# Couple questions

Discussion in 'General Electronics Chat' started by DavS13, Jun 16, 2014.

1. ### DavS13 Thread Starter New Member

Jun 16, 2014
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I'm wondering what do you gain when building a circuit with capacitors or inductos (or both?). I understand that they store charge, and when an AC current flows through them, they are constantly charging and discharging. What is the point of this? Why not just let current pass through without using these circuit elements? How does an inductor differ from a capacitor in this regard?

Also when powering the rails of an op-amp, you must have a positive and negative voltage applied to the respective terminals. When the input to an op-amp receives an AC voltage, I was told something like: When the input swings negative, the negative voltage of the rail cancels so you must do something like make your ground the point between 24V (if you had +12V and -12V for example). What does this mean, and why does it work this way?

I know these are alot of questions, but I'm very curious;Thanks!

2. ### #12 Expert

Nov 30, 2010
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Time. Capacitors and inductors let you modify time relationships.

Op-amps need more voltage for their power supply than the signal they have to work with. If you want to amplify a 10 volt signal by 2, you can't get 20 volts out of an op-amp if it doesn't have 20 volts to work with.

3. ### MrChips Moderator

Oct 2, 2009
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5,700
Whoever has been giving you advice is giving you faulty information.

Before providing you with correct answers it would help us to know your age and level of education.

The first question about inductors and capacitors is usually covered in around the 2nd year level of college/university physics.

The second question makes no sense what so ever. Power supplies, op-amps and basic analog electronics are usually covered in around 2nd/3rd year of college/university electrical/electronics engineering.

You can read up on some of this in the All About Circuits on-line Tutorials:

4. ### crutschow Expert

Mar 14, 2008
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Inductors and capacitors have a varying impedance with AC frequency. The impedance of an inductor increases with frequency and the impedance of a capacitor decreases with frequency. They are used for many different tasks in a circuit including filtering, noise decoupling, dc blocking (capacitors), short term energy storage, etc., far too much to describe in a couple paragraphs. Look here (chapters 13 and 15 in particular) for more info.

A standard op amp must have input voltages smaller than the supply rail voltages so if the input AC signals goes above and below zero then you need the power supplies to be plus and minus also.

5. ### DavS13 Thread Starter New Member

Jun 16, 2014
21
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@MrChips

I'm 21, starting my senior year in the electrical engineering program. I'v been taught all this stuff; I know how to solve all these kinds of problems, and have done labs involving them. But we were never taught from a design perspective WHY these things work the way they do.

For the second question, I don't know how else to explain it but that. If I simply put a + voltage and - voltage at the respective terminals I was told that the op amp is actually receiving +2V; the negative part gets cancelled with the AC input. So we had to work around that.

6. ### MrChips Moderator

Oct 2, 2009
18,154
5,700
For senior year, as in one more year before graduating with a degree in electrical engineering, did you not study that

C = dQ/dV

and

v = L di/dt

Capacitors and inductors are as fundamental as resistors when applied to electrical/electronic circuit theory.

Sorry to say, but your school has failed you terribly in providing you with the proper knowledge for your degree.

Last edited: Jun 16, 2014
7. ### DavS13 Thread Starter New Member

Jun 16, 2014
21
0
@MrChips

Yes, like I said before I know all that. All I'm asking is what purpose they serve from a design perspective. So far all we have done is solve problems but we were never told, for example, design a circuit that does "this". I'm wondering what what make you put a capacitor or an inductor in a certain place for example. @crutschow thank you for giving me some examples in where they are used.

8. ### #12 Expert

Nov 30, 2010
18,076
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My answer was rather simplistic. My only excuse is that I was guessing about you level of education.

9. ### DavS13 Thread Starter New Member

Jun 16, 2014
21
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#12

Yes, but thank you for pointing out time relationships, that makes a bit more sense now.

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
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The why is covered in physics course(s) and electromagnetics course.

I don't have any of my physics textbooks handy.

My electromagnetics textbook was: Fundamentals of Engineering Electromagnetics by David K. Cheng. Capacitances and Capacitors is Chapter 3. Inductances and Inductors is Chapter 5. If I remember right, this was 3rd year course. So you get some of it from physics in 2nd year and more of it in 3rd year.

11. ### shteii01 AAC Fanatic!

Feb 19, 2010
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All filters in your second year courses would required RL, RC and RLC circuits. If I ask you to design a low pass filter with 20 kHz cutoff, how are you going to do it?

