Capacitors for power filtering

[stevie1556]

I've always wondered about this since I started SMD about 2 years ago and never been able to find an answer.

A lot of pedals have a power filtering section, and some use a 47uF cap and others use a 100uF cap. I've always wondered if there is any difference between them, and if the 100uF cap can be changed to 47uF with no adverse effects. I've dug out my old electronic books from school and couldn't really find any answers for it. The 47uF caps work out around £30 cheaper per 100 so it's quite a big potential saving.

[galaxiex]

With a resistor the cap forms a low pass filter to remove (potential) high(er) frequency noise from the power source.
Using a LP filter calculator we can determine the -3db cutoff frequency of the filter.

http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php

Just subbing the 47uF cap and not changing the resistor will raise the cutoff frequency.
Playing around with the calculator shows that raising the resistance value lowers the cutoff frequency.

[Lubdar]

Don't larger caps like a 220uf provide better power filtering anyway?

[galaxiex]

Don't larger caps like a 220uf provide better power filtering anyway?

Larger caps can provide more power "reserve" if there is a sag in the supply, or the circuit suddenly demands more power and the supply cannot provide that instant demand.

As far as better filtering... it depends... If the larger cap and circuit resistance are "tuned" to reduce noise in a power supply, and the frequency of that noise is below the cutoff frequency of the R/C filter, then yes.

So it depends on what the circuit needs.
Does it need specific frequency filtering because of a noisy supply?
Or does it need some "reserve capacity" because of circuit demands and a marginal supply?
Or maybe both?

[stecykmi]

you can always put two 47uF in parallel. putting several caps in parallel is very common in commercial power supplies, often because it's cheaper than buying 1 very large cap (at the expense of taking up more board space). caps in parallel often work "better" than a single larger cap as well, they tend to have lower equivalent series resistance, which makes them faster to charge and discharge.

you remember when working out the total capacitance of parallel caps, it's simple addition: 47uF + 47uF = 100uf (almost)