head_spaz said:
It is not like we can use anything for suitable insulators.
There are 3 things, at least. Qualities needed.
1. electrical insulation factor
2. can take high temperature
3. mechanically strong
These special insulators we buy,
often have got some figures for #1 and #2.
They are tested for electrical insulation
and not to be damaged and keep insulation at higher temp, say 150-250 degrees.
Aluminum Oxide brick are very mechanical strong.
They wont get holes or get damaged by age.
Some Plastics, even if laminated with some minerals
can in some cases change with age
and when they are put under high mechanical pressure and stress.
We all know about rubber, sunlight (UV) and age. (Old Bands of Rubber! And very old CAR TIRES. )
and that not all plastic film is suitable for use to protect photographies.
Because they can do bad effects over time.
Teflon as you mention is a very good electrical qualities insulator material,
used often for wires and connectors. For example RCA.
But here we deal not only with electrical factor
but also with possibly, even if sometimes for a very short time,
heat, high temperatures,
as well as the MECHANICAL STRESS factor.
There is also a certain limit to insulator THICKNESS.
Therefor such silicon rubbers that can be used with good results
ARE NOT ALLOWED to be TOO THIN.
Because then safety and good quality can not be GRANTED.
So we hardly find any such insulators being <0.3mm thick.
This limit can be very different for different materials.
lineup
XEAGLEKEEPER said:
Actually your reply and mine is saying, partly,
the same thing.
Your statement is in the short form.
My statement is 'the school teacher' in action.
But the bottom line advice and recommendation ends up in the same:
It is not that easy, we can use whatever material for heatsink insulation,
and expect it to work at all or give very nice result.
There are in fact not very many materials
that can fulfill all the requirements for a good safe insulator
with low thermal resistance!
This is why they do not very often come at budget price ...
lineup
Workhorse said:
I can not say I have any practical experience of using Kapton.I do not even know exactly what this is.
But i have read some info on different option
and so far, I can only see 2 options for me,
(my country is small and not many chances to find suppliers).
1. AluminiumOxide bricks - they are the best, both in mechanical reliability
and low thermal resistance.
Perfect choice for TO3 big transistors at for example Class A operation. High currents, lot of heat.
(I do not count the poison BerylliumOxiode as an option
- FORBIDDEN to use in my country, and I almost sure within EU
You may NO longer even sell a device, an amplifier that has got this substance!)
2. Enforced Silicon Rubber.
Easier to use than AlO, as it needs no thermal grease.
And you just stick it onto the transistor - self-adhesive, like tape.
A good option for low power, lower heat for example TO220 transistors.
Notice the big difference in thickness.
Aluminum Oxide bricks are 1.5-3.0 mm.
While reinforced Silicone Rubber is only 0.3 mm
More info and/or ordering within EU:
http://www.elfa.se/elfa-bin/dyndok.pl?dok=2008961.htm
--------------
Another recent topic where we discussed almost same topic ...
Aluminium Oxide thermal resistance post by lineup
17th October 2006
http://www.diyaudio.com/forums/showthread.php?postid=1032056#post1032056
An externally hosted image should be here but it was not working when we last tested it.
head_spaz said:
While it may be a good insulator, it has no creep resistance. When you put it into compression, it will slowly cold flow. This will cause shorts, and it will cause the device being held down to loosen over time.
I do not recommend it's use.
Workhorse said:
Kapton is an excellent, stable plastic. It has a dielectric withstanding capability of 6 Kilovolts per thousanth of an inch thickness. Absolutely amazing.
It is reasonably strong, but once nicked, will tear easily.
Sharp edges and burrs will cut through it as well. But once assembled with caution, will last forever.
It's also the only reasonable plastic which is flexible in liquid helium, should you need that..
But as with all plastics, the thermal resistance is still kinda high. I would go with alumina.
I've used those plastic/rubber type insulators... I assembled a pair of SWTPC tigersaurus amps back in '81, and they are still in service today. (every decade or so, I have to re-tighten the TO-3 screws). So while I do not prefer them, I have had good luck with them.
Cheers, John
I wouldn't use it. It seems its thickness is too unpredictable when stretched and its thermal conductivity may be limited.
Otherwise, try an experiment with it. Maybe use a three-terminal regulator, use teflon tape to mount it to a large heatsink, then flog it with lots of power to see how well it works. An LM7805 is cheap, is fully protected, and while dissipating 25 watts, will stress any thermal film it's attached to. If you do, please report the test parameters and results here. I'm sure lots would be interested to see abusive test results and if it works.
One insulator I've used with good success and I've seen used many, many times in car audio is to simply use the anodizing of heatsink as the insulator. It's very thin, mechanically solid, and electrically insulates well while being a thermal conductor. Caution and care must be used along with silicon thermal paste, but otherwise, it works great. Some don't recommend it, but if it can survive a car environment (temperature extremes and vibration) for many years, it will do well for home hi-fi use. Just make sure the heatsink surface is very flat, everything is clean and burr-free, and the mounting hardware is properly implemented.
Otherwise, try an experiment with it. Maybe use a three-terminal regulator, use teflon tape to mount it to a large heatsink, then flog it with lots of power to see how well it works. An LM7805 is cheap, is fully protected, and while dissipating 25 watts, will stress any thermal film it's attached to. If you do, please report the test parameters and results here. I'm sure lots would be interested to see abusive test results and if it works.
One insulator I've used with good success and I've seen used many, many times in car audio is to simply use the anodizing of heatsink as the insulator. It's very thin, mechanically solid, and electrically insulates well while being a thermal conductor. Caution and care must be used along with silicon thermal paste, but otherwise, it works great. Some don't recommend it, but if it can survive a car environment (temperature extremes and vibration) for many years, it will do well for home hi-fi use. Just make sure the heatsink surface is very flat, everything is clean and burr-free, and the mounting hardware is properly implemented.
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