Heat Sink gets heat up on LM7808

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Jan 9, 2016.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    I have 12V input on LM7805, & output load current is aroung 600-700mA.
    I have connected a large aluminium heat sink with fins on it. Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.
    Can my circuit works for continuous 1 day for that? or I should go for some dc-dc.
     
  2. nerdegutta

    Moderator

    Dec 15, 2009
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    Hi.

    Please upload you schematic diagram, and a picture of your circuit. Did you solder it yourself, or is it on a bread board?

    Is it a 7808 or a 7805?
     
  3. hp1729

    Well-Known Member

    Nov 23, 2015
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    LM7805?
    Is dropping 7 Volts at 800 mA. Yep, might get warm. It could also be oscillating, but I suspect you would know that.
     
  4. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    Its simple LM7805. Output voltage comes around 4.80V when loaded. I have soldered the board myself. Will post schematic also.
     
  5. Picbuster

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    Dec 2, 2013
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  6. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    P=7*.8=5.6Watt
    use big heat sing or low the input voltage if possible.
    yes it heat at high current
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Did you use decoupling capacitors ?
     
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  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The 7V drop x 700 mA is 5 watts. That is a big heat sink. It should also get forced air cooling from a fan.
    You could use a DC/DC converter and that will reduce the heat.

    You could also get a different SMPS supply to power your project (e.g. Use a cell phone charger for the 5v source) or any other DC source that is not 12 V

    Finally, you can make sure that your circuit needs 5 V. Some CMOS chips can handle higher, LEDs can be powered with higher if you use appropriately sized current limiting resistors. They way, all the heat is dissipated over more components instead of one poor little 7805.
     
  9. Roderick Young

    Member

    Feb 22, 2015
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    If you want to burn less power in the same footprint, try the muRata OKI-78SR-5 . You may not need a heatsink.
     
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  10. R!f@@

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    Where can I buy these. I can use them
     
  11. ISB123

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    May 21, 2014
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  12. GopherT

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  13. dl324

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    Mar 30, 2015
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    Prices at places like Mouser and Digikey are 2-3X that!
     
  14. dl324

    Distinguished Member

    Mar 30, 2015
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    Being too hot for you to touch isn't a practical indication of a component being stressed.

    LM7805 will operate at a junction temperature of 125C. Thermal resistance from junction to case is 5C/W, so it won't start temperature throttling until the case is 100C. Unless you're a professional chef, you'll find that temperature too hot to tolerate even briefly; but the part will be within it's specs.
     
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  15. GopherT

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    Nov 23, 2012
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    What makes them special? Were they born with fingers made of Nomex?
     
  16. dl324

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    Mar 30, 2015
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    No. When it comes to handling things that are hot, many of them have conditioned themselves to tolerate a higher level of pain.
     
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  17. crutschow

    Expert

    Mar 14, 2008
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    What is the size of this "large" heatsink?
     
  18. bertus

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    Apr 5, 2008
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  19. R!f@@

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    Dang....that is expensive.
    I stick with 7805 and a sink
     
  20. hp1729

    Well-Known Member

    Nov 23, 2015
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    That sounds like it is loaded down more than 800 mA.
     
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