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Tubing for high voltage wire.
Tubing for high voltage wire.
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Old 14th September 2019, 11:42 AM   #1
eduard is offline eduard  Netherlands
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Default Tubing for high voltage wire.

Hello,
I am planning to use a 900-0-900 transformer. It has 30 cm/ 1 foot ''outputwires '' The two 900 volts need to be twisted on their way to the 5R4GYB rectifier. I wanna cover the existing wire with a teflon? tube or a heat shrink tube just to create some extra safety. What is the best way to accomplish this. Not all PTFE seem to have the same voltage rating. I do have a large coil of teflon tubing but i dont know its electrical properties.
Many thanks in advance, Eduard
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Old 14th September 2019, 11:47 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The wires from the transformer will be enough to insulate electrically otherwise the transformer would fail a flash test.
The wires only need twisting together for aesthetics, the current drawn through them is insufficient to cause disruption to the rest of the circuit, unlike heater wiring, if laid out away from sensitive electronics.
By all means use the sleeve of your choice.
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Old 14th September 2019, 12:20 PM   #3
eduard is offline eduard  Netherlands
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Hello,
The current going to the rectifier will be around 80/90 mA so the advice to always use twisted wires will not have that much of a profit.
The R core transformer i received as a kind of exchange from a friend in Hongkong so probably it is Chinese made. The wires used to make a two coils are connected at the '' inside '' ( anyway covered by the paper cover showing the specs) to a few coloured wires to seperate the different voltages. These wires have a printing with 300/500 V on them. I asked for several secundairies so have 975-950-925-900-0-900-925-950-975. As far as i can see now i am going to use the 900 volt ones. Once the amp is finished i could just cut the wires i wont be using or solder them all to one of these ceramic binding posts but these have also voltage ratings. 975-950-925 on one post and the other 925-950-975 on another terminal block . Both block well isolated from the chassis because the centre tap will be connected to the chassis.
Or i could use a thick teflon tube for all individual wires and fix them close to the transformer with a perfect '' mounting solution)
I could look at Mouser if they sell some tubing that has a link to a datasheet.
Greetings, eduard
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Old 14th September 2019, 12:37 PM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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I find braided PET or Fibre glass sleeving is excellent to protect the cables.
Cable Sleeves | Cable Sleeving | RS Components


Hope that helps.
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Old 14th September 2019, 07:37 PM   #5
Mark Tillotson is offline Mark Tillotson
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PTFE tubing is very stiff and would need care with strain-relief at the ends of the tube if passing fine wires through it, perhaps some heat shrink on the ends.


PTFE is a very good insulator and doesn't absorb moisture. Any kind of braid is little use preventing flashover, not really any point with these voltages, solid tube is better. I think PTFE is a good choice. You've got 2.5kV peak between the ends of that secondary, that's very serious voltage.
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Old 14th September 2019, 07:43 PM   #6
PRR is offline PRR  United States
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The fiberglass sleeve is good.

I would trust the 'rubber' on that not-old outdoor extension power cord that I hit with the weed-wacker. It's good thick PVC, over-sized for mechanical robustness (oops) rather than applied supply voltage. Cut-back the frayed end, cut a nice clean 1-foot(whatever) length, and pull the cores out. I've used it inside chassis.

Depending what you use inside your walls, scrap in-wall cable may be rated for higher temperature. US/Can types NM and UF are rated much hotter than I would touch (90c for wires, and the jacket should not drip much before that; in tests it doesn't).
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Old 16th September 2019, 06:04 PM   #7
RJM1 is offline RJM1  United States
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Silicon rubber has some of the best insulating properties (used in 30KV anode leads for CRT's). When I worked for a high definition video projector manufacturer in the '90s we used to cover the silicon anode lead(35KV) with Tygon tubing for added insulation and to keep dust and oils from being attracted to the silicon wire. The only problem is that it might be a bit thick for you.
Tygon is a very soft PVC.

Last edited by RJM1; 16th September 2019 at 06:22 PM. Reason: Forgot the u on you
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Old 20th September 2019, 12:05 AM   #8
trobbins is offline trobbins  Australia
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Eduard, it is sad that a 975-0-975V transformer has possibly under-rated lead out wires.

From a safety perspective, any type of standard solid insulation applied as an additional layer of insulation over the existing wire insulation should achieve a suitable outcome, whether it be bland heatshrink tubing or some exotic tubing. This is not about high-temperature performance, but more about total insulation thickness and so a braided tube may not be as applicable as a solid tube.

Given a multitude of tap wires, I would carefully loop/bundle each unused tap wire and put heatshrink tube over the bundle, to isolate it from accidentally being in contact with chassis or parts or other wires that may be at up to 1900Vrms difference.

You may not need to twist the HT wires going to the valve base, but I would certainly run then as close together as possible.

900-0-900V is right at the limit of 5R4GYB, indicating you need to be quite aware of its peak current limitations due to filter capacitance, even if the continuous current is well below datasheet maximum. The valve base also needs some care in selection, with nothing connected to the spare terminals.

Protection should imho also be considered. 250VAC fusing of the CT is allowable for PT secondaries up to quite a high voltage level, but 900VAC may be exceeding that confidence level, which likely precludes the use of secondary side fuse protection. Adding multiple series ss diodes in series with each 5R4 anode provides a worthy level of protection if the 5R4 ever goes gassy or leaky (as will happen at some time).
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