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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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22nd Oct 2016, 6:04 pm | #1 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
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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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22nd Oct 2016, 6:23 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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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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22nd Oct 2016, 6:28 pm | #3 | |
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Re: encapsulating wire wound resistors
Quote:
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22nd Oct 2016, 6:36 pm | #4 |
Heptode
Join Date: Jul 2009
Location: Stoke-on-Trent, Staffordshire, UK.
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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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22nd Oct 2016, 6:36 pm | #5 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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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. |
22nd Oct 2016, 6:50 pm | #6 |
Dekatron
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Location: Chesterfield, Derbyshire, UK.
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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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22nd Oct 2016, 6:53 pm | #7 |
Dekatron
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Re: encapsulating wire wound resistors
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22nd Oct 2016, 7:05 pm | #8 |
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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. |
23rd Oct 2016, 12:30 pm | #9 |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
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Re: encapsulating wire wound resistors
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23rd Oct 2016, 2:03 pm | #10 |
Dekatron
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Location: Leominster, Herefordshire, UK.
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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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23rd Oct 2016, 2:18 pm | #11 |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
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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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23rd Oct 2016, 3:10 pm | #12 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
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Re: Encapsulating wire wound resistors
What do they use in MICC (mineral insulated cables)?
A material with good heat conduction would be desirable. |
23rd Oct 2016, 6:35 pm | #13 |
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Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
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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. |
23rd Oct 2016, 7:37 pm | #14 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
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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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23rd Oct 2016, 10:32 pm | #15 |
Dekatron
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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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23rd Oct 2016, 11:34 pm | #16 |
Heptode
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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 ?). |
24th Oct 2016, 12:15 pm | #17 |
Dekatron
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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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24th Oct 2016, 12:39 pm | #18 |
Heptode
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Location: Melbourne Australia
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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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24th Oct 2016, 3:39 pm | #19 |
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Re: Encapsulating wire wound resistors
Dry plaster of paris jammed in, after a while it will absorb moisture and set.
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24th Oct 2016, 6:50 pm | #20 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
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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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