I'm designing a circuit (or trying to :-) ) that will run off 12V car
bateries, and am wondering where smoothing (right word?) capacitors come
into play? I've already got some on my 5V reg either side going to ground
and was wondering where else I should put them. Please can someone either
suggest a link or tell me a way how to work the values.....So far googling
hasn't proved to be very productive.
I don't know much about when they are needed or how you calculate their
values, but it might help others to know what type of circuit you are
creating to help them answer your questions.
The capacitors are used to short circuit any noise generated by your
circuit to ground. High speed digital circuits can generate a lot of noise
on the power lines as they switch and that noise can generate RF signals
that radiate from your supply wires and cause interference both in parts of
your own circuit as well as in things around it (like your car radio, cell
phones, etc). How many you need and where you need them is a complex issue
of of how much noise your device generates, and how much you care about
what RF noise it generates. Some designs will put small capacitors next to
almost every digital IC on a circuit board design to help reduce radiation
of noise though the power lines.
Most regulator circuits need them on the regulated output but you can
normally find answers to what is needed with the specs or application notes
of the regulator. On thing to keep in mind is that they normally need two
capacitors in parallel, a large one, and a small one. In theory, the small
one wouldn't be needed, but large electrolytic capacitors don't actually
work well at high frequencies so you need to small capacitor to deal with
the high frequencies that the large one can't handle.
I'm fairly sure that you can't put too much capacitors on the power lines.
It's mostly just an issue of making sure you have enough in the right
places to keep the noise levels down to whatever level is required.
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