electrical component questions

[thesnare77]

I've been watching a series of lectures here, on basic electronics here:

nptelhrd - YouTube

Here's what I'm confused about

1. Capacitors- To charge one, you connect to one of the plates to a negative terminal of a battery and other plate to the positive terminal. I can understand how the plate gets positively charged, the battery takes electrons away from the plate, but what about how it adds electrons to the other plate? With the positive terminal has holes in which the electrons from the wire and plate rush to fill, why would electrons go to fill the plate which you want to negatively charg? That plate and wire is electrically neutral, as far as I know, it's atoms already have the amount of electrons to equal the amount of protons, so how would electrons travel from the negative terminal to the plate to give it extra electrons?

And, it is said here

nptelhrd - YouTube

at 46:42 that capacitors in series add up. But, I don't understand this diagram. You see those two plates? Those are the capacitors I'm guessing. With the one on the left, the left-most plate is either the positive or negative plate, and the next left- most plate is it's opposite. A capacitor is a path between the two plates, one negative, one positive. So, how would the electricity get from the negative to the positive plate if there's no connection between them? I don't see how you can have them in series

Variable capacitors- one way to make one is to have the plates be adjustable, so you could for example, move them away from each other. How does this work? By moving them away from each other, so they have a shorter path to follow to the other plate via the wire , so if you connect the two plates, it has a lower resistance- a shorter route to get there, and thus more current, or does effect how much electricity is stored (farads) somehow?

A variable capacitor is also a component in an oscillator. How would it work specifically if say, I wanted to tune into 98.5FM on my radio, how varying the capacitor would allow me to do it. Then, perhaps furthermore, how the pattern of information (whatever form it takes) is decoded and produced as audible sound, yes I've been trying to understand radio for a while as well.

And, finally (for right now) there's rectifiers. Here's a picture of a halfway recifier, the first one shows, with one diode.

http://www.play-hookey.com/ac_theory/ps_rectifiers.html

As explained in the lectures, it is considered half because the current can only go one
way. The inductor gets charged, sends the electricity both up and down the circuit, each one- the one which has the diode and the one that doesn't (this is somewhat confusing to me, I'm not sure whether to call it DC or AC, since the inductor sends it TWO ways) and the current can not go past the diode the other way since a diode only lets current go one way. Easy enough to understand, full wave recifiers are where I have a little trouble.

In the first diagram under it's section on that website, this one has 2 diodes, it can not go back either way because of them, but what is the purpose of the center tap and how does that work?

 

[War_Spigot]

First of all, THANK YOU for the descriptive post, as opposed to "How capacitor work I really need help guys! Thx!"

Capacitors are not connected in the middle, they store a charge between the two plates. If they are in series like this: --||---||--, then the two inner plates are effectively connected, and the two outer ones form the plates of a capacitor. The outer two plates make a capacitor, but there is effectively more space between them, resulting in a lower capacitance. If they're in parallel, they effectively have larger plates with the same distance between them.

The center tap(I'm not sure about this) I think is so you can have a ground in between the upper and lower voltages, splitting them. That way, one voltage will be positive and one negative, as opposed to one being 0V and the other positive.

I don't think I completely understand exactly HOW the capacitor stores its charge, so I won't try to explain it.

 

[thesnare77]

See this capacitor

You can see when the switch bridges the capacitor's circuit that the electricity is free to flow from one plate to the other. So, I don't understand how the capacitor in the middle would effect the other capacitor's plates (Yes, I did suspect that perhaps the two plates in the middle was it's own capacitor, it's positive and negative plates, and the two outer ones were a separate capacitor) Since that capacitor is isolated from the other one, there's no connection between the two, no circuit, except the diaelectric- which is meant to keep the charges seperate, until you ready to let the electrons flow

 

[jackorocko]

As explained in the lectures, it is considered half because the current can only go one
way. The inductor gets charged, sends the electricity both up and down the circuit, each one- the one which has the diode and the one that doesn't (this is somewhat confusing to me, I'm not sure whether to call it DC or AC, since the inductor sends it TWO ways) and the current can not go past the diode the other way since a diode only lets current go one way. Easy enough to understand, full wave recifiers are where I have a little trouble

That inductor is just part of a transformer, the secondary side. The only thing it is doing is either stepping up or down the voltage. Its output will still be an AC voltage sine wave. You can ignore the fact that it is an inductor and that it's output will be the same sine wave as the input to the transformer, but just at a lower/higher voltage.

