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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Entertaining experiments: get acquainted with the diode. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Diode - the simplest semiconductor device that passes electric current in one direction - from the anode to the cathode. Nevertheless, it is very interesting and is widely used in radio electronics. The proposed experiments will serve as confirmation of the foregoing. Let's make a reservation right away that for experiments we will take two types of diodes - germanium and silicon, the most common series: D9 and KD105 (Fig. 1). Their characteristics - the dependence of the forward current (Ipr), i.e., the current through the diode in the forward direction (from the anode to the cathode), on the forward voltage (Upr) applied to the diode (measured between the anode and cathode terminals), are somewhat different. The silicon diode begins to open at a higher voltage compared to the germanium one (see Fig. 1), so the characteristic of the germanium diode is much smoother - this feature is sometimes used in the design of certain devices.
Electronic security. Start with a simple experiment (Fig. 2a): take a GB1 battery with a voltage of 4,5 V (type 3336) and connect a voltmeter PV1 to it (the Ts20 avometer should work in this mode) through a silicon diode VD1. What did the voltmeter pointer show? A voltage close to the battery voltage, but not equal to it (more on the reason for this later). When you turn on the germanium diode instead of the silicon voltmeter, the voltmeter will show a voltage that is almost equal to the battery voltage.
In both versions, the diode is connected in the forward direction, a current of about two tens of microamperes flows through it, the forward voltage falling across the diode is small compared to the battery voltage. Now reverse the polarity of the battery leads. The anode of the diode will be connected to the negative terminal of the battery, i.e. the diode will be turned on in the opposite direction. If it is silicon, the voltmeter needle will not move, since its resistance with this inclusion is almost infinite. With germanium, the situation is different. For example, the diode of the D9 series has a reverse resistance of about 2 MΩ, and the input resistance of the Ts20 in the 10 V range is 200 kΩ. Therefore, the voltmeter needle will record a voltage about 10 times less than the voltage of the power source. But it is worth switching to a smaller measurement range, as the voltage measured by the voltmeter will also drop - after all, the input resistance of the device will become smaller, which means that the transfer coefficient of the divider formed by the reverse resistance of the diode and the input resistance of the voltmeter will change. What conclusion follows from this experiment? The diode is able to protect the load from accidentally applying reverse polarity voltage to it. Many years ago, radio amateurs built a diode into the power circuit in some designs, in particular in small-sized transistor radios. As a result, it was possible to avoid trouble (failure of transistors) if the power supply was connected incorrectly. Such protection can be used by you in various developments. However, the question arises: why can't you find such protection in modern designs? An experiment will help answer it, for which you will need a 4,5 V battery, a diode (germanium and silicon) and two voltmeters (Fig. 2, b). Voltmeter PV1 controls the voltage of the power supply, and PV2 - the voltage on the load, which is protected by the diode. As long as the load resistance (in this case, the input resistance of the voltmeter) is high, little current flows through the germanium diode and there is practically no voltage drop across it. The voltmeters will read the same. Connect a constant resistor with a resistance of 2 kOhm in parallel with the PV1 voltmeter - the voltmeter needle will record a decrease in voltage at the load. And when you connect a resistor with a resistance of 430 ohms, the voltage will become even less due to the greater forward voltage across the diode. When you install a silicon diode in place of VD1, the voltage on the PV2 voltmeter will be less than on PV1, even without a connected resistor. It is not difficult to explain this if we compare the characteristics of diodes (see Fig. 1). With the same even weak forward current, the forward voltage on a germanium diode is less than on a silicon one. Connecting the resistance causes an increase in the forward voltage of the diode, and therefore a decrease in the voltage across the load. True, the forward voltage does not exceed 1 V with an increase in the forward current through the silicon diode of the KD105 series up to 300 mA (for D9 - from 10 to 90 mA, depending on the specific type of diode). And yet, its loss when the structure is powered by a voltage of 9; 4,5 and especially 3 V is noticeable. That is why this method of protection has not found wide application. In amateur radio practice, it may be necessary to protect the input circuits of devices operating with small signals from accidental high voltage. In such cases, we have to remember the silicon diode, which begins to pass current only from a certain voltage. Indeed, on its characteristic, the initial section runs along the horizontal axis. This property of the diode is used to operate it as an element of electronic protection. The experiment (Fig. 2, c) will make sure of the foregoing, for which, in addition to a silicon diode, constant and variable resistors, a 3336 battery, a switch and a DC voltmeter with a measurement range of, for example, 3 V (Ts20 avometer) will be needed. Having first set the engine of the variable resistor R1 to the lower position according to the diagram, the supply voltage is supplied by the switch SA1. By smoothly moving the resistor slider upwards, one observes a smooth increase in the voltage across the diode by the deviation of the voltmeter needle. At a voltage of approximately 0,6 V, the voltage increase on the voltmeter will begin to decrease, and soon the arrow of the device will practically stop (at a voltage of approximately 0,7 ... 0,8 V) and remain in this state even when the variable resistor slider is in the upper according to the position diagram, i.e. 4,5 V will be applied to the protection device. What happened? Up to a certain voltage, the diode was closed and the voltmeter measured the voltage taken from the variable resistor engine. And then the diode began to open and shunt the voltmeter, which in this case imitates the protected circuit. As the voltage increased, the current through the diode increased, which means that its shunting effect also increased. Soon the diode opened so much that it completely shunted the voltmeter. The voltage on the diode remains stable despite changes in the external voltage (taken from the variable resistor engine) due to the excess voltage drop across the resistor R2. In this case, the diode protects against an accidental increase in voltage of a certain polarity. If you need to protect the circuit from power surges of different polarities, put two diodes connected in parallel - one in the forward direction and the other in the opposite direction. It is possible that protection is required that "triggers" at a higher voltage than a single diode provides. Then they put two or more series-connected diodes (Fig. 2, d). Test this option and see for yourself. Brightness control. As you know, the flat flashlight uses a 3336V 4,5 battery and a 3,5V lamp. When the battery is fresh, the lamp is very bright. If necessary, the brightness can be somewhat reduced by including a silicon diode VD1 and an additional switch SA1 in its circuit (Fig. 3, a). Mount this node on a breadboard and see how it works.
