Measuring mini-laboratory. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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What measuring instruments does a beginner radio amateur need? Voltmeter? - Yes. Ohmmeter? - Yes. Low frequency generator? - Yes. A pulse generator to test the operation of cascades on integrated circuits? - Undoubtedly! Probe for "dialing" installation? - Absolutely. And, of course, the dream of a radio amateur is an oscilloscope, on the screen of which one can observe the "life" of electronic cascades and nodes.

These devices are united in one building by Arthur Mesropovich Piltakyan, an avid radio amateur from school, the developer of many amateur radio and industrial designs in the field of television, measuring and other equipment, the author of dozens of publications in periodicals, including the Radio magazine, and popular books for radio amateurs.

When developing a mini-laboratory, the task was to simplify the measuring instruments included in it as much as possible, but at the same time to provide parameters sufficient for the practical activity of a novice radio amateur. The appearance of the laboratory is shown in fig. 1, and a peculiar block diagram - in fig. 2.

One of her important instruments is an oscilloscope. Its input resistance is approximately 70 kOhm, the smallest input signal amplitude is 0,1 V. With an amplitude of more than 5 V, it is permissible to apply the signal directly to the deflecting plates of the cathode ray tube. Sweep frequency ranges - 60...600 and 600...6000 Hz.

The audio frequency generator (3H) operates at a fixed frequency of about 1 kHz and produces a sinusoidal signal with a voltage of up to 1,5 V. The pulse generator also operates at a fixed frequency, its maximum output amplitude reaches 15 V. The ohmmeter allows you to measure resistance in the 50 Ohm range. ..40 kOhm and 500 Ohm...400 kOhm.

All of these devices are powered by a common unit. Only a voltmeter with a probe does not require mains power. It is designed to measure DC voltage within 10, 100 and 1000 V. When using a voltmeter as a probe, an autonomous power source - a battery - comes into operation.

Let's analyze the device and operation of all units of the mini-laboratory according to its concept (Fig. 3).

(click to enlarge)

Oscilloscope (node ​​A1). Its basis is a cathode ray tube (CRT) VL1. It has a filament (terminals 1, 14), a cathode (2), a control electrode or modulator (3), a focusing electrode or first anode (4), a second anode (9) and two pairs of so-called horizontal (10, 11) and vertical (7, 8) deflecting plates arranged mutually perpendicularly along the CRT axis.

A high voltage is applied between the cathode and the second anode, in our case 600 V. The heated cathode emits electrons, which, under the influence of a positive voltage, rush towards the second anode, passing sequentially through the holes in the modulator and the focusing electrode. Having gained speed, by inertia they pass through the hole of the second anode and, moving between the deflecting plates, finally hit the CRT screen, causing it to glow in the form of a bright spot. Negatively charged electrons tend to repel each other, so the spot has no clear boundaries.

In order to get a luminous point instead of a blurry spot, the electron flow must be focused. For this purpose, a constant voltage is applied to the focusing electrode from a variable resistor R8 - by moving its engine, the desired focusing is achieved.

To adjust the brightness of the spot (hereinafter referred to as the image), a modulator is used, applying a negative voltage to it from the variable resistor R9 engine. The larger it is, the fewer electrons will hit the screen, the lower the brightness of the point.

If there is no voltage on the plates, the dot will be located approximately in the center of the screen. But it is necessary to apply voltage, say, to horizontal plates (with a variable resistor R5), the point will move horizontally towards the plate with positive voltage. The point will behave similarly when voltage is applied to the vertical plates (with a variable resistor R1) - it will move up or down.

When a 1 Hz alternating voltage is applied to the horizontal plates, a dot on the screen moves every second from the leftmost position to the rightmost position and back. Increasing the voltage frequency will result in the appearance of a solid horizontal line on the screen, the length of which depends on the amplitude of the applied voltage. A similar picture can be observed when the same signal is applied to the vertical deflecting plates. The presence of two pairs of plates allows you to move a point on the screen in any direction, that is, "draw" any shape.

