+ Post New Thread
Results 1 to 12 of 12
  1. #1
    Member level 5
    Points: 1,922, Level: 10
    Achievements:
    7 years registered

    Join Date
    Feb 2011
    Posts
    87
    Helped
    2 / 2
    Points
    1,922
    Level
    10

    7805: when to use a heatsink???

    Hi all,
    same circuit, new question ;)
    I am using a 7805 to power a circuit using around 200mA but let's say 300mA to be on the safe side.
    The input to the 7805 is a 9V transformer (after rectification obviously). Since 9V is effective voltage, the power dissipated by the 7805 should be 4*300 = 1.2W.

    Does that require a heatsink? I would tend to say so, as I read somewhere, but can't remember where that the thermal resistance of the 7805 is 65K/W.
    But how big should it be?

    The project is to be contained in a plastic derivation box.

    thanks

    Davide

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 12,636, Level: 26
    Achievements:
    7 years registered

    Join Date
    Apr 2010
    Posts
    1,918
    Helped
    411 / 411
    Points
    12,636
    Level
    26

    Re: 7805: when to use a heatsink???

    yes. it requires a heatsink.



  3. #3
    Administrator
    Points: 38,285, Level: 47
    Achievements:
    Created Album pictures Created Blog entry 7 years registered
    alexan_e's Avatar
    Join Date
    Mar 2008
    Location
    Greece
    Posts
    11,895
    Helped
    2093 / 2093
    Points
    38,285
    Level
    47
    Blog Entries
    6

    Re: 7805: when to use a heatsink???

    There is a graph in the datasheet

    Click image for larger version. 

Name:	LM78xx_power_dissipation.gif 
Views:	129 
Size:	26.9 KB 
ID:	55989

    As you can see you can dissipate about 1.25W without heatsink even at 70 degree ambient temperature.

    Alex



  4. #4
    Member level 5
    Points: 1,922, Level: 10
    Achievements:
    7 years registered

    Join Date
    Feb 2011
    Posts
    87
    Helped
    2 / 2
    Points
    1,922
    Level
    10

    Re: 7805: when to use a heatsink???

    Quote Originally Posted by alexan_e View Post
    There is a graph in the datasheet

    Click image for larger version. 

Name:	LM78xx_power_dissipation.gif 
Views:	129 
Size:	26.9 KB 
ID:	55989

    As you can see you can dissipate about 1.25W without heatsink even at 70 degree ambient temperature.

    Alex
    Thanks Alex,
    then I wouldn't need one... right?

    ---------- Post added at 18:11 ---------- Previous post was at 18:10 ----------

    Quote Originally Posted by srizbf View Post
    yes. it requires a heatsink.

    thanks, but can you be a little more specific? According to the graph above I wouldn't.



  5. #5
    Administrator
    Points: 38,285, Level: 47
    Achievements:
    Created Album pictures Created Blog entry 7 years registered
    alexan_e's Avatar
    Join Date
    Mar 2008
    Location
    Greece
    Posts
    11,895
    Helped
    2093 / 2093
    Points
    38,285
    Level
    47
    Blog Entries
    6

    Re: 7805: when to use a heatsink???

    Assuming that your ambient temperature in the box will be below that temperature, no you don't need a heatsink.

    Alex



    •   AltAdvertisement

        
       

  6. #6
    Member level 5
    Points: 1,922, Level: 10
    Achievements:
    7 years registered

    Join Date
    Feb 2011
    Posts
    87
    Helped
    2 / 2
    Points
    1,922
    Level
    10

    Re: 7805: when to use a heatsink???

    well if the 7805 doesn't heat it above, then it won't ... what does it mean exactly that it can dissipate power?



  7. #7
    Administrator
    Points: 38,285, Level: 47
    Achievements:
    Created Album pictures Created Blog entry 7 years registered
    alexan_e's Avatar
    Join Date
    Mar 2008
    Location
    Greece
    Posts
    11,895
    Helped
    2093 / 2093
    Points
    38,285
    Level
    47
    Blog Entries
    6

    Re: 7805: when to use a heatsink???

