why high voltage capacitors are used in low voltage circuits...

[kasumyku]

...beside the fact that is safer to use a higher capacitor voltage than the circuit voltage, what other reasons would be?
Ex:100v capacitor in a 12v circuit...

thanks!

[firewalker]

I guess you are referring to non electrolytic capacitors. I believe that the most of the other kind of caps are "high voltage" by the nature of the materials used/design.

Alexander.

[mariush]

Some capacitors types (ceramic  capacitors) in particular lose capacitance at large voltages. For example, a 10uF 50v ceramic capacitor only means it can tolerate 50v, doesn't mean it will have 10uF when 50v goes through it. In fact, it may even be 8uF at 5v, or 1uF at 16-25v

Sometimes larger electrolytic capacitors are used due to their lower ESR  or because due to bigger volume they can tolerate heat better and are rated for more hours and high temperatures. For example, a 100uF 16v might be 8x11.5 mm and rated for 500-1000h@105c  while a 100uF 50v at 12x15 mm might be rated for 2000-4000h @ 105c

In other cases, the capacitor may not be used for its property of handling big voltages, but for some other characteristic like 0.5-1% precision instead of 20% for electrolytics, or low leakage or other things...

[JackOfVA]

And sometimes the 100V part is used in other products and it's cheaper to buy one part type in volume and use it in both applications.

[SeanB]

Most film capacitors are rated at 63V or 100V minimum due to the plastic foil used having to be strong enough to withstand the manufacturing process and the metal coating in a vacuum. A thinner film will tear during production or melt. The lowest voltage ones were polystyrene capacitors, where you could get thin films that were rated for 16V, and you could get a capacitor of 10uF 16V that was not much bigger footprint wise than a 0.1uF 250V unit but a lot taller. Of course it was hard to solder, and melted at 70C, but they are pretty stable units even with a high ESR due to the thin metal foils used.

[vk6zgo]

Some high value Polystyrenes look like notch filters at high frequencies.

We were refurbishing a Marconi TV Transmitter Sub modulator for the advent of colour TV back in the '70s,& 'replaced the rather cruddy looking 0.47uF  metallised paper caps with nice new "Styroseals".
When we swept the thing, the sweep looked like a fish tail---ERKKK!!!

Pulled 'em out,fitted old style Ducon wax dipped paper caps,& all was well!!

I've looked askance at Polystyrenes ever since.

[Dave]

Some capacitors types (ceramic  capacitors) in particular lose capacitance at large voltages. For example, a 10uF 50v ceramic capacitor only means it can tolerate 50v, doesn't mean it will have 10uF when 50v goes through it. In fact, it may even be 8uF at 5v, or 1uF at 16-25v

I have never heard about this before.
Do you maybe know any website, where I could read more about this effect?

[mariush]

See http://www.maximintegrated.com/app-notes/index.mvp/id/5527

[michaelym]

Iv'e seen a 220pF ~4kV cap used to connect a ground plane of isolated RS232 to another ground plane. I also saw it used in a Nokia cell phone for a similar purpose.

[JuKu]

Polypropylene caps are typically 400V or even 1kV. They are used in audio circuits not because of the high voltage but because of their low distortion in low impedance circuits.

[Neilm]

I've used One Hung Low high voltage capacitors (well tried them out as my boss didn't want to pay for decent ones). Spec was 10nF, 4kV ceramics.

Tested them with an LCR meter about 10nF (as near as makes no odds). Tested them with a higher voltage LCR tester (can't remember the voltage but it is a very expensive bit of kit in our calibration lab) it had dropped to 9nF. Tested them by charging them to 3kV and discharging them through a known resistance. Result was 2nF - 80% drop. And that was before adding the temperature effects of the dielectric (Z5U so would drop 50% due to temperature).

Needless to say I didn't use them.

Neil

[Smokey]

Ya the jacked up thing about capacitance changes due to DC bias is that it's not a data sheet spec usually.  So how do you know how bad it's going to be?  You don't until you test it.  And that is why we qualify parts first and keep an approved vender list and don't let the purchasing people add new parts based solely on price

[lgbeno]

Iv'e seen a 220pF ~4kV cap used to connect a ground plane of isolated RS232 to another ground plane. I also saw it used in a Nokia cell phone for a similar purpose.

This is probably a unique case for line transients where there could be huge common mode differences across the isolation barrier.  I see this very typically on Ethernet jacks which are isolated.

In fact for regulated industries like medical, the cap also needs to be UL marked as a "Y1" cap to be considered a means of protection.

In general for a few nF and lower capacitance, I think engineers make a practice of just speccing say 50V caps because their is no consequence (size or cost) to picking such a part and it becomes a standard part.

[lorth]

Some high value Polystyrenes look like notch filters at high frequencies.

What do you mean by this? The impedance?

[vk6zgo]

Some high value Polystyrenes look like notch filters at high frequencies.

What do you mean by this? The impedance?

Yes,in the case I quoted,nothing else was changed in the circuit except for the capacitor.
It seems like it looked like a high impedance at mid video frequencies.

Maybe the construction looks inductive at those frequencies,& was parallel resonant with other circuit capacitances.
It may be a limitation in how high in capacitance you can make them-----I can't remember seeing such high value polystyrenes used since.

[SeanB]

Polystyrene caps are made by winding a long length of foil into a sandwich, with the leadout of one foil being at the opposite end to the lead of the other foil. this makes for a rather high self inductance, which is only fixable by using the extended foil construction, where each winding is deliberately made long on the one side so it overlaps the dielectric on the side, so that you can spray metal onto it to short all the turns together. The high self inductance makes it a very good series LC circuit and can be a problem.

This method though is hard with polystyrene, as it has such a low melting point, so it is not used much for them, but is very popular for making the MKS polycarbonate capacitors which are made in a large ring then cut to size when finished. For polystyrene they make the long foils then simply crimp them together with the leadout, then shrink the outer covering over it for stress relief, often additionally placing the finished capacitor in a plastic case and potting it as well, as the leadout connections are very fragile.