Question on capacitors

Hello guys,

I have a doubt about capacitor's functioning. I've been reading quite a lot on the internet, so I know the whole history about these little things, how are they used, different types, etc. But I still have a doubt. As far as i've understood, they're more useful when they're used with A/C circuits as they're used as "stabilizers". With D/C circuits, they accumulate voltage and dont let any power go through. But then, i've seen circuits like this: http://www.adrirobot.it/elettronica/modulo_sonoro/miniamplificatore.htm that are working with a continuos 5V battery but still have the usage of capacitors. Plus, I don't get the C1 capacitor: wont it block the flow of energy?

I know this might sound like a stupid question, but it's a conceptual issue and so far I still haven't found the response to this. I'd be more than happy if someone would explain this (simple?) concept to me.

Cheers :)

E.g. from "http://farside.ph.utexas.edu/teaching/302l/lectures/node60.html" :

"Capacitors do not play an important role in DC circuits because it is impossible for a steady current to flow across a capacitor. If an uncharged capacitor is connected across the terminals of a battery of voltage then a transient current flows as the capacitor plates charge up. However, the current stops flowing as soon as the charge on the positive plate reaches the value . At this point, the electric field between the plates cancels the effect of the electric field generated by the battery, and there is no further movement of charge. Thus, if a capacitor is placed in a DC circuit then, as soon as its plates have charged up, the capacitor effectively behaves like a break in the circuit."

The explanations make me even more confused..... =(

I haven’t had the time to look into the schematic you specified however it is just wrong to say that capacitors are not important in DC circuits. In most circuits which use some kind of ICs there are several capacitors used for de-coupling the signals (see this link De-coupling thanks to the ever Grumpy_Mike for that guide).

It is true that a capacitor is a barrier for current (meaning there will be no current flowing through a capacitor) however it will “pass” a voltage that is supplied. In physics terms this means that there won’t be any electrons moving through a capacitor however there will be a potential (voltage) between the capacitor and ground.

Edit: I just skimmed the circuit you supplied and have a further addition. I’m not an electronics expert therefore this is only guess-work…
I already said that there can be no current flowing through a capacitor. The circuit contains an op-amp and in theory there is no current flowing into the input of an op-amp (see http://en.wikipedia.org/wiki/Op-amp#Ideal_op-amps ). The op-amp needs a voltage difference between both inputs to work therefore the cap won’t actually block the input signals.

But then, i've seen circuits like this:

The circuit you linked to is an audio amplifier device and circuit. As such it has both DC voltages (power for the device) and AC voltages (audio is just AC voltages in the 20hz to 20khz range). Capacitors do block DC but do allow AC voltages to pass through. Your understanding will improve if you progress with the normal subject order of electronic fundamentals, DC theory, then AC theory, etc. In AC theory you will learn about capacitors and there functions including how to compute the capacitance reactance of a capacitor for a given value at a specific frequency.

Lefty

Thanks to both of you. You were both helpful!

I already said that there can be no current flowing through a capacitor.

That is not technically correct. A current can flow through a capacitor if it's being driven by a AC voltage source. The cap will have a certain impedance (capacitance reactance) measured in ohms that will allow a AC current to flow through it using ohms law, I=E/Xc where I is current in amps, E is the AC RMS voltage value and Xc is the capacitance reactance of the cap at the AC frequency of the voltage source.

http://www.allaboutcircuits.com/worksheets/xc.html

Lefty

HI, One way to think about this that works for me:

Capacitors oppose a change in VOLTAGE

What this means for DC Circuits:

  • A capacitor will try to keep the voltage constant, when there are changing currents in a circuit
  • This is what is called a "Bypass Capacitor", because it conducts any changes in voltage to ground.
  • Bypass capacitors DO conduct current when some circuit tries to make the voltage change, such as changing currents drawn by Arduino chips on their circuit board. That's why there are "Bypass Capacitors" on the Arduino boards.
  • If there is a constant changing AC signal "trying to change the voltage" like in a power supply, a capacitor can act as a "Filter Capacitor", and "smooths" the voltage so that it is more like pure DC.