    It can handle that amount of power consumption

    Answers.com - What is Power dissipation

    Alex


    1 members found this post helpful.

  8. #8
    Advanced Member level 5
    Points: 12,636, Level: 26
    Achievements:
    7 years registered

    Join Date
    Apr 2010
    Posts
    1,918
    Helped
    411 / 411
    Points
    12,636
    Level
    26

    Re: 7805: when to use a heatsink???

    datasheets give the correct figures.
    provided you operate in the region you see.

    on the otherhand ,
    in practice , you put a heatsink when Pd exceeds approximately 800 to 900mW(close to 1W).

    you can have a safe and longlife device.



  9. #9
    Member level 5
    Points: 1,922, Level: 10
    Achievements:
    7 years registered

    Join Date
    Feb 2011
    Posts
    87
    Helped
    2 / 2
    Points
    1,922
    Level
    10

    Re: 7805: when to use a heatsink???

    Quote Originally Posted by alexan_e View Post
    It can handle that amount of power consumption

    Answers.com - What is Power dissipation

    Alex
    Yep, I knew this, but what I find hard is to predict the resulting temperature of the device, let alone of the box ;)

    ---------- Post added at 18:26 ---------- Previous post was at 18:25 ----------

    Quote Originally Posted by srizbf View Post
    datasheets give the correct figures.
    provided you operate in the region you see.

    on the otherhand ,
    in practice , you put a heatsink when Pd exceeds approximately 800 to 900mW(close to 1W).

    you can have a safe and longlife device.
    so how should I choose the heatsink? Should I just bolt it to the device or do I need thermal paste?



    •   AltAdvertisement

        
       

  10. #10
    Administrator
    Points: 38,285, Level: 47
    Achievements:
    Created Album pictures Created Blog entry 7 years registered
    alexan_e's Avatar
    Join Date
    Mar 2008
    Location
    Greece
    Posts
    11,895
    Helped
    2093 / 2093
    Points
    38,285
    Level
    47
    Blog Entries
    6

    Re: 7805: when to use a heatsink???

    Can't help you on the temperature calculation.
    A thermal paste will always give lower thermal resistance between the semiconductor case and the heat sink, this will improve the heat transfer and will keep the device at a lower temperature.
    It is up to you to use it or not since this is a very low consumption application.

    Alex



  11. #11
    Member level 5
    Points: 1,922, Level: 10
    Achievements:
    7 years registered

    Join Date
    Feb 2011
    Posts
    87
    Helped
    2 / 2
    Points
    1,922
    Level
    10

    Re: 7805: when to use a heatsink???

    Quote Originally Posted by alexan_e View Post
    Can't help you on the temperature calculation.
    A thermal paste will always give lower thermal resistance between the semiconductor case and the heat sink, this will improve the heat transfer and will keep the device at a lower temperature.
    It is up to you to use it or not since this is a very low consumption application.

    Alex
    yep guess so ;) I am going to shop for a heatsink tomorrow and see what they have as thermal paste.

    thanks a lot!



  12. #12
    Advanced Member level 1
    Points: 3,875, Level: 14

    Join Date
    Mar 2006
    Location
    Culiacn, Sinaloa, Mexico
    Posts
    479
    Helped
    71 / 71
    Points
    3,875
    Level
    14

    Re: 7805: when to use a heatsink???

    Well the ROJA = 65C/W

    If your ambient temperature = 25C
    Tj = 25C+65C/W*(.3A * (9V-5V)) = 103C that is below the 150C max for Tj

    And datasheet gives a 2W operation without heatsink.

    However in practice there is a protection circuit inside the 7805 that avoid the operation for more than 0.5W that is for certain circuits not for all.

    Regards



--[[ ]]--