Simple probes, attachments, meters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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For many years, the Radio magazine has published descriptions of the simplest designs for beginner radio amateurs, which, either by themselves or together with well-known avometers, made it possible to check radio components, measure transistor parameters if necessary, "ring out" the installation for correct circuit connections, or simply expand the possibilities of using an avometer. Some of these devices are described in the proposed article.

Probe for "dialing" installation

Before proceeding with the adjustment of the assembled structure, you need to "ring out" its installation, that is, check the correctness of all connections in accordance with the circuit diagram. For these purposes, radio amateurs often use an ohmmeter or avometer. operating in resistance measurement mode.

Often, such a device can replace a compact probe, whose task is to signal the integrity of a particular circuit. Probes are especially convenient for "ringing" multi-wire bundles and cables. One of the possible probe circuits is shown in Fig. 1. It has three low power transistors, two resistors, an LED and a power supply.

In the initial state, all transistors are closed, since there is no bias voltage on their bases relative to the emitters. If you connect the conclusions "To the electrode" and "To the clamp", a current will flow in the base circuit of the transistor VT1. Its value depends on the resistance of the resistor R1. The transistor will open and a voltage drop will appear on its collector load - resistor R2. As a result, transistors VT2 and VT3 will open and current will flow through the HL1 LED. The LED will flash, which will serve as a signal that the circuit under test is working.

The probe is made somewhat unusually: all its parts are mounted in a small plastic case (Fig. 2), which is attached to a watch strap (or bracelet). From the bottom to the strap (opposite the case) a metal plate-electrode is attached, connected to the resistor R1. When the strap is fastened on the arm, the electrode is pressed against it. In this case, the fingers of the hand act as a probe probe. When using a bracelet, no additional electrode plate is needed - the output of the resistor R1 is connected to the bracelet.

The probe clamp is connected, for example, to one of the ends of the conductor, which must be found in the bundle or "ringed" in the installation. Touching the ends of the conductors on the other side of the bundle with your fingers in turn, the desired conductor is found by the appearance of the glow of the LED. In this case, not only the resistance of the conductor, but also the resistance of the hand turns out to be included between the probe and the clamp. The current passing through this circuit is enough for the probe to “trigger” and the LED to flash.

Transistor VT1 is used by any of the KT315 series with a static coefficient (for short, simply a coefficient) of current transfer of at least 50; VT2 and VT3 - also any low-power low-frequency ones, of the appropriate structure and with a current transfer coefficient of at least 60 (VT2) and 20 (VT3).

The AL102A LED is economical (consumes a current of about 5 mA). has a low luminosity. If it is insufficient for our purposes, install the AL 1025 LED. The power source is two D-0.06 or D-0.07 batteries connected in series. There is no power switch on the probe. since in the initial state (with the base circuit of the first transistor open), the transistors are closed and the current consumption is negligible - it is commensurate with the self-discharge current of the power source.

The probe can be assembled on transistors of the same structure, for example, according to the one shown in Fig. 3 scheme. True, it contains a few more details than the previous design, but its input circuit is protected from external electromagnetic fields, sometimes leading to false flashing of the LED.

Silicon transistors of the KT315 series with a current transfer coefficient of at least 25 work in this probe. Capacitor C1 eliminates false indication from external interference.

As in the previous case, in the initial mode, the device practically does not consume energy, since the resistance of the HL1R4VT3 circuit connected in parallel to the power source in the closed state of the transistor is 0,5 ... 1 MΩ. The current consumption in the indication mode does not exceed 6 mA. The brightness of the LED can be changed by selecting the resistor R3.

Probes with sound indication can cause no less interest. The scheme of one of them, attached to the arm with a bracelet, is shown in fig. 4.

It consists of a sensitive electronic key transistors VT1. VT4 and an audio frequency generator (34) assembled on transistors VT2, VT3 v in a miniature phone BF1. The oscillation frequency of the generator is equal to the frequency of the mechanical resonance of the phone. Capacitor C1 reduces the effect of AC interference on the operation of the indicator. Resistor R2 limits the collector current of transistor VT1. and hence the current of the emitter junction of the transistor VT4. Resistor R4 sets the highest volume of the phone sound, resistor R5 affects the stability of the generator when the supply voltage changes.

