Selecting a capacitor for a bigger voltage

[ljille]

Several times I and I think a lot of people have used a capacitor rated for 50 V when the specification is 25 V. But if you have a 500 V rated capacitor, for instance 2.2 nF, for a circuit of 5 or 12 volts, can we use it without problems or an impact on the circuit function?
That is, I understand the voltage specification refers to the isolation capability of the device, so, if it is bigger, it shouldn't affect the capacitance, but do somebody can answer if when this data is too different, do the 5 volts on the circuit be too low to achieve the capacitance rated by the manufacturer?

Thanks for your time
Luis

 

[barry]

The applied voltage will affect the capacitance of some devices, electrolytic e.g. You need to consider the particular capacitor type.

 

[ljille]

Voltage will affect as it is the 'strenght' of electrons to be moved, but it is not the question (I don't explained myself enough), who is, if the exagerated difference in the rate of a capacitor (the isolation capacity), for instance, from 50 to 500, could affect the circuit behavior? That is, if the much bigger capacity of isolating could do that a capacitor with a 500 V rating, feeded at a regulated just 5 volts (instead of the maybe 220 V for wich it could have thought) could give us for instance a real 1.9 nF, instead of a 2.2 nF marked in it's specification, making a different oscilating time for the circuit than the designed one, or, on the other hand, it will remain offering the same capacitance of its specs. independently of the so different isolating rate.

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The associated question could be that if there are two type of capacitors, for instance, electrolitic and tantalum, both specified as 1uF, or maybe tantalum and ceramic at maybe 1nF both, what difference could it do in an given oscilator?

 

[barry]

I think I did answer your question. And I have no idea what you mean by "isolation capability". The voltage rating is the maximum voltage that can be applied to the capacitor.

 

[KlausST]

Hi,

Does this discussion really make sense?
* if you want to build an oscillator, then use an appropriate capacitor type. A tantalum surely isn't.
* if the voltage rating of the same type of capacitor is 10 times, then you may expe t it's volume to be 100 times.
* if you use an appropriate capacitor for a dedicated application, then you will find the important parameters in it's datasheet.
Example:
* for oscillator: parameters like capacitance drift over time, temperature, applied voltage...
* for a supply rail bulk capacitor: ESR, lifetime, ripple current rating...

--> therefore: first select the correct capacitor type for your application, then read it's datasheet.

Klaus

 

[ljille]

In other words: what happen if I have already and use a capacitor rated at 1nF, 500 V, instead of the 1nF 50 V. I supposedly should use, and I use it in a 12 V op amp circuit wich is adding two signals about 20 KHz?
As a secod question:
Is there a document where I can find a sinthesys about selecting capacitors?
Thank you
Luis

 

[KlausST]

Hi,

In other words: what happen if I have

Again: read their datasheets if you want to know how they behave.

There are many capacitor selection guides.
And there are many documents dicussing different dielectric materials.

Klaus

 

[FvM]

For most commonly used capacitors, the answer to the question is simple and clear, I don't understand why you are hesitating to answer it.

Except for the possibly larger form factor, there's nothing against using capacitors with higher rated voltage.

 

[Pjdd]

It depends on the dielectric type and the intended application. At 2.2nF/500V the OP is obviously not considering an electrolytic. Plastic types will not show any significant change in capacitance with voltage but some ceramics do. This may or may not matter depending on the application.

 

[ljille]

Thank you. It is not going to be connected to 500 V, the question is if one can use a 500 V rated cap where the voltage is just 5 V, and the 2.2 nF will remain being the same capacitance specified. I suppose yes.

 

[Pjdd]

It is not going to be connected to 500 V, ..........

I know. I understood your question. My answer meant that, for all practical purposes, a plastic capacitor will have the same capacitance whether you apply 5V or 500V. Some ceramic capacitors will exhibit different capacitances at 5V and at 500V.

 

[FvM]

I think, the answers are still more vague than they should. Answering too general is another way to miss a clear answer.

Non-linearity (voltage dependent capacitance) is a property of high permittivity dielectrica, e.g. X7R. As an extreme case, see the below characteristic of a 100V capacitor, which has much more than 50% of nominal capacitance at 50% rated voltage. Most X7R capacitors still have 70-80% capacitance at 50% rated voltage.



The capacitance is effectively unchanged below 10 % of rated voltage, which should answer the initial question.

As previously mentioned, the answer is similar for most commonly used capacitors. You see a different behavior e.g. with polarized high Er capacitors like Epcos/TDK CeraLink series.