Author Topic: What is the back of a T0-220 transistor connected to?  (Read 551 times)

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Offline corrado33Topic starter

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What is the back of a T0-220 transistor connected to?
« on: July 31, 2011, 04:32:17 PM »
I have a circuit where I'm using a bunch of T0-220 package transistors that will need heatsinks.  Now, I remember reading on a circuit somewhere that they could only use the same heatsink for the ones that the.... _____________.... are connected to the same thing.  I believe that blank was collector, but I can't find the site again.  So, what IS the back (metal part) of those connected to? 

Is it the same for all T0-220 transistors, or will it vary device to device?

Thanks!

Offline Soeren

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Re: What is the back of a T0-220 transistor connected to?
« Reply #1 on: July 31, 2011, 04:54:02 PM »
Hi,

I have a circuit where I'm using a bunch of T0-220 package transistors that will need heatsinks.  Now, I remember reading on a circuit somewhere that they could only use the same heatsink for the ones that the.... _____________.... are connected to the same thing.  I believe that blank was collector, but I can't find the site again.  So, what IS the back (metal part) of those connected to? 

Is it the same for all T0-220 transistors, or will it vary device to device?
Guessing that your DVM died?   :P  ;D

The metal tab is usually connected to the middle pin (which is usually the collector).
You can get a variant of the TO-220 where the tab is isolated (encased in the plastic housing), but they have worse temperature transfer than the metal tabbed ones.
When you have transistors of different polarity, just add an isolation washer/shim between the transistor and heat sink. The ones made in thermally conductive silicone means you don't need heat sink compound (although the best heat transfer is still by the oldfashioned way, with a mica washer smeared thinly wit H.S. compound.

Remember to isolate the heatsink from an eventual box or what you mount it to, if each of the transistors aren't isolated, or you may short cut something to eg. common ground.
Regards,
Søren

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Offline corrado33Topic starter

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Re: What is the back of a T0-220 transistor connected to?
« Reply #2 on: July 31, 2011, 06:07:13 PM »
Thanks!

Another question though...

If the tabs are, in fact, connected to the middle (collector) pin, and you had two transistors with their collectors connected to the same thing, could you use a heatsink as an electrical connection?  I'm assuming that'd be... bad practice to say the least. 

Offline Soeren

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Re: What is the back of a T0-220 transistor connected to?
« Reply #3 on: July 31, 2011, 09:47:53 PM »
Hi,

If the tabs are, in fact, connected to the middle (collector) pin, and you had two transistors with their collectors connected to the same thing, could you use a heatsink as an electrical connection?  I'm assuming that'd be... bad practice to say the least. 
Very bad practice... If it's not in Murphy's Law, it should be...
When you want it to isolate, it conducts. When you want it to conduct, it isolates.

Initially, it will conduct of course (to a degree determined by how well the screw breaches the oxide).
When it heats up and cools down (alu and steel expands and contracts at different rates), even the best screw connection will get oxidized. Aluminum oxide is an isolator.


And by reading this (the expanding and contracting part), you hereby swear to never mount a transistor to a heatsink without a suitable spring washer under the screw, at the peril of burned parts, right?  ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline corrado33Topic starter

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Re: What is the back of a T0-220 transistor connected to?
« Reply #4 on: August 01, 2011, 02:58:03 PM »
I hereby solemnly swear to never mount a transistor to a heatsink without a suitable spring washer under the screw, at the peril of burned parts.   ;D ;D

 


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