In practice, the horizontal plates are fed with a voltage resembling saw teeth in shape (it is called "sawtooth"), and the investigated signal is fed to the vertical plates, say, a sinusoidal shape. With the same frequency of both signals, an image of one period of a sinusoidal voltage will appear on the screen. With an increase in the frequency of the voltage under study, there will be two periods, three times - three, etc. In order to be able to select the required number of observed periods, the frequency of the sawtooth voltage is tuned, making it a multiple of the frequency of the signal under study.

And now for one clarification. Although the story was and will be about horizontal and vertical plates, in fact they were deliberately swapped relative to their usual position, since in the real design the tube is rotated 90 ° to provide a larger image of the signal under study.

The source of the sawtooth voltage, often referred to as the sweep voltage, is a frequency-controlled oscillator made on a transistor VT1. It works like this. After the power is turned on, the collector voltage of the transistor is zero. Capacitors C4 and C5 begin to charge (or C4 and C6, depending on the position of the movable contact of switch SA2), the transistor is closed. The charging rate of capacitors depends on their total capacitance and the resistance of resistors R12, R13. As soon as the voltage on the collector reaches a certain value, the transistor will open like an avalanche and the capacitors will be discharged almost to zero through the collector-emitter section. The collector voltage drops to almost zero, the transistor closes, and the process repeats.

Capacitors charge almost linearly, but they discharge much faster. As a result, a sawtooth voltage is formed on the collector of the transistor, the frequency of which is set stepwise by the switch SA2 and smoothly variable resistor R13. If capacitor C5 is turned on, the frequency can be changed from 600 to 6000 Hz, when capacitor C6 is turned on, it can be adjusted from 60 to 600 Hz.

But the amplitude of the sawtooth voltage is still not enough to supply it to the deflecting plates. Therefore, it enters through the decoupling capacitor C7 and the limiting resistor R14 to the amplifying stage, made on the transistor VT2. Through the resistor R15, voltage is supplied to the base of the transistor from the divider R16, R17, which together with the resistor R18 determines the operating mode of the transistor.

From the load resistor R19 sawtooth voltage is supplied to switch SA3. In the left according to the scheme position of the movable contact of the switch, voltage is applied to the horizontal plates. In the right position, an external signal can be applied to the plates from the X5 socket.

On the vertical plates, the signal under study with an amplitude of more than 10 V is fed through the X2 socket, the variable resistor R20 and the SA1 switch (its moving contact must be in the position shown in the diagram). Part of the signal is taken from the engine of the variable resistor R2 and fed to the base of the generator transistor - this is a synchronization circuit that allows you to "stop" the image on the CRT screen.

When studying signals of much lower amplitude, they are fed from the variable resistor engine through switch SA1 (its moving contacts should now be in the lower position according to the diagram) to the input of an amplifier made on transistors VT3, VT4. To increase the input resistance of the first stage of the amplifier, resistors R21, R24 are introduced. The output stage of the amplifier is made in the same way as the analogous stage of the sweep generator. From the load resistor R31, the amplified signal is fed through the capacitor C10 to the switch SA1. Capacitor C15 prevents self-excitation of the amplifier.

If the signal is large, it is fed to the X4 socket, and the image span on the screen is regulated by a variable resistor R25. This option is used, for example, when measuring the resistance of resistors with an ohmmeter (more on that later).

Power supply (node ​​A2). It contains two rectifiers that provide a voltage of 600 V to power the CRT, a stabilized voltage of 240 V to power the stages on transistors VT1, VT2, VT4, as well as a voltage of 15 V to power the stage on the transistor VT3, generators and external tested structures connected to the socket X1 (and, of course, to the X16 socket or X17, XXNUMX).

The power supply transformer T1 contains four windings: network I, step-up II, filament III and step-down IV. The voltage of 600 V is removed from the rectifier, made according to the doubling scheme on diodes VD3, VD4 and filter capacitors C16, SP. Half of the voltage of this rectifier is supplied to a parametric stabilizer from resistors R32, R33 and zener diodes VD1, VD2. As a result, a stabilized voltage of 240 V is obtained. Using the VD5 diode bridge and the C19R35C18 filter, a voltage of 15 V is obtained - only in the case of the position of the moving contacts of the SA5 switch shown in the diagram. If these contacts are set to a different position, the alternating voltage from the IV winding will be applied to the ohmmeter. In this option, the signal LED HL1 goes out.

