N Cook wrote:
Ross Herbert wrote in message
...
On Sun, 2 Mar 2008 20:55:26 -0000, "N Cook" wrote:

:N Cook wrote in message
...
: whit3rd wrote in message
:
...
: On Mar 2, 1:23 am, "N Cook" wrote:
: Not the usual nylon or whatever ones that I never trust for high
: temperatures...
:
: Mounting 'kits' for power transistors cost more than many of
: the transistors themselves (over a dollar for a mica gasket
: and some screws and washers). You can cut a small length of
: PTFE (Teflon) tubing for the machine screw shaft, and use a
: piece of fiberglass-reinforced plastic (FRP) as a washer to hold off
: the head, I've used polyimide (Kapton) tape for the heatsink
: insulation barrier.
:
: I'm not sure where my FRP washers came from, but the
: Teflon tubing, about gauge #9 for 4-40 screws, is a common
: laboratory supply item.
:
: reply:
: I've plenty of sources of PTFE suitable for the insert part but no
known
: sourse of suitable FRP. I wonder if fibre reinforced phenolic board
would
:be
: suitable for the washer, I'll try heating some. Of course that washer
part
: does not have to be circular and cutting it square with a hole would be
: easier.
:
:
:
:The washer part could be a disc or 2 of that stiff plastic material found
:inside power supplies where there is a chance that some component will
touch
:the casing, next to a steel washer under the bolt/screw head.
:For that matter I don't see a problem of using a mica disc or 2 then a
steel
:washer for that part.


Richco make a high temperature shoulder washer designated MNI-HT
http://www.richco-inc.com/group_deta...&grpcode=A2390

The #4-220 is designed for the TO220 transistor mounting hole


But that is also nylon, just because they call it high temperature, does not
necessarily mean it does not compress/creep in normal but highly elevated
temperatures, leading to premature failure of device.
The point is I'm after some insulated and mechanically strong enough
alternative to plastic without having to drill into heatsinks for spring
retention.

MEC make thermoplastic transistor insulators that are rated up to 180c
continuous. Farnell sells them.

Ron(UK)


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