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How to design analog circuit

ElectronicsR

Mar 23, 2016
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Hello,
For learning electrical engineering there is importance of analog please tell where to start analog engineering?
for example we have charge 2200uf capacitor of rating 50V
but i applies a 12v source.
and there is load R how to find time it will store charge?
 

BobK

Jan 5, 2010
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dv means delta v, change in v over time.

That equation says that if there is a current I into (or out of) the capacitor with capacitance C, the the change in voltage over time will be C dt.

For example, a 1F capacitor charging at a rate of 1A will rise by 1V each second.

Do you know any calculus?

Bob
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Hello,
For learning electrical engineering there is importance of analog please tell where to start analog engineering?
for example we have charge 2200uf capacitor of rating 50V
but i applies a 12v source.
and there is load R how to find time it will store charge?
You can start by learning physics and math. Learn about the passive components used in electronics: resistors, capacitors, and inductors. Learn how resistors behave in circuits by learning to use Ohm's Law and Kirchoff's Laws to analyze the currents in, and the voltage drops across, resistors in increasingly complicated series and parallel circuits with one or more voltage and/or current sources. Learn about voltage and current sources, both DC as well as AC, and their Thevenin and Norton equivalents. Learn how capacitors store and release energy. Learn how inductors store and release energy. Learn basic calculus skills of differentiation and integration. Learn how to examine a passive circuit and set up and solve differential equations describing the currents and voltage drops in the circuit.

Once you have mastered circuit analysis using passive components you can begin a course of study in analog circuit engineering using active components. Start with the bi-polar junction transistor (BJT) and master it's biasing and use in common-emitter, common-base, and common-collector circuits. Move on to field-effect transistors (FETs) and do the same thing for common-source, common-gate, and common-drain circuits. This should keep you busy for the first year. Do a lot of lab work, measuring voltage and currents in actual (real) circuits to compare the measured results against your circuit analysis.

Avoid using SPICE simulators until you fully understand how circuits work. Simulators don't teach you anything. They can be a big help in performing "what if" analysis, such as "what happens if I change this resistor value?" But you need to understand the circuit first. Like spreadsheet programs, there is a lot of hidden processing going on in a simulation program. Unless you understand the models used in the simulation, and the limitations of the processing, simulator results can lead you down a prim-rose path to disaster.

i(t) = C dv(t)/dt

what is this dv?
It is derived from the equation that defines capacitance: C = Q/V, where C is the capacitance in farads, Q is the charge separation on the capacitor in coulombs, and V is the voltage across the capacitor in volts. Assuming the capacitance is constant, you can write this as Q = CV and differentiate both sides with respect to time to get dQ/dt = C dV(t)/dt. But dQ/dt (the derivative of charge with respect to time or the rate of change of charge) is a definition of current, I(t), as a function of time. So by substitution you get I(t) = C dV(t)/dt.

Looking at the right side of this equation, if dV(t)/dt is a constantly increasing voltage (a ramp) a constant current will be charging the capacitor. Or, dividing both sides by C to get I(t)/C = dV(t)/dt you will find that if I(t) is a constant current, the voltage across the capacitor will be a linearly changing ramp. The capacitor acts as an integrator of the current input to the capacitor.

In your first post, you asked how long a capacitor will store charge if a load, R, is attached to the capacitor. The load will continuously discharge the capacitor until (eventually) no charge remains. Theoretically, this takes forever, but the discharge current decreases exponentially as the capacitor voltage decreases. Using calculus and circuit analysis you can derive the equation for the voltage, Vcap, remaining across a capacitor, C, with parallel-connected resistor, R, initially charged to voltage Vinit. It is Vcap = Vinit [e^(-t/RC)], where "e" is the base of natural logarithms (about 2.71828) raised to the negative power of the quotient t/RC, where t is the elapsed time of the discharge. If you plug in values for t, R, and C and perform the calculation, you will see that Vcap approaches zero, and is nearly zero, when t = 5RC. This is the "practical definition" of "fully discharged" but in reality the discharge continues as long as the resistor is connected, until eventually the voltage across the capacitor is just noise... the noise voltage being generated thermally by the discharge resistor. See Johnson Noise for more information on that.

Good luck with your studies!
 

ElectronicsR

Mar 23, 2016
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I have done experiment of capacitor where we have to find capacitor capacitance.
what is the best dielectric to have more capacitance than air and mica?
which is best air or mica??
 