When you have a full wave rectifier, it just simply means that one of the two diodes will always be conducting during both phases of the sine wave. The positive going part and the negative going part. A diode will only conduct when the anode of the diode is 0.7V more positive then the cathode, they call this forward bias.

If you look at that page you linked two again, draw out the path of conduction when the sine wave is positive and then when it is negative. While one diode is conducting the other is blocking. Vice versa for the negative phase of the sine wave. The result is a DC voltage.

 

[thesnare77]

That inductor is just part of a transformer, the secondary side. The only thing it is doing is either stepping up or down the voltage. Its output will still be an AC voltage sine wave. You can ignore the fact that it is an inductor and that it's output will be the same sine wave as the input to the transformer, but just at a lower/higher voltage.

Maybe I was confusing it with a oscillator, which is a capacitor (or a generator, I don't think it makes a difference does it?) is hooked up to an inductor coil, the capacitor tries to discharge, it gets to the inductor, which then sends the current back and forth, until the resistance kills the current completely.

You're saying this doesn't exactly happen with a transformer (two inductors next to each other) I thought that when an inductor is energized it will send the current back and forth, if it's part of either a transformer or an oscillator. With a transformer, both will be energized (though the voltage will be changed) and send the current both ways down the wire the inductor connects to. Yeah, inductors confuse me.

If you look at that page you linked two again,

Which? This one? http://www.play-hookey.com/ac_theory/ps_rectifiers.html

draw out the path of conduction when the sine wave is positive and then when it is negative. While one diode is conducting the other is blocking. Vice versa for the negative phase of the sine wave. The result is a DC voltage.

Yeah, it's my confusion with inductors that's confusing me That might clear some of this up, if what I was saying about is or isn't true.

 

[War_Spigot]

The current through the conductor makes a magnetic field around it. This magnetic field influences other electrons in other conductors. The inductor will have a field around it, and when it is changing, the field will also change. The changing field will make the electrons in the other conductor move also. Moving electrons, sounds like current to me. So, the magnetic field from the current in one inductor has induced(inductor= something that induces) a current in the other one.

For diodes/rectifiers, etc... look at this video from Afrotechmods. It, as well as his other videos, have very good explanations.

 

[thesnare77]

Ok, let's go through this a little slower. Inductors are a little confusing for me, probably the simplest circuit you can have with one inductor is an oscillator, and this what an oscillator is, correct?

http://electronics.howstuffworks.com/oscillator2.htm.

So, the simplest circuit you can have with one inductor is this, it includes a power source, either a charged capacitor, a battery, or an AC or DC generator, correct?

And, as that link states, the way it works, is the inductor starts to build a magnetic field when capacitor tries to discharge through it, the charges up the other plate, the electricity flows the other way, charges the inductor again, it charges up the plate, the energy goes back and forth until the electricity stops moving due to resistance in the wire, right?

Now, this would be the same with either a battery (except the way it would work with that after the battery dies, it would continue to oscillate again until the electricity stops moving- like with the capacitor)

With an AC or DC generator I'm not really sure how this would work. Assuming it was just the generator and just the inductor,

With a DC generator it would travel one way through the inductor, the inductor would build up a magnetic field, try to discharge it onward, but there's no capacitor or battery at the other end to charge up it would go towards the end where the generator didn't charge it, would the electricity just stop?

 

[War_Spigot]

This might be a more relevant article to read.

The "simplest inductor circuit" would just be an inductor across a battery(although you'd want a resistor or you'd probably burn the inductor and/or the battery). It would allow current to go through it, and a magnetic field would build up around it. So if you had, for example, a compass near it, you would see the compass needle move. It wouldn't oscillate though. I think there would be a reverse voltage across the battery after it is depleted, but it won't recharge the battery, so the process would never repeat. The same with an AC-DC generator or a DC generator. It's all DC, it's all the same.