What this means for AC Circuits:

  • Capacitors act as "Coupling Capacitors" between AC circuits, by charging up once to the average DC voltage, but then coupling changing voltages (signals) to the following circuit.
  • Capacitors can be used in circuit where they are constantly charged and discharged to produce needed waveforms like Triangle or Sawtooth shaped changes of voltage. Often this is used for timing purposes like in a 555 timer, or changing pulse widths in a PWM circuit.

What might you use Capacitors for with typical Arduino/Microcomputer circuits??

  • Added "bypass capacitors" to smooth power supplied to external circuits. Locate them close to the circuit.
  • Same thing when putting IC Chips on a breadboard. Connect "Bypass Capacitors" from the Power/Vcc pin of the chip to the chip ground pin, very close to the chip. A good value is .1uF
  • Timing capacitor for a 555 timer to set it's frequency, along with a resistor. Maybe setting 38KHz to drive IR LEDs.. -

Anyone have other good examples to add? I'll add this stuff to the arduino-info WIKI here: http://arduino-info.wikispaces.com/

Fabiuccio, this is one of the non-obvious things in electronics, and there are few obvious examples in everyday life of what capacitors do.

@Lefty: True and to make that statement truly valid: Of course even in a DC circuit there is current flowing "through" the capacitor while charging it. But in both cases (AC and DC) it is not current flowing through the cap but it is a so called displacement current. There are no electrons moving from one side of the cap to the other but the (current) flow that is charging and discharging the cap can be measured in the other parts of the circuit.

Edit: @Terry: Great summary; you should definitively add this to your wiki! I have one more example which might be useful, the charge pump ( http://en.wikipedia.org/wiki/Charge_pump ) which is a DC-DC-converter that uses capacitors. I know most DC-DC-converters use inductors but when it has to be cheap and speed is not that important a charge pump can be useful (see also http://www.maxim-ic.com/app-notes/index.mvp/id/725 ).

Hello all,

to those who're "ignorant" like me about this topic, I found a quite interesting document about by-pass capacitors:

http://www.intersil.com/data/an/an1325.pdf

It contains examples and images.

Nevertheless, thanks again to all of you. It's clearer now, and I've also realized that the concept behind this is not as trivial as one might expect.

Tschus.

But in both cases (AC and DC) it is not current flowing through the cap but it is a so called displacement current. There are no electrons moving from one side of the cap to the other but the (current) flow that is charging and discharging the cap can be measured in the other parts of the circuit.

The same can be said for current flowing through a transformer. Actual electrons don't pass through, but through magnetic coupling, real effective AC current passes through and powers the load attached.

Lefty

elemakil: It is true that a capacitor is a barrier for current (meaning there will be no current flowing through a capacitor) however it will "pass" a voltage that is supplied. In physics terms this means that there won't be any electrons moving through a capacitor however there will be a potential (voltage) between the capacitor and ground.

No, currents do pass through a capacitor - but only alternating currents. Large numbers of electrons move in and out of the terminals and build up charge on the capacitor plates during the AC cycles. No actual electrons (in an ideal capacitor) pass across the "gap", but the electric field in the gaps changes and that is physically equivalent to a current according to Maxwell's equations (indeed electromagnetic waves depend on this equivalence). Such displacement current happens even in a vacuum - something many people find hard to accept, nature has no such qualms!

The fundamental equation for a capacitor is I/C = dV/dt (or put another way I.dt = C.dV) - in words; "current divided by capacitance = rate of change of voltage"

@MarkT: Quite true but then again if you had read the whole thread you'd know that we have been talking about a DC-circuit and also there would have been no need to state the same I wrote three posts before you (...okay you mentioned Maxwell and added the equation). I'm sorry to be such a dick about this but have your really read more than the first two posts?

My aim was to correct an erroneous post as much for the benefit of that contributor as for the original questioner... No harm restating things already said in slightly different ways - often helps some people to 'get it' I think.

Then again I'm sorry for having been such a dick about it.