Voltmeter with probe (node ​​A3). The voltmeter is made according to the usual scheme with a dial indicator RA1 and additional resistors of measurement subranges. To simplify the process of calibrating the voltmeter, each additional resistor is made up of two connected in series - a constant and a trimmer. The measured voltage is applied to socket X9 and one of the sockets X6-X8, depending on the desired subrange.

When using a voltmeter as a probe, the probes are included in sockets X9 and X10. The indicator pointer is set to the final division of the scale - conditional reference zero - with a variable resistor R36. Since the resistance range of this resistor is large, the probe is able to work with a significant discharge of the battery G1.

Ohmmeter (node ​​A4). It is made according to the classical bridge circuit, when the tested resistor (or other part with resistance) is included in the shoulder of the diagonal of the bridge (sockets X14, X15), voltage is applied to one diagonal (extreme terminals of the variable resistor R46), and on the other (the engine of the resistor R46 and socket X14 - common wire) - removed. The bridge is balanced with a variable resistor, and the resistance value is measured on its scale. The balance indicator is an oscilloscope, the X4 socket of which is connected to the X12 socket of an ohmmeter. When the bridge is balanced, the image on the screen will turn into a dot.

The ohmmeter range is set by switch SA6, which includes either resistor R44 (range 500 Ohm ... 400 kOhm) or R45 (50 Ohm ... 40 kOhm) in the bridge arm.

AF generator (node ​​A5). One VT5 transistor turned out to be enough to build this generator, which produces sinusoidal oscillations of one fixed frequency. Oscillation generation occurs due to feedback between the collector and the base of the transistor through a chain of resistors R47 - R49 and capacitors C20, C21, C23.

From the generator load resistor R52, sinusoidal oscillations are fed through the capacitor C24 to the variable resistor R51 (output signal amplitude control), and from its engine to the socket X11. A probe is included in this socket, with the help of which a signal is sent to the structure being tested. Of course, the common wire of the generator (say, socket X16) is connected to the same wire of the structure. Power is supplied to the generator by the SA7 switch.

Pulse generator (node ​​A6). It is assembled according to the scheme of a symmetrical multivibrator on transistors VT6, VT7, therefore, pulses with the same duration and pause (the so-called "meander") will be observed at the output of the generator (on resistor R56). From the variable resistor slider, the adjustable output signal is fed to the X13 socket. As in the previous generator, a remote probe is connected to the socket. Power is supplied to the rectangular pulse generator by the SA8 switch.

Details and construction. The network transformer is homemade, made on a magnetic circuit W 18x32. Winding I contains 1670 turns of PEV-1 0,25 wire, II - 1890 turns of PEV-1 0,15, III - 49 turns of PEV-1 0.75. IV - 100 turns of PEV-1 0.35.

Oxide capacitors - K50-31 (C8. C14). K50-32 (C16, C17). K50-12 (C 18. C19). Capacitor C9 - paper for a voltage of at least 500 V. C20-C27 - any for a voltage of at least 15 V, the rest of the capacitors - film, metal film or paper for a voltage of more than 200 V. Variable resistors R13, R46 - type SP-1, respectively, with a power of 2 and 1 W. the remaining variable and tuned resistors are SPO-0.5, fixed resistors are MLT not lower than the power indicated on the diagram.

Instead of MD217, it is permissible to use MD218, KD105G. KD209V and other rectifier diodes with a reverse voltage of at least 800 V, and KD906A will replace any diode bridge designed for a reverse voltage of more than 50 V. Instead of 2S920A, other series-connected zener diodes are suitable, the total stabilization voltage of which is about 240 V at a maximum stabilization current of 30. ..42 mA.

The GT320B transistor can be replaced with another from the GT308, GT313, GT320, GT321 series, the rest - with similar parameters.

