Shop OBEX P1 Docs P2 Docs Learn Events
"transistor case" and "ground" question? — Parallax Forums

"transistor case" and "ground" question?

Zap-oZap-o Posts: 452
edited 2011-09-11 22:29 in General Discussion
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.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-2612-5-ND

Why am I thinking something is wrong?

Comments

  • LeonLeon Posts: 7,620
    edited 2011-09-11 08:56
    The collector is usually the outer part of the die, it is much easier to connect that to the case than the emitter.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-09-11 09:02
    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.
  • Zap-oZap-o Posts: 452
    edited 2011-09-11 09:03
    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.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-09-11 09:04
    Like I posted, source the proper mounting kit.
  • Mike GreenMike Green Posts: 23,101
    edited 2011-09-11 09:07
    See Planar 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.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-09-11 09:11
    NTE413 is (still) available from Mouser, but there are no pics included.
  • Zap-oZap-o Posts: 452
    edited 2011-09-11 09:11
    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.
  • kwinnkwinn Posts: 8,697
    edited 2011-09-11 09:16
    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.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2011-09-11 22:29
    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.
Sign In or Register to comment.