View Full Version : "transistor case" and "ground" question?

09-11-2011, 03:51 PM
I want to use this transistor and as you can see in the data sheet that the case is tied to the collector. I am a bit confused or its early morning fog thats got me wondering:

Why is the case not tied to emitter? You would think that the case tied to the emitter would kill 2 birds with one stone.

1. If the transistor is coupled to my "electronics gadgets chassis" heat dissipation would be improved while running the transistor as a amplifier.

2. Earth ground and common would be connected together.


Why am I thinking something is wrong?

09-11-2011, 03:56 PM
The collector is usually the outer part of the die, it is much easier to connect that to the case than the emitter.

PJ Allen
09-11-2011, 04:02 PM
It can still be heatsinked to your/a chassis.
You need to source the proper mounting kit (shoulder bushings, mica insulator or "silpad".)
That's the way it's done.

09-11-2011, 04:03 PM
So I would have to electronically isolate the transistor collector from the chassis of my device? What ? How would this be possible to couple the transistor to the chassis for heat dissipation yet not allowing a short?

Opps I posted and PJ answered at the same time.

PJ Allen
09-11-2011, 04:04 PM
Like I posted, source the proper mounting kit.

Mike Green
09-11-2011, 04:07 PM
See Planar Transistor (http://en.wikipedia.org/wiki/Diffusion_transistor) for a picture

Yes, you would have to electrically isolate the case from the chassis of your device. There are insulators specifically made for this purpose. Often they're made of a thin layer of mica. Sometimes they're made of anodized aluminum where the aluminum oxide layer on the outside acts as an insulator. Usually you'd use a thermally conductive grease between the layers.

PJ Allen
09-11-2011, 04:11 PM
NTE413 is (still) available from Mouser, but there are no pics included.

09-11-2011, 04:11 PM
Okay thanks guys.

I think I could use a PNP instead and pull this off with out using a isolation kit. Otherwise Ill use a kit as mentioned and hope it wont short out.

09-11-2011, 04:16 PM
As Mike says you need an insulator, and heat sink compound would also be a good idea. See http://search.digikey.com/scripts/DkSearch/dksus.dll

The other reason the collector is connected to the case is to maximize heat transfer from the transistor to the case. Most of the heat is produced at the collector.

Loopy Byteloose
09-12-2011, 05:29 AM
It is rather naive to change your design to avoid using isolation. After all, if you are going to learn to use power transistors and power mosfets properly, you can not always avoid isolating them.

You need 3 items.
1. silicone paste
2. a shouldered washer for electrically isolating the screw
3. a barrier or mica, 'silpad', or another product

All of these items are readily available at parts suppliers. I got into building some Class A audio amplifiers and acquired a small inventory that I keep on hand. If you want a good explanation of heat sinks and isolation, you might Google "Elliot Sound Products" as this Aussie really writes very clear explanations of what to do and how to do. His contributions to 'why' are also well informed.