't seen any posts by yourself for a while.

Andy.
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Old 26th May 2018, 9:37 pm   #17
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Alright Andy, I am about just not much posting on any forum, life, work and such like getting in the way.
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