Powering Arduino, Rpi and DC motors from battery

Discussion in 'The Projects Forum' started by seky, Oct 10, 2013.

  1. seky

    Thread Starter Member

    Sep 19, 2013
    31
    1
    Hi guys,
    I bought a 12V battery 6800mAh and I want power from this battery Arduino and Raspberry Pi and possibly DC motors. I built a schematic as I show at my upload image. I know that arduino has its own regulator and 12V for it is not problem, but for Raspberry Pi I need range from 4.75 to 5.25V. When I use this regulator at ouput I show about 4.9V. But to the Rpi I have connected USB WiFi dongle, USB camera and Ethernet cable and when I use 5V 1A adapter from my HTC mobile all is OK. But when I use 4.9V it was reseting, but when I measure voltage at output of battery there was 4.7V and output of regulator 4.9V :D I mean it is impossible, for this reason I go to charge this battery and next I will post a reaction. But I need to know, that is possible powering Arduino and Raspberry Pi via this way. And then when all will be ok, what I need when I go to power DC motors?...Now I power it from 6.7V another battery, it only powers DC motors, but I want rebuild it only at one battery, what I need? 6V regulator or what?...but I think, that I need there something more or not?...Thanks for reply..

    Schematic: http://prntscr.com/1wggzf
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,151
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    I speculate that you did not measure the voltage when all of those things were drawing current. The 7805 cannot supply enough current for all that and was likely dropping voltage to protect itself from overheat and over-current. It's great they can do that without destroying themselves. It was probably quite hot.
     
  3. seky

    Thread Starter Member

    Sep 19, 2013
    31
    1
    Yes It was hot, but for a few minutes it goes, but maybe after that battery has depleted, are there any solutions?...Or I need to buy better, or what type of regulator?..Thanks
     
  4. wayneh

    Expert

    Sep 9, 2010
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    There are many options but I think a DC-DC converter, like the cheap ones you can find on e-bay, is worth a look. It should be far more efficient than the linear 7805 - which wastes as heat more than half the power you pass through it - and you can get one rated to handle your current needs.
     
  5. seky

    Thread Starter Member

    Sep 19, 2013
    31
    1
  6. wayneh

    Expert

    Sep 9, 2010
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    Yup, or this or this. Not endorsements, just links.
     
  7. seky

    Thread Starter Member

    Sep 19, 2013
    31
    1
    So, I tested a regulator 7805 with full charged battery. Battery output shown 12.28V and stabilizator`s output with no connected Raspberry Pi 4.99V. When I connected Rpi with all devices (WiFi,camera and ethernet). It showed at multimeter about 4.98-4.97, I had regulator without heat sink it it got soo hot and after 10 sec voltage went down to 4.92 and then I switched-off my battery with switch on it. I have idea that for this high temperature 7805 cant hold a regular voltage or what?...I will mount a fan but it will help?...oder I bought DC-DC converter and all will be ok...Can I build DC-DC converter on my own? (I think it is hard?)...because it is my project to school and I want build things on my own (those that are possible).

    Thanks
     
  8. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
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    You may not have a current overload on that 7805. When you say "It was hot" you're suggesting that it may be overheating instead, and reducing its current output for that reason. What kind of heat sink is this device on, and for that matter, is it actually the TO-220 package?
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could boost the 7805 output current by using a pass transistor; but it will need a large heatsink. Might be better to use a different regulator, or a DC-DC converter as suggested above.
    I would power any motors from a separate supply if possible, to avoid spurious resetting of the RPi by interference.
     
  10. seky

    Thread Starter Member

    Sep 19, 2013
    31
    1
    Thanks for all responses, but I have another question. Its my project to a school and I want buy things as little as possible and for this reason I want to build DC-DC converter on my own. I checked that 7805 is not right way and I find a schematic as this http://www.eleccircuit.com/12v-to-5v-3a-dc-converter-step-down-regulator I know that I didnt create this schematic but still beter than buy DC-DC converter as that: http://www.ebay.co.uk/itm/Car-Power..._Measurement_Equipment_ET&hash=item20d556437b ...But I want to know how differencies are between that 2 things? ..Thanks
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The first circuit is a 7805 with a pass transistor (as I mentioned in post #9). The second is a switch-mode regulator, which is much more efficient.
     
  12. seky

    Thread Starter Member

    Sep 19, 2013
    31
    1
    Yes, yes I know difference but I want build it on my own maybe by following schematic because its for my school project, what would be maybe the best way?..Or I can buy DC-DC from ebay and only say why I bought and didnt build...
     
  13. seky

    Thread Starter Member

    Sep 19, 2013
    31
    1
  14. wayneh

    Expert

    Sep 9, 2010
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    I think this would actually display some wisdom on your part. Why struggle to reinvent the wheel? You will see farther standing on the shoulders of giants. (Paraphrasing Isaac Newton.)
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Then why did you ask "But I want to know how differencies are between that 2 things?"
     
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