The BF1 sound emitter can be any miniature telephone (for example, TM-2) with a resistance of 16 to 150 ohms. The power source is a D-0,06 battery or an RTS53 element. Transistors - any other silicon, p-np (VT1) and npn (VT2-VT4) structures. with the highest possible current transfer coefficient and reverse collector current of not more than 1 μA.

The parts of the probe are mounted on an insulating bar or board made of one-sided foil fiberglass. The bar (or board) is placed, for example, in a metal case in the form of a watch, to which a metal bracelet is connected. Opposite the radiator, a hole is cut out in the housing cover, a miniature socket of the X2 connector is fixed on the side wall. into which an extension conductor is inserted with an X1 probe (it can be an alligator clip) at the end.

A slightly different probe circuit is shown in Fig. 5. It uses both silicon and germanium transistors.

Capacitor C2 shunts the electronic key in alternating current, and capacitor C3 is the power supply.

It is desirable to select a transistor VT1 with a current transfer coefficient of at least 120, VT2 - at least 50. VT3 and VT4 - at least 20 (and a reverse collector current but more than 10 μA). Sound emitter BF1 - capsule DEM-4 (or similar) with a resistance of 60 ... 130 Ohm

Probes with audible indication consume slightly more current than the previous ones, so it is advisable to turn off the power source during long interruptions in operation.

RC Meter

As you probably guessed, the story will be about a device that measures the resistance of resistors and the capacitance of capacitors. It is based (Fig. 6) on a bridge measuring circuit, known from the school physics course and widely used in engineering for accurate measurements of various parameters.

The left side of the circuit is an alternating voltage generator, the right side is a measuring bridge. The device is designed to measure the resistance of resistors from 10 Ohm to 10 MΩ and the capacitance of capacitors from 10 pF to 10 μF.

The alternating voltage generator is assembled on one MP39 transistor (any of the MP39-MP42 series or another low-frequency transistor will do). The primary winding of the transformer T1 is included in the collector circuit of the transistor, its secondary winding is connected to the base of the transistor. The bias voltage is applied to the base from the divider R1R2. The emitter circuit includes a feedback resistor R3. stabilizing the operation of the generator when the ambient temperature changes and the supply voltage decreases. Generation (excitation) occurs due to positive feedback between the collector and base circuits. An alternating voltage is taken from the collector of the transistor and fed to the bridge through the capacitor C1.

Switch SA2 to the measuring bridge connect the reference resistors and capacitors. Balance the bridge with a variable resistor R7. You will connect the tested parts to the "C, Rx" terminals, and to the "Tf" sockets include headphones with high resistance (TON-1, TON-2 and others, with a resistance of at least 2 kOhm).

Take fixed resistors MLT, BC, and R4-R6 with a tolerance of at least 5%. Capacitors C1-C3 can be paper (types MBM, BMT, KBGI and others), and C4 mica, the capacitance of capacitors C2 - C4 must also be with a tolerance of 5% Transformer T1 must have a ratio of turns of the collector and base windings of approximately 3: 1. Any matching transformer from industrial transistor receivers is suitable here. In extreme cases, wind the transformer yourself on a magnetic core made of permalloy W-shaped plates with a cross section of at least 30 mm2 (for example, Sh5 iron, set thickness 6 mm). Winding I must contain 2400 turns of PEV or PEL wire with a diameter of 0.06 ... 0.08 mm. winding II - 700 ... 800 turns of the same wire.

Assemble the device in a wooden or metal case (Fig. 7). Mount the switch SA1 on the front wall. switch SA2, variable resistor R7, clamps and sockets for connecting the tested parts and headphones.

Against each fixed position of the switch, write the nominal value of the reference part, as shown in the figure. Draw a circle around the handle of the variable resistor and apply two risks for the time being, corresponding to the extreme positions of the handle.

After checking the installation, turn on the device and listen to the headphones. If there is no sound, swap the leads of one of the windings of the generator transformer.

