transistor requirement

Can you recommend a surface mount transistor with a thermal resistance of 1mW/ºC for use as a flowmeter

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LevyOlson
13 posts since
Aug 15, 2011

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Aug 7, 2012 9:29 AM (in response to Caz)
Re: transistor requirement

Hey Caz,

Do you have any other specifications? Transistor manufacturers usually spec thermal resistance as a function of RθJA.

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Caz
4 posts since
Aug 7, 2012

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Aug 7, 2012 9:10 AM (in response to Caz)
transistor requirement

That's correct. I would like to raise the transistor temperature by 25C using as little a power as possible, hence 25mW dissipation.
I am monitoring VBE to assess the cooling effect of the airstream. A surface mount device has very small thermal mass so the response should be quite fast.

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- LevyOlson
  13 posts since
  Aug 15, 2011
  
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  Aug 7, 2012 11:02 AM (in response to Caz)
  transistor requirement
  
  That being your application, I think I would have you look at part number BC857CT-7-F. http://www.diodes.com/datasheets/ds30275.pdf
  
  It has a high thermal resistance, while haveing one of our lowest power dissipation ratings. So I would think it will produce the most amount of heat for the smallest amount of power.
  
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  - Caz
    4 posts since
    Aug 7, 2012
    
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    Aug 8, 2012 4:57 AM (in response to LevyOlson)
    transistor requirement
    
    Thanks a lot, Levy, that looks exactly what we were looking for. One more question, is the thermal barrier on the underside, in which case we should invert it to get best exposure to the airflow?
    
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    - LevyOlson
      13 posts since
      Aug 15, 2011
      
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      Aug 8, 2012 10:23 AM (in response to Caz)
      transistor requirement
      
      Hey Caz, I don't know if there is room in your application, but how about soldering it to wire and have it floating? That should minimize the heat sinking done by the PCB and give you more surface area on the chip.
      
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      - Caz
        4 posts since
        Aug 7, 2012
        
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        Aug 8, 2012 10:28 AM (in response to LevyOlson)
        transistor requirement
        
        Thanks Levy, great idea
        
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Aug 7, 2012 8:23 AM

transistor requirement

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