# How do capacitors work

Im starting the arduino and im using the starter kit, by far im on the 5th project and asks me to use a servo and 2 capacitors each with 100uf. What i wanna knnow is in the future how will i know how many uf will i need if i wanna use it in some other project. I searched all over google but none explained specifically about the ammount of UFS or what type do i need. Basically im just asking is how do i know the right capacitor. Is there a formula or something?

capacitor blocks DC by charging up to supply level, passes AC by charging and discharging with the signal. the level of charge is indicated by Faraday units, the level of charge is determined by the size of the 'plates' of which the capacitor is constructed (if you manage to break open an electrolytic cap, you will see that it is composed of two strips of what looks like very thin aluminum foil on a thin mylar. you want to know more, go google and wiki. a certain size cap will pass AC at a certain rate, determines by it's dynamic resistance (Capacitive reactance), which changes with frequency (impedance= Xc = 1/ 2 * PI * Frequency * capacitance (in Farads). That's how you find out what size cap you need for passing AC at a certain frequency. A cap is usually used in DC to filter out any transients (by filling in the gaps or passing the AC element to ground). Filter wise, bigger is better on the supply side(but be careful that inrush current, at power on, is not high enough to damage the supply). You will usually see a 10-100uF where the supply enters the circuit, and a .01-.1uF at each IC, as filters in a digital circuit.

That's all more and less than you need to know. look up basic electronics tutorials on the web. There lots of good ones out there.

All of the above is from school 40+ years ago. the detail may be flawed, but the concepts should be correct.

A capacitor stores electrical charge, a bit like a battery but the charge stored is proportional to the voltage across it and the value of the capacitance. Current can flow into it but can't cross the gap between the plates so a charge builds up.

In algebraic terms; Q = C x V where Q is the charge, C is the capacitance, and V is the voltage.

The current flowing is given by I = C dV/dt in simple terms that means that the current flowing is given by the capacitance times the rate of change of the voltage.

Russell

Is there a formula or something?

There are lots of formula associated with capacitors, which one to use depends on what you are doing with the capacitor.

Basically im just asking is how do i know the right capacitor.

You learn about electronics, get to know what a circuit does, then you will know what to use where.

This is basically like saying, I have bought some flour at the mart, how much do I use? --- Well what are you going to do with it, pancakes, bread, sauce or cake?

Simplified -

Capacitors in parallel with the load (like the capacitors used with your servos) "resist changes in voltage". You'll see large capacitors across (in parallel with) power supply voltage to "smooth" the rectified AC voltage.

Capacitors in series with the load "block DC" (non-changing voltage) and "pass AC" (changing voltage). Or, they block/reduce low frequencies (slowly changing voltage) and pass high-frequencies.

For example, a high-pass RC audio filter (to filter-out the bass) has the capacitor in series to block/reduce the low frequencies.

A low-pass filter (to block out high-frequencies) has the resistor in series and the capacitor in parallel.

When it comes to choosing a capacitor (or resistor), most of the time the value is not that critical... It's like choosing a nail when you're building something with wood... You just need something that works...

Frequently with capacitors there's a minimum (or minimum recommended) value. I'm pretty sure you could use 200uF or 500uF capacitors in your servo circuit, but you might not get away with using 50uF.

But of course, there are cases (like audio filters) where the value is critical and you have to make a calculation.

A lot of engineering has to do with understanding when a value is critical and when it's not.... That requires knowledge and experience.

Sometimes the IC manufacturer (or servo manufacturer) recommends a particular value, and you just go with that.

Sometimes the type of capacitor is important. For example, electrolytic capacitors don't behave-electrically like capacitors at very-high frequencies. So, sometimes you'll see a low value ceramic capacitor (say 0.1uF) in parallel with a high value electrolytic capacitor (say 1000uF). (And, you can't buy a 1000uF ceramic capacitor.)

The simplest calculation you can make is the RC time constant - This is simply the capacitance multiplied by the resistance... A 1F capacitor charged through a 1 Ohm resistor charges to 63% of the applied voltage in 1 second. Or 1uF through 1 meghohm will also charge to 63% in 1 second.

Actually you can buy a 1000uF ceramic, but it will likely cost \$1000's and be a stack of smaller caps.

The only equation you really need to remember for capacitors is Q=VC, everything else follows, compare to I=V/R and dI/dt=V/L

Grumpy_Mike: There are lots of formula associated with capacitors, which one to use depends on what you are doing with the capacitor.

You learn about electronics, get to know what a circuit does, then you will know what to use where.

This is basically like saying, I have bought some flour at the mart, how much do I use? --- Well what are you going to do with it, pancakes, bread, sauce or cake?

I vote cake.

No pancakes every time :)

I yield to your superior intellect and good taste. Can we have raspberry sauce?

From a hobby perspective, the places you'll likely use caps are: 0.1uf ceramic caps between Vcc and Gnd of integrated circuits (you need one on most IC's) 18-22pf ceramic caps used as loading caps for crystal oscillators (when making your own microcontroller board) 1-10uf caps on input and output of voltage regulators, as specified in datasheet 1-1000 uf caps of type appropriate for the amount of capacitance you need, for power supply filtering (to smooth out the power supply voltage to prevent sudden changes in current from flinging the supply voltage around) 0.1uf ceramic cap used for the DTR autoreset trick.

MarkT: Actually you can buy a 1000uF ceramic, but it will likely cost \$1000's and be a stack of smaller caps.

The only equation you really need to remember for capacitors is Q=VC, everything else follows, compare to I=V/R and dI/dt=V/L

I haven't seen any (those stacks of caps digikey sells seem to cap out at 220?).... but you can get 100uf ceramics in useful voltages in 1210 package, up to 330uf for 2.5 volts (I think 220 for 4v?).

Ceramic caps are advancing at an incredible pace, while other types of caps have been barely improving

123Splat: I yield to your superior intellect and good taste. Can we have raspberry sauce?

Yes but only if you have a Raspberry Pi :) :)