Capacitors

[Burner]

Can someone explain to me about the values a capacitor has?
I'm building a board and trying to decide what capacitors to use. I'm interested in how uF and ESR relate and what kind of things I should be looking for in a capacitor.
I need capictors both for power supply filtering and to go off a voltage rail that will have motors/servos connected to it.
Also if you can explain what kind of arrangements you should put capacitors in that are in parallel. Like 1uF and 1000uF in parallel, why would I do that. Or if 2 470uF and 1 1uF capacitor in parallel is bad.
Thanks

 

[ScottMac]

The basic value for a Cap is FARAD. Most common values is "Microfarads," you may also see some that are "picofarad" (pronounced "peekofarad").

A microfarad is one millionth of a farad, a picofarad is one billionth of a farad.

Capacitors add in parallel, divide in series. Two 10mfd caps in parallel is 20mfd; in series 5mfd.

Depending on the quality of the raw DC voltage, you can probably start at ~500mfd for the power supply filter ...less if it's clean, more if you're using something like a bridge rectifier (big bumps = needs more "capacity" to fill in the throughs)

Religious preferences vary but, personally, I like to go --at least-- 100% over the voltage rating (a 16V cap for a 5V line).

There are a bunch of different kinds of caps - Mylar film, electrolytic, tantalum, ceramic ... some are better at some rols than others. Tantalums are comparable to small electrolytics, but tend to be higher quality ( less "leakage"), most power supply / filter caps are going to be some flavor of electrolytic.

Glitch / noise caps tend to be smaller values (10Mfd or less, down to the tens-to-hundreds of picofarads), depending on the anticipated frequency of the noise you are trying to filter out. Glitch caps tend to be ceramic disks or mylar film.

For relay / motor kickback, something ~10mfd should work OK, up to ~47mfd.

MOST capacitors are polarized - (especially electrolytics & tantalums) there is a plus & minus lead. Failure to observe polarity can get to smelling pretty nasty real quick.

If you describe what you need to do and what components you have to do it with, I'm sure someone can jump in and give you some specific recommendations. There's just too many areas to cover in a post or two.

Good Luck

Scott

 

[Fencer128]

Hi,

I'm not sure on your numbering convention - but a picofarad (pF) is 10^-12 farad, as opposed to a nanofarad (nF) which is 10^-9. Given that, I would say a nanofarad is a billionth of a farad and not a picofarad.

Cheers,

Andy

 

[Mark R]

Another couple of points:

Most capacitors are non-polarised - they work equivalently when connected either way around. Electrolytic capacitors are almost always polarised (this includes traditional aluminium electrolytics and tantalum electrolytics). An incorrectly powered polarised capacitor will not operate correctly - electrolytics tend to explode if reverse biased.

The other points are well made, and I shan't repeat them.

ESR stands for equivalent series resistance - if you imagined the capacitor actually having a resistor in series with it, the resistor would have this value. The lower it is, the faster the capacitor can charge and discharge.

ESR is important in high-power, and high-frequency circuits because it may be the dominant factor which limits charging and discharging. In a switching power supply (e.g. the CPU voltage regulator on a PC mainboard) the ESR is often more important than the actual capacitance value. The reason that the CPU socket is often surrounded by 6 or 8 electrolytics in parallel is to get a suitably low ESR.

Different types of cap have different ESR (although it is normally only relevant for electrolytic caps). Tantalum caps are hugely better than aluminium electrolytic, and much smaller. They are, however, much more expensive.

 

[DoubleUhat]

No wonder, couple of years ago i tried to repair a switching PSU by replacing a faulty 16v47uF cap with a 50v47uF and found out it don't work that way, only when i finally got a 16v47uF then it works.

This ESR thing must be it.

 

[Burner]

Thanks for the responses.
I have several specific needs.
My input voltage is somewhere between 9 and 16 volts.
I have two 470uF 16v panasonic F package VS capacitors on it in parallel.
It then goes to a 5v linear voltage regulator
I have 1(more capacitance in other areas) 1000uF 6V panasonic F package VS capacitor on this.
Next it goes to a 3.3v linear voltage regulator
This has 1 1000uF 6V panasonic F package VS capacitor
then there are ferrite beats for both gnd, vdd, and another 1000uF 6V panasonic F package VS capacitor
For each rail there is also a 0.1uF 0402 capacitor on it. I heard you should put dual caps of different values that are significantly different to filter out all the noise. I'm concerned about the dual 470uF cap and 0.1uF cap on the input rail though.

The 5V rail can be hooked up to servo motors which vary quite a bit on their current needs. There is another 1000uF 6V panasonic F package VS capacitor hooked up by where the servos are.

the input rail is hooked up to 4 motors, these would probably ramp somewhat smoothly in current draw, so I'm not too worried about them. Also I don't think the rail will drop below 7volts from motor draw killing the 5v rail after the regulator.

On the ICs I have surface mounted 0402 1uF and 0.1uF capacitors on most vdd inputs. It is connected such that the main vdd trace goes to the capacitor lead and from the capacitor lead to the pin through a via. Most of the traces are 8mils. Gnd is connected in a simliar manner.

The board is only 2 layers. I tried to make most of the power traces 80-120mils, except the final traces to the pins.

Right now I think I have enough capacitance around the regulators. Should I reduce the capacitance in any areas?
Any other information that would help you? I know almost nothing about analog electronics. My board does boot up seems to work alright but I haven't tried any motors or servos on it. If someone wanted to look at the whole design I could send it to them if they wanted to give me pointers.

Also I have some questions about voltage regulators if anyone is interested in tha discussion.
Thanks for the help.

 

[Demon-Xanth]

Why you'll see 0.1uF caps in paralell with 100uF caps:

As frequency increases, the inductance found in large caps makes thier impedance INCREASE after a certain frequency. Smaller caps are often made from different materials that have much less inductance and thus have a higher frequency where they become inductors. So in effect, the large capacitor passes lower frequencies, and the smaller passes higher. Check some datasheets on caps and you'll see curves that look kind of like a V, with frequency on the bottom.