Then start grading the scale. Since the scale is general, it can be graduated on any measurement range. But for this range, pick up a few parts with known denominations. For example, you have selected the range "x10k" and put the switch SA2 in this position. Stock up on resistors from 1 to 100 kOhm First, connect a 1 kOhm resistor to the terminals and turn the knob of the variable resistor until the sound disappears in the phones. The bridge is balanced, and on the scale in this place you can put the risk with the inscription "0.1" (1 kOhm: 10 kOhm = 0,1). By connecting resistors with a resistance of 2, 3, 4 ... 10 kOhm to the terminals in turn, put risks from 0.2 to 1 on the scale. Risks from 2 to 10 are also applied. Only resistors in this case should be 20. 30 kOhm, etc. d.

Check the operation of the device on other ranges. If the measurement results differ from the true value of the part rating, select more precisely the resistance of the corresponding reference resistor or capacitor capacitance.

When using the device, follow the following sequence. Connect the measured resistor to the terminals and first set the switch to the "x1 M" position. Try to balance the bridge by turning the variable resistor knob. If this fails, set the switch sequentially to the following positions. In one of them, the bridge will be balanced. Calculate the resistance of the measured resistor by multiplying the readings of the scales of the switch and the variable resistor. For example, the switch is in the "x10 k" position, and the variable resistor knob is against the "0.8" risk. Then the measured resistance will be 10 kOhm x 0.8 = 8 kOhm. Similarly, measure the capacitance of the capacitor.

If, when working with the device, the sound volume is not enough, you can plug in the X3 socket instead of phones a constant resistor with a resistance of 2 ... . The amplifier must be powered from a separate source.

How to test a transistor...

To check the performance of transistors, you can use the radio broadcasting network by assembling a prefix for this, the diagram of which is shown in Fig. 8. The tested transistor VT and the parts shown in the diagram form an amplifier, the input of which is supplied with a voltage of the AF signal of the radio broadcasting network, greatly weakened by the divider R1R2. If the mains voltage is 30 V., the resistor R2 will be only 0,08 V, and even less at the base of the transistor. With a good transistor in BF1 phones, a loud sound will be heard. According to him, however, roughly, they judge the amplifying properties of the transistor. When checking transistors of the npn structure, you need to swap the connection of the terminals of the battery GB1 and capacitor C1.

As a BF1 sound indicator, it is better to use a DEMSh, DEM-4M telephone capsule or a small-sized dynamic head (for example, 0.1GD-3 or 0.1GD-6), but it should be turned on through an output transformer from a small-sized receiver. Its primary winding (with a large number of turns) is included in the collector circuit, and the head is connected to the secondary.

All resistors - MLT-0,25, capacitor C1 - K50-6, power source - battery 3336.

In another probe (Fig. 9), the transistor under test operates in the generation mode and a sound of a certain tone is heard in BF1 headphones. If the transistor is faulty, there will be no sound.

High-resistance telephones (TON-1, TON-2), resistors - MLT-0,25, capacitors C1, C2 - BM. MBM. C3 - K50-6, connector X2 - two-socket block. Terminals X2-X4 for connecting a transistor - any design, battery - 3336. As in the previous case, if necessary, check the transistors of the npn structure, you should swap the connection of the terminals of the battery and the oxide capacitor.

To test the transistors of both structures (pn-p and npn), a device is suitable, the circuit of which is shown in fig. 10. If both transistors are working, the device turns into an asymmetric multivibrator, the operation of which is controlled by the sound in the headphones. If the transistor is faulty, there will be no sound. Thus, to check transistors using this device, you need to have one serviceable transistor of each structure, which are used as exemplary ones.

As telephones, capsules DEM-4M, DEMSH are used. microtelephone TM-2. Power supply G1 - one of the elements 316,332,343 or 373. There is no power switch in the device - when the transistors are not connected, there will be no current consumption from the source.

The procedure for working with the device is as follows. When checking a transistor, for example, a pnp structure, it is connected to the corresponding terminals of the device, and a known-good transistor of a different structure, npn, is connected to other terminals. After that, a phone plug is inserted into the two-socket block and the operation of the multivibrator is controlled.

You can also check low-power transistors of any structure using a probe (Fig. 11), in which the transistor being tested is paired with an exemplary one (previously tested and specially selected for the probe), but of a different structure. If, say, a pnp structure transistor is checked, its leads are inserted into the sockets of connector X1, and the leads of an exemplary npn structure transistor are inserted into the sockets of socket X2. Then you get a generator that generates audio frequency oscillations - they are heard in the BF1 headphone. The sound will be only if the tested transistor is in good condition. The moment of occurrence of generation depends on the position of the slider of the variable resistor R3 "Generation".

In addition to two serviceable exemplary transistors of different structures, for the probe you will need a TM-2A miniature telephone, a G1 power supply - elements 316, 332, 343, 373, a variable resistor of any type and MLT fixed resistors with power up to 0,5 W. Connectors can be transistor sockets, sockets or clips.

The transmission coefficient of the tested transistor is easy to determine by the position of the variable resistor slider - the larger the range of its movement the sound is stored in the phone, the greater the transmission coefficient the transistor has.

... and measure its parameters

Like other radio components, transistors have their own parameters that determine their use in certain devices. But before putting the transistor into the design, it needs to be checked. To check all the parameters of the transistor, a complex measuring device is required. It is almost impossible to make such a device in amateur conditions. Yes, it is not needed: after all, for most designs it is enough to know only the static current transfer coefficient of the base, and even less often - the reverse collector current. Therefore, it is better to do with the simplest instruments that measure these parameters.

How can you judge the static current transfer coefficient of the base? Look at fig. 12. The transistor is connected to the power supply G1, and a current flows in its base circuit, which depends on the resistance of the resistor R1. The transistor amplifies this current. The value of the amplified current is shown by the arrow of a milliammeter connected to the collector circuit. It is enough to divide the value of the collector current by the value of the current in the base circuit and you will find out the static current transfer coefficient.

There are two slightly different current transfer coefficients - h21, h21e.

The first is called the dynamic current transfer ratio and shows the ratio of the collector current increment to the base current increment that caused it. It is difficult to measure this coefficient in amateur conditions, therefore, in practice, the second coefficient is often determined. This is a static current transfer ratio indicating the ratio of collector current to a given base current. At low collector currents, both coefficients are close.

And more about the current transfer coefficient. It largely depends on the collector current. In some measuring instruments, the circuits of which were published in the popular radio technical literature of past years, the current transfer coefficient of low-power transistors was measured at a collector current of 20 and even 30 mA. This is wrong. At such a current, the transistor gain drops and the device shows an underestimated value of the current transfer coefficient. That is why it is sometimes heard that the same transistors, when tested on different devices, show transfer coefficients that differ by two or even three times. The readings of any meter will be close only if the maximum collector current during measurements does not exceed 5 mA. Such a limit is adopted in the simple constructions described below. In more complex meters for the transistor, the collector current is set at which the transistor will operate in the structure - it will determine the real value of the transfer coefficient.

On fig. 13 shows the simplest diagram of a practical device for testing transistors of the pn-p structure. The device works like this. To the terminals (or sockets) "E", "B", "k" connect the outputs of the transistor (emitter, base, collector, respectively). When the SB1 button is pressed, the supply voltage from the GB1 battery is applied to the transistor outputs. In this case, a small current begins to flow in the base circuit of the transistor. Its value is determined mainly by the resistance of the resistor R1 (since the resistance of the emitter junction of the transistor is small compared to the resistance of the resistor) and in this case is chosen to be 0,03 mA (30 microamps)

The current amplified by the transistor registers the PA1 milliammeter in the collector circuit. The milliammeter scale can be calibrated directly in h21E values. If the device uses a milliammeter designed to measure current up to 3 mA (there is such a limit in the Ts20 avometer), then the deviation of the arrow to the final division of the scale will correspond to a current transfer coefficient of 100. For milliammeters with other currents of the deviation of the line to the final division of the scale, this value will be different. So, for a milliammeter with a scale of 5 mA, the limit value of the current transfer coefficient at the above base current will be about 166.

Parts of the device do not have to be placed in a case. They can be quickly connected to each other and test the batch of transistors you have. Resistor R2 is designed to limit the current through a milliammeter if a transistor with a broken emitter-collector junction accidentally comes across.

But what if you need to check transistors of a different structure - p-pn? Then you have to swap the leads of the battery and the milliammeter.

Another attachment to the avometer is a transistor tester (Fig. 14), which allows you to measure two parameters of low-power bipolar transistors: h21e - static base current transfer coefficient, 1KBO - collector reverse current. The tested transistor VT is connected with the leads to the corresponding terminals "E", "B" and "K". Depending on the structure of the tested transistor, switch SA2 is set to the "pnp" or "npn" position. This changes the polarity of the power supply connection, as well as the outputs of the PA1 indicator.

As in the previous attachment, the Ts20 avometer is used as an indicator. When measuring the h21E coefficient (switch SA1 in the right position according to the diagram), resistor R1.3 is connected in parallel to the indicator through section SA2, as a result of which the indicator needle deviates to the final division of the scale already at a current of 3 mA. In the same position of the switch, through the SA1.2 section, a resistor R1 is connected to the output of the base of the transistor under test, providing a base current of 10 μA. In this case, the indicator scale will correspond to the coefficient h21E=300 (3 mA: 0.01 mA=300).

In the left position of the switch SA1 according to the diagram, the base of the tested transistor VT is connected to the power source, and the shunt resistor R2 is disconnected from the indicator. This position corresponds to measuring the collector reverse current, and the indicator scale corresponds to a current of 300 μA.

All measurements are carried out by pressing the push-button switch SB1.

Resistor R1 type MLT-0,25, trimmer resistor R2 of any type. Switches - sliding, push-button switch - self-returning (bell button is applicable).

The clamps for connecting the transistor are any, it is only important that they provide reliable contact with the terminals of the transistor. Self-made clamps have proven themselves well (they can be used in other meters and probes), shown in Fig. 15. The clip consists of two bent strips of springy brass or bronze. Holes for the transistor output are drilled in the outer 1 and inner 2 strips. The inner strip is necessary to increase the reliability of the device and the spring properties of the clamp. The strips are fastened to each other and attached to the set-top box with screws 3. To fasten the transistor output, press the upper part of the strips until the holes align, insert the transistor output into the holes and release the strips. The output of the transistor will be firmly pressed against the strips at three points.

A possible design of this attachment is shown in Fig. 16. The top panel is made of insulating material (getinaks, textolite), the bottom (the GB1 battery is fixed on it) and the side walls are made of aluminum or other sheet metal.

Establishing the attachment comes down to setting the resistor R2 to a given measurement limit equal to 3 mA. To do this, set the SA1 switch to the "h21E" position and, without connecting the transistor, connect a constant resistor with a resistance of 1,5 kOhm between the "E" and "K" terminals (choose exactly). Turning on the power with a push-button switch, resistor R2 sets the arrow of the indicator RA1 to the final division of the scale.

To test transistors with rigid short leads (for example, the KT315 series), you need to cut a small bar from the foil material and cut several grooves in the foil to make three tracks. The width of the tracks and the distance between them must match the size of the transistor pins. Segments of a stranded mounting wire are soldered to the tracks, which, when checking the transistor, are connected to the corresponding terminals of the device. The transistor leads are applied to the tracks and the SB1 button of the device is pressed.

Before mounting transistors of medium and high power, it is also necessary to know their static current transfer coefficient, and sometimes the reverse collector current. Of course, it would be possible to introduce an additional switch into the previous set-top boxes and test high-power transistors on them. But such a check is not often required, and additional switching would complicate the design of set-top boxes. Therefore, it is easier to make another attachment to the avometer - only to test high-power transistors. The scheme of such a prefix is ​​shown in Fig. 17.

As in the previous set-top boxes, the tested transistor VT is connected to the terminals "E", "B" and "K", and the necessary polarity of the power source and the inclusion of the indicator RA1 for transistors of different structures is set by the switch SA1. The h21E coefficient is measured at a fixed base current of 1 mA. This current depends on the resistance of the resistor R1. The indicator scale (the avometer is switched on for measuring direct current up to 300 mA) turns out to be calculated for the coefficient h21E=300.

After connecting the transistor and setting the switch to the desired position, press the SB 1 button and determine the h21E parameter on the avometer scale. However, it should be taken into account that the measurement duration should be as short as possible, especially for transistors with a large (over 100) h21E value. If necessary, measure the reverse current of the collector, disconnect the emitter output from the attachment and press the button.

Switch - sliding, button and clips - any.

The attachments described here can become the basis for an independent design of a measuring device using a microammeter in it with a full deflection current from 100 to 300 μA. In each case, depending on the indicator, you will have to select the appropriate resistors. It is also easy to combine all attachments into a single independent measuring device.

High resistance DC voltmeter

The Ts20 avometer, as you know, is designed to measure direct voltage. However, it is not always possible to use it as a voltmeter. This, in particular, concerns voltage measurements in high-resistance circuits of radio devices. After all, the relative input resistance of its DC voltmeter is small - about 20 kOhm / V, and when measuring voltage, a significant part of the current of the measured circuit flows through the device. This leads to shunting of the measuring circuit and the appearance of an error (sometimes significant) in the measurements. Therefore, one of the first tasks to improve the combined measuring device Ts20 is to increase its relative input resistance when measuring voltages.

A diagram of a relatively simple prefix that allows solving this problem is shown in Fig. 18. The prefix is ​​​​a DC measuring bridge, in one diagonal of which the G1 power supply is connected, and the PA1 indicator (Ts20 avometer, included in the DC measurement limit of 0,3 mA) is connected to the other diagonal. The shoulders of the bridge form the emitter-collector sections of transistors VT1 and VT2, resistor R10 with the upper (according to the scheme) part of the variable resistor R11 from the engine, and resistor R12 with the lower part of resistor R11. The bridge is balanced with a variable resistor R11 ("Set 0"); trimmer resistor R8 change the bias voltage at the bases of the transistors and thereby equalize the resistance of the emitter-collector sections.

The measured voltage is applied to the transistor bases through one of the additional resistors R1-R5. In this case, a voltage drop forms on the resistors R6-R9, and the base of the transistor VT2 is under a more negative voltage (relative to the emitter) than the base of the transistor VT1. The bridge is out of balance, and the indicator needle deviates. The angle of its deviation will be the greater, the greater the measured voltage on the selected subrange. Moreover, the current through the indicator will be tens of times greater (this depends on the static current transfer coefficient of the transistors) than through the input circuit of the set-top box.

The relative input resistance of a voltmeter with such an attachment can be about 300 kOhm / V, but it is obviously reduced to 100 kOhm / V by introducing a tuned resistor R6. This is done in order to simplify the selection of transistors and, in addition, use additional resistors R1-R5 of standard ratings (and not select them). Fixed resistors - with a dissipation power of at least 0,25 W, and it is desirable to use additional resistors R1-R5 with a tolerance of ± 5%. Trimmer resistors R6, R8 and variable resistor R11 - SPO-0,5, SP-1.

It is desirable to select transistors with the same static current transfer coefficient equal to 50 ... 80.

Power supply G1 - elements 332, 343 or 373 with a voltage of 1,5 V. Input sockets XI-X6, as well as clamps X7, X8 - any.

Attachment parts can be placed in any suitable ready-made or home-made case (Fig. 19). On the top panel of the case there are sockets, clamps, a power switch and a variable bridge balancing resistor.

Before setting up the set-top box, the sliders of resistors R8 and R11 should be set to the middle position according to the scheme, and the resistor R6 to the top position (this is necessary so that the outputs of the bases of the transistors are short-circuited). The terminals are connected to the probes of an avometer, switched on for a DC measurement limit of up to 0,3 mA. Then turn on the power of the set-top box and with the resistor R11 set the arrow of the avometer to zero, i.e. balance the bridge. The engine of the resistor R6 is set to the lower position according to the diagram and the bridge is additionally balanced with a tuning resistor R8. If at the same time it turns out that the engine of the resistor R8 is installed close to one of the extreme positions, you will have to select the resistor R7 or R8. If, for example, the engine of the tuned resistor is close to the top position in the circuit, the resistor R7 should be of less resistance or the resistor R9 of a larger one. Such an adjustment only indicates that the transistors used differ in the static current transfer coefficient.

The next stage of adjustment is setting the desired relative input impedance of the set-top box. To do this, between sockets X6 and X2, a 1,5 V source (for example, element 343) should be turned on and the trimmer resistor R6 set the arrow of the indicator PA1 to the final division of the scale. By applying appropriate voltages to the other input sockets, they check the correctness of the indicator readings at other measurement limits. If discrepancies are found, an additional resistor of the corresponding measurement limit is selected.

Author: B.S. Ivanov

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