How to calculate what capacitors to use?

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

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!

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

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!

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...)

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.

majenko:
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).

The often recommended two caps setup is more prone to oscillation than just one large cap.

With the exception of low drop out regulators I would have to disagree with that.

Does the smaller cap really do anything? If anything wouldn't a lower capacitance have more impedance to a higher frequency?

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.

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?

And for the inductance of a capacitor, the wires powering the device probably have more inductance

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. De-coupling

But wouldn't such a small inductance have a much higher impedance to a frequency?

No the lower the inductance the lower is the impedance at any given frequency.

And so would basically do nothing to help smooth high frequencies?

Inductance stops the capacitance from working (smoothing) so you want it as low as possible. Using a ceramic capacitor has a low inductance. However, you need a large capacitance for the low frequency noise. These are not ceramic and have a much higher inductance, so you need both.

And for the inductance of a capacitor, the wires powering the device probably have more inductance

No.
The leads will have an inductance that is why they have to be as short as possible but the inductance of the capacitor swamps the leads.

Turns out, 10 uF ceramic capacitors aren't that expensive anyway. In your experience, would a single 10 uF guy be sufficient for the HF dampening, given that ceramic inductance is lower?

Has anyone done any tests of the difference between 10uf and 10 + 0.01 uf filters?

Turns out, 10 uF ceramic capacitors aren't that expensive anyway.

Yes but the dielectric used causes the capacitance to change with applied voltage and also temperature. The current rating is also a lot lower. So while it might suffice it is not as good.

This is a good summery:-

Read the datasheets carefully. One linear voltage regulator I used was not happy when it only encountered a ceramic capacitor on its output. Not only did I blow a voltage regulator, but the downstream SD card too (4.5VDC vs. 3.5VDC max allowable will do that for you). Other voltage regulators I have used subsequently specifically stated that they are safe for use with ceramic output capacitors. But they tend to sell at a premium and you have to confirm (via datasheet or inquiries with the manufacturer) that the regulator will be stable using only a (or several) ceramic output capacitor(s).

More often than not, linear voltage regulators are specified to use a large Alumimum Electrolytic or Tantalum capacitor along with a 100nF ceramic one. Data sheets and application examples by the manufacturer are invaluable to beginners like myself, they take a lot of guesswork (and magic smoke) out of the equation.

Luckily, I have some 33uF tants laying around, so good news for me :slight_smile:
Thanks for sharing your experience; I find that kind of contribution invaluable!