capacitor across battery..

[extreme.aly]

I'm making a robot that will have two 12v batteries... One of the two will be stepped down to 5v (using 7805) to power up the ICs (microcontroller, sensors, leds, etc) and the other 12V will be used to power up the motors...

I need to put a capacitor across each battery to have a smooth supply... Please suggest which capacitor should I use? And please also tell the reason for using that particular rated capacitor...

Thank you

[newInRobotics]

If You are going to use 2 separate batteries to power motors and electronics, I see no need for filtering caps, as high current draw by motors won't affect control circuitry. If somehow Your electronics get affected by noise from motors, You can put 3 caps across motor terminals and casing.

Also, to the best of my knowledge, filter caps are put as close as possible to electronic device one wants to protect and not across battery terminals.

Here's a very simplistic article --> Reducing Motor Noise

[jlizotte]

Thanks for the link, New...a useful starting point for me on this particular topic.

[jwatte]

You can probably use the same 12V supply for driving your electronics. Just put a nice, big capacitor across the input to the regulator, close to the regulator. Something between 100 and 1000 microfarad would be good, and rated at at least 25 volts.
Also, you'll probably want to use a step-down DC/DC converter rather than a linear regulator like the 7805, to save some battery juice. Else you'll be burning off more power in the regulator than your electronics actually consume!

[extreme.aly]

You can probably use the same 12V supply for driving your electronics. Just put a nice, big capacitor across the input to the regulator, close to the regulator. Something between 100 and 1000 microfarad would be good, and rated at at least 25 volts.
Also, you'll probably want to use a step-down DC/DC converter rather than a linear regulator like the 7805, to save some battery juice. Else you'll be burning off more power in the regulator than your electronics actually consume!

can you give me a link for the dc/dc converter ure talking about? on internet, they say 7805 is also a dc/dc converter..

[newInRobotics]

can you give me a link for the dc/dc converter ure talking about? on internet, they say 7805 is also a dc/dc converter..

What he meant was a Switching Mode DC/DC Converter (a.k.a. switching step down DC converter) as opposed to linear DC regulator. Switcher is much more efficient as it does not (in theory) dissipate voltage difference as heat. Why not to use more reasonable voltage for microcontroller (say 6V)? Why go with 12V and then have voltage stepped down?

[jwatte]

In daily speech, I've typically heard "DC DC converter" used for switching converters, and "regulator" used for linear regulators, but I'm not sure if this is clearly ambiguous.

Anyway, there are plenty of high quality DC DC converters available. For example:

http://www.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=811-2692-ND - 5V, 1.5A
http://www.pololu.com/catalog/product/2119 - fixed 5V, about 1A
http://www.pololu.com/catalog/product/2118 - adjustable, about 1A
http://www.pololu.com/catalog/product/2110 - adjustable, 3.5A

[Gertlex]

One more option (same as first one listed by JWatte, but different supplier ):

http://www.mouser.com/ProductDetail/Murata-Power-Solutions/OKI-78SR-5-15-W36-C/?qs=sGAEpiMZZMtwaiKVUtQsNemMZL4TplJBqOl7845nWHA%3d

Capacitor advice given by others is better than I can give.