Connecting a Transistor 2N3055 with Heat Sink

That all depends on the rest of the circuitry. One certainly can use this transistor in a circuit that has only a negative supply and attach this collector to a chassis connected to ground. Typically the collector of this transistor would be electrically isolated from the chassis, as you propose. Now one can have an electrically isolated from ground heatsink in direct contact with this transistor case to improve thermal transfer from case to heatsink. Even in this case a thin thermal compound paste should be applied between heatsink and case that will fill tiny voids between surfaces.

Well I wouldn't ground the 2N3055 case as it is the collector.

You don't need to isolate the case (collector) if you can ensure that the heatsink won't accidentally be shorted or come into contact with anything else. We would advise isolation however as general practice dictates.

Maybe this will help

http://www.youtube.com/watch?v=hNRacVQRgBc

There used to be a "rubber" insulator that did not require any grease. I was more familiar with a mica insulator plus thermal compound, and Teflon shoulder washers that were inserted from the back side of the heat sink for the mounting bolts.

That socket he is using is pretty slick.

There are still companies that make rubber like insulating pads. Bergquist's Sil-Pad is one such product. The video has a common misconception about thermal grease. As the transistor case heats up the case will warp. The rate of heat transfer from transistor to heat sink is dependent on the contact surface area between solids. A warped transistor case will have less thermal conduction to the heat sink. Thermal grease or a thick flexible insulator will fill the voids between solids as they warp.

Fully agree with GW....

Last I remember working with this transistor, we ALWAYS had mica to isolate the body, and plastic to isolate the legs/pins.....

'Course I've been away from college and TVs for xxx years........

http://www.youtube.com/watch?v=mPGusBmm3XE

P-N thermal . . . basics re-visited

The case of your TO-3 package is the collector of the transistor. If this is always at the same potential as your case or mounting surface, you don't need to isolate it electrically but you should still provide a low thermal resistance path to the case. This is normally done with thermally conductive grease that you can get at any electronic supply store. If you require electrical isolation, you can obtain a kit with a mica or thermally conductive gasket to electrically isolate the transistor from your chassis. You should still use the thermally conductive grease.

http://www.learnabout-electronics.org/Amplifiers/amplifiers51.php

(?TO-3)

http://www.interfacebus.com/temperature-derating-curve-2N3055.html

(!TO-3)

http://www.ieeta.pt/~alex/docs/applicationnotes/dc%20power%20supply%20handbook.pdf

(nidnightfind muhaha - a quck P-sup how to / how not to - Hi, my name is Justin Case)

If in debate...

ISOLATE

It depends on the circuit and the physical topology of the circuit.

Bill