Understanding Capacitor Applications in AC and DC Circuits

[jaredogden]

We are going over capacitors in AC and DC circuits in my circuits class and I was reading over the applications of capacitors. It is mentioned that they are used for filtering in power supplies and there is some stuff about full-wave and half-wave rectifiers. I am confused on how all of this works, if anyone has any resources to better explain capacitors and how they function in this way and their application I would appreciate any help.

Thank ahead of time.

 

[Bassalisk]

Well if you want to understand rectifiers, you need to understand the diodes, because that is the main component. Capacitor is there to make the output voltage more desirable for the device.

Consider this circuit.

A diode is in a nutshell a device that let's the current only one way. This is AC input. So if you let the current only one way, you basically cut off half of the sinusoid.

Now, put a capacitor in parallel with this resistor. What happens?
During one small time interval called \DeltaT, a current runs through diode in one way and it runs until capacitor is fully charged. Capacitor is max charged at peak of the sinusoid.
Now after that, AC input starts to drop, but you have that charge in that capacitor right? So it slowly starts to give away that charge.

[URL]http://physicsarchives.com/course/electrictransmission_bestanden/image268.gif[/URL]

That almost flat line represents capacitor discharge.

This is a basic idea, of course there are more formulas and stuff behind this, and this circuit that you see is just a trivial example of how these rectifiers work. More complex structures with diodes and capacitors are used in actual electrical engineering.

 

[jaredogden]

Oh.. That makes perfect sense. So then is it a number of capacitors that flattens off AC to DC or is the discharge lines slope infinitesimally small so it can just be considered DC? I don't know if my question makes sense. I guess my question is how do you get the almost flat lines to a flat DC line?

Thanks for the help that really makes a lot of sense.

 

[jaredogden]

Also your example is of a half-wave rectifier correct? How does a full-wave rectifier differ? I think I see the answer of flattening off the lines, it says that after the rectifier there is a power supply filter if I'm not mistaken. So something in the power supply filter flattens off the waves to DC?

 

[Bassalisk]

Well of course your question makes sense.

You see that peak of sinusoid and that last point, before capacitor starts to charge up again?

That difference is called ripple voltage. And you can achieve that infinitesimally small difference by decreasing ripple voltage almost to 0.

U_r=U_dc*T/RC

T is the sinusoid period.
Udc is output voltage

Well you do the math, you can achieve this 2 ways. Either get a very very large capacitor OR make the omega of the sinusoid very large. That is in my intuition behind this simple example.

 

[Bassalisk]

Well the full wave rectifier is basically a more combination of diodes. Principle with the capacitors stays the same.
[PLAIN]http://pokit.etf.ba/get/251490510a9552a64e17861f0eda712c.jpg

That c) graph is a graph of the circuit above but with a capacitor in parallel with R.

b) is without capacitor.

 

[jaredogden]

Well of course your question makes sense.

You see that peak of sinusoid and that last point, before capacitor starts to charge up again?

That difference is called ripple voltage. And you can achieve that infinitesimally small difference by decreasing ripple voltage almost to 0.

U_r=U_dc*T/RC

T is the sinusoid period.
Udc is output voltage

Well you do the math, you can achieve this 2 ways. Either get a very very large capacitor OR make the omega of the sinusoid very large. That is what in my intuition behind this simple example.

Great this makes a lot more sense now. I feel like I just read about ripple voltage somewhere. I have one more question though. If you decrease the ripple voltage to almost 0 would you still have a negatively sloping voltage line because of the discharging capacitors or can you solve this by somehow increasing frequency so that all of the very small ripple voltages are very close flattening off the line? I don't know if that is even possible or makes sense, I am a Mechanical Engineering major so I am not good with circuits, however I am intrigued and really love it.

 

[Bassalisk]

I don't think you can really flatten that line up. But you can make it very short and very close to flat. More complicated schematics come in case. I think you should wait PhD answer for this one, I think sophiecentaur will be of very much help here.

 

[jaredogden]

I don't think you can really flatten that line up. But you can make it very short and very close to flat. More complicated schematics come in case. I think you should wait PhD answer for this one, I think sophiecentaur will be of very much help here.

Right I meant get the line as close to flat as would be allowed. Great thanks so much for your help you cleared a lot up for me.

 

[Bassalisk]

I think that these days you don't actually flatten that line up, but you use a special diode called Zener diode, that gives a constant voltage drop, but the a story for another time.
A circuit is a complex one.

 

[Bassalisk]

http://pokit.etf.ba/get/d2c97a33cc3e0d07c863cb611f6d169a.jpg

here is a link that should probably give an answer. And an equivalent circuit.

http://pokit.etf.ba/get/0b97d32a0c3f1f5ab9287c60ded47c24.jpg