capacitor question.

Hi All,

Sorry in advanced for the noob question but I am still learning. so here goes.

I am working on a project that calls for a 10uf 16 volt capacitor, but all I have in are 10uf 35 Volt capacitors.

My question is will it work and if so why wouldn't I alway use a higher voltage rated capacitor.

Thanks

-KC

Yes it will work. Using higher voltages than you need results in a physically larger and more expensive capacitor.

Thank you so very much

The voltage rating of a capacitor is the voltage which you mustn't exceed. If you apply a higher voltage to it than the rated value the capacitor will explode. And when a capacitor explodes you know about it :)

Hi, I agree with all previous answers, BUT don't go to to high a voltage rating over your working voltage. Using a 35V in the place of a 16V is fine, but don't use a 50V or higher and expect to get the same performance.

An electrolytic works by having an oxide layer as the dielectric, this oxide layer is produced by the voltage that it is working with. If you use a cap that is say 100V in a circuit that is only using 15V then you may over time find that the capacitor will not perform to its rated capacity because the dielectric has changed.

That is why any circuits working off 12V have 16V or 25V caps, 24V ciruits use 35V.

A good question.

Tom

TomGeorge: That is why any circuits working off 12V have 16V or 25V caps, 24V ciruits use 35V.

No, not really. The reason is much more practical than that. The higher the voltage (regardless of the dielectric system), the lower the capacitance you will generally get for a given volume. Plus, for the same cap-rating, generally the price is higher for higher rated voltage.

Generally when using an aluminum-electrolytic the highest rated voltage you can use (again generally limited by cost and physical volume) is desired since you'll get more service life out of the cap. The thicker the oxide, the more time you have for the electrolyte to breakdown the oxide.

As with all capacitor discussions, we need to be careful. Not all statements about one type of dielectric system apply to others. You don't de-rate the voltage on a ceramic for the same reason you de-rate the voltage on a wet-electrolytic, nor do solid-electrolytics work anything like their wet counterparts.

Regarding wet electrolyte (not dielectric, that's the oxide layer) aluminum electrolytic capacitors, what TomGeorge said is accurate. In normal use, the oxide layer is breaking down and being reformed constantly. If you start with a much higher voltage rating, the oxide layer may end up uneven, concentrating ripple current in some areas, causing local heating. A bad thing. It is bad engineering practice to select a wet electrolyte aluminum or tantalum electrolytic capacitor with a voltage rating much higher than the maximum voltage it must withstand in circuit.

James C4S, you said not to make general statements right after making a general statement.

If it were true that using a much higher voltage rating with a wet eletrolyte meant longer life, then why do unused old capacitors require reforming? At 0V, they should have a very long life, instead they develop high leakage currents.

http://www.eetimes.com/document.asp?doc_id=1279791

"Your first thought might be to increase the capacitor’s voltage rating to minimize the possibility of a dielectric failure. However, doing so can lead to a capacitor with a higher equivalent series resistance (ESR). Because the capacitor typically has a high ripple current stress, this higher resistance leads to extra internal power loss and increased capacitor temperature. The failure rate increases with the increased temperature. In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage."

Rules of thumb for capacitors always get engineers into trouble. Each dielectric system makes them very different devices and even within dielectric systems there is huge variance in how they perform.

polymorph: If you start with a much higher voltage rating, the oxide layer may end up uneven

Actually, the oxide is already quite uneven. In order to increase the surface area of the dielectric, we acid etch the foil sheets to create valleys. Within those valleys, the oxide will always grow an uneven amount--either when we form it or when it is reformed during application use. It turns out that de-rating past 30% of the rated voltage generally doesn't increase the life of the part because of these areas constantly re-growing due to the localized heating that causes them to break down. (Just as you said, except that we already know it happens anyway.)

polymorph: It is bad engineering practice to select a wet electrolyte aluminum or tantalum electrolytic capacitor with a voltage rating much higher

Again, you cannot apply the same reasoning to a wet-aluminum and a solid-tantalum, or even a solid polymer aluminum. For example, solids like mno2-ta, polymer-ta, and polymer-al don't have a situation where the oxide is breaking down over time.

I didn't mean for my argument to be that you you should use a 100V al-lytic in a 15V application. I was making the more general statement "use the highest rated voltage you can for your application." Which might mean considering many of the points you've brought up. Along with the practicality of most applications limits that rated voltage anyway.

polymorph: If it were true that using a much higher voltage rating with a wet electrolyte meant longer life, then why do unused old capacitors require reforming?

You're mixing use conditions with shelf-life, which is important distinction. While sitting on a shelf unpowered, the oxide is being broken down by the electrolyte. Until voltage is applied and current can flow, the oxide can't regrow itself. So no matter what the voltage the dielectric was formed with, eventually the oxide will breakdown completely allowing enough leakage current to cause catastrophic failure when powered again.

So your example of 0V doesn't really apply because that isn't allowing the oxide to regrow.

polymorph: http://www.eetimes.com/document.asp?doc_id=1279791 "Your first thought might be to increase the capacitor’s voltage rating to minimize the possibility of a dielectric failure. However, doing so can lead to a capacitor with a higher equivalent series resistance (ESR).

Yes, I agree with Rob's statements. However keep in mind that he doesn't not work for a capacitor company, and I do.

I took issue with

Generally when using an aluminum-electrolytic the highest rated voltage you can use (again generally limited by cost and physical volume) is desired since you'll get more service life out of the cap.

Then you said

I didn't mean for my argument to be that you you should use a 100V al-lytic in a 15V application. I was making the more general statement "use the highest rated voltage you can for your application." Which might mean considering many of the points you've brought up. Along with the practicality of most applications limits that rated voltage anyway.

So I don't really think we disagree. Keep in mind that there is a very high percentage of newbies and medium-newbies here. So saying "highest rated voltage you can use", you and I and a dozen people here may know the limitations, but the other 99% do not. And someone may have their dad or grandfather or neighbor's old TV technician surplus and therefore have a bunch of wet Al-electro caps >100V.

I believe I did specify, and it is in the portion you quoted, that I was speaking of wet electrolyte capacitors.

I don't think Rob's doesn't not working ;') for a capacitor company disqualifies him from having an informed opinion.

polymorph: So I don't really think we disagree.

I agree we don't disagree. ;) Later I thought "You know, we're talking the same thing from different points of view." My bad.

;')