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I am trying to understand a RTC module from Sparkfun.

I don't understand the purpose of the capacitor in this circuit.

Schematic's Link

Here are the procduct's page and the schematics link.

Can some one please explain its purpose.

BharathYes
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2 Answers2

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This is a slightly silly and simplistic analogy: -

The capacitor is like a flywheel on an engine - it smooths the rather sporadic pulses of energy associated with the movements of pistons and produces a more constant drive to the wheels of your vehicle.

Without a flywheel, the jolts and impulses from the pistons would be felt all over the vehicle and things might rattle and break or even fall-off in the road. In other words, without a flywheel, all parts of the vehicle feel the effect of the pistons.

If your chip didn't have a local decoupling capacitor, all the pulses of current it draws from the supply would "shake" the power supply voltage around and cause interference on all other circuits sharing the same power supply.

Andy aka
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The capacitor is there as local decoupling for U1.

Please read. Why need the capacitors be as close as possible to the device?

Community
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  • So what would happen if I removed that capacitor ???? Would it still work. – BharathYes Jan 19 '14 at 18:31
  • Also I don't understand the **coupling** effect. Can you please explain it a little more ??? I tried searching wikipedia. It says : _decoupling is the prevention of undesired coupling between subsystems._ – BharathYes Jan 19 '14 at 18:34
  • The next sentence on Wikipedia answers this: "**Noise** caused by other circuit elements is shunted through the capacitor, **reducing the effect it has on the rest of the circuit**.". – David Jan 19 '14 at 18:46
  • That is even more confusing. I am very new to the electronics stuff. I am a software student. Can you please point me to a site where I can learn about these more deeply and from the basics. – BharathYes Jan 19 '14 at 18:50
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    http://en.wikipedia.org/wiki/Decoupling_capacitor "The decoupling capacitor works as the device’s local energy storage. The capacitor cannot provide DC power because it stores only a small amount of energy but this energy can respond very quickly to changing current demands. " – kenny Jan 19 '14 at 19:09
  • In very layman's terms: it is your local grocery, where you can walk in and buy a banana each day. In contrast to the shop's supplier (the battery), which can deliver banana's in crates of 100kg, but only on a week's notice. – Wouter van Ooijen Jan 19 '14 at 19:50
  • Think of it in water terms. BATTERY: hose/pipe, IC: plughole/tap, CAPACITOR: bucket/bath. The IC (plughole/tap) is going to be drawing very pulsy loads (especially if it is digital), this will reduce the local voltage (pressure) and will upset the operation of the IC (tap) and worse still, any nearby IC's (or taps) will be affected. The local, VERY VERY close capacitor (bucket) allows those burst to be locally drawn before being topped up –  Jan 19 '14 at 20:13
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    There are plenty of questions on this site about decoupling capacitors. Do a search and read some of the answers. – geometrikal Jan 20 '14 at 01:33