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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Entertaining experiments: some professions of the transistor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur [an error occurred while processing this directive] The transistor amplifies electrical signals - you were convinced of this, for example, when you built the simplest intercoms. But a transistor can become a temperature sensor, light sensor, an electronic key device - it is easy to verify this by doing the proposed experiments. Transistor - temperature sensor (Fig. 1). One of the transistor parameters that you sometimes have to pay attention to is the reverse collector current. The reliability of the designed device sometimes depends on its stability. This current appears when the source is connected to the collector junction in the opposite direction, i.e. when the collector of the transistor of the p-n-p structure has minus voltage, and the base has plus (or the collector of the npn transistor has plus voltage, and the base has minus ). To see how stable this current is when the ambient temperature changes, stock up on at least two transistors, one of which is silicon and the other is germanium. You will also need an ohmmeter and a glass of warm (50 ... 60 ° C) water.
If you have a germanium transistor MP39B (pnp structures). connect an ohmmeter to the terminals of its collector and base. so that the positive probe of the ohmmeter is connected to the base terminal. The ohmmeter needle will fix the reverse resistance of the collector junction, which is determined by the reverse collector current. The resistance can be very large - several hundred kilo-ohms. Observing the readings of the ohmmeter, lower the transistor with the "hat" down into a glass of warm water so that the base of the transistor with the leads is 2 ... 3 mm above the water level. In just a few seconds, you will notice that the controlled resistance will begin to decrease. After about a minute, it can drop to 50 kOhm - it all depends on the temperature of the water.
It is worth removing the transistor from the water, as after a while the ohmmeter needle will return to its original position. If the transistor is placed in a refrigerator, the reverse resistance will increase compared to the initial one. Do the same experiment with a silicon transistor, such as KT315. You will make sure that its collector reverse current will not be detected by the dial indicator of the avometer. It is units and even fractions of a nanoampere (1 nA = 10-9A). Therefore, oscillations of the collector reverse current are less noticeable by cascades made on silicon transistors compared to the same cascades on germanium ones. From this it is easy to understand why silicon transistors are preferred in the development of radio equipment.
And one more conclusion. Since the collector return current depends on the ambient temperature, the germanium transistor can become a sensor that can measure, for example, the temperature of the outside air. This solution is sometimes found in amateur radio practice. Transistor - photosensitive sensor (Fig. 2). From the transistors you have, select a low-power germanium with the highest possible transfer coefficient. Suppose you settled on the MP39B transistor. Remove the cap from it by first sawing off the “bottom” of the case or carefully breaking it off with wire cutters. Then connect an ohmmeter to the terminals of its collector and emitter in the polarity indicated on the diagram and cover the transistor with a sheet of paper so that light does not fall on it. The ohmmeter needle will mark a very high resistance between the indicated terminals. Now open the transistor and direct the light of a table lamp at it from a distance of a meter or two. The ohmmeter will record a decrease in resistance. As the lamp approaches the transistor, i.e., as its illumination increases, the resistance value measured with an ohmmeter will drop. So, a light-sensitive photo sensor was obtained from a transistor. The more light falls on the sensor, the less its resistance. It is not difficult to guess about the possible use of such a sensor in an illumination meter, an automat for switching on lighting at dusk on the street, a photoelectronic dash, an optical telephone, etc. large transfer ratio.
