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I'm building my first biped robot, using a Raspberry Pi as the brain. My problem is, I want to decouple the battery before I use it to power the Pi.

The reason I want to decouple the battery is because I will have 32 servos connected to a servo controller which is also powered from the same battery, I expect it will cause a lot of fluctuations in the power source.

I understand what a decoupling capacitor does but not how to use it or what types of capacitors I can use.

So I guess my questions are, how do I use a decoupling capacitor and can I just use any type of capacitor?

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Decoupling capacitors -- usually around 100 nF -- are usually placed as close as possible between the power and ground pins of an IC to absorb spikes on a power rail from interfering with the operation of the IC.

Your Raspberry Pi should already have these capacitors added to all noise-sensitive components, so you don't need to add any.

The battery voltage is the next concern. The Raspberry Pi is expecting to be supplied from a USB charger providing 5v @ 700 mA or more. While this is fed into three linear regulators to supply internal 3.3v, 2.5v, and 1.8v rails which are used by virtually all IC's on the board, the 5v rail is also brought out to several I/O connectors.

So you should not input a voltage higher than 5v to the power jack of the Raspberry Pi if you have devices connected to either General Purpose I/O headers P1 and P5, the HDMI connector, or any of the USB connectors. And even if you are not using any of these devices, inputting a much larger voltage than 5v (e.g. 12v) will cause the three linear regulators (3.3v, 2.5v and 1.8v) to get hot since they will have to drop a lot more voltage.

So depending on what voltage your battery is (I assume it is not 5v), you need to provide an external regulator (either linear or preferably a switcher) to bring it down to 5v. At the output of that regulator, you can add a couple hundred μF capacitor.

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    \$\begingroup\$ I want to pick a nit with your first paragraph. The primary purpose of a capacitor placed close to an IC is to supply the short-term current needs of that IC, reducing the voltage fluctuations that would otherwise be caused by (i.e., "decoupling" it from) the impedance of the power distribution network. Its ability to reduce noise generated elsewhere on the power bus is more limited, and depends strongly on the impedance between the noise source and the capacitor (which can be increased through the use of a series resistor or choke). \$\endgroup\$ – Dave Tweed Apr 7 '14 at 10:44
  • \$\begingroup\$ @DaveTweed "reducing the voltage fluctuations" -- that's what I meant by "to absorb spikes on a power rail". I think I was trying to say the same thing you are, just worded it differently. \$\endgroup\$ – tcrosley Apr 7 '14 at 11:51
  • \$\begingroup\$ @tcrosley the battery is a 7.5v lithium battery, rated at 2500mah, I have a 5v 1a regulator on the circuit before the Raspberry Pi but after the servo controller so input voltage will be fine. However I have no capacitors either side of the regulator. \$\endgroup\$ – Luke Alderton Apr 7 '14 at 13:05
  • \$\begingroup\$ Do I need to add capacitors either side of the regulator? \$\endgroup\$ – Luke Alderton Apr 7 '14 at 14:06
  • \$\begingroup\$ @LukeAlderton: What kind of regulator is it? \$\endgroup\$ – Dave Tweed Apr 7 '14 at 14:36

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