The reason the capacitor one oscillates is because, as you said, the capacitor discharges through the inductor. Once the capacitor's charge is depleted, there's no more current flowing. The magnetic field then collapses. When it collapses on the inductor, the inductor produces a voltage, but in the reverse polarity that it was the first time. This current flows to the capacitor, charging it(again, reverse what it was the first time). Then the process repeats. So it's oscillating.

The process of the field breaking down and producing a voltage is the reason for flyback diodes. The energy from the magnetic field can translate to a very high voltage(although it will be at a low current). This high reverse voltage can destroy components(LED's for example), so the diode allows the current to flow back through the other way.

 

[thesnare77]

The "simplest inductor circuit" would just be an inductor across a battery(although you'd want a resistor or you'd probably burn the inductor and/or the battery). It would allow current to go through it, and a magnetic field would build up around it. So if you had, for example, a compass near it, you would see the compass needle move. It wouldn't oscillate though. I think there would be a reverse voltage across the battery after it is depleted, but it won't recharge the battery, so the process would never repeat. The same with an AC-DC generator or a DC generator. It's all DC, it's all the same.

The reason the capacitor one oscillates is because, as you said, the capacitor discharges through the inductor. Once the capacitor's charge is depleted, there's no more current flowing. The magnetic field then collapses. When it collapses on the inductor, the inductor produces a voltage, but in the reverse polarity that it was the first time. This current flows to the capacitor, charging it(again, reverse what it was the first time). Then the process repeats. So it's oscillating.

Ok, good! Now that we have that out of the way, I know that it won't oscillate with anything but a capacitor. But, what exactly is an AC/DC generator? I thought a generator can be one of the other, not both. In the case of AC, the coil rotates, it sends it down one wire, the on the half of the rotation it sends it down the other wire.

http://www.generatorguide.net/howgeneratorworks.html

Here's a DC generator

http://www.sciencejoywagon.com/physicszone/otherpub/wfendt/generatorengl.htm

It sends it up, through the voltmeter and back to itself, the electricity only goes one way.

So, with this
http://www.play-hookey.com/ac_theory/ps_rectifiers.html

With the first diagram of the half wave rectifier, what happens. The AC generator sends the electricity one way through the inductor, the inductor magnetizes the other one, drops or lower the voltage, but now which way to does the electricity travel? Does the second inductor send it up and through the diode,

Or does the second inductor send it BOTH up and down, one goes through the inductor, the other goes the other way and gets blocked by the diode, because it is forward biased and can't go any further when it gets to it?

 

[War_Spigot]

In the generator, it produces AC. With the commutator though, it also makes the contacts points(where the wires are touching) switch when the current directions switches. This results in current flowing only one way.
Whether it is a generator or not doesn't matter, DC from a battery, capacitor, or a generator is all the same. An AC/DC converter would basically be (optionally a transformer) a diode, and there'd probably be capacitors and a voltage regulator as well.

In the diagrams on the other page, don't think about the transformer as two back to back inductors. To simplify, just think of it as an AC voltage source. How the voltage gets there does not matter, it's still AC(just like battery/capacitor/generator with commutator is all DC). In a half wave rectifier, the voltage goes one direction, is allowed to pass by the diode, and goes on to the output. When it goes the other direction, the diode basically acts as a break in the circuit, so no current can flow. So, no current flows to the output. When it goes one way, you get current, when it goes the other way you get none.

 

[thesnare77]

Ok, good. I might have a lot more questions though (I already have some) don't want to make this a full time job for you.

 

[War_Spigot]

I recently got started in electronics, too. I know how hard it can be to understand some things. That's why I like to help.

 

[thesnare77]

I recently got started in electronics, too. I know how hard it can be to understand some things. That's why I like to help.

Ok, so suppose with a half way rectifier, in the diagram, the electricity goes down the circuit that doesn't have a diode and hits the opposite end of the other diode- and can't go any further- there must be some purpose the electricity is going down that way (the cycle with the diode) I presume there some kind of load it would power. Say there's a light bulb in the path the current takes on the path without the diode. Then it would first go through the light bulb, then hit the diode, and can't go any further. You wouldn't just be sending current just to send current there would be some kind of load to power. So wouldn't the current on the other cycle without the diode reach that load too?

