Asymmetric multivibrator on imported transistors circuit. Symmetrical multivibrator

Schematic diagram of a powerful transistor multivibrator with control, built on transistors KT972, KT973. Many radio amateurs began their creative journey by assembling simple direct-amplification radios, simple audio power amplifiers, and assembling simple multivibrators consisting of a pair of transistors, two or four resistors, and two capacitors.

The traditional symmetrical multivibrator has a number of disadvantages, including a relatively high output impedance, long pulse edges, limited supply voltage, and low efficiency when operating on a low-resistance load.

circuit diagram

On fig. 1. shows a diagram of a controlled symmetrical two-phase multivibrator operating at audio frequencies, the load to which is connected via a bridge circuit the load would be included in one of the arms of the multivibrator.

In addition, a "real" AC voltage is supplied to the load, which significantly improves the working conditions of the dynamic head connected as a load - there is no effect of indentation or protrusion of the diffuser (depending on the polarity of the speaker). There are also no clicks when turning the multivibrator on or off.

Rice. 1. Schematic diagram of a powerful multivibrator based on transistors KT972, KT973.

A symmetrical two-phase multivibrator consists of two push-pull arms, the voltage on which alternately changes from low to high. Assume that when the power is turned on, the composite transistor VT2 opens first.

Then the voltage at the terminals of the collectors of transistors VT1, VT2 will become close to zero (VT1 is open, VT2 is closed). A composite p-n-r transistor VT5 is connected to the connection point of their collectors through a current-limiting resistor R12, which will open. A voltage of about 8 V will be applied to the load at a multivibrator supply voltage of 9 V. With the recharge of capacitors C2, C4, the multivibrator will switch - VT1, VT6 will open, VT2, VT5 will close.

The same voltage will be applied to the load, but in reverse polarity. The switching frequency of the multivibrator depends on the capacitance of the capacitors C2, C4, and, to a lesser extent, on the set resistance of the tuning resistor R7. With a supply voltage of 9 V, the frequency can be tuned from 1.4 to 1.5 kHz.

When the resistance R7 decreases below the conditional value, the generation of sound frequencies breaks down. It should be noted that after starting the multivibrator can work without resistors R5, R11. The voltage shape at the output of the multivibrator is close to rectangular.

Resistors R6, R8 and diodes VD1, VD2 protect the emitter junctions of transistors VT2, VT6 from breakdown, which is especially important when the multivibrator supply voltage is more than 10V. Resistors R1, R13 are necessary for stable generation; in their absence, the multivibrator may “wheeze”. The VD3 diode protects powerful transistors from supply voltage reversal. If it is absent and if the power supply is sufficient, when the voltage is reversed, the built-in protective diodes of the transistors may be damaged.

To expand the functionality of this multivibrator, it has the ability to turn on / off when a voltage of positive polarity is applied to the control input. If the control input is not connected anywhere or the voltage on it is not more than 0.5 V, the VТЗ, VT4 transistors are closed, the multivibrator is working.

When a high level voltage is applied to the control input, for example, from the TTLSH output. CMOS microcircuits, a sensor of electrical or non-electrical quantities, for example, a humidity sensor, transistors VTZ, VT4 open, the multivibrator slows down. In this state, the multivibrator consumes less than 200 µA of current, excluding the current through R2, R3, R9.

Details and installation

The multivibrator can be mounted on a printed circuit board with dimensions of 70 * 50 mm, a sketch of which is shown in fig. 2 Fixed resistors can be used any small. Trimmer resistor RP1-63M, SP4-1 or similar imported. Oxide capacitors K50-29, K50-35 or analogues Capacitors C2, C4 - K73-9, K73-17, K73-24 or any small film capacitors.

Rice. 2. Printed circuit board for a powerful multivibrator circuit on transistors.

Diodes KD522A can be replaced by KD503. KD521. D223 with any letter index or imported 1N914, 1N4148. Instead of diodes KD226A and KD243A, any of the series KD226, KD257, KD258, 1 N5401 ... 1 N5407 is suitable.

