First Post, Basic Questions ...

[budidharma]

Hello All,
This is my first post. I'm new to electronics; I'm trying to teach myself through books and internet resources, so, if your opposed to answering very basic questions, read no further.

I've a few questions, really.

The first: I understand that a capacitor is two electrodes, separated by a non-conducting dialectric. The purpose of a capacitor is to store electrons, right? I don't understand how this works, really. Typically, in order for electrons to flow through a circuit, they must have a complete path (closed circuit) from the negative terminal of the power source to the postive terminal. Why then, if you put a capacitor in the middle of the circuit (which effectively breaks the circuit, as far as I've learned), will they still flow into the capacitor, and stay there until discharged?

The second: I've seen several schematics to build a benchtop power supply - mostly from books and I'm not sure as how to post schematics, so I'll describe it, I'm sure you'll understand. The most simple was a standard 9V power supply, it used AC IN to a 120V->6.3V transformer @ 60hz, to a 4-way bridge rectifier, connected to DC + and -, which had a 3300uF capacitor. I hope this makes sense.

I don't understand the purpose of the capacitor. I believe it's got something to do with smoothing current spikes, but im not positive.

I think that's all for now, I'd greatly apprectiate any helpful answers on the subject though. Thanks!

 

[bmcculla]

1) current does flow into and out of a capacitor. Since the two plates are so close together as charge is pushed onto one plate the electric field from this charge pushes the charge off the other plate. In the gap between the plates the current is transmitted through the Electric field between the two plates - no electrons cross the boundary but they get stuck on the plate create the E-field which pushes the electrons off the other plate. This process continues until the voltage between the plates is equal to the applied voltage. When you remove the external voltage there is nothing holding the charge on the plates so the charge is dumped back into the circuit. There's more to it but this is close enough to give you some intuition for why caps work.

2) The AC input swings between two voltages. That means in between each swing there is a point where there is no voltage- the zero crossings on the sin wave. The bridge rectifier moves both voltage swings to the positive side but there are still the 0 voltage spots. The cap stoers charge when the voltage is high and supplies the circuit durring the low voltage times between cycles.

Hope this helps.
Brent

 

[Juglenaut]

Caps in DC from a DC input do nothing more than stiffen the supply voltage, they opposed changes in voltage keeping it more steady.

AC when it comes to Caps shorts them like a closed switch basically. Coupling, bypassing and such use AC anaylisis where as DC is unaffected so this is where swap resistors in DC come into play.

Caps play a part majorly in all electronics from very large to very small. Used for resonant LC tanks, AC signal grounds, low, high, band-pass, and band-stop filters, filtering noise, coupling loads to AC while blocking DC, and many more etc.

In rectifers the caps filters the waveform and produce a doubling RMS or AVE factor and reduce the wave form to a ripple form. The more uF the more charge which yields less ripple, but use of a regulator to stem the voltage below input will clean up the DC to produce a clean offset.

 

[samcheetah]

The purpose of a capacitor is to store electrons, right?

no, it stores energy. the electrons are always there whether the capacitor is charged or not!!!!!!!!!!!

the current through a capacitor is called the displacement current and bmcculla has explained how it works.

for a detailed explanation of how capacitors work, go to http://amasci.com/emotor/cap1.html

and yes the capacitor after the bridge smooths out the spikes.

 

[budidharma]

Thank You, Thank You, that sorted me out a bunch.

 

[grrr_arrghh]

worth a mention perhaps, is that a common misconception is that current is electronics flowing through a medium. Not only does the effect we know as 'current' flow the oposite way to the electronics themselves, but the electrons travel quite slowly. they effectivly bash into each other, passing on energy down the line in a chain. the effect of the energy transfer is the effect we know as electrcity, and this effect travels at nearly the speed of light (approx 3*10^8 m/s).

probably doesn't answer your question, but felt it was worth mentioning.

 

[budidharma]

worth a mention perhaps, is that a common misconception is that current is electronics flowing through a medium. Not only does the effect we know as 'current' flow the oposite way to the electronics themselves, but the electrons travel quite slowly. they effectivly bash into each other, passing on energy down the line in a chain. the effect of the energy transfer is the effect we know as electrcity, and this effect travels at nearly the speed of light (approx 3*10^8 m/s).

probably doesn't answer your question, but felt it was worth mentioning.

I have seen similar information mentioned a few times, referring to current flow and electron flow, and that they travel in different directions. Now, please excuse me if I am wrong, but this is the way I have interpreted this explanation: Current flow refers to "hole flow," in a conductor, electrons would be shifting from one atom to another to fill the empty bond, thereby causing the hole to be shifting in the opposite direction across the atoms.