When the switch contacts are closed, the brightness of the EL1 lamp is the highest. It is worth setting the switch to the position of open contacts, as the diode comes into operation. A forward voltage on it reduces the voltage on the lamp, and its brightness decreases. A diode works more efficiently in an alternating current circuit (Fig. 3, b), which can be powered, say, by a night lamp. Here, when the contacts of the SA1 switch are opened, there is a greater decrease in voltage (average voltage) on the lamp due to the manifestation of the property of the diode - to pass current in one direction, in this case only with positive half-cycles of the alternating voltage at the anode of the diode. The transformer should be selected so that the voltage on the winding II does not exceed the voltage for which the incandescent lamp is designed. Control lamps on two wires. What if you need to turn on two lamps separately, located at a distance from the switch and connected to it only by a two-wire line? Think of the diode in this case. When supplying the line with direct current (Fig. 4, a), two diodes will be needed - each of them is connected to the circuit of its "own" lamp, but in different directions: one in forward, the other in reverse. When the switch SA1 is in the position shown in the diagram, the current flows through the diode VD1 and the lamp EL1 - it lights up. When the switch is set to another position, the current will flow only through the VD2 diode and the EL2 lamp. Lamp EL1 will go out and EL2 will light up.
If the wiring is powered by alternating current, two diodes cannot be dispensed with, since each of them, although it will work at its “own” half-cycle, the lamps will flash simultaneously. Therefore, you will have to add two more diodes (Fig. 4, b) and put a separate switch in the circuit of each of them. To light the EL1 lamp, you need to close the contacts of the SA2 switch, and to ignite only the EL2 lamp - the SA2 switch. When the contacts of both switches are closed, all the lamps will light up. Simple and convenient. True, the lamps will shine half-heartedly, since the current flows through each of them only during one half-cycle of the alternating voltage on the secondary winding of the transformer T1. To maintain the same brightness of lighting (such that it would be with a direct connection of the lamp to the transformer), it is possible to recommend the use of lamps of higher power. voltage doubler. The device, the scheme of which is shown in Fig. 5, a, - half-wave rectifier. The constant voltage U1 on the capacitor C1 will exceed the alternating voltage measured by the alternating current voltmeter on the secondary winding of the transformer by about 1,4 times, i.e., it will correspond to the amplitude value of the half-wave of the alternating sinusoidal voltage.
It is not difficult to almost double the constant voltage at the rectifier output (Fig. 5b) by adding one more diode (\/02) and a capacitor (C2). Now you get a rectifier that works with both half-waves of alternating voltage. During positive half-waves, the capacitor C1 will be charged on the upper terminal of the winding II of the transformer according to the scheme, and during negative ones, C2. Since the capacitors are connected in series, the voltages across them (U1 and U2) will add up and the final voltage (U3) will be twice as much as on each of the capacitors. Therefore, such a rectifier is called a voltage doubling rectifier. It is implemented in cases where the step-down transformer has only one secondary winding. For the experiment, any step-down network transformer with a voltage on the secondary winding of 6 ... 10 V is suitable. Diodes can be, in addition to those indicated in the diagram, any rectifier, silicon or germanium (even any of the D9 series will do). Capacitors - any oxide, with a capacity of at least 10 microfarads per rated voltage of at least twice the AC voltage on the secondary winding of the transformer. diode probe. How to determine the ends of a two-wire communication line laid, say, between two rooms of an apartment?
Of course, you won’t use an ohmmeter here, because the length of its probes is not enough. The diode comes to the rescue again (Fig. 6). It is connected to the ends of the wires of the line (it can be simulated by a two-wire network wire assembled into a ball) in the same room and mark the wire to which the anode of the diode is connected. In the other room, to the ends of the wires, first in one and then in the other polarity, the probes XP1 and XP2 of the signal device assembled from a 3336 battery and an incandescent lamp for a voltage of 3,5 V are connected. In one of the connection options, the lamp will flash, which will indicate the passage of current through the communication line and the diode. And this, in turn, will make it possible to testify that the ends to which the anode of the diode and the positive terminal circuit of the battery are connected belong to the same wire. The diode for the experiment can be any silicon or germanium, designed for the passage through it of a current exceeding the current of an incandescent lamp. Author: V.Polyakov, Moscow
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