Capacitor and RC circuits

A device consisting of two plates, between which there is a dielectric that allows the accumulation of electrical charge. The larger the area of the plates and the smaller the distance between them, the better the parameters of the capacitor and its capacity. In fact, a capacitor is very similar to a rechargeable battery, it also accumulates charge and releases it, but unlike it, a capacitor can instantly release all the accumulated charge, while the battery releases charge gradually
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Виды конденсаторов.jpg

Delay scheme

One simple example of an RC circuit is a delay circuit that can be used, for example, when turning on the flame in a burner with a delay so that the gas has time to pass through. The circuit controls the load, which can be a relay, which in turn controls a more powerful consumer.
When voltage is applied, it passes through the resistor and begins to charge the capacitor, i.e. part of the voltage goes to the capacitor and only after it is fully charged the full voltage goes further. As a result, we get a microsecond signal delay
The delay circuit can be used for both on-delay and off-delay
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Differentiating RC circuits

Unlike the delay circuit in the differentiating RC circuit, the capacitor and resistor are in reverse, which allows us to rely on the fact that the current flowing through the capacitor is directly proportional to the rate of change of voltage. Those. the higher the rate of change of voltage, the higher the current and vice versa, if the voltage does not change, then there is no current, since the voltage becomes constant, and a capacitor, as we know for a constant voltage, is an open circuit.
If there is a rectangular signal at the input as in the figure, then at the output we will get only the areas where the voltage changes, i.e. in essence we only get markers or voltage change points.
The car differential or rear axle shown in the picture will give you a better understanding of the principle of operation, in essence we have the rotation of the shaft from the engine, which is transmitted to the axle, the direction and speed of rotation of the axle can change depending on the design, but the beginning of rotation of the axle always occurs with the start of rotation shaft
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Integrating RC circuit

An integrating circuit is very similar to a delay circuit. As we see in the figure, we have an input voltage in the form of a square wave (rectangles) which, due to our integrating circuit, we can convert into a sawtooth signal.
Depending on the period of the signal, we see a change in shape, so with a short period the input and output signals are almost no different, since there is enough time to charge and discharge the capacitor, but as the period increases, the capacitor will not have time to completely discharge/charge< br /> Those. if the input voltage is represented by rectangular pulses from 0 to 5 volts, then at the output with a high period we will get 2.5 volts
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Polar and non-polar capacitors

The capacitance of a capacitor is the amount of charge that a capacitor can absorb. The basic characteristic of a capacitor, which is measured in Farads. 1 F is a very large value and such capacitors are extremely rare (for example, in car audio, when you need to listen to the “bottom” very loudly), but in practice, capacitors with a capacitance of micro or pico farads are mainly used.

The main difference between a capacitor is polar and non-polar, but there are also variable capacitors, for example in radios, where to adjust the frequency we turn the knob, changing the capacitance of the capacitor, but in practice this is rarely used.
Рис 9. Обозначение полярности конденсатора на плате.jpg

Capacitor polarity designation

If the capacitor is polar, then its polarity must be indicated, as well as the rated voltage. In the diagram, a capacitor is indicated by two plates; if there is no “+” symbol on any of the plates, then the capacitor is non-polar and vice versa. There may also be a trimmer or variable capacitor, which are indicated by arrows in the diagram.

It is worth remembering that in alternating current circuits the use of polar capacitors is unacceptable (there are rare exceptions), i.e. If you see a polar capacitor, then most likely you are looking at a circuit with a constant current direction. And at the same time, in high-frequency circuits, the capacitance of the capacitor changes, often downward
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Rated voltage

The rated voltage of the capacitor must be at least 30% higher than the voltage in the circuit, i.e. if you have a 12 volt circuit, then the minimum capacitor should be 16 volts, you can, of course, put it at 25 V (many craftsmen do this) if it fits the dimensions.

