Insulating hardware, esp.washers

[[email protected]]

Wavy washers are functionally about the same as Bellevilles, but they
won't gouge surfaces like Bellevilles and split rings will.

Handy--I passed on some, thinking they wouldn't be stiff enough.

Thanks.

James

 

[[email protected]]

On Saturday, December 21, 2013 1:34:12 PM UTC-5, [email protected] wrote:


Hi James, Nice art work! So are we talking about the washer between the back side nut and the heatsink?

I was mostly wondering what people do between the bolt and the
semiconductor--sorry I didn't make that very clear. I saw the
various offerings at DigiKey, but some of them looked cheesy.

(not shown in drawing) How about an anodized aluminum washer? There's a washer company up in Idaho that has all sorts of options. (Do you do any prep to the to-220 surface so it doesn't scratch the anodization*?

I didn't prep them. Trapping grit would be my main concern, but even
that shouldn't make much difference as long as the grit's non-conductive.

What about where the screw goes through the hole in the heat sink?)

That depends. If the screw's not touching the TO-220, it doesn't matter.

Re: nylon and crumble at ~80C.. have you seen this?

I've seen old nylon, yes, yellow-brown, and brittle.

(Does your device run 24/7?)

It won't run continuously, and in normal operation it shouldn't
even get hot. But, abused, it will, and the customer's hammering
the heck out of it.

So, I'm trying to protect the semis under fault conditions.

It already has instantaneous current limiting with a limit inversely
proportional to switch temperature. That and the heat sinks were really
just meant so the unit would survive long enough to take out its input fuse.

I might make that thermal overload cutoff a lot more aggressive. Or
possibly not--blowing sacks of fuses might get old, and enforce a
certain amount of discipline.

I've used teflon shoulder washers for ~120C heater operation.. and nylon at lower temps.

Per John, Teflon cold-flows. Nylon would probably work here. As it is
I'm using Kapton, 'cause that was handy locally, intruding into the hole
to keep the bolt clear of the TO-220 tab.

*I've had issues with burrs left on a threaded hole punching through sil pads and shorting the to-220. I don't know if the same thing can happen with hard anodization.

Removing burrs is essential--burrs are NOT allowed. Having done that,
the Al2O3 works great--no problems at all.

Cheers,

James Arthur

 

[Paul E Bennett]

I was mostly wondering what people do between the bolt and the
semiconductor--sorry I didn't make that very clear. I saw the
various offerings at DigiKey, but some of them looked cheesy.


I didn't prep them. Trapping grit would be my main concern, but even
that shouldn't make much difference as long as the grit's non-conductive.



That depends. If the screw's not touching the TO-220, it doesn't matter.

I've seen old nylon, yes, yellow-brown, and brittle.


It won't run continuously, and in normal operation it shouldn't
even get hot. But, abused, it will, and the customer's hammering
the heck out of it.

So, I'm trying to protect the semis under fault conditions.

It already has instantaneous current limiting with a limit inversely
proportional to switch temperature. That and the heat sinks were really
just meant so the unit would survive long enough to take out its input
fuse.

I might make that thermal overload cutoff a lot more aggressive. Or
possibly not--blowing sacks of fuses might get old, and enforce a
certain amount of discipline.


Per John, Teflon cold-flows. Nylon would probably work here. As it is
I'm using Kapton, 'cause that was handy locally, intruding into the hole
to keep the bolt clear of the TO-220 tab.


Removing burrs is essential--burrs are NOT allowed. Having done that,
the Al2O3 works great--no problems at all.

Cheers,

James Arthur

One check always worth doing when you have assembled everything is an
insulation integrity check to ensure what should be insulated remains
insulated.

--
********************************************************************
Paul E. Bennett IEng MIET.....<email://[email protected]>
Forth based HIDECS Consultancy.............<http://www.hidecs.co.uk>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-510979
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************

 

[Lasse Langwadt Christensen]

Den lørdag den 28. december 2013 05.02.28 UTC+1 skrev John Larkin:

They come in a wide range of stiffnesses.

and can be stacked in "parallel" or "serial"

-Lasse

 

[josephkk]

Nice parts. Those would make for an easy retrofit on this
project.

Thanks.

Cheers,

James Arthur

A new material for my list, cool. I was going to recommend diallyl
phthalate for the job, i have used it enough at temperatures as high as
125C for years in a test lab. No problems.

?-)