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This question already has an answer here:

In a very concise description, what does a capacitor do for a voltage regulator (for example a 3.3v). I have used the regulator with and with out the capacitors and no difference (that I can tell).

Also what does a Capacitor do in general? I understand why most electronics work, but I don't really understand the uses that I have seen for the Capacitors for.

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I am hoping others can also use this to understand it a bit more.

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marked as duplicate by JRE, Peter Smith, Robherc KV5ROB, Leon Heller, laptop2d Aug 22 '16 at 16:17

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A capacitor is (in simplistic terms) a small reservoir of electricity.

A voltage regulator is quite a slow device. When more current is needed the regulator has to respond to that and make more current available. That takes it (on the scale of things) quite a long time to do. It pretty much has to go down to the corner store to get more current for you.

The capacitor, though, has a reserve of extra electricity it can use to meet your demands. It gives you the extra current you need from its reserves, then the voltage regulator can go off and get more for you, which it then uses to replenish the reserves in the capacitor.

This is called decoupling because you have decoupled the demand from the source.

If the capacitor is too small it can't provide enough current fast enough (or if the capacitor isn't there at all) and this is seen as ripple on the voltage. As more current is requested the voltage drops to compensate until the regulator can make the required adjustments. When less current is required the voltage rises, again until the regulator can adjust itself accordingly.

If the capacitor is too big it can take too long to respond to high frequency demands for current.

It is normal to have multiple capacitors at different locations in a power system - large ones at the power supply, and places like where power enters a board. Medium sized ones to help distribute the power around the place more smoothly, and small ones right by where the electricity is used to respond to the small bug high frequency switching demands of things like microcontrollers and logic chips.

There are other more technical uses for capacitors as well - signal filtering, resonant circuits, etc. and I am sure someone else will cover those uses.

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  • \$\begingroup\$ So really, I don't need a capacitor at all, but it will help the devices running on my 3.3v start quicker when or as needed, right? \$\endgroup\$ – Bill Daugherty Aug 22 '16 at 14:34
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    \$\begingroup\$ You do need a capacitor. Devices constantly make variable demands on a circuit. Without the capacitor things will go awry. You will get brownouts, overvoltages, all sorts of horrible things. MCUs crashing, components overheating... you name it. But why quibble over what is most often the cheapest component in a circuit? They are (quite literally) ten-a-penny. \$\endgroup\$ – Majenko Aug 22 '16 at 14:36
  • \$\begingroup\$ Thanks, and now I fully understand. I only quibble :) over this as I want to make sure that I understand what I am putting in it. So now that I understand: THANK YOU, THANK YOU, THANK YOU! \$\endgroup\$ – Bill Daugherty Aug 22 '16 at 14:39
  • \$\begingroup\$ ALSO: Why is a capacitor needed at all for a crystal? I have been using my Arduino IC's just fine with a crystal, but with out 22uf capacitors hooked up to it, and all is OK. And yes, I am using the external clock (the crystal). \$\endgroup\$ – Bill Daugherty Aug 22 '16 at 14:41
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    \$\begingroup\$ Read the datasheet for the device you are using. \$\endgroup\$ – Majenko Aug 22 '16 at 14:56

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