In Reply to: Interesting reading! posted by jorisdallaire on September 5, 2014 at 20:11:08:
"I didn't care much about this, figuring the thickness of the insulating sheet was enough to prevent contact. Now that I think of it, that was a pretty dumb reasonning :-)"
No, you're correct. The shoulder washer in the tab is enough to prevent electrical contact to the screw, regardless of whether the heatsink is tapped. In addition, the hole in the mica is supposed to be the same size as the screw. That prevents the mica from moving laterally and uncovering the body of the transistor. Again, if you're measuring a change in resistance over temperature, the compound is conducting. Nothing else in this simple system will do that.
I think the point made by Russ57 about not using a tapped heatsink is important to consider if high voltages are involved. The thin lip on the shoulder washer seems a little risky unless there's another one at the other end of the screw. Installing a shoulder washer at both ends would electrically float the screw, making flash-over much less likely.
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- You're Not Wrong - Triode_Kingdom 23:37:34 09/05/14 (0)