Sizing capacitors for smoothing signals

[sherrellbc]

In general, by what considerations guide the decision to size a capactior to smooth voltage signals?

For example, if I were to use a 3.3V regulator in a circuit design, what size cap should be set in place to ensure a smooth regulated voltage? My attempt at answering my own question would bet to get an appropriate voltage rating (~10V, for this case maybe?) and, presummably, larger capacitance is better? I know that smaller capacitances will discharge very quickly if the need ever arises (i.e. fluctuations in the output voltage require cap discharge).
-What keeps a capacitor from discharging back back into the regulator?

However, at the same time, I know that discharge rates of capacitors depend largely on the load across its terminals. That being said, is a direct analysis per design in order for capacitance sizing?

What about the use of dielectric capacitors versus ceramic (non-polar) capacitors?

 

[mfb]

What keeps a capacitor from discharging back back into the regulator?

The regulator (or the connection between regulator and capacitor), hopefully.

and, presummably, larger capacitance is better?

Usually, yes.

what size cap should be set in place to ensure a smooth regulated voltage?

That depends on the fluctuations you expect.
Maximal current for the capacitors is also a design choice, and depends on the load.

 

[sherrellbc]

The regulator (or the connection between regulator and capacitor), That depends on the fluctuations you expect.
Maximal current for the capacitors is also a design choice, and depends on the load.

Where would such fluctuations originate from? I was considering placing smoothing capacitors across the terminals of the a voltage regulator to ensure output is kept very clean and any high frequency fluctuations (inherent circuit noise?) is kept filtered out.

For example, say mains voltage is stepped down to around 12V, which is then in turn clamped to 5V and again to 3.3V for various applications. What order of fluctuations are to be expected in a configuration like this? If each clamping stage is filtered by such a capacitor that is the purpose of this thread.

What about if the initial supply were to come from a battery. Are capactiors required in this case, or is the voltage supplied by batteries very clean to begin with?

 

[mfb]

Where would such fluctuations originate from?

That depends on your setup.
A variable load, a variable input voltage for the regulator (no regulator is perfect...), induced electric fields in the circuit, noise in the circuit components, ...

For example, say mains voltage is stepped down to around 12V, which is then in turn clamped to 5V and again to 3.3V for various applications. What order of fluctuations are to be expected in a configuration like this? If each clamping stage is filtered by such a capacitor that is the purpose of this thread.

It is impossible to answer it without analyzing the whole circuit.

What about if the initial supply were to come from a battery. Are capactiors required in this case, or is the voltage supplied by batteries very clean to begin with?

I would not expect significant variations from the source, but what about your load?

 

[alphy]

The decision to size a capacitor for smoothing voltage signals is based on several considerations. These include the required voltage rating, the capacitance value, and the type of capacitor used.

The voltage rating of the capacitor should be chosen to be higher than the maximum voltage expected in the circuit. In this case, a 10V rating would be appropriate for a 3.3V regulator. This ensures that the capacitor can handle any voltage spikes or fluctuations without being damaged.

The capacitance value is determined by the amount of ripple voltage that can be tolerated in the circuit. A larger capacitance value will result in smoother voltage output, as it can store more charge and therefore reduce the effects of any fluctuations. However, too large of a capacitance can also cause issues such as longer charging times and increased cost. A balance must be struck between the desired smoothness of the signal and practical considerations.

The type of capacitor used can also affect the smoothing of voltage signals. Dielectric capacitors, such as electrolytic capacitors, have a higher capacitance value and are better suited for filtering low frequency signals. Ceramic capacitors, on the other hand, have lower capacitance values but are more effective at filtering high frequency signals. The choice between the two will depend on the specific requirements of the circuit.

To prevent a capacitor from discharging back into the regulator, a diode can be placed in parallel with the capacitor. This will allow current to flow out of the capacitor but not back in. Additionally, the load across the capacitor should be considered when selecting the capacitance value. A higher load will result in a faster discharge rate, so a larger capacitance may be needed to maintain a smooth voltage output.

In conclusion, the decision to size a capacitor for smoothing voltage signals requires careful consideration of the voltage rating, capacitance value, and type of capacitor used. It is important to strike a balance between the desired smoothness of the signal and practical considerations, such as cost and load requirements.