Switches - galetnye. sliders or toggle switches. Pointer indicator RA1 - M4248 or another small-sized one with a full deflection current of the arrow 100 μA. Power source G1 - battery or galvanic cell with a voltage of 1,5 V.

The frame of the measuring laboratory with dimensions 240x200x150 mm is made of aluminum corners 15x15 mm. The front panel is hinged and can be rotated 90° (Fig. 4).

On this panel, a CRT with a light-protective frame, an arrow indicator, controls and sockets are reinforced. Part of the parts of the sweep generator is mounted on one board (Fig. 5), the amplifier - on the other (Fig. 6), the generators - on the third (Fig. 7), the power supply - on the fourth (Fig. 8). All boards are cut out of textolite, and metal racks or mounting tabs are riveted on them.

The details of the voltmeter, probe and ohmmeter are placed on a strip of insulating material attached with a metal corner to the front panel from the inside of the case. To install the battery, a simple holder (Fig. 9) is used, made from a plastic cap from an ordinary medicine bottle.

The diameter of the cap should be slightly larger than the diameter of the battery. Two strips 35 ... 40 long and 4 ... 5 mm wide are cut out of thin tin and soldered to them along a segment of a stranded installation wire in insulation. Then a heated strip is pierced through the cap in its lower part. After cooling, the strip is securely fixed in the cap. Next, they put the battery on the strip, pierce the cap over it with the second heated strip, press it against the battery with force and hold it in this position until the strip cools down. The holder is glued to the board.

To place the parts of the device inside a relatively small case, two levels are used - the base and the shelf (Fig. 10). A network transformer, a 3-hour and pulse generator board, as well as a power supply board are placed on the base - they are placed on racks about 15 mm high from the base.

Two wooden planks with a section of 15x15 mm and a length of 140 mm are attached to the bottom of the base - they replace the legs of the case.

The boards of the sweep generator and amplifier are placed on the shelf.

To make it more convenient to use the oscilloscope, a transparent scale with a scale grid is installed in front of the CRT screen. It is made of organic glass with a thickness of 1.5 ... 2 mm according to the internal dimensions of the frame in such a way that it is inserted into the frame with a certain force. With a pointed object, for example, a thick needle, 10 horizontal marks are applied to the scale at an equal distance from each other. To avoid parallax, the same risks are applied on the opposite side. Black paste from a ballpoint pen is rubbed into the risks.

And another home-made device - an ohmmeter scale (Fig. 11), made of thick paper. It is pressed with a variable resistor nut R46 to the front panel. At the time of calibrating the ohmmeter, the same "draft" scale is set, the resistance values ​​\uXNUMXb\uXNUMXbof "reference" resistors are applied to it, and then they are transferred to the main scale.

The connections between the boards and parts are made with a stranded installation wire in isolation. Since it is difficult to purchase a socket for a CRT, 11 contacts are made of copper foil instead. A thin mounting wire of the appropriate length is soldered to each contact. While the contact is heated, a PVC tube about 25 mm long is pulled over it. The contact must be put on the pin with force.

Before proceeding with the adjustment, you should carefully check the installation and the strength of all connections. Then, without including the device in the network, the voltmeter measurement limits are set with trimmers R41 - R43, supplying the corresponding limit voltage to its input sockets and controlling it with a "exemplary" voltmeter. At the limit of "1000 V", it is enough to apply, say, 200 V and, with the resistor R41, set the indicator needle to the corresponding division of the scale. After closing the sockets X9 and X10. set with a variable resistor R36 the indicator arrow to the final division of the scale.

Now with a probe you can check the high-voltage and low-voltage power circuits - if there are any short circuits in them. Only after that it is possible to turn on the laboratory in the network and measure the voltage between the upper terminal of the capacitor C16 according to the diagram and the common wire. Moreover, special care and safety requirements must be observed, since the voltage reaches several hundred volts! They also check the voltage between the anode of the zener diode VD1 and the common wire, and between the positive terminal of the capacitor C18 and the common wire. If the voltages correspond to those indicated in the diagram, they begin to check and adjust the oscilloscope.

