capacitor ratings and frequencies they cut

[tjmicsak]

Is there any chart or cross reference for knowing what frequencies will be cut/low passed for a given capacitor rating?
Let's say to know what difference there would be between a 4.7 uf and a 10 uf?
If the cut off frequency is based on the time for a charge/discharge cycle to reach completion- or not, then isn't there a direct cross reference for any given value?

[Ripthorn]

If you search I am sure you will find something.  Also, usually capacitors are paired with a resistor to make high or low pass filters.  The cutoff frequency is 1/(RC) in rad/s I believe.  If you mean in/out caps, then that depends on the circuit.

[R.G.]

A cap has no frequency by itself. It has only an impedance, and that is inversely proportional to frequency:
Impedance Xc = 1/(2*pi*F*C), where pin is the irrational 3.14159..., C is the capacitance and F is the frequency.

However, the combination of a resistor and a capacitor has a characteristic frequency. That frequency where the cap and the resistor have equal impedances is the rolloff frequency for that R and that C whether used as a high pass or low pass filter.

The frequency for a given R and a given C is F = 1/(2*pi*R*C).

This is very simple to do on a calculator: input R, hit the times key, input C, hit the equal key, hit the times key, input 6.28, hit the equals key, then hit the inverse (1/x) key.

This also lets you know some obvious things. To change the frequency of an R/C filter, doubling the capacitor lowers the frequency by 2 (an octave down) and halving the capacitor raises the frequency by a factor of two (i.e. an octave higher). A 4.7uF cap has very close to a 2:1 effect  compared to a 10uF.

The reason the relationship is not direct is that the cap's impedance is proportional to 1/capacitance, so it's inverse.

[solderman]

^ R-C filter calculator for us lazy builders 
http://www.muzique.com/schem/filter.htm

[R.G.]

^ R-C filter calculator for us lazy builders 

IMHO, if you use an online calculator before you understand what the numbers mean, you are probably preventing yourself from ever understanding what you're doing.

It's your mind. Use it however you want to.
Or not.

[Mark Hammer]

Value = the amount of capacitance the unit provides

Rating = the operating voltage range within which the component is assured to meet its specification

A 10uf/25v cap has a value of 10uf (plus or minus some tolerance), and is rated to have that value as long as the operating voltage is 25vdc or less.  It may well continue to maintain that value at higher voltages, but it is not guaranteed above the rating.

Not being picky here or scolding; just trying to bring you up to speed with respect to use of tech terms that will get you the best answers from others (like the people at the Mouser order desk!! ).

If the cut off frequency is based on the time for a charge/discharge cycle to reach completion- or not, then isn't there a direct cross reference for any given value?

No, precisely for the reason you noted.  When the cap is "charged", it has acquired current and is holding it.  How long did it take to charge that cap?  Well, think of it like filling a piggy bank.  How quickly it gets "filled" depends on the size of the piggy bank (capacitance), and how much money you are able to keep sticking in the slot.  The relevant resistance associated with that capacitor sets the rate at which any money (current) is deposited.  A small piggy bank could be filled more slowly than a big one.

That's why RG refers you to a formula that incorporates both R and C.

[tjmicsak]

Thanks for the information. It's gonna take some time to get this sorted out but what got me going on this was adding the missing .01uf cap to my MkII tonebender clone and finding .02uf was better so I thought it would be nice to have some direct cross reference on this EQ effect, not just for this but also for guitar tone pots. Often times I see cap values recommended but corrolating those capacitence values to an actual frequency roll off would seem to me to be alot more usable. If coupled with a specific pot resistance (250K or 500K) it would be nice to know the range of the available "EQ" those configurations would allow and be able to visualize them in a graphic sense.

[solderman]

^ R-C filter calculator for us lazy builders 

IMHO, if you use an online calculator before you understand what the numbers mean, you are probably preventing yourself from ever understanding what you're doing.

It's your mind. Use it however you want to.
Or not.

Well. I meant use the site instead of a calculator. Besides, this site explains this better than most calculators I've seen.

[R.G.]

Thanks for the information. It's gonna take some time to get this sorted out but what got me going on this was adding the missing .01uf cap to my MkII tonebender clone and finding .02uf was better so I thought it would be nice to have some direct cross reference on this EQ effect, not just for this but also for guitar tone pots. Often times I see cap values recommended but corrolating those capacitence values to an actual frequency roll off would seem to me to be alot more usable. If coupled with a specific pot resistance (250K or 500K) it would be nice to know the range of the available "EQ" those configurations would allow and be able to visualize them in a graphic sense.

Good man - you're thinking.

Doing what you're trying to do requires some learning and thought. Changing a 0.01 to a 0.02 changes things by 2:1. But what is the "1"? It depends on the resistances that are connected to the cap. If the resistances are in series and the cap a shunt to ground, then the change will move the point at which treble starts being lost down by an octave. If the cap is in series and the resistance goes to ground, this moves the frequency at which bass is being lost down by an octave.

If you're combining a cap with a pot, you have to know whether the pot and the cap are being used as a high pass filter or a low pass filter, and whether the pot is the only resistance involved, and whether the pot is used as a variable resistor (that is, a two-terminal pot) or as a voltage divider (i.e. three terminals used). This is because a pot as variable resistor can go from maximum to almost zero ohms, and a voltage divider pot has a complex resistance if the cap is connected to the wiper. All the details of the connection matter.

To get the full understanding, you need to first understand a single resistor and capacitor, then series-parallel combinations of resistors to figure out what those are equivalent to; at that point, you can do the simple calculation for the frequency. An online R-C calculator can only do this last part for you, it can't figure the effective resistance.