Mixing voltage regulators

Discussion in 'General Electronics Chat' started by drkblog, May 10, 2013.

  1. drkblog

    Thread Starter Member

    Oct 4, 2012
    109
    0
    (Hello guys, I'm back again)

    I'm working on a circuit which is going to interface with an Arduino UNO board (this board has its own LM7805 regulator). My circuit needs 5V source, but I can't use the 5V from the Arduino because the regulator get's too hot and placing a dissipation sink there, isn't easy.

    My question is: Can I put my own 7805 and mix the 5V from my regulator with the 5V from the Arduino?

    Of course, both circuits will have a common ground.

    A secondary question is if this is correct: I can choose to source a LM7805 with 9V DC or 12V DC. Assuming there is no parameter to use one over the other, I think it's better the 9V because this will lower the power dissipation on the regulator.
     
  2. Andreas

    Active Member

    Jan 26, 2009
    68
    4
    Drkblog,

    Not sure of your exact wiring (a schematic could help) but in a nutshell I can't see why mixing two power supplies (such as the 7805 regs you propose) in the one system should be a problem. After all, just consider your PC's motherboard.

    The sharing of the common ground should be OK too.

    A 9V battery should drop about 4V across the regulator and so long as the current demands don't exceed the specification of the device then it shouldn't need heatsinking. The LM7805 has internal current limiting and thermal shut down so is pretty indestructible.

    Use an appropriate filter cap on the output side to improve stability. You may or may not need an input filter cap but I always put one in, matter of habit.
     
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  3. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    You can using three different methods.
    1. Using common ground, and two independent +5V Power from 7805.

    2. If you want to mix two 7805 became one +5V, you can using two 0.22Ω/3W or 5W resistors, in series a 0.22Ω/3W with each 7805, and then mix the output of 7805 together.

    3. This is a little complicated from Arduino board, because you need to using the clamping voltage, a) in series a 1N4001~4007 with GND of 7805, b) in series a 1N4001~4007 with output of 7805, c) mix two negtive polarity of 1N4001~4007 together.
     
  4. drkblog

    Thread Starter Member

    Oct 4, 2012
    109
    0
    First of all, thank you for the replies.

    As a matter of fact, I don't need to connect the output from each regulator together, but the digital input/output from Arduino and my own circuit. Just like Andreas mentioned, it's like a PC motherboard. And it's the case 1 from Scott's answer. I'll source Arduino's 7805 with 9V via Vin pin. Arduino will use its own regulator. And I'll source my 7805, and use that out put to source my own ICs (gates, counters, etc).

    I didn't see it clearly this morning, but I don't need to connect regulators' outputs between them. The problem I experienced in the past with Arduino is its regulator dissipates heat to the board. So, when you put a high DC level in the input (over 12V DC) and you have some extra shield like an Ethernet board, the regulator get's really hot.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    If you really care about the heat, then you can change to LM2575(1A)--5V,LM2576(3A) --5V, using switching mode may reduce a lots of heat.

    LM2575, LM2576, datasheet.
     
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  6. drkblog

    Thread Starter Member

    Oct 4, 2012
    109
    0
    Scott, the regulator which overheats is the one on the Arduino's board. I don't care about regulators I use in my own power sources overheating, because I can use a heat sink on them.

    Yet, your suggestion is very interesting for replacing LM7805. The only obstacle I see is the inductor. I don't like the idea of messing with them :)
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    Use an LM323 3Amp 5Volt ,,, datasheet

    or it may be cheaper to build an external 5V psu, what is the supply voltage that feeds your Arduino pcb, and how much current is it drawing?
     
    Last edited: May 19, 2013
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