 

[War_Spigot]

If the diode is put in reverse-bias(i.e. "against" the current) it basically acts like an open circuit. Like a break in the wire. In forward bias(i.e. "with" the current) it acts like a short circuit. Like a normal wire(not entirely, there is a voltage drop). So, when the voltage is reversed across the diode, there is no circuit, no loop, for current to go through. There is no current.

 

[thesnare77]

If the diode is put in reverse-bias(i.e. "against" the current) it basically acts like an open circuit. Like a break in the wire. In forward bias(i.e. "with" the current) it acts like a short circuit. Like a normal wire(not entirely, there is a voltage drop). So, when the voltage is reversed across the diode, there is no circuit, no loop, for current to go through. There is no current.

I know that, I'm just saying, the electricity would go around the loop, hit the other side of the diode, and not be able to go any further, but I'm wondering what kind of load the rectifier would be powering. Obviously, it's not sending current just to send current, there's no reason to connect a wire from one terminal of a battery to the other, that will just drain the battery very quickly. It would really depend on well..what load you want to power. In the diagram here for the half wave rectifier.

http://www.play-hookey.com/ac_theory/ps_rectifiers.html,

See that resistor?

Suppose it were a
light bulb instead- the load we wish to power, on the other half of the AC cycle, the current would move through it on it's way to the diode, kinda defeating the purpose if you don't want the current to get to it. It's just confusing me where we are sending the current, it appears to be somewhat incomplete, has me confused.

 

[War_Spigot]

The current will only flow if there is a complete circuit. With the diode there, there is not a complete circuit when it it reverse biased. The current won't go around until it reaches the diode, it just won't go anywhere. So on the negative half of the wave, there is no current through the circuit with the diode. If the voltage were high enough, it would be able to cause a breakdown of the diode and current would flow.

 

[davenn]

.................
http://www.play-hookey.com/ac_theory/ps_rectifiers.html,

See that resistor?

Suppose it were a
light bulb instead- the load we wish to power, on the other half of the AC cycle, the current would move through it on it's way to the diode, kinda defeating the purpose if you don't want the current to get to it. It's just confusing me where we are sending the current, it appears to be somewhat incomplete, has me confused.

OK you didnt comment on which circuit you were referring to ??

Yes that resistor represents the load, be it a light globe, a complex cct whatever.

the first cct shows a half wave rectifier....

half wave rectified output before smoothing.....

the load only sees a halfwave rectified DC voltage ( current) and a really rough DC at that, it needs lots of smoothing with a large value capacitor. The lower output side of the transformer is a 0V (GND) reference. There is no current flow in that direction as the diode inhibits that flow.

further down you see a full wave rectifier using 2 diodes, still very rough before smoothing but better than a half wave rectifier, at least now both halves of the AC voltage are being rectified

or a fullwave bridge rectifier

and the waveform output before smoothing....

the smoothing removes the deep "valleys" between each half of the rectified AC cycles
and you end up with .....

That little difference in voltage level between each peak and where it drops a little before the next peak of the smoothed output is called the ripple voltage.

cheers
Dave

 

[jackorocko]

If the voltage were high enough, it would be able to cause a breakdown of the diode and current would flow.

... and magic smoke would appear and then the circuit would stop working. diodes are not made to breakdown in the reverse direction, unless they are zener diodes.

 

[thesnare77]

Thanks. I came across something that could be real helpful the other day. I went to a website, looking up electronics, and it was linked to tutorcircle.com. I'm not sure if it was automated or not. I don't think there was a real person typing in that windows (as I typed exactly what I was looking for and it said "Oh, you need help with physics?)

But, that could really me helpful in my position here. Not that I don't appreciate your help, but I probably could have gotten all my questions answered in a most I'd say 20 minutes if I could talk to someone with my mic with screen sharing so I could show them exactly what I mean, instead of typing something waiting hours for a reply (again, it's no one's fault here, I think everyone here is a volunteer) and then responding, waiting hours or day, etc. But, I'm not taking a class or anything, is there any way to get a service like this for free?.

 

[War_Spigot]

MIT has many of their lectures online for free. Google "MIT OCW electronics" or physics, or just "MIT OCW." Also Google "MITx."