LEDs are installed on the multivibrator board
LED flasher - symmetrical multivibrator The application of the symmetrical multivibrator circuit is very wide. Elements of multivibrator circuits are found in computer technology, radio measuring and medical equipment.
A set of parts for assembling LED flashers can be purchased at the following link http://ali.pub/2bk9qh . If you want to seriously practice soldering simple structures, the Master recommends purchasing a set of 9 sets, which will greatly save your shipping costs. Here is the link to purchase http://ali.pub/2bkb42 . The master collected all the sets and they started working. Success and growth of skills in soldering.
Multivibrator.
The first circuit is the simplest multivibrator. Despite its simplicity, its scope is very wide. No electronic device is complete without it.
The first figure shows its circuit diagram.
LEDs are used as a load. When the multivibrator is working, the LEDs switch.
For assembly you will need a minimum of parts:
1. Resistors 500 Ohm - 2 pieces
2. Resistors 10 kOhm - 2 pieces
3. Electrolytic capacitor 47 uF for 16 volts - 2 pieces
4. Transistor KT972A - 2 pieces
5. LED - 2 pieces
KT972A transistors are composite transistors, that is, their housing contains two transistors, and it is highly sensitive and can withstand significant current without heat sink.
Once you have purchased all the parts, arm yourself with a soldering iron and start assembling. To conduct experiments, you don’t need to make a printed circuit board; you can assemble everything using a surface-mounted installation. Solder as shown in the pictures.
Let your imagination tell you how to use the assembled device! For example, instead of LEDs, you can install a relay, and use this relay to switch a more powerful load. If you change the values of resistors or capacitors, the switching frequency will change. By changing the frequency you can achieve very interesting effects, from a squeak in the dynamics to a pause for many seconds..
Photo relay.
And this is a diagram of a simple photo relay. This device can be successfully used wherever you want, to automatically illuminate the DVD tray, to turn on the light, or to alarm against intrusion into a dark closet. Two schematic options are provided. In one embodiment, the circuit is activated by light, and in the other by its absence.
It works like this: when light from the LED hits the photodiode, the transistor will open and LED-2 will start to glow. The sensitivity of the device is adjusted using a trimming resistor. As a photodiode, you can use a photodiode from an old ball mouse. LED - any infrared LED. The use of infrared photodiode and LED will avoid interference from visible light. Any LED or a chain of several LEDs is suitable as LED-2. An incandescent lamp can also be used. And if you install an electromagnetic relay instead of an LED, you can control powerful incandescent lamps or some mechanisms.
The figures show both circuits, the pinout (location of the legs) of the transistor and LED, as well as the wiring diagram.
If there is no photodiode, you can take an old MP39 or MP42 transistor and cut off its housing opposite the collector, like this:
Instead of a photodiode, a p-n junction of a transistor will need to be included in the circuit. You will have to determine experimentally which one will work better.
Power amplifier based on TDA1558Q chip.
This amplifier has an output power of 2 X 22 watts and is simple enough for beginner hams to replicate. This circuit will be useful for you for homemade speakers, or for a homemade music center, which can be made from an old MP3 player.
To assemble it you will need only five parts:
1. Microcircuit - TDA1558Q
2. Capacitor 0.22 uF
3. Capacitor 0.33 uF – 2 pieces
4. Electrolytic capacitor 6800 uF at 16 volts
The microcircuit has a fairly high output power and will need a radiator to cool it. You can use a heatsink from the processor.
The entire assembly can be done by surface mounting without the use of a printed circuit board. First, you need to remove pins 4, 9 and 15 from the microcircuit. They are not used. The pins are counted from left to right if you hold it with the pins facing you and the markings facing up. Then carefully straighten the leads. Next, bend pins 5, 13 and 14 up, all these pins are connected to the power positive. The next step is to bend pins 3, 7 and 11 down - this is the power supply minus, or “ground”. After these manipulations, screw the chip to the heat sink using thermal conductive paste. The pictures show the installation from different angles, but I will still explain. Pins 1 and 2 are soldered together - this is the input of the right channel, a 0.33 µF capacitor must be soldered to them. The same must be done with pins 16 and 17. The common wire for the input is the minus power supply or ground.
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.
A traditional symmetrical multivibrator has a number of disadvantages, including a relatively high output resistance, long pulse rises, limited supply voltage, and low efficiency when operating with a low-impedance load.
