LM7812 and L78L33 for 12v and 3.3v

Discussion in 'General Electronics Chat' started by us202000, Feb 5, 2013.

  1. us202000

    Thread Starter New Member

    Jul 5, 2011

    I am making a hobby robot which will need power supply at 12v and 3.3v with low current (source is a battery at 12-24v). I am thinking using LM7812 and L78L33 to connect in series like this:

    Vin -C1- LM7812 -C2->12v out-C3-> L78L33 -C4-> 3.3v out

    Where C1 and C3=0.33uF capacitors; C2 and C4=0.1uF capacitors. The capacitors are based on the datasheet from LM7812 and L7833 when used individually.

    My question is whether this circuit makes sense and do I need this many capacitors? Or is there a easier way to achieve the same (such as zener diode)? The 12v is for the Mosfet driver while the 3.3v is for the microcontroller.

    Thanks in advance.
  2. timescope


    Dec 14, 2011
    If the battery is 12v, you can not use 7812 because it needs the input voltage to be at least 2v above the output voltage.

    You need to specify the currents.

    The capacitors are needed for stability as the regulators may oscillate without them if the input wires are long.

  3. antonv


    Nov 27, 2012
    Also, are you aware of how much power will be dissipated in each regulator?
  4. atferrari

    AAC Fanatic!

    Jan 6, 2004

    Learn the hard way, very recently, that you cannot escape from taking that into consideration before starting to decide on something else!! :(
  5. us202000

    Thread Starter New Member

    Jul 5, 2011
    Hi, thanks for the reply. The current in the 3.3v section is about 0.2mA. In the 12v section, power dissipation is 1 W according to the spec.

    the output voltage required for the 12 v is actually 10-20v. If the supply voltage is 12v, will the LM7812 produce 10v?

    Also, what are the advantages of using zener diode for voltage regulation?
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    Yes, you can cascade LMxx regulators as long as you take into account that the one at the head of the line is working for all that follow it. The .2mA is inconsequential as an added load on the 7812.

    No, if the input to the LM7812 is less than about 14V (check the datasheet) the LM7812 will not regulate.

    IIRC correctly the pinout on the low power 78xx regulator in the TO92 package is exactly reversed from the
    larger regulator in the TO220 package. Check the datasheet to be sure.
  7. SPQR


    Nov 4, 2011
    I mention this only to raise another possibilty for you.
    Recently I've started using SMD devices and I found the AMS1117 series.
    They come in fixed and variable voltage types.

    It was a bit scary at first, since I'm so used to the 78XX series, but they seem to work great,
    and they would be smaller and lighter on a robot.

    Obviously you could make a PCB, but because leads are 0.1 spaced, they would also work on the prototype PCB strip boards.

    But if you choose them, be careful the pin out is NOT like the 78XX series.