Switch SA1 is switched to the "Amplifier" position, SA3 to the "Expanded" position, the resistor R13 slider is set approximately to the middle position, and the resistor R20 is set to the lower position according to the scheme. When you turn the sliders of the resistors R9 "Brightness" and R8 "Focus", a scan line should appear on the CRT screen. Check the action of the "Offset X" (R5) and "Offset Y" (R1) regulators - when you turn their sliders, the line should move left-right and up-down. The sweep line should be preserved when the SA1 switch is set to the "Plate" position.

It may happen that instead of a line on the screen there will be a dot. Then re-check the installation of the sweep generator. If no problems are found, check the cascade on the transistor VT1. To do this, the output of the capacitor C7, left according to the scheme, is disconnected from the generator and a conductor connected to the X5 socket is connected instead, and the SA3 switch is switched to the "In. X" position. Of course, for the period of all soldering and connections, the device is turned off from the network.

By moving the engine of the resistor R13 from one extreme position to another, they try to get a scan line on the screen. If, at any position of the resistor slider and switch SA2, a dot remains on the screen or a sweep line (it should be 5 ... 10 mm long) appears only at the extreme right position of the slider according to the diagram, replace transistor VT1.

When the cascade starts to work, restore the connection of the capacitor C7 and set the switch SA3 to the "Developed" position. In the absence of a scan line, the installation and serviceability of the parts of the cascade on the transistor VT2 are checked.

Checking the vertical deflection amplifier is easy with a 3H generator (it usually starts working right away). Socket X2 is connected with a short conductor to socket X11, power is supplied to the generator with the SA7 switch, the resistor R51 slider is moved to the upper position according to the diagram, the SA1 switch is moved to the "Amplifier" position, the gain is set with the resistor R20 so that the image of the "picture" of chaotically moving lines took up the entire screen. Then the regulators "Frequency smoothly" and "Synchronization" achieve a fixed image of several sinusoidal oscillations at both positions of the switch SA2.

In the low-frequency range of the generator (the movable contact of the SA2 switch is in the right position according to the diagram), more compressed sinusoids can be observed in the left side of the image compared to the right side - the result of a non-linear sweep. Of course, you can slightly reduce the nonlinearity by more accurate selection of resistors R14. R16 - R18, but in most cases this is not necessary.

The action of the regulator "Strength U2" is checked as follows. Connect the X4 and XI2 sockets with a short conductor, switch the SA3 switch to the "In X" position, and the SA5 switch to the "Ohm" position. A vertical line should appear on the screen, the length of which can be changed by variable resistors R25 and R46. The adjustment and verification of the oscilloscope ends here.

Now, using an oscilloscope, you can check the waveform of the 3H generator by connecting sockets X4 and X11. A more correct shape of the sinusoid can be obtained by selecting the resistor R50.

Similarly, the shape of the rectangular oscillations of the pulse generator is checked by connecting sockets X4 and X13. If you wish, the symmetry of the "meander" can be refined by selecting resistors R53 - R55.

The final stage in establishing a laboratory is the calibration of an ohmmeter. Connect the X4 and XI2 sockets with a conductor. switch SA1 is set to "Amplifier", SA3 - "In. X". SA5 - "Ohm", SA6 - to the bottom according to the diagram. A "draft" scale is attached to the front panel, a "beak" handle with a thin risk is put on the protruding shaft of the resistor. Plugs are inserted into sockets X14, X15, connected by mounting wires with crocodile clips. Resistors are selected with an exact or possibly close resistance of 50,100,200, etc. up to 40000 ohms. By connecting the "crocodiles" in turn to each resistor, they achieve the balance of the bridge with the resistor R46 - along the shortest length of the vertical line on the CRT screen. On the scale against the risks of the "beak" note the value of resistance. Similarly, the ohmmeter is calibrated on the second subrange (SA6 - in the upper position according to the diagram), stocking up with resistors of the corresponding resistances, after which the graduation is transferred to the "finish" scale.

And the last. When the oscilloscope is operating, the CRT heats up. So that its heat does not affect the mode of transistors of nearby nodes, it is advisable to put a cylinder made of cardboard on the tube.

Author: A. Piltakyan, Moscow

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