Best way to power MCU's off batteries?

Discussion in 'Embedded Systems and Microcontrollers' started by jdraughn, Dec 14, 2010.

  1. jdraughn

    Thread Starter Active Member

    Jan 30, 2009
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    I have always had trouble wondering how I should power my 5v PIC's when using batteries. Like for instance, the datasheet for my PIC18F4321 says 5.5v max, but 4 1.2v nicad batteries will have a voltage closer to 6v when full charged.

    I don't know much about LDO regulators, but they still have a drop out voltage so when the batteries are at 4.8v to 5v, what does the regulator do, just bypass it self and output the voltage straight out?

    So I have looked at using buck boost regulators so I can just use 2 or 3 cells and the regulator will boost the voltage to 5v. This kinda seems the way to go but it just seems to me to be unnecessarily complex and introduces switching noise and more expense.

    I am also planning on trying to make the switch to 3.3v microcontrollers, there so many IC's I would like to use, but they all use 3.3v power and logic and 5v will burn them out, sounds like a 3.7v LiPO with an LDO regulator would be perfect for these.

    Can anyone recommend any good solutions? I want to try building a few different projects that require long battery life and/or high effeciency so my projects will run for weeks or months at a time, or with a small solar panel.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    If you want high efficiency than you should consider everything.
    Not just the LDO but the type of IC's too
     
  3. tyblu

    Member

    Nov 29, 2010
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    A LDO regulator can only output it's rated voltage if the input is that rated voltage plus the dropout voltage. At source voltage decreases below that that, the output also drops. Note that many micros designed to run at 5V can run at lower voltages without any issue.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Which PICs are you using? Many of them will run fine at 3.3V, but the LF/low voltage PICs can only run at 3.3v and not at 5.

    Check the datasheet first for minimum voltage.

    I usually power simple things with either 4 AA Alkaline or 2 3V Coin cells, both with a 78L05, which is good to 5.5V. By the time the AA batteries hit 5.5V, they are fully dead.

    I've seen some use just a diode on a 6V battery supply and not have big issues, but I wouldn't recommend since there is no regulation. A steady voltage keeps all the analog parts (ADC, Comparators, etc) running happy, and the internal oscillator speed stable.

    For a bit more money, 2AA batteries run for a long time with a boost converter bumping the voltage to a regulated 5V.
     
    Last edited: Dec 14, 2010
  5. nerdegutta

    Moderator

    Dec 15, 2009
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    My PICs run on 9v or 12v with a .1uF cap on the in pin of a 78L05 and a 100uF electrolytic cap on the out. Both caps goes to GND.

    I use the good old 16F628. :)
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    same here...I just use a 4.7V 1W zener across the F628 for a 12V :D
     
  7. nerdegutta

    Moderator

    Dec 15, 2009
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    Saves time, components and pcb space. Hmmm nice! :)
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    The best thing is to not use any regulator. Circuits like 7805, LM317 and simple Zener regulators are not very battery friendly. If you post your design we may come up with some design desing ideas.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Why are u saying not battery friendly...

    I use zener method for the flashers I built
     
  10. t06afre

    AAC Fanatic!

    May 11, 2009
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    In order to regulate a zener must conduct some current. Typical 2-3 mA. In battery apps this bleading may be to much.
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Aaah! that....I used mine in cycles..

    You are correct. :D
     
  12. maxpower097

    Well-Known Member

    Feb 20, 2009
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    Last edited: Jan 16, 2011
  13. jdraughn

    Thread Starter Active Member

    Jan 30, 2009
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    Thanks for all the responses guys. I have made the switch to 3.3v pics so I can use the wireless MRF24J40MA 2.4Ghz tranceivers and have found that either a single lipo cell or 3 "AA" or "AAA" batteries are perfect with a 3.3v LDO.
     
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