Composite transistors KT972A can be replaced by any of this series or from the KT8131 series, and instead of KT973 by any of the KT973, KT8130 series. If necessary, powerful transistors are installed on small heat sinks. In the absence of such transistors, they can be replaced by analogues of two transistors connected according to the Darlington circuit, Fig. 3. Instead of low-power p-p-p transistors KT315G, any of the KT312, KT315, KT342, KT3102, KT645, SS9014 and similar series are suitable.

Rice. 3. Schematic diagram of the equivalent replacement of transistors KT972, KT973.

The load of this multivibrator can be a dynamic head, a telephone capsule, a piezoceramic sound emitter, a pulse step-up / step-down transformer.

When using a driver with an 8 ohm winding impedance, be aware that with a supply voltage of 9 V, 8 watts of AC power will be supplied to the load. Therefore, a two ... four-watt dynamic head can be damaged after 1 ... 2 minutes of operation.

Establishment

The operating frequency of the multivibrator is significantly affected by the load capacitance and supply voltage. For example, when the supply voltage changes from 5 to 15 V, the frequency changes from 2850 to 1200 Hz when working on a multivibrator for a load in the form of a telephone capsule with a winding resistance of 56 ohms. In the region of low supply voltages, the change in the operating frequency is more significant

By selecting the resistances of resistors R5, R11, R6, R8, you can set the shape of the pulses to be almost strictly rectangular when the multivibrator operates with a specific connected load at a given supply voltage.

This multivibrator can be used in various signaling devices, sound warning devices, when, with a small available voltage of the power source, it is required to obtain significant power at the sound emitter. In addition, it is convenient to use it in low-voltage to high-voltage converters, including those operating at a low frequency of the lighting network of 50 Hz.

Butov A. L. RK-2010-04.

FET multivibrator

Novice radio amateurs, of course, know that multivibrators (balanced and unbalanced) perform on bipolar transistors. Unfortunately, such multivibrators have a drawback - when working with a sufficiently powerful load, for example, incandescent lamps, large base currents are required to fully open the transistors.

If the arms of the multivibrator switch with a frequency of 3 ... 0.2 Hz, it is necessary to install high-capacity oxide capacitors in the frequency-setting circuits, and hence large dimensions. We should not forget about the relatively high saturation voltage of open transistors.

The proposed multivibrator (see figure) uses domestic field-effect n-channel transistors with an insulated gate and an induced channel. Inside the case, between the gate and source terminals, there is a protective zener diode, which significantly reduces the likelihood of a transistor failure if it is handled ineptly.

The switching frequency of the multivibrator transistors is about 2 Hz, it is set by capacitors and resistors. Multivibrator transistor load - incandescent lamps EL1, EL2.

Resistors connected between the drain and the gate of the transistors provide a soft start of the multivibrator. Unfortunately, they "tighten" the switching off of the transistors a little.

Instead of incandescent lamps, it is permissible to include LEDs with 360 Ohm limiting resistors or a telephone capsule, for example, TK-47, into the transistor drain circuit (for this option, the multivibrator must operate in the audio frequency region). In the case of using only one capsule, it is necessary to include a resistor with a resistance of 100 ... 200 Ohm as a load in the drain circuit of another transistor.

Resistors R1, R2 of the ratings indicated in the diagram can be made up of several smaller resistances connected in series. If this option is not available, install smaller resistors and larger capacitors.

Capacitors can be non-polar ceramic or film, for example, series KM-5, KM-6, K73-17. Incandescent lamps are used from a Chinese-made "flashing" Christmas tree garland for a voltage of 6 V and a current of 100 mA. Small-sized lamps for a voltage of 6 V and a current of 60 or 20 mA are also suitable.

Instead of transistors of the specified series, withstanding direct current up to 180 mA, it is permissible to use keys of the KR1064KT1, KR1014KT1 series designed for higher current. If you use a multivibrator with a more powerful load, say, car incandescent lamps, you will need other transistors, for example KP744G, which allow a drain current of up to 9 A. But with this option, you need to install protective zener diodes for a voltage of 8 ... 10 V between the gate and the source ( cathode to the gate) - KS191Zh or similar. At high load currents, transistors will have to be installed on heat sinks.

The multivibrator is adjusted by selecting capacitors until the desired switching frequency of the transistors is obtained. To operate the device at audio frequencies, the capacitors must be 300 ... 600 pF. If you leave the capacitors of the capacitance indicated on the diagram, you will have to select resistors of lower resistance - up to 47 kOhm.

The multivibrator is operational at a supply voltage of 3 ... 10 V, of course, with an appropriate load. If it is supposed to be used as some node in the design being developed, a blocking capacitor with a capacity of 0.1 ... 100 μF is installed between the power wires of the multivibrator.

Symmetrical and asymmetric multivibrators for various purposes can be built not only on bipolar transistors, but also on field ones. One example of this can be found in . Considering that field-effect transistors have a number of advantages over bipolar ones, the main of which is the extremely low current in the control circuit when operating at low frequency or in static mode, it can be assumed that a conventional two-transistor multivibrator, but only on field-effect transistors, will be in a winning position in front of similar nodes assembled on their bipolar counterparts.

You can see the scheme of the first mulvibrator in fig. 1. Its operation is in many ways similar to the operation of a multivibrator on pnp bipolar transistors - the LEDs will also wink. The difference is that to close each of the transistors VT1.1, VT1.2, it is necessary to apply a positive gate-source voltage, which must exceed the cut-off voltage of these transistors (about 4 V). This happens every time the multivibrator arms are switched, due to the presence of time-setting capacitors C1, C2. That is why there is no need for a bipolar power supply.

The switching frequency of transistors in this generator is once every 6 s. When installing high-quality electrolytic capacitors (with a low leakage current), with a capacity of 100 ... 4700 μF, it is possible to achieve transistor switching with a period of several tens of minutes, which is unattainable for simple devices based on bipolar transistors.

The resistances of resistors R2 and R3 can differ by several thousand times, for example, R2 can be taken as 30 MΩ, and R3 as 10 kΩ. The multivibrator will then become asymmetrical. The capacitances of the capacitors change in the same way. Having properly selected these elements, it is possible to obtain very short pulses at the drain output of one of the transistors, following with a large duty cycle (100 ... 10000). If in a device made according to the scheme of Fig. 1, instead of ordinary LEDs, turn on blinking transistors as a load of transistors, for example, L-36BSRD, then any of them, blinking several times, will rest while its neighbor blinks. If you need to operate the multivibrator at audio frequencies, then the resistance of resistors R2 and R3 reduce by 10 ... 20 times, and take capacitors with a capacity of several hundred picofarads.

Instead of conventional resistors R2, R3, you can install photoresistors (FSK, SF2-x, SFZ-x, FR117, etc.). In this case, the switching frequency of transistors will change several thousand times depending on the level of illumination. It should only be noted that if the resistance of the resistors R2, R3 is less than 3 kΩ, the generation may fail.

A multivibrator made according to the scheme shown in fig. 1, requires the use of field-effect transistors with a large initial drain current (10 ... 30 mA). In the absence of such assemblies from the KR504 series, it is possible to assemble a similar multivibrator according to the scheme shown in Fig. 2. Here, field-effect transistors operate with a lower drain current, and in order to obtain sufficient brightness of the LEDs, current amplifiers are installed on bipolar transistors VT1, VT4. The switching frequency of this multivibrator is about 1 Hz. If you install powerful composite transistors from the KT829 series in place of transistors VT1, VT4, then incandescent lamps can be used as their load. In this case, R2, R6 are not installed, since transistors of the KT829 type contain their own built-in resistors.

If this multivibrator "refuses" to work, then resistors R3, R7 should be more accurately selected. In a node assembled according to the scheme shown in Fig. 1, it is possible to use microassemblies of matched pairs of field-effect transistors of the KR504, (K504, 504) series with an initial drain current of more than 10 mA. KR504NT4V, KR504NTZV are most suitable, but you can try with indexes A, B. When changing the polarity of the supply voltage and connecting LEDs, instead of a transistor assembly, you can use two separate field-effect n-channel transistors from the KP302, KP307 series. If they have a large cutoff voltage, then the supply voltage can be increased to 15 V.

For the node, the scheme of which is shown in Fig. 2, KR504NT1, KR504NT2 microcircuits with any letter index are suitable, and when selecting resistors R3, R7 - KR504NTZ, KR504NT4. In addition, many field-effect transistors of the KP103, KP101 series will also work without tuning. It is better to use non-polar capacitors, for example, small-sized K73-17 for 63 V. "Regular" LEDs can be any of the AL307, KIPD21, KIPD35, KIPD40 series, as well as 1-1513, L-934, etc. Flashing - L-816BRSC-B, L-769BGR, L-56DGD, Т1ВК5410 and others.

Since the field-effect transistors of the KR504NT (1 ... 4) assemblies allow a maximum source-drain voltage of not more than 10 V, the supply voltage of the multivibrators should not exceed 10 ... 12 V.

Literature

A. Butov. Multivibrator on field-effect transistors. - Radio, 2002, N4, p.53.

Microcircuits and their application. - M.: Radio and communication, 1984, p.73.

Publication: www.cxem.net

Perfection is not achieved when there is nothing to add,
and when there is nothing to remove.
Antoine de Saint-Exupery

Many radio amateurs, of course, have come across SMT (Surface mount technology) printed circuit board surface mount technology, met SMD (Surface mount device) elements mounted on the surface and heard about the advantages of surface mounting, which is rightfully called the fourth revolution in electronic technology after the invention lamp, transistor and integrated circuit.

Some find surface mounting difficult to implement at home due to the small size of the SMD elements and ... the lack of holes for the leads of the parts.
This is partly true, but upon closer examination, it turns out that the small dimensions of the elements simply require accuracy during installation, of course, provided that we are talking about simple SMD components that do not require special equipment for installation. The absence of reference points, which are holes for the leads of parts, only create the illusion of difficulty in making a printed circuit board drawing.

You need practice in creating simple designs on SMD elements in order to gain skills, self-confidence, and make sure that surface mount is promising for yourself. After all, the process of manufacturing a printed circuit board is simplified (no need to drill holes, form the leads of parts), and the resulting gain in mounting density is noticeable to the naked eye.

The basis of our designs is an asymmetric multivibrator circuit based on transistors of various structures.

We will assemble a flasher on an LED, which will serve as a talisman, and also create a reserve for future designs by making a prototype of a chip popular with radio amateurs, but not quite accessible.

Asymmetric multivibrator on transistors of different structures

(Fig. 1) is a real "bestseller" in amateur radio literature.

Rice. 1. Scheme of an asymmetric multivibrator

By connecting certain external circuits to the circuit, you can assemble more than a dozen structures. For example, a sound probe, a Morse code generator, a mosquito repellent, the basis of a monophonic musical instrument. And the use of external sensors or control devices in the base circuit of the VT1 transistor allows you to get a watchdog, an indicator of humidity, light, temperature, and many other designs.

--
Thank you for your attention!
Igor Kotov, editor-in-chief of Datagor magazine

List of sources

1. Mosyagin V.V. Secrets of amateur radio skill. – M.: SOLON-Press. – 2005, 216 p. (pp. 47 - 64).
2. Shustov M.A. Practical circuitry. 450 useful schemes for radio amateurs. Book 1. - M .: Alteks-A, 2001. - 352 p.
3. Shustov M.A. Practical circuitry. Control and protection of power supplies. Book 4. - M .: Alteks-A, 2002. - 176 p.
4. Low-voltage flasher. (Abroad) // Radio, 1998, No. 6, p. 64.
5.
6.
7.
8. Shoemaker C. Amateur control and signaling circuits on the IS. - M: Mir, 1989 (scheme 46. Simple battery discharge indicator, p. 104; scheme 47. Drawer marker (flashing), p. 105).
9. Generator on LM3909 // Radio scheme, 2008, No. 2. Diploma specialty - radio engineer, Ph.D.

The author of the books “To a young radio amateur for reading with a soldering iron”, “Secrets of amateur radio skill”, co-author of a series of books “For reading with a soldering iron” in the publishing house “SOLON-Press”, I have publications in the journals “Radio”, “Instruments and Experimental Techniques”, etc. .

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annotation

This explanatory note presents a description of the circuit and timing diagrams, calculation methods for a multivibrator based on field-effect transistors. In accordance with the task, the necessary parameters of the circuit were calculated.

THE SUMMARY

In the given explanatory note the description of the circuit and time diagrams, settlement techniques of the multivibrator on field transistors are submitted. According to the task necessary parameters of the circuit are designed.

pulse repetition period T: 200 μs

duration

: 10 µs

slice duration

: 1 µs

pulse amplitude U out. u: -10V

Title page

annotation

Technical task

Introduction

1. Description of the scheme of the device of a fantastic sawtooth generator

2. Calculation of the fantastic oscillator sawtooth voltage

2.1 Electrical calculations

2.2. Choice of substantiation of the element base

Conclusion

Bibliographic list

Specification

Timing diagrams

INTRODUCTION

Electronic computing technology is a relatively young scientific and technical direction, but it has the most revolutionary impact on all areas of science and technology, on all aspects of society. The constant development of the computer element base is characteristic. The elemental base is developing very quickly; new types of logical circuits appear, existing ones are modified. There are many different electronic devices: logic elements, registers, adders, decoders, multiplexers, counters, frequency dividers, triggers, generators, etc.

Generators convert the energy of the power source into the energy of periodic or quasi-periodic electrical oscillations. The main purpose of generators in electronics is the formation of initial setting and synchronization pulses, control signals of various shapes and durations.

The whole variety of generators can be divided into the following types:

Rectangular pulse generators;

Linear voltage generators (LIN);

Step voltage generators;

Sinusoidal generators

Typical square wave shapes are shown in Figure 1

Rectangular pulse generators with energy-accumulating elements in the feedback loop are called multivibrators.

Multivibrators are divided into two groups:

Self-oscillating multivibrators;

Waiting multivibrators or single vibrators.

The main difference between these multivibrators is that self-oscillating multivibrators form a pulse sequence when the supply voltage is applied to the circuit, since they have two feedback circuits with energy storage devices, and standby multivibrators form a single pulse with specified parameters for external triggering, since one the feedback loop has no energy storage. A single vibrator is something between a multivibrator and a trigger.

There are soft and hard modes of excitation of multivibrators. In soft mode, any voltage changes in the feedback circuit at the time of power-up lead to the occurrence of generation mode; in hard mode, generation occurs when the voltage in the feedback circuit reaches a certain threshold.

Multivibrators are divided into restartable and non-restartable. In the first case, when a trigger pulse is applied, the generation of output signals starts anew from the initial state. Restarts allow you to unlimitedly increase the duration of the output pulse, regardless of the parameters of the multivibrator circuit. Non-restartable multivibrators do not respond to external trigger pulses

1. Description of the multivibrator circuit on field-effect transistors

The high input impedance of field-effect transistors (FETs) makes it possible to design multivibrators for very low pulse repetition rates with small capacitances of time-setting capacitors. Due to this, the shape of the output pulses is less distorted, and the duty cycle is greater than that of multivibrators based on bipolar transistors.

For self-oscillating multivibrators, FETs with a control p-n junction are most suitable, since during the charging of capacitors, the voltage in the gate-source section is applied in the forward direction and therefore the resistance of this section is small and the charge time of the capacitors becomes small.

The scheme of multivibrators from FETs with a control p-n junction and a p-type channel is shown in Fig. 2. In this multivibrator through resistors

a small negative voltage is applied to the gate relative to the source, which increases the stability of the oscillation period and the duration of the output pulses. Unlike a multivibrator on BP transistors, the operation of the device is not disturbed if resistors are connected between the gate and the common point (circuit with a "zero" gate).

Timing diagrams of the operation of an asymmetric multivibrator are shown in Fig.3. In basic terms, the principle of operation of this multivibrator is the same as that of a tube multivibrator. It differs from the BT multivibrator in that in temporarily stable equilibrium states, the discharge of capacitors occurs almost exclusively through resistors

and not to zero voltage, but to a value at which the gate voltage becomes equal to the cutoff voltage (usually 1-6 V)

2.1. ELECTRICAL CALCULATION

I. Choice of transistor. To ensure temporarily stable equilibrium states, it is necessary to choose transistors in which

- the maximum allowable drain-source voltage, - cutoff voltage.

According to the reference book, we select PT KP103L, which has the following parameters:

At voltages

= 10 V and = 0 drain current = 3 - 6.6 mA, slope of characteristic S = 1.8 - 3.8 mA / V; gate current 20 nA, input capacitance pF, through capacitance pF and power dissipation on the collector P = 120 mW. Calculate the average cutoff voltage and input resistance.

For calculation we take