# Any analogy as to how decoupling works ?

Discussion in 'General Electronics Discussion' started by Reprovo, Jan 10, 2014.

1. ### Reprovo

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Jan 5, 2014
I understand how a capacitor blocks DC but I can't grasp how a decoupling capacitor blocks AC.What changes occur that bring about the opposite effect of blocking AC ?

2. ### Harald KappModeratorModerator

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Nov 17, 2011
To get the wording clear: a decoupling capacitor is placed between Vcc and GND e.g. near an IC. Are we on same terms?

A decoupling capacitor doesn't block AC.
In fact, it blocks DC, but is a low impedance for AC ripple on the power supply lines, therefore short-circuiting AC to GND.
That's one way to see it.

Another way is that the capacitor is a local storage to supply short pulses of current when an IC needs it (e.g. when an output is switching).

3. ### Reprovo

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Jan 5, 2014
Thanks.That helped. If I understood correctly , this works because the capacitors are In parallel to the IC.

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Dec 18, 2013
Blocks AC what do you mean. A capacitor only passes leakage current because it is a very good insulator, nothing else. Tell me what you don't understand giving an example and I will try and help. Remember a capacitor is neither charged or discharged, a capacitor always has the same amount of total charge if that helps.

5. ### Reprovo

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Jan 5, 2014
I believe I understand It now.In a decoupler arrangement , AC Is short circuited to ground so It doesn't reach the IC etc.

6. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
I created an analogy that explains capacitors and resistors fairly well. At least, I think it does. It's had a mixed reaction here and I'd like to get more feedback on it. I call it the DTS Model. There's an introduction at https://www.electronicspoint.com/newbie-questions-t248766.html#post1470672

Once you've read that, come back here.

If you can visualise a capacitor as a cylinder with a piston and restricted air flow, so the piston resists being moved quickly (voltage across the capacitor resists being changed quickly), you can see that when you attach it across the power rails of a device that draws brief heavy currents, it will act as a local voltage reservoir.

Short spikes of current (force) will not cause much movement of the piston (not much change in voltage), and the piston is returned to its original position by current from the supply.

The DTS Model also provides an easy way to understand the action of a capacitor in a coupling (not decoupling) application, but I haven't written that up.

Do you find the DTS Model useful?