Schematic diagram
In 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. Due to this, the amplitude swing of the signal across the load is almost twice the supply voltage of the multivibrator, which makes it possible to obtain a significantly higher volume compared to the load would be included in one of the arms of the multivibrator.
In addition, the load is supplied with “real” AC voltage, which significantly improves the operating 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 using transistors KT972, KT973.
A symmetrical two-phase multivibrator consists of two push-pull arms, the voltage on which alternately changes from low to high. Let's 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 pnp transistor VT5 is connected to the connection point of their collectors through the current-limiting resistor R12, which will open. A voltage of about 8 V will be applied to the load when the multivibrator supply voltage is 9 V. With the recharging 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 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 adjusted from 1.4 to 1.5 kHz.
When the resistance R7 decreases below the conventional value, the generation of sound frequencies is disrupted. It should be noted that after startup, the multivibrator can operate 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 power supply voltage reversal. If it is absent and the power supply is of sufficient power, the built-in protective circuits of the transistors may be damaged when the voltage is reversed.
To expand the functionality of this multivibrator, it has the ability to turn on/off when a positive polarity voltage is applied to the control input. If the control input is not connected anywhere or the voltage on it is no more than 0.5 V, transistors VT3, VT4 are closed, the multivibrator works.
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 VT3, VT4 open, the multivibrator is inhibited. In this state, the multivibrator consumes a current of less than 200 μA, excluding the current through R2, R3, R9.
Parts and installation
The multivibrator can be mounted on a printed circuit board measuring 70*50 mm, a sketch of which is shown in Fig. 2 Fixed resistors can be used in any small size. Trimmer resistor RP1-63M, SP4-1 or similar imported one. Oxide capacitors K50-29, K50-35 or analogs Capacitors C2, C4 - K73-9, K73-17, K73-24 or any small-sized film.
Rice. 2. Printed circuit board for a powerful multivibrator circuit using transistors.
KD522A diodes can be replaced with 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 with analogues of two transistors connected according to a Darlington circuit, Fig. 3. Instead of low-power pnp transistors KT315G, any of the KT312, KT315, KT342, KT3102, KT645, SS9014 and similar series are suitable.
Rice. 3. Schematic diagram of equivalent replacement of transistors KT972, KT973.
The load of this multivibrator can be a dynamic head, a telephone capsule, a piezoceramic sound emitter, or a pulse step-up/step-down transformer.
When using a dynamic head with a winding resistance of 8 Ohms, it should be taken into account that with a supply voltage of 9 V, 8 W of AC voltage power will be supplied to the load. Therefore, a two...four-watt dynamic head can be damaged after just 1...2 minutes of operation.
Setting up
The operating frequency of the multivibrator is significantly influenced 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 operating on a multivibrator with 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 operating frequency is more significant
By selecting the resistances of resistors R5, R11, R6, R8, you can set the pulse shape to be almost strictly rectangular when the multivibrator is operating with a specific connected load at a given supply voltage.
This multivibrator can find application in various signaling devices, sound warning devices, when, with a small available voltage of the power source, it is necessary to obtain significant power at the sound emitter. In addition, it is convenient to use in low-to-high voltage converters, including those operating at a low lighting network frequency of 50 Hz.
Butov A. L. RK-2010-04.
In this article we will talk about the multivibrator, how it works, how to connect a load to the multivibrator and the calculation of a transistor symmetrical multivibrator.
Multivibrator is a simple rectangular pulse generator that operates in self-oscillator mode. To operate it, you only need power from a battery or other power source. Let's consider the simplest symmetrical multivibrator using transistors. Its diagram is shown in the figure. The multivibrator can be more complicated depending on the necessary functions performed, but all the elements presented in the figure are mandatory, without them the multivibrator will not work.
The operation of a symmetrical multivibrator is based on the charge-discharge processes of capacitors, which together with resistors form RC circuits.
I wrote earlier about how RC circuits work in my article Capacitor, which you can read on my website. On the Internet, if you find material about a symmetrical multivibrator, it is presented briefly and not intelligibly. This circumstance does not allow novice radio amateurs to understand anything, but only helps experienced electronics engineers remember something. At the request of one of my site visitors, I decided to eliminate this gap.
How does a multivibrator work?
At the initial moment of power supply, capacitors C1 and C2 are discharged, so their current resistance is low. The low resistance of the capacitors leads to the “fast” opening of the transistors caused by the flow of current:
— VT2 along the path (shown in red): “+ power supply > resistor R1 > low resistance of discharged C1 > base-emitter junction VT2 > — power supply”;
— VT1 along the path (shown in blue): “+ power supply > resistor R4 > low resistance of discharged C2 > base-emitter junction VT1 > — power supply.”
