المكثف والمفاعلة السعوية - قوانين الدارة ووحدات قياسها .. الإلكترونيات

Capacitor and Capacitive Reactance - Circuit Laws and Units of Measurement .. Electronics

Capacitor and Capacitive Reactance

A capacitor consists of metal plates insulated from each other. If the capacitor is connected to an electromotive force, it stores an electric charge inside it. If the electromotive force is disconnected and a metal wire is connected instead across the capacitor, we find that the stored charge has been discharged immediately (see page 28). If the capacitor is connected to a source of alternating current, it is charged and discharged whenever the polarity changes in it, i.e. in every half cycle. The process of charging and discharging in the capacitor leads to obstruction of the flow of current, and this obstruction is called the capacitive reactance of the capacitor and is also measured in ohms.

Figure (127) shows a source of alternating current connected to a capacitor through which a current passes. In it, we find that the intensity of the current passing through the capacitor increases with the increase in the capacitor's capacity (27B (0). This is due to the decrease in the value of the capacitive reactance of the capacitor with the increase in frequency. If the capacitor is connected to a source of alternating current with a frequency of 50 Hz (128) and then the same capacitor is connected to another source of alternating current with a frequency of 100 Hz (28B), we find that the value of the capacitive reactance of the same capacitor in the second case is less than in the first case. What does this affect?

Figure (29) shows a transformer (see page 40) whose primary coils (N) are fed with audio signals issued by an amplifier and whose secondary coils (N2) feed a loudspeaker. A capacitor (S) is connected across the primary coils. We choose its value so that high-frequency signals pass through it easily, while they cannot pass through the primary coils because

27 - The capacitor's reactance decreases with the increase in capacitance.
28 - The capacitor's reactance decreases with the increase in frequency.

The increase in its impedance resulting from the increase in frequency. Therefore, this device is used to cut off high frequencies and to control the mining process. Both the primary transformer coils and the capacitor are used as a "blocking" circuit for high frequencies. As for Figure (30), the capacitor (S A) is connected to the amplifier tube's riser circuit, and passes the high-frequency signals to the ground. The cutting of signals with frequencies within a certain range of high frequencies and their passing to the ground can be controlled using a variable resistor (MA) connected in series with the capacitor.

29 - The decrease in the capacitor reactance leads to the "cutting" of high frequencies.
30 - Loudness control circuits.

This resistor works as a variable resonant controller. The switch (H) is used to select one of the different capacitors S2, S3, or S4. It is worth noting that choosing the low-capacity capacitor S? allows only high frequencies to pass easily to the next stage. As for choosing the medium-capacity capacitor S3, it allows signals with medium frequencies (less than high frequencies) to pass easily, while the large-capacity capacitor S4 allows signals with low frequencies to pass easily. Resonant controllers are used in receivers and amplifiers, and most of them depend on the fixed fact that the capacitive reactance of the capacitor decreases with increasing frequency.

31 - Using inductive reactance.

Inductive reactance has various uses. Figure (31) shows a rectifier that feeds a power filter, and it also shows how to obtain a direct current free of ripples from alternating current. In this figure, L is a choke with high inductance that also prevents the passage of high-frequency ripples, while S1 is a About its tank, while the capacitor S2 works to pass a direct current free of ripples.

In Figure (132) the radio frequency signals appear with the audio frequency signals at the collector M, and the choke coil impedes the passage of the radio frequency signals, so they are forced to pass through the capacitor SA with a small capacity. Since the inductive reactance of the creator coil is small at low frequencies, it allows the audio frequency signals to pass through it to take their way through the capacitor S? with a large capacity.

In Figure (32b) the choke coil is placed as a hiss filter in a circuit for audio signals. This filter works to prevent the passage of signals with

32 - Stopping the passage of high frequencies.
33 - Feeding the blocking network.
34 - Cathode bias problems.

High frequencies to the audio stage, thus reducing the hiss resulting from the friction of the phonograph needle on the surface of the record when listening to it. Figure (33) shows a pentode used as an amplifier, and it is noted that the supply line is at 250 volts, while the blocking network is fed with a voltage of only 150 volts. To obtain this voltage, the supply line voltage must be reduced by 250 - 150 = 100 volts. Therefore, the resistor M1 is used with the blocking network circuit to reduce this voltage by 100 volts. The value of the resistor M is known from measuring the current intensity passing through it or through the data specific to the valve. If this current intensity is 6 milliamps, then M1 = G / T = 100 / 2005 = 20000 ohms.

What is the power lost in the resistor M?
Power lost = T X C = 205 0 × 100 = 0.5 watts. Therefore, a 20 kΩ, 1/2 watt resistor must be used. In addition, the shielding network must be connected to the ground at the operating frequency, i.e. "grounded", and this is done by a capacitor with a capacity of 0.05 microfarads and capable of withstanding 250 volts. We find that at the output stage of the receiver (34), the value of the cathode voltage must be equal to 15 volts. Since the current passing through the cathode is equal to the sum of the currents passing through the anode and the network, i.e.
0.25 + 0.005 = 0.03 amperes, therefore, what becomes:
M1 = G / T = 15 / 0.03 = 500 ohms

To pass the signals with audio frequencies at a voltage of 15 volts, the capacitor S A, which has a capacity of 25 microfarads, is connected in parallel

35 - Value of the resistors in the voltage meter

36 - Operating conditions of the DC transistor amplifier

With the resistor, at an operating voltage of 25 volts. Figure 35 shows a test device that operates on more than one voltage range: zero - 10 only, zero - 100 volts. Suppose the value of the current passing through it is 1 milliamp, (001 amperes), then two resistors must be connected in series, one of them is

M1 10 / 0.001 = 10,000 ohms and the other is M2 = 100,000 ohms

Figure (36) shows a transistor circuit whose collector current may not be correct even though all the circuit's resistances and its supply voltage are correct. If the correct value of the collector current is 2 milliamps, then the simplest way to test the transistor is to measure the voltage of the resistor between point S and point Y. This voltage should be equal to V = V x M = 0.002 × 1000 = 2 volts.

If the device reads only 0.5 volts, then the current drawn becomes V = V / M = 0.5 / 1000 = 0005 amperes, i.e. only 0.5 milliamps.