can i connect 7805 to a 12v battery?

Discussion in 'The Projects Forum' started by onlyvinod56, Oct 3, 2009.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    hello all,

    iam using 89c51 micro controller to design a speedometer for my byke.
    i didnt get the idea of maximum Vcc in datasheet.
    the battery voltage of my byke is 12v.
    can i use 7805 to run my micro cont?

    or can i directly connect my micro controler to the battery?

  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    The maximum input to the 7805 is around 30V so you can connect the 12V input directly to the input of the regulator. That being said, you may need to provide a heat sink to the device as it may get overheated depending on the amount of current that you are running through it. The power that the 7805 will dissipate is :

    Power = (12 - 5) * Current

    And, that will be turned into heat...

    Do not connect the micro directly to the 12V battery as it will destroy the part. Look at the specifications for the part you are using for the input voltage. A 5V part can only accept around 4.5 - 5.5 V without being damaged.
  3. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    thankyou stayat....
  4. mik3

    Senior Member

    Feb 4, 2008
    Also, connect a 0.1 uF ceramic capacitor at the input (input-ground) and output (output-ground) of the regulator as close to the chip as possible. This is to suppress any oscillations due to the long wires you will have.
  5. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    ok thanQ mik3....

    and for my speedometer, iam going to design a large 7segment display unit using few LEDs..

    that is 5 leds (5 leds are connected in series) in each segment connected in series with 47ohm. the supply iam using is 12v.

    now micro controller will give a BCD data with 5v magnitude.
    this 5v logic pulses will be given to 4511 (BCD to 7 SEG).

    the 7 outputs of the 4511 should excite the large 7 seg display.

    my question is..... each segment requires 20ma current (each led current rating is 20ma and 5 leds are in series / segment). totally there are 7 segments.
    so..... if the display shows the digit "8"....all the seven segments requires 140ma......

    can the 4511 drives this 140 ma through Vcc and its 7 outputs?

    check my incomplete attachment