7805 Current

Discussion in 'General Electronics Chat' started by Dritech, Jan 16, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,

    I want to power a load which consumes 1A using the 7805. The datasheet states that it can handle a peak current of 2.2A. Is it safe to connect a 1A load or is it better to use current booster transistor?
     
  2. #12

    Expert

    Nov 30, 2010
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    If you keep it cool, a 7805 will exceed its specs.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi, but is the 7805 rated at 2.2A or at 1A? Because on the simulator it is marked as 5V 1A. If so, is it safe to work at maximum rating?
     
  4. #12

    Expert

    Nov 30, 2010
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    There are several 7805's. Their current rating is different, depending on the package they are in. 78L05, 78M05, etc. You did not tell which 7805 you are talking about so I have to assume it is the "standard" 7805. A 7805 will deliver more than the rated current if you keep it cool and fail to deliver the rated current if you let it get to hot. So, the answer is yes, if you use enough heat sink.
     
  5. matty204359

    Member

    Apr 6, 2011
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    With proper heat sinking I don't see why not. You may want to look for an comparable ic which has higher spec if you are really worried. I have used and abused the 7805. It should be fine. I obviously wouldn't recommend running an ic at its absolute maximum in a production run of a product. 80% is a good margin when shopping for components. One off prototypes typically have less stringent design requirements.
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the information. Another quick question: When using a variable voltage regulator, how can I make it to go down to 0V instead of 2.5V?
     
  7. #12

    Expert

    Nov 30, 2010
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    You can't. You have to either waste 2.5 volts with a zener or give the chip a negative supply of 2.5 volts for its reference pin.

    (Don't most of the regulator chips use 1.25 volts?)
     
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  8. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Sorry I meant 1.25V. Thanks for the info.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The current is based on input-output differential voltage. There used to be a typical curve showing this on the data sheet, but it is a TYPICAL curve not a guaranteed spec. The 1A is what they guarantee. It does not hurt to run 1A through a 78XX regulator as long as it has an adequate heatsink. Read the data sheet, page 17 top right curve:

    http://www.fairchildsemi.com/ds/LM/LM7805.pdf
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Some regulators, such as the LT3083, can be adjusted down the 0V output. It is also a low-dropout type regulator.
     
  11. #12

    Expert

    Nov 30, 2010
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    Speaking from the nit-picking gallery, you have to have some voltage out to get 1ma minimum current, so absolutely zero volts out is not achievable.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Picky, picky...:D
     
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  13. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the replies.

    Another question: If the input voltage is coming from a source which vary from 14V to 50V (which is greater than the absolute maximum one can apply to the 7805), how can I drop the voltage to keep it below 35V which is the maximum input?
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What current would be needed?
    You could have a look at the LM317HV.
    That would allow the input voltage to go upto 60 Volts.

    Bertus
     
  15. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    the 7805 isn't well suited for large voltage differences.

    It is not just the 1A limit. Also you get a limitation from the product of the current * voltage difference (which is Watts).

    For a small TO220 cooler this is just 6 Watts, with larger coolers you can go as high as 20W.

    the theoretical limit is about 40 to 50 Watts.

    Why do you want 5 volts from a 14 to 50 volts input?

    You should use a buck switching regulator.
     
  16. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi,
    Can I use a pre-regulator circuit using a zener diode and transistor? or will that be a problem when the voltage is 12V?
     
  17. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    You could use a preregulator circuit as given in the LM317HV datasheet:

    [​IMG]

    That way the dissipation of the second regulator will be almost constant.

    A better solution has already been mentioned, the use of a buck regulator.
    The LM5116 would be a perfect match for your application.
    On page 22 of the datasheet there is a schematic that allows 7 to 60 Volts input for 5 Volts output voltage.

    Bertus
     
    Last edited: Jan 18, 2014
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