This is the “unsteady” mode of operation of the multivibrator. It lasts for a very short time, determined only by the speed of the transistors. And there are no two transistors that are absolutely identical in parameters. Whichever transistor opens faster will remain open—the “winner.” Let's assume that in our diagram it turns out to be VT2. Then, through the low resistance of the discharged capacitor C2 and the low resistance of the collector-emitter junction VT2, the base of the transistor VT1 will be short-circuited to the emitter VT1. As a result, transistor VT1 will be forced to close - “become defeated”.
Since transistor VT1 is closed, a “fast” charge of capacitor C1 occurs along the path: “+ power supply > resistor R1 > low resistance of discharged C1 > base-emitter junction VT2 > — power supply.” This charge occurs almost up to the voltage of the power supply.
At the same time, capacitor C2 is charged with a current of reverse polarity along the path: “+ power supply > resistor R3 > low resistance of discharged C2 > collector-emitter junction VT2 > — power source.” The charge duration is determined by the ratings R3 and C2. They determine the time at which VT1 is in the closed state.
When capacitor C2 is charged to a voltage approximately equal to the voltage of 0.7-1.0 volts, its resistance will increase and transistor VT1 will open with the voltage applied along the path: “+ power supply > resistor R3 > base-emitter junction VT1 > - power supply.” In this case, the voltage of the charged capacitor C1, through the open collector-emitter junction VT1, will be applied to the emitter-base junction of transistor VT2 with reverse polarity. As a result, VT2 will close, and the current that previously passed through the open collector-emitter junction VT2 will flow through the circuit: “+ power supply > resistor R4 > low resistance C2 > base-emitter junction VT1 > — power supply.” This circuit will quickly recharge capacitor C2. From this moment, the “steady-state” self-generation mode begins.
Operation of a symmetrical multivibrator in “steady-state” generation mode
The first half-cycle of operation (oscillation) of the multivibrator begins.
When transistor VT1 is open and VT2 is closed, as I just wrote, capacitor C2 is quickly recharged (from a voltage of 0.7...1.0 volts of one polarity, to the voltage of the power source of the opposite polarity) along the circuit: “+ power supply > resistor R4 > low resistance C2 > base-emitter junction VT1 > - power supply.” In addition, capacitor C1 is slowly recharged (from the power source voltage of one polarity to a voltage of 0.7...1.0 volts of the opposite polarity) along the circuit: “+ power source > resistor R2 > right plate C1 > left plate C1 > collector- emitter junction of transistor VT1 > - - power source.”
When, as a result of recharging C1, the voltage at the base of VT2 reaches a value of +0.6 volts relative to the emitter of VT2, the transistor will open. Therefore, the voltage of the charged capacitor C2, through the open collector-emitter junction VT2, will be applied to the emitter-base junction of the transistor VT1 with reverse polarity. VT1 will close.
The second half-cycle of operation (oscillation) of the multivibrator begins.
When transistor VT2 is open and VT1 is closed, capacitor C1 is quickly recharged (from a voltage of 0.7...1.0 volts of one polarity, to the voltage of the power source of the opposite polarity) along the circuit: “+ power supply > resistor R1 > low resistance C1 > base emitter junction VT2 > - power supply.” In addition, capacitor C2 is slowly recharged (from the voltage of the power source of one polarity, to a voltage of 0.7...1.0 volts of the opposite polarity) along the circuit: “right plate of C2 > collector-emitter junction of transistor VT2 > - power supply > + source power > resistor R3 > left plate C2". When the voltage at the base of VT1 reaches +0.6 volts relative to the emitter of VT1, the transistor will open. Therefore, the voltage of the charged capacitor C1, through the open collector-emitter junction VT1, will be applied to the emitter-base junction of transistor VT2 with reverse polarity. VT2 will close. At this point, the second half-cycle of the multivibrator oscillation ends, and the first half-cycle begins again.
The process is repeated until the multivibrator is disconnected from the power source.
Methods for connecting a load to a symmetrical multivibrator
Rectangular pulses are removed from two points of a symmetrical multivibrator– transistor collectors. When there is a “high” potential on one collector, then there is a “low” potential on the other collector (it is absent), and vice versa - when there is a “low” potential on one output, then there is a “high” potential on the other. This is clearly shown in the time graph below.