Is that correct?

Also, reffering back to my earlier question regarding capacitors; Using the same example, AC->transformer->4 way bridge rectifier-> capacitor regulating DC current ... The capactitor is smoothing the current, but how? I don't understand the principles. When is the capacitor storing energy, and when is it discharging it? Can anyone relate the way in which a capacitor stores energy to the above discussion of current flow?

Thanks much.

 

[Nigel Goodwin]

Also, reffering back to my earlier question regarding capacitors; Using the same example, AC->transformer->4 way bridge rectifier-> capacitor regulating DC current ... The capactitor is smoothing the current, but how? I don't understand the principles. When is the capacitor storing energy, and when is it discharging it? Can anyone relate the way in which a capacitor stores energy to the above discussion of current flow?

"Capacitance is the storing of energy in the form of an electrostatic field".

In your power supply example, the capacitor stores energy (just like a small battery) on the rising edge of each half cycle (with a full wave rectifier). As the falling side of the half cycle takes place the capacitor partially discharges into the load - thus maintaining a fairly constant output voltage. The larger the capacitor, the smaller amount it will discharge, and the lower the ripple on the supply.

 

[samcheetah]

Current flow refers to "hole flow," in a conductor

there is no such thing as hole flow in conductors. holes dont exist in conductors. according to the energy band theory holes can never form in conductors. holes are formed in semi-conductors.

and in semi-conductors hole flow is described as you have said. when an electron moves in one direction a hole moves in the opposite direction. as i have already said that a hole is basically the abscence of an electron. so when an electron move from one place to the other, the state of abscence of an electron moves in the opposite direction.

 

[budidharma]

"Capacitance is the storing of energy in the form of an electrostatic field".

In your power supply example, the capacitor stores energy (just like a small battery) on the rising edge of each half cycle (with a full wave rectifier). As the falling side of the half cycle takes place the capacitor partially discharges into the load - thus maintaining a fairly constant output voltage. The larger the capacitor, the smaller amount it will discharge, and the lower the ripple on the supply.

Hmm. I can understand this. The power supplied to the circuit from AC occurs in the form of a sine wave. I've read this, and the way I interpreted it, was that the current from one side of the circuit is applied on one side of the circuit from 0A to the maximum amount of Amps the circuit will supply and then back to 0. It then flip-flops polarities (ie changing the direction of the voltage through the circuit), and applies current this way through the circuit from 0A - the max - back to 0A again, and then switches polarities again. Is this right? If it is, by the "rising edge of each cycle" which part of each cycle would you be talking about?

Also, when does it "know" when to discharge? ... After the AC current has been rectified into a DC current, I guess it won't be completely smooth (though I don't really understand why). I don't understand why their would still be a "rising edge" on the DC current.

 

[Nigel Goodwin]

Also, when does it "know" when to discharge? ... After the AC current has been rectified into a DC current, I guess it won't be completely smooth (though I don't really understand why). I don't understand why their would still be a "rising edge" on the DC current.

Right, you understand what a sinewave looks like - that's a good start, it goes from 0V up to +ve, back down through 0V and down to -ve, then back up to 0V again - and repeats.

There are two basic rectifier types, half wave and full wave, first half wave, which is just a single rectifer.

All this does is completely block one half of the sinewave (we'll choose the negative half). So what comes out of the rectifier is a positive sine pulse followed by half a cycle of nothing - the capacitor provides energy during the time between the pulses.

With a full wave rectifier the negative pulses are inverted, this gives a continuous stream of positive pulse, with only very brief times of zero volts - but it's still just a series of half sine pulses - just twice as many. The capacitor fills the gaps left between the tops of the pulses as one half cycle drops down towards zero, and before the other one builds up.

 

[samcheetah]

remember that a capacitor will always try to keep its voltage changing. the direction of change (whether increasing or decreasing) doesnt matter. when you charge a cap the voltage changes from 0 to some voltage. now the moment the charging source is disconnected from the cap it will try to retain the change of voltage as before. but this time the change will be negative. it will go from some voltage back to zero. now how will the capacitor discharge?? it will discharge through the load. and the time it takes for the capacitor to discharge will depend on the time constant of the circuit.

simillarly an inductor will try to keep its current changing. if you have read about tank circuits you will have an idea about the behaviour of caps and inductors. the capacitor tries to keep its voltage changing and the inductor tries to keep its current changing and hence energy is transferred back and forth between the capacitor and the inductor at the resonant frequency.