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Author Topic: How to calculate what capacitors to use?  (Read 1656 times)
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Gothenburg, Sweden
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Hi,

I'm just about to make my first standalone Atmel/Arduino application and there's one thing I'd like to know before I go ahead and solder things in place.
In Tom Igoe's example on standalone Atmega/Arduino and in the book "Making things talk" he uses capacitors togheter with the 7805 voltage regulator and in a number of other places. These are referred to as "smoothing" out voltage drops. When I've googled around I've seen a 7805 voltage regulator with a 10µF cap across input and ground and 10µF between output and ground. But there has also been an instance with a 1µF between output and ground. And in the 5v circuits all sorts of small caps are used.

So my questions is how do I calculate what capacitor to use when I want to smooth things out?

Preemptive thanks to everyone who can help me or point me towards good info elsewhere.

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So my questions is how do I calculate what capacitor to use when I want to smooth things out?

Preemptive thanks to everyone who can help me or point me towards good info elsewhere.


You don't calculate the values, you go to the manufacturer's datasheet for their recommended value or range of values. The output capacitor in not for 'smoothing' but rather to prevent device oscillation under certain input voltage and load conditions. The values actually used is not all that critical, more critical is where you mount them, they should be as physically close to the actual devices as possible.


Lefty
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Thanks for the quick reply. Just to see if I got things straight, I'm looking at my 78XX datasheet from ST and on page 28 there is an example circuit "Fixed output regulator " using with two caps. The input bypass capacitor is specified at 0.33µF and the output as 0.1µF. The thing that confuses me is that these value seem very far from the two 10µF caps in Tom Igoes example.

You state that the values are not that critical but really 10µF and 0.33µF seems like quite a stretch!
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Manchester (England England)
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No it is not a streatch. The data sheet shows the minimum.
You can have too little but it is hard to have too much
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Ok, I think I better check with you guys anyway before I actually try and do some estimating on my own. But I think I got the general idea now.

Thank you!
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Linear regulators are a lot more forgiving than switching regulators. The switching guys need specific ESR values in addition to capacitance. Most linear regulators will see people use one "big" electrolyte, and one "small" ceramic on the output, though, because of the different behaviours of the components.
Myself I just add a 10uF or 22uF or whatever I have laying around, when I breadboard, only on the output. Works well enough for most cases (not building 120 dB S/N paths here...)
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Personally I tend to use 2 capacitors on the output instead of one.

I use the typical 10µF, 22µF or whatever cap.  I alsu add a much smaller cap in the range of 100nF.  This allows for filtering out of high frequency noise which the big capacitor doesn't.

Also, you should choose capacitors which have a low ESR.
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Personally I tend to use 2 capacitors on the output instead of one.

I use the typical 10µF, 22µF or whatever cap.  I alsu add a much smaller cap in the range of 100nF.  This allows for filtering out of high frequency noise which the big capacitor doesn't.

Also, you should choose capacitors which have a low ESR.

Actually no, not always, some regulators (low-dropout ones) specifically specify a non-low-ESR capacitor to prevent oscillation - go to the datasheet for your particular regulator to be certain (a generic "7805" type shouldn't be that fussy though).
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The often recommended two caps setup is more prone to oscillation than just one large cap.
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With the exception of low drop out regulators I would have to disagree with that.
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Does the smaller cap really do anything?  If anything wouldn't a lower capacitance have more impedance to a higher frequency?
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Does the smaller cap really do anything?
Yes.
All capacitors have inductance, this limits the frequency response. The smaller cap has lower inductance and so suppresses higher frequencies than the larger cap.
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But wouldn't such a small inductance have a much higher impedance to a frequency? And so would basically do nothing to help smooth high frequencies?
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And for the inductance of a capacitor, the wires powering the device probably have more inductance
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How I pick Parts....

No I'm kidding.  

I too follow what the Manufacturer recommends in the data sheet.  It is one of those cases where once you add what is *needed* you can still embellish a little, such as adding a decoupling type 0.01uF at the 3 terminal regulator (as well as others close to your other IC's) with little ill effect but you can gain a slight benefit as outlined in G M's tutorial. http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

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