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I'm designing a system where an arduino (3.3v) will run continuously, and occasionally turn a MG996R servo - this servo is pretty strong and can draw more than 2 amps.

The servo runs fine off 4 AA batteries - I want to make the arduino run off these batteries as well. Is this a good idea? Or should the servo and arduino batteries be separate?

If possible, I'm guessing I'll need a capacitor from what I see in other posts. If so, is there a formula for a capacitor size that I need for this kind of servo?

Thanks for your help, any tips or pointers would be greatly appreciated

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  • \$\begingroup\$ You might want to tell more about how things are connected and how you are going to convert the 3.3V from whatever input voltage you have (if you have all batteries in series I wonder what batteries you are going to use to be able to draw more than 2 amps) \$\endgroup\$
    – PlasmaHH
    Commented Apr 25, 2016 at 11:49
  • \$\begingroup\$ While it is possible to draw 2A from LR6 cells, it comes at a price: you're reducing the nominal capacity by 3. I would use at least 12 cells (assuming your servo needs 6V/2A) \$\endgroup\$
    – Dmitry Grigoryev
    Commented Apr 25, 2016 at 12:00
  • \$\begingroup\$ Use different source for power and control section. How will be sure battery level is critical ? You drop all control rules and safety chain. Did you want know "what's going on ?" use separate source ! \$\endgroup\$
    – dsgdfg
    Commented Apr 25, 2016 at 13:00

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Since Arduinos have modest current consumption, any linear regulator will do. I own a few Atmel boards which have a 662K regulator on them, these can handle 6V just fine.

I'm not sure whether adding a large cap to protect from voltage drops will do any good. By the time your battery drops from 6V to 3V under load it's probably unsuitable to run the servo anyway. If you need your Arduino running while the servo is struggling to start, I'd add a small ultracap (0.22 - 0.5F) to Adruino power lines. You'll need to carefully evaluate the power consumption to know how much time exactly you can run off this backup power.

If you want to maximize battery life, I suggest you look into low power modes provided by Atmel controllers. Take a look at this library for example.

Using a switching regulator will probably worsen battery life: these regulators are only efficient when substantial power is needed. At currents below 1mA linear regulators beat switching regulators by orders of magnitude in terms of efficiency - the 662K I have mentioned consumes only 1μA, while an LM2575 wastes 5mA.

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  • \$\begingroup\$ Thanks for the replies everyone. Looks like a linear regulater and small ultracaps is the way to go. Regarding the design, this will run of AA disposable batteries (or rechargable ones if people choose to do so). The servo will open/close a valve every few hours, so most of the time it will be unused and unpowered (power will be cut with a relay). I ran a test and can get well over 1000 open/close cycles from the servo with the battery still going, so I'm hopeing that my 3.3v arduino + servo running off these 4 batteries will last for months if not over 1 year \$\endgroup\$
    – MG123
    Commented Apr 26, 2016 at 0:22
  • \$\begingroup\$ Hey Dmitry, I have one more question - to maximize the battery life should I use a 3.3v switching regulator? Or is a linear regulator enough? I believe that the pro mini has a built in regulator which is not efficient for long battery life - is this because it's a linear regulator and should be replaced with a switching regulator? Or is the problem that the linear regulator is crap and I should use a better linear regulator? Thank you for your help :) \$\endgroup\$
    – MG123
    Commented Apr 26, 2016 at 0:45
  • \$\begingroup\$ If you want to maximize battery life, I suggest you look into low power modes. Take a look at this library for example. At idle currents below 1mA linear regulators beat switching regulators by orders of magnitude - the 662K I have mentioned consumes only 1μA, while an LM2575 wastes 5mA. \$\endgroup\$
    – Dmitry Grigoryev
    Commented Apr 26, 2016 at 8:03
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    \$\begingroup\$ Thank you Dmitry, that's very kind of you to take the time to help me. In that case, I will use an arduino pro mini with it's built in linear regulator, and will use low power modes while it's sleeping. I'll also remove the power LED to save some juice there. \$\endgroup\$
    – MG123
    Commented Apr 27, 2016 at 0:25
  • \$\begingroup\$ Sounds like a good plan. Good luck. \$\endgroup\$
    – Dmitry Grigoryev
    Commented Apr 27, 2016 at 6:17

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