The multivibrator load must be connected in parallel with one of the collector resistors, but in no case in parallel with the collector-emitter transistor junction. You cannot bypass the transistor with a load. If this condition is not met, then at a minimum the duration of the pulses will change, and at a maximum the multivibrator will not work. The figure below shows how to connect the load correctly and how not to do it.
In order for the load not to affect the multivibrator itself, it must have sufficient input resistance. For this purpose, buffer transistor stages are usually used.
The example shows connecting a low-impedance dynamic head to a multivibrator. An additional resistor increases the input resistance of the buffer stage, and thereby eliminates the influence of the buffer stage on the multivibrator transistor. Its value should be no less than 10 times the value of the collector resistor. Connecting two transistors in a “composite transistor” circuit significantly increases the output current. In this case, it is correct to connect the base-emitter circuit of the buffer stage in parallel with the collector resistor of the multivibrator, and not in parallel with the collector-emitter junction of the multivibrator transistor.
For connecting a high-impedance dynamic head to a multivibrator a buffer stage is not needed. The head is connected instead of one of the collector resistors. The only condition that must be met is that the current flowing through the dynamic head must not exceed the maximum collector current of the transistor.
If you want to connect ordinary LEDs to the multivibrator– to make a “flashing light”, then buffer cascades are not required for this. They can be connected in series with collector resistors. This is due to the fact that the LED current is small, and the voltage drop across it during operation is no more than one volt. Therefore, they do not have any effect on the operation of the multivibrator. True, this does not apply to super-bright LEDs, for which the operating current is higher and the voltage drop can be from 3.5 to 10 volts. But in this case, there is a way out - increase the supply voltage and use transistors with high power, providing sufficient collector current.
Please note that oxide (electrolytic) capacitors are connected with their positives to the collectors of the transistors. This is due to the fact that on the bases of bipolar transistors the voltage does not rise above 0.7 volts relative to the emitter, and in our case the emitters are the minus of the power supply. But at the collectors of the transistors, the voltage changes almost from zero to the voltage of the power source. Oxide capacitors are not able to perform their function when connected with reverse polarity. Naturally, if you use transistors of a different structure (not N-P-N, but P-N-P structures), then in addition to changing the polarity of the power source, you need to turn the LEDs with the cathodes “up in the circuit”, and the capacitors with the pluses to the bases of the transistors.
Let's figure it out now What parameters of the multivibrator elements determine the output currents and generation frequency of the multivibrator?
What do the values of collector resistors affect? I have seen in some mediocre Internet articles that the values of collector resistors do not significantly affect the frequency of the multivibrator. This is all complete nonsense! If the multivibrator is correctly calculated, a deviation of the values of these resistors by more than five times from the calculated value will not change the frequency of the multivibrator. The main thing is that their resistance is less than the base resistors, because collector resistors provide fast charging of capacitors. But on the other hand, the values of collector resistors are the main ones for calculating the power consumption from the power source, the value of which should not exceed the power of the transistors. If you look at it, if connected correctly, they do not even have a direct effect on the output power of the multivibrator. But the duration between switchings (multivibrator frequency) is determined by the “slow” recharging of the capacitors. The recharge time is determined by the ratings of the RC circuits - base resistors and capacitors (R2C1 and R3C2).
A multivibrator, although it is called symmetrical, this refers only to the circuitry of its construction, and it can produce both symmetrical and asymmetrical output pulses in duration. The pulse duration (high level) on the VT1 collector is determined by the ratings of R3 and C2, and the pulse duration (high level) on the VT2 collector is determined by the ratings R2 and C1.
The duration of recharging capacitors is determined by a simple formula, where Tau– pulse duration in seconds, R– resistor resistance in Ohms, WITH– capacitance of the capacitor in Farads:
Thus, if you have not already forgotten what was written in this article a couple of paragraphs earlier:
If there is equality R2=R3 And C1=C2, at the outputs of the multivibrator there will be a “meander” - rectangular pulses with a duration equal to the pauses between pulses, which you see in the figure.
The full period of oscillation of the multivibrator is T equal to the sum of the pulse and pause durations:
Oscillation frequency F(Hz) related to period T(sec) through the ratio:
As a rule, if there are any calculations of radio circuits on the Internet, they are meager. That's why Let's calculate the elements of a symmetrical multivibrator using the example .
Like any transistor stages, the calculation must be carried out from the end - the output. And at the output we have a buffer stage, then there are collector resistors. Collector resistors R1 and R4 perform the function of loading the transistors. Collector resistors have no effect