# pcb - capacitor usage

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#### er_r_karthik

##### Newbie level 4
capacitor usage

can i have the exact difference between a bypass capacitor & decoupling capacitor? where does they are used & at which stages?

#### rts

##### Newbie level 4
capacitor usage

bypass and decoupling, imho, the same.
The main destination is supply power energy
concrete point (ex. supply pin of IC), when
current pick of consumption take place.
Because power trace has some impedance,
it can be much enough for short current pick, then capacitors provide energy for this pick and voltage drop not happens. In calculus terms decoupling
capacitors reduce power bus impedance in
concrete point of PCB.

### er_r_karthik

Points: 2

#### GESELAW

##### Newbie level 2
Yes, and it is good practice to put small decoupling cap near every IC and one big decoup cap near power regulator.

#### centiago

##### Full Member level 3
Bypass capacitors, it's used when ac signal is to be passed. placing bypass capacitor in series acts as shot for a/c signals & acts a open (blocks) for d/c signal.

The filter capacitor you place to the suppy pin in case of multilater boards. In case of single or double side board you can place it anywhere between supply & vcc pin.

The reason for this is in multilayer boards you have dedicated ground place spread throught out the board. The return path is the plane.

In single/double sided board you have track that connects the vcc & gnd pins. It's like placing a capacitor across battery. It's immateral whether you place close to vcc or gnd pin.

Apply concept of circuit theory.

Good luck

#### kender

Check out this thread. It's about decoupling capacitors, and ut explains why sometimes you need several different decoupling caps at the same time.

#### dunn

##### Full Member level 3
Just a note.

If the device has a +15v and a -15V then you require 2 bypass capacitors.

(one from +15V to 0v and the other -15V to 0V.)

#### tranzorb

##### Newbie level 4
Hi,

Basically, bypass and decoupling capacitors are the same. Decoupling cap is placed across the power pins of a circuit, device, etc...
You can have a look at this very explanatory link if you like :