Transistor - electronic switch. You can demonstrate this property of the transistor on a toy model, which we will call "electronic swing". Like a real swing, our toy is functional. It is set in motion ... by an electric current. And some figure will swing on them. Pay attention to the swing diagram shown in fig. 3. An electronic key is assembled on the transistor VT1, through which power is supplied to the winding of the coil L2 of the electromagnet. The control signal to the key comes from the winding of the coil L1. placed on the same frame as L2. When the SA1 switch is closed, the supply voltage will be applied to the transistor. The transistor will be closed, since its DC base is connected to the emitter through the inductor L1 and there is no bias voltage on the base. A relatively small reverse collector current will flow in the emitter circuit of the transistor. But it is worth quickly bringing a permanent magnet close to the core of an electromagnet (say, with the north pole), as an electromotive force (EMF) will begin to be induced in the winding of the coil U1. A negative bias voltage will appear at the base of the transistor, which will increase as the magnet approaches. The transistor will turn on and current will flow through coil L2. A magnetic field is formed around the core, which will begin to attract a permanent magnet. The highest bias voltage will be when the permanent magnet is above the core of the electromagnet. With its further advancement above the core, the other pole of the magnet will appear and the EMF will change its direction. A positive voltage will appear at the base of the transistor, and the transistor will close. The current through the electromagnet winding will stop So, at a certain position of the permanent magnet relative to the core of the electromagnet, a force appears that pushes the magnet. She makes the toy swing swing. Diode VD1. shunting the winding of the coil L2. prevents the occurrence of oscillations in it with a frequency determined by the inductance of the electromagnet, the capacitance of the installation and the transistor. The fact is that when the transistor is opened, an oscillatory process occurs, which, due to the strong connection between the base and emitter circuits, can be undamped. The control action of the permanent magnet in this case will stop and the swing will stop. Diode or. cutting off the positive half-wave already of the first oscillation, prevents the occurrence of such a phenomenon. Transistor - any of the MP39-MP42 series. diode - also any of the D9 ... D226 series. The power supply is 4.5 V or 9 V, depending on the strength of the permanent magnet used. It is not necessary to put the power switch SA1. because when the permanent magnet is against the core of the electromagnet (the swing is stopped), the transistor is closed and the device draws negligible current. Coils are wound on a frame (Fig. 4.a). glued from thick cardboard or machined from a suitable insulating material. The windings are wound simultaneously (Fig. 4.b), putting together two PEL wires. PEV or PELSHO with a diameter of 0.1 ... 0.15 mm, until the frame is filled. A core is inserted inside the frame (Fig. 4.c). machined from mild steel, and glued to the frame. To improve the magnetic properties of the core and prevent its residual magnetization, it is advisable to anneal the core blank (heat it, for example, in the flame of a gas stove burner), and then cool it at room temperature The details of the electronic device are placed inside a small case (Fig. 4.e). while the swing is reinforced on its top bar. The electromagnet is attached to panel 3 (Fig. 4.d) so that the core 4 is flush with the surface of the panel or protrudes slightly above it. To attach the swing, two racks are attached to the same panel, and a crossbar is installed between them. Two wire brackets are driven into it and pieces of thick sewing thread are passed through them. The ends of the threads are tied to the board 2 swings, on which the figure is strengthened. A small permanent magnet is glued to the bottom of the board 1. Please note that the stronger the magnet, the better the electronic key works. It can be made up of two magnets from an unusable micro-electric motor - they are glued like this. so that the north poles are in the middle. A magnet from a magnetic latch (such latches are used in modern furniture) or from other devices is also suitable. If the existing magnet is large, do not try to break it with hammer blows, otherwise it will be demagnetized. It is best to separate part of the magnet by squeezing it in a vise or breaking it off without hitting it. The magnet is attached to the board like this. so that when the swing is stopped, it is exactly opposite the core of the electromagnet and at a distance of 2 ... 3 mm from it (this distance is adjusted using the board's thread hangers) After turning on the power of the toy, swing the board with the figurine. If it stops soon, the probable cause is the incorrect inclusion of the winding of the coil L1 of the electromagnet. Swap its conclusions. The operation of the electronic key can be checked in this way. After turning off the power, connect a 100 mA milliammeter in parallel with the switch terminals (in other words, in the collector circuit of the transistor). When the board swings or the permanent magnet approaches the core of the electromagnet, the milliammeter needle will sharply deviate. If it deviates weakly, install a stronger permanent magnet or increase the supply voltage.
According to the principle of operation of this toy, the pendulum of many electronic-mechanical clocks, for example "Glory" is built - inside they also have an inductance coil, two permanent magnets, a transistor (Fig. 5). Author: B.S. Ivanov
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