Pages: [1]   Go Down
Author Topic: What does this capacitor do? (decoupling cap?)  (Read 634 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
God Member
*****
Karma: 2
Posts: 711
a, b = b, a+b
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

And why does it not short out the circuit?

I am referring to the capacitor placed between Vin and Ground in this schematic:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P180ACT-ND

I am basically copying this schematic for a similar brakoutboard I am building, but I dont really know *why* i am placing a capacitor there.

*

Is this a decoupling capacitor? From wikipedia:

Quote
One common kind of decoupling is of a powered circuit from signals in the power supply. Sometimes, for various reasons, a power supply supplies an AC signal superimposed on the DC power line. Such a signal is often undesirable in the powered circuit. A decoupling capacitor can prevent the powered circuit from seeing that signal, thus decoupling it from that aspect of the power supply circuit.

Is this whats happening? If yes, then why use a capacitor instead of a diode? (I realize the diode would short the circuit if connected in the same way as the cap, but why not just place a diode somewhere in the circuit instead of that capacitor?)

*

If someone could also point me to a hands on guide on how to use decoupling capacitors and when to use them, I would apreceate it. Wikipedia is a bit abstract, and I am allways reluctant trusting a random source on the internet that someone else has not recommended to me.

Thanks

P.
Logged


Phoenix, Arizona USA
Offline Offline
Faraday Member
**
Karma: 39
Posts: 5551
Where's the beer?
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

From our own Grumpy_Mike:

http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
Logged

I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17259
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
Is this whats happening? If yes, then why use a capacitor instead of a diode? (I realize the diode would short the circuit if connected in the same way as the cap, but why not just place a diode somewhere in the circuit instead of that capacitor?)

Well one very useful feature of a capacitor is that it will block DC voltages but pass AC voltages. This is perfect for use as a power supply ripple (or noise) filter as the capacitor will allow the small AC component riding on the steady DC voltage to flow directly to ground, but will not allow any DC current to flow to ground. So the cap is acting like a very small resistance to ground for AC voltages but acts like a open circuit to the normal DC voltage of the power supply rail.

 A diode does not have that same property as once it is at or above it's forward voltage drop value it conducts both AC and DC, and below it's forward voltage drop it allows no current to flow AC or DC.

Lefty

Logged

Offline Offline
God Member
*****
Karma: 2
Posts: 711
a, b = b, a+b
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

but why is it not shorting the circuit in this case?

Vcc ------||-----Ground


edit: thanks for the link. its exactly what I was hoping for.
« Last Edit: July 19, 2011, 03:13:47 pm by fkeel » Logged


Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 597
Posts: 33292
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
but why is it not shorting the circuit in this case?
Because a capacitor blocks DC. It is basically a frequency dependent resistor, at DC it is so high it is open circuit and at high frequencies it is so low it is a short circuit. So all the high frequency noise gets shorted out (or filtered as it is called).
Logged

Offline Offline
God Member
*****
Karma: 2
Posts: 711
a, b = b, a+b
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

got it. thanks.
Logged


Dubuque, Iowa, USA
Offline Offline
Edison Member
*
Karma: 44
Posts: 2458
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

but why is it not shorting the circuit in this case?
Vcc ------||-----Ground
If you were to treat the capacitor like a resistor then the amount of DC current that would leak through would be based on the "insulation resistance" of the cap. That would be in the 100s or 1000s of megaohms.
Logged

Pages: [1]   Go Up
Jump to: