1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

LM7812 and LM7805 Linear voltage regulator

Discussion in 'General Electronics Chat' started by Kranthikkenoch, Nov 28, 2017.

  1. Kranthikkenoch

    Kranthikkenoch New Member

    Joined:
    Jun 28, 2017
    Messages:
    10
    Likes:
    0
    Hi all,

    I have a 36V 10amp battery pack and a small switching circuit. I need 5V to power my microcontroller and 12V to power my mosfet.
    Can anyone suggest me if I can connect my 36V battery to LM7812 and get 12V required or my input voltage is too high?
     
    Last edited: Nov 28, 2017
  2. kubeek

    kubeek Well-Known Member

    Joined:
    Mar 11, 2006
    Messages:
    1,510
    Likes:
    189
    Location:
    Prague, Czechia (not Chechnya)
    What kind of microcontroller needs 5W of power? :eek:
    What current do you need for the 12V? Please show your circuit, it may be possible that you only need the 5V.
     
  3. Kranthikkenoch

    Kranthikkenoch New Member

    Joined:
    Jun 28, 2017
    Messages:
    10
    Likes:
    0
    sorry to mention 5V 1amp, I'm using atmega32 MCU. My question is I need 12V for mosfet and I have 36V battery pack.
    Can I use LM7812 for which input is 36V will the IC survive if the current is 500mA?
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. JLNY

    JLNY Active Member

    Joined:
    Mar 5, 2011
    Messages:
    223
    Likes:
    36
    Location:
    New York, USA

    The LM78XX series is rated to a maximum input voltage of 35V, so yes, your 36V battery pack is too high. On top of that, all of that voltage drop from 36V to 12V or 5V will be burned off as heat, so the power dissipation could also be a concern depending on how much current the circuits will draw.

    Would there be any way to configure the battery pack for a lower voltage, such as 18V? 15V might also work depending on how much the voltage of the pack drops over its discharge range. If the voltage drops any lower than 14V, you may run into issues with the dropout voltage of the 7812.

    Edit:
    okay, so for 500mA @ 12V, that would be a voltage drop of 24V, which would be 12W of heat dissipated at the regulator. This would probably overheat unless you had reasonably good heatsinking on the 7812. For an 18V pack, the dissipation @500mA drops to 3W.
     
    Last edited: Nov 28, 2017
    • Like Like x 1
  6. Kranthikkenoch

    Kranthikkenoch New Member

    Joined:
    Jun 28, 2017
    Messages:
    10
    Likes:
    0
  7. kubeek

    kubeek Well-Known Member

    Joined:
    Mar 11, 2006
    Messages:
    1,510
    Likes:
    189
    Location:
    Prague, Czechia (not Chechnya)
    You can turn on some mosfets with 5V or even less, are you sure you need 12V?
     
  8. tomizett

    tomizett Active Member

    Joined:
    Aug 11, 2013
    Messages:
    449
    Likes:
    47
    Location:
    UK
    Some LM78 regulators have a max. input of 35V, and for some it's 30V... always check the manufacturer's datasheet for the exact part you're using!
     
  9. JLNY

    JLNY Active Member

    Joined:
    Mar 5, 2011
    Messages:
    223
    Likes:
    36
    Location:
    New York, USA
    Actually, thinking about it some more, depending on the voltage range/regulation requirements of your MOSFET circuit, it might be possible to use a 12V pack directly and eliminate the 7812 regulator altogether. Depends on the application, though.
     
  10. Pommie

    Pommie Well-Known Member Most Helpful Member

    Joined:
    Mar 18, 2005
    Messages:
    10,161
    Likes:
    340
    Location:
    Brisbane Australia
    Just but two of these of your local ebay.

    Mike.
     
  11. cowboybob

    cowboybob Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2011
    Messages:
    3,052
    Likes:
    479
    Location:
    James Island, SC
    Those are outstanding buck PSs. I've used them for all manner of projects, especially battery driven units. And their efficiency is pretty much as stated (≈92%).

    They do take up a fair amount of space (vs. a 7012 or '05), but that never bothered me, given its advantages...
     

Share This Page