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Old 22nd Oct 2016, 6:04 pm   #1
vinrads
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Default Encapsulating wire wound resistors

I am thinking of fitting two resistors in a metal tube, any thoughts regarding what filler to use. Mick.
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Old 22nd Oct 2016, 6:23 pm   #2
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Default Re: encapsulating wire wound resistors

What sort of power will they be dissipating? That you're talking wirewound makes me think quite a bit - in which case the 2 materials that come to mind are Fireclay, or the "gun-gum" putty used to bodge-up rusty car exhausts. Both will survive the sort of amounts of heat that your resistors probably won't!
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Old 22nd Oct 2016, 6:28 pm   #3
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Default Re: encapsulating wire wound resistors

Quote:
I am thinking of fitting two resistors in a metal tube any thoughts ,regarding what filler to use
Why not leave them in free air?
 
Old 22nd Oct 2016, 6:36 pm   #4
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Default Re: encapsulating wire wound resistors

This is often done to replace "Standee" resistors on Leak amplifiers, the "accepted" filler there is exhaust putty as used for cars.
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Old 22nd Oct 2016, 6:36 pm   #5
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Default Re: encapsulating wire wound resistors

If they're in a tube, allow for de-rating, if the tube is filled with stuff then allow even more.

Lawrence.
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Old 22nd Oct 2016, 6:50 pm   #6
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Default Re: encapsulating wire wound resistors

It sounds like exhaust putty is the way to go ,thank's for the reply's . Mick.
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Old 22nd Oct 2016, 6:53 pm   #7
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Default Re: encapsulating wire wound resistors

Quote:
Originally Posted by merlinmaxwell View Post
Why not leave them in free air?
I need to replicate a resistor fitted in a Rock Ola amplifier .
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Old 22nd Oct 2016, 7:05 pm   #8
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Default Re: encapsulating wire wound resistors

There is a fireproof silicone now that stands 1200 degrees,
if you need to know, pm me and I can give you the ebay contact.
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Old 23rd Oct 2016, 12:30 pm   #9
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Default Re: encapsulating wire wound resistors

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Originally Posted by Boater Sam View Post
There is a fireproof silicone now that stands 1200 degrees,
Is that Fahrenheit or Centigrade?
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Old 23rd Oct 2016, 2:03 pm   #10
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Default Re: Encapsulating wire wound resistors

It's 1200C. Just search for fireproof silicone.

This stuff looks even hotter:

http://www.ebay.co.uk/itm/Silicate-C...AAAOSwDk5UBGNG

NFI etcetc
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Old 23rd Oct 2016, 2:18 pm   #11
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Default Re: Encapsulating wire wound resistors

Depending on where you look, 1500'C (the limit for the material you linked) is either 'brilliant white' or 'dazzling white' hot. Either way, looks like there's some margin for this application .
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Old 23rd Oct 2016, 3:10 pm   #12
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Default Re: Encapsulating wire wound resistors

What do they use in MICC (mineral insulated cables)?

A material with good heat conduction would be desirable.
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Old 23rd Oct 2016, 6:35 pm   #13
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Default Re: Encapsulating wire wound resistors

Magnesium oxide I believe.
Silicate sealer though standing more heat is a cement type, made from silica, sand in other words. Sets hard.
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Old 23rd Oct 2016, 7:37 pm   #14
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Default Re: Encapsulating wire wound resistors

You could probably get a length of Pyrotenax cable (it's used for wiring up fire alarms -- you don't want the alarm not going off because the fire has melted the insulation and the wires to the control panel have shorted out) out of a builders' skip, anywhere offices are being refurbished; then just cut it open and extract the mineral insulation. It would benefit from a good bake in the oven, to drive out any moisture it may have absorbed, but it would be The Real Deal.

There must be somewhere being rewired in Sheffield .....
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Old 23rd Oct 2016, 10:32 pm   #15
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Default Re: Encapsulating wire wound resistors

But would anybody chuck Pyro in a skip? I'd think someone would grab it for the copper scrap before it landed!
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Old 23rd Oct 2016, 11:34 pm   #16
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Default Re: Encapsulating wire wound resistors

For starters, the heat has to egress somehow, otherwise even the best of resistors will burn up/degrade. It sounds like the resistor leads are soldered together and that joint is 'in the middle', and that there is some separation between the resistor casings and the ID of the metal tube, and that there needs to be functional insulation between the resistor cases and wiring and the metal tube of perhaps a few hundred volts?

One thought is to put the resistors in a tube of heatshrink (to provide the functional insulation) and then use a good thermal conductor paste to fill the void between heatshrink and tube ID (perhaps use heatsink goob ?).
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Old 24th Oct 2016, 12:15 pm   #17
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Default Re: Encapsulating wire wound resistors

Normal thermal conductor paste isn't a very good thermal conductor, it's just rather better than air so a thin layer which just fills in the small voids between two "flat" surfaces is useful.

It would probably be worth sacrificing a cheap thermocouple to check that the resistor temperature inside the final assembly is within ratings.
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Old 24th Oct 2016, 12:39 pm   #18
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Default Re: Encapsulating wire wound resistors

Yes it would have to be a special paste to have a highish conductivity. Anything that can cope with the temperature would be better than air.
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Old 24th Oct 2016, 3:39 pm   #19
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Default Re: Encapsulating wire wound resistors

Dry plaster of paris jammed in, after a while it will absorb moisture and set.
 
Old 24th Oct 2016, 6:50 pm   #20
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Default Re: Encapsulating wire wound resistors

I think the original part was a ceramic tube filled with a cement, probably like a fire cement. If you just want to replace a dead one why not leave it in place and fit new parts under the chassis.
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