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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th Oct 2020, 7:23 pm   #1
jamesinnewcastl
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Default Small Insulated Tubes - Any Good Source?

Hi All

Can anyone direct me to a source for small tubes of an insulating material please? I'm trying to make my own dog-bone resistors. I need 4mm, 5mm, and 6.7mm or very close with an ID of 3mm minimum. The method is to put a new resistor inside and connect to some meaty outer wires and leads - easy!

I've Googled and the only thing near it I can find are Carbon Fibre tubes, but obviously only an idiot would make up a good number of resistors with this material as it is conductive. If someone were to do this they would be surprised to find that the resistor they made was substantially lower in resistance to the resistor they had thought was in the tube. They might try again with a definite, resistance checked 100K to be certain of no mistake. Finding a seemingly random resistance they might use another meter to find out what was wrong with the first rubbish meter that had always been right before. Then, they would realise their stupid mistake and measure some of the tubing directly to find out that carbon - even in its fibre form - is conductive.

Just so that doesn't happen to me, the tube needs to be of an insulating material!

Cheers
James
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Old 25th Oct 2020, 7:33 pm   #2
Cobaltblue
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Default Re: Small Insulated Tubes - Any Good Source?

Just an off the wall thought how about PVC printed circuit board spacers.

Not suggesting this source just an example of what I had in mind

https://www.ebay.co.uk/itm/Essentra-...IAAOSw3xJfZKgp

I think you can get these in a hex shape as well as well some of the old dogbones

Cheers

Mike T
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Old 25th Oct 2020, 7:37 pm   #3
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Default Re: Small Insulated Tubes - Any Good Source?

Any particular colour? I suppose you will be painting it body, tip, dot colouring. Try B and Q if there is one nearby or a modelmaker's shop. I guess ceramic tube would be best but it would be pricey. Les
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Old 25th Oct 2020, 7:48 pm   #4
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Default Re: Small Insulated Tubes - Any Good Source?

Model shops or suppliers carry a range of styrene tubing. For example, not a recommendation.

https://modelshop.co.uk/Shop/Strip-S...d-tube/ITM1719

Watch for heat build up of course.

Ken
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Old 25th Oct 2020, 8:00 pm   #5
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Default Re: Small Insulated Tubes - Any Good Source?

Modelmakers seem to use quite a lot of ABS tube. I think it's stable up to about 80 centigrade. It can be difficult to get paint to stick reliably to plastics though.

The problem with all carbon comp resistors, of which dogbones are one type, is that they drift in value, typically upwards. There are still a fair few dogbones around, but the resistance of most of them won't match the paint colours. If, say, a 10-year life would be acceptable you could save yourself a good deal of fuss and just buy a dogbone one or two steps down the E12 range from the value you want and repaint that.

Cheers,

GJ
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Old 25th Oct 2020, 8:00 pm   #6
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Default Re: Small Insulated Tubes - Any Good Source?

Hi James, I have a selection of "fishbone" ceramic spacers here in various diameters and from 3 to 10mm long or so that could be bonded together.

Ed
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Old 25th Oct 2020, 8:29 pm   #7
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Default Re: Small Insulated Tubes - Any Good Source?

Plastic straws? get them soon as they seem to be doomed.
 
Old 25th Oct 2020, 8:29 pm   #8
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Default Re: Small Insulated Tubes - Any Good Source?

I needed a high voltage replacement - this was the first attempt - paint too thick.
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Old 25th Oct 2020, 8:34 pm   #9
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Smile Re: Small Insulated Tubes - Any Good Source?

I remember a valve based television project in a 50's practical television magazine. The insulating tube for an HT wire passing through an earthed screen was a piece of Macaroni pasta. Hmmm....

Christopher Capener
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Old 25th Oct 2020, 8:48 pm   #10
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Smile Re: Small Insulated Tubes - Any Good Source?

If you have access to a machine shop then RS Stock No.225-5482 might be worth a look.

This is a white machinable ceramic rod 6mm diameter although it is rather expensive at £30 for a 100mm length!! It would be ideal if the resistor you want to create is a high power one that will get hot. Should be relatively easy to paint as well.

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Old 25th Oct 2020, 10:31 pm   #11
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Default Re: Small Insulated Tubes - Any Good Source?

Possibly too large in diameter but the fibreglass tube used for flexible tent poles might work, or maybe a scrap fibreglass fishing rod?
Is glass tubing still available?
or drill down a wooden dowel?
You'd have to be careful though.
A.
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Old 26th Oct 2020, 12:01 am   #12
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Default Re: Small Insulated Tubes - Any Good Source?

modelmaking supplies (plenty online) do 'styrene tube'. It's plastic, but styrene keys quite well with solvent based paints. The one below is 'plastruct' brand.
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Old 26th Oct 2020, 9:26 am   #13
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Default Re: Small Insulated Tubes - Any Good Source?

Plastic rod rather than tube and find someone with a small lathe to drill it through.

gmb
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Old 26th Oct 2020, 9:31 am   #14
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Default Re: Small Insulated Tubes - Any Good Source?

James, google SRBP tube. Tufnol and Attwater are good, although minimum quantities may apply.
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Old 26th Oct 2020, 9:32 am   #15
M0FYA Andy
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Default Re: Small Insulated Tubes - Any Good Source?

B&Q certainly used to sell plastic tubing, I've got a couple of lengths somewhere I bought maybe a decade ago with the intention of replicating old-style capacitors, but never put it to use. Whether they still sell it I don't know, but worth a look.

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Old 26th Oct 2020, 9:55 am   #16
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Default Re: Small Insulated Tubes - Any Good Source?

Hi James, one thing to bear in mind is that whatever tubing yiou use it will reduce the power handling capabilities of the resistors inside, which may then get a lot hotter than normal

Ed
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Old 26th Oct 2020, 11:08 am   #17
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Default Re: Small Insulated Tubes - Any Good Source?

I had this idea a while ago, and having heard horror stories about badly made Chinese fuses where the end caps twisted off, i bought a load of the cheapest boggo Chinese eBay fuses I could lay my hands on, obviously planning to strip them and use the ceramic tubes. Naturally the ones I got weren’t sufficiently badly made, resisting all my attempts to take them apart. Still, a good idea in theory..

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Old 26th Oct 2020, 11:47 am   #18
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Default Re: Small Insulated Tubes - Any Good Source?

I've got loads of old fuse bodies if they are any use. These are 23.58mm long, 5.72mm wide and the hole is just over 3.5mm; a 3.5mm drill bit will pass through but a 4mm won't.

Last edited by ThePillenwerfer; 26th Oct 2020 at 12:16 pm. Reason: Added measurements.
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Old 26th Oct 2020, 2:15 pm   #19
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Default Re: Small Insulated Tubes - Any Good Source?

With some types of plastic there is also a fire risk if a fault causes the resistor to heat up rapidly.
Fuse bodies sound like they will be much safer.
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Old 26th Oct 2020, 2:26 pm   #20
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Default Re: Small Insulated Tubes - Any Good Source?

Are there any pottery enthusiasts on this forum?

Could porcelain clay or stoneware clay be wrapped around a glass rod, air dried, turned on a lath and then 'fired' to produce ceramic tubes?

Could a Portland cement and fine paving sand mixture be used to cast some tubes?

Soda glass tubes used by chemists might be the easiest to make but will paint stick to it?
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