Electrolytic capacitors

Electrolytic capacitors consist of two plates, between which there is paper impregnated with electrolyte as a dielectric. The plates are twisted into a tube to increase the area, and therefore the capacity. On top of the electrolytic capacitor there is a cross, in the form of an almost cut-out housing, which allows the housing to burst in the right place, without severe deformation and major consequences. Sometimes capacitors do not burst in a special place, but swell, which creates the expression “Pregnant capacitor” among repairmen.
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Capacitor ESR

ESR (Equivalent Serial Resistance or equivalent series resistance) is also an important parameter that takes into account that a capacitor is not an ideal device; in any case, a current passes through its dielectric, although it is negligible, but it exists. Just as there is inductance of twisted foil, although it appears only at high frequencies, it is there and cannot be completely discarded, just as one must not forget about the resistance of the conductors connecting the plates to the board. If we sum up all these parameters, we get ESR, which should not be higher than the values ​​​​in the table, but not more than 4-5 Ohms.
Таблица 3. ESR электролитических конденсаторов.jpg

Capacitor quality factor

Vloss (Voltage loss) is the loss of voltage after exposure to a charging pulse, i.e. Let's assume that we connected the capacitor to 9V, if we disconnect the capacitor from the current source and measure the voltage in an ideal capacitor it should be the same 9 V, but in fact this number will be slightly lower. A deviation of 3-5% is allowed, anything more is defective. The second name for Vloss is the quality factor of the capacitor.
Рис 11. Измерение конденсатора LCR метром.jpeg

Ceramic and tantalum capacitors

Ceramic capacitors have no polarity and can be either in DIP package (with legs) or in SMD surface mount package

Tantalum capacitors are a capacitor very similar to a regular ceramic capacitor, but it has a mark (WHICH MEANS +) and markings on the case; by this marking you can determine the capacitance, voltage and even the date of production.
Рис 12. Внутрисхемное измерение SMD конденсатора.jpeg

RC circuit

Now that we've learned about the resistor and capacitor, it's time to learn about RC circuits. Which just consists of a capacitor and a resistor. As we know, a resistor is a static element whose characteristics are subject to Ohm's law, while a capacitor can be charged and discharged.
While charging the capacitor, the current tends to infinity and the voltage is 0, which indicates a short circuit, and after charging, the current stops and the voltage becomes equal to the source voltage. In real conditions, charging and discharging of a capacitor does not occur instantly.

Let's calculate how long it will take to charge a capacitor with a capacity of 10 uF to 95% through a 100 Ohm resistor:
T= C*R = 0.000001 * 100 = 0.001c
3T = 0.003s = 95% (three times the time constant)

High-pass filters and low-pass filters are based on the fact that a capacitor is an ideal conductor for high frequencies, and a resistor is the opposite. For the DC component, the capacitor is an open circuit, and the resistor is a conductor.
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Дополнительно

A capacitor is a device that has two main parameters: VOLTAGE and CAPACITY. Capacitors do not dissipate energy because their voltage and current are offset by 90 degrees relative to each other.


If we remember the lesson with resistors, we will remember that the current passing through a resistor is directly proportional to the voltage, but in a capacitor everything is not so simple, because the current is proportional not just to the voltage, but to the rate of change of this voltage. So if you change the voltage on a capacitor with a capacity of 1 farad by 1 volt in 1 second, you will get a current equal to 1 ampere.

By design, a capacitor is two conductors located very close to each other, but not touching. The larger the area of ​​the conductors and the smaller the distance between them, the higher the capacitance of the capacitors.

Depending on the material of the dielectric (insulator) between the conductors and design features, capacitors are divided into the following groups

- Electrolytic: filtration in power supplies
Tantalum: circuits where large capacity is required
Ceramic: non-critical circuits
More (Mica, Polyester, Teflon, Glass, etc.)

When two capacitors are connected in parallel, their total capacitance is equal to their sum, while when connected in series, the formula is a little more complicated

С=С1*C2/(C1+C2)

So, for example, let’s take a running capacitor of 470 μF, which is installed in the power supplies of control modules of washing machines; with a parallel connection we get 940 μF, but with a series connection 235 μF, and here everything is obvious, since this is half of their capacity. But let’s take two capacitors of 470 and 680, also connected in series and we get approximately 278 uF, and to make it completely clear, let’s take 10 and 680, (10*680/(10+680)=9.855) or LESS THAN SMALL of the capacitors for any layouts.



There are two main characteristics of AC circuits
Change in voltage U and current I over time
Change in amplitude when the signal frequency changes
To begin with, we will touch on the first option (time), and a little later we will analyze the frequency response

If we consider the simplest RC circuit, then a 1 μF capacitor charged to 1 volt and connected to a 1 kOhm resistor will be discharged, and the current in the circuit will be 1 mA.
The capacitor is discharged or charged in 5 times the product of RC or in our case 5 ms
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