BobK

Jan 5, 2010
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It is for a test question. No practical use.

Bob
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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what is this circuit for? ...
It is the starting point for building a logarithmic amplifier.

Isn't this a little off the original topic of how to design an analog circuit, which I tried to answer in post #4?

We are not here to guide you through all the analog circuits posted in your text book(s). The text book should provide ample explanation. If not, try another text book. You may have to read several hundred books before you succeed at becoming an analog circuit designer. And don't forget to do the lab work to confirm your knowledge of what you read! It is especially helpful to set up your own electronics lab at home, gradually acquire components and test equipment over your lifetime, and actually build, test, and debug circuits on your own. Learning theory from reading text books (or online resources, including Electronics Point) is essential, but "hands on" experience is also vital to the learning experience.

Many of the members who post regularly here are also electronics hobbyists, well-seasoned with experience in real electronics. Some of us also have extensive educational backgrounds, but it is not the primary purpose of this forum to educate you. That is your responsibility. We are simply here to help you, and we do so gratis on our own time because of our love and passion for electronics. Plus, I often get ideas from this forum and go off into left field to explore them, reporting back here from time to time with whatever I find that might be interesting to others here. EP is an electronics social network, so go off and learn something, build something, and come back here to talk about it. We (mostly) love newcomers to the hobby.
 

pathinteractive

May 17, 2016
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The dielectric strength for air is approximately 3 megavolts per meter. In comparison, the dielectric strength for mica is approximately 120 MV/m. The choice of dielectric material is very important in some applications where high voltages are expected, or when the thickness of the dielectric is very small. Air has a lower dielectric constant than water.
Formula for the capacitance of a parallel-plate capacitor:
CodeCogsEqn-13.png

where C is the capacitance, εr is the relative permittivity of the material, ε0 is the permittivity of vacuum, A is the area of the plates and d is the distance between the plates. It becomes clear that the larger εr is, the larger the resulting capacitance becomes.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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The dielectric strength for air is approximately 3 megavolts per meter. In comparison, the dielectric strength for mica is approximately 120 MV/m. The choice of dielectric material is very important in some applications where high voltages are expected, or when the thickness of the dielectric is very small. Air has a lower dielectric constant than water.
Formula for the capacitance of a parallel-plate capacitor:
CodeCogsEqn-13.png

where C is the capacitance, εr is the relative permittivity of the material, ε0 is the permittivity of vacuum, A is the area of the plates and d is the distance between the plates. It becomes clear that the larger εr is, the larger the resulting capacitance becomes.
Water is an excellent dielectric... if you can keep it clean and pure. But water is also a highly polarized molecule that dissolves a lot of stuff, becoming an electrolyte instead of an insulator rather quickly. Try measuring the conductivity of various water samples (tap water, distilled water, de-ionized filtered water, bottled water, etc.) to see the huge variations in conductivity. We used to make triple-distilled water in the lab and noticed an increase in conductivity (which is very low for triple-distilled water) when the freshly distilled water was transferred to a test tube, beaker, or flask for actual use. No one I know sells capacitors with water dielectrics.

As for air versus mica... you have to support the capacitor somehow and even a mica capacitor will arc across its terminals in air. No one makes meter-thick mica capacitors that will hold off megavolts in air. To do that you surround the capacitor (of whatever dielectric) with pressurized sulfur hexafluoride gas and use suitably spaced, long, ceramic insulators for the exterior terminals. If you need a high-voltage variable capacitor, a vacuum dielectric is the usual choice.

Air-insulated variable capacitors are used for tuning RF circuits, but at high power levels there will be arcing between opposing plates. Vacuum doesn't arc, so that is the type preferred for tuning high-power RF. Mica is not often used for capacitors although it does have good stability and excellent power-handling capability. I like dipped-mica capacitors for use in RF by-pass applications.

The main thing affecting practical capacitor construction is the area of the plates and the effective separation between plates. So-called super-capacitors get the separation down to a molecule or so thickness but sacrifice voltage hold-off to do so. Most practical for high capacitance AND a reasonably high voltage hold-off are electrolytic capacitors.

The OP asked:
which is best air or mica??
@davenn gave the correct answer in post #4: you have to specify how the capacitor is used before you can decide which dielectric is "better." Then you have to find a manufacturer who makes a capacitor that meets your specifications. The classic capacitance formula you posted does not take into account electrostatic field-fringing edge effects. There is more to selecting a capacitor than just an equation.
 
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