12. ### MrChips Moderator

Oct 2, 2009
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Here is a circuit taken at random with lots of resistors, capacitors and inductors.

I would expect a graduating electrical engineer to be able to explain the function of every component in this circuit.

13. ### shteii01 AAC Fanatic!

Feb 19, 2010
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If you put +1 volt to the op amp positive supply voltage pin and you put -1 volt to the op amp negative supply voltage pin, you get: 1-(-1)=1+1=2 volts. What this has to do with the input voltage? Did you notice that input voltage was not mentioned at all? Not even once!

14. ### Veracohr Distinguished Member

Jan 3, 2011
668
101
Here's an example of a capacitor use: power supply decoupling.

Near the power input to an IC, it's common to put a capacitor to filter out variations in the supply voltage. High-frequency noise gets shunted to ground because the capacitor appears as a low-impedance path for high frequencies. It also serves to reduce sagging voltage because if the supply voltage starts to fall, the energy stored in the capacitor can make up some of the difference.

An example of an inductor use: switching power supplies.

A switching power supply turns on and off a switch connecting the input power to the circuit being supplied. Because the input is off and on, the energy supplied to the circuit comes from an inductor. The energy stored goes up and down as it charges and discharges during the switching. This is done because it's more efficient than a linear power supply, which wastes power through heat.

Capacitors and inductors together can be used to form an oscillator. The energy stored goes back and forth between the two at the resonant frequency.

But you're aware that capacitors and inductors aren't just used for energy storage, right? The frequency-dependent impedance they have is an important aspect that is much used.

15. ### DavS13 Thread Starter New Member

Jun 16, 2014
21
0
@veracohr thank you for the information, I haven't learned about oscillators yet. Yes now that you mention it, their impedance does depend on frequency and I'm sure that plays a big role in circuit design.

@shteii01 I haven't learned much about filters yet, I'm they were to be covered shortly next academic year.
http://electronics.stackexchange.com/questions/46916/op-amps-where-to-split-the-rail

I think this post covers what I'm talking about I guess their called rail splitters? This is what I had to do except I didn't use resistors as a voltage divider, I used a power source that had two parts to it (I would plug one in for the +V and the other for the -V). Then someone came up to me and said that was wrong; told me hook the power sources together and make the point between them be the ground.

16. ### DavS13 Thread Starter New Member

Jun 16, 2014
21
0
@MrChips

I'm about to be a senior; I should have made that clear, I apologize. Starting in autumn I will be a senior. There still alot of stuff that hasn't been covered yet, and we have just been solving problems so far not designing, so I could tell you what that does at all.

17. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Yes, I have done that too. A normal bench power supply, like the one we used in circuits and logic labs, have two positive outputs, Port A and Port B.

Dual Power Supply:

Set Port A and Port B to Independent. Then connected Port B + to Port A -, this is the point between them and it is ground for them. Now you have positive voltage from Port A +, negative voltage from Port B -.

I think one time in one of the labs we ended up using three bench power supplies. The reason it is funny is because the bench power supplies are chained to the bench so getting everything arranged, dragging them as close as possible to our breadboard, it was a hassle at the time, it is funny now, a few years later.

Also. I think you said you are senior. It might be a language problem. If you are in 4 years electrical engineering program, then you are freshman. Senior is somebody in their last, 4th, year, they finish the 4th year, they get their engineering diploma.

18. ### DavS13 Thread Starter New Member

Jun 16, 2014
21
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Yes this is exactly what I'm talking about. Do you know the reason behind this? The bench power supply I use also has a ground in each port. Why couldn't I simply connect the -V from port B to the -V for the op amp and then ground, and similarly with port A for +V of the op amp. Why do I have to connect them?

Also I am a senior, now that this academic year is over; I receive my engineering diploma in about 1 year.

19. ### crutschow Expert

Mar 14, 2008
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6,116
I assume you know that current always goes in a loop, thus the current out of each of the power supply terminals must have a return. Since most power supplies do not have a common ground you must connect them all to a common ground point (the negative of the positive supply to ground and the positive of the negative supply to ground). Is there something about that you don't understand?

20. ### DavS13 Thread Starter New Member

Jun 16, 2014
21
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@crutshow No I think I get it now. I was confused because the power supply bench I use has a ground terminal for each port, so I didn't see the point of connecting the ports together to a common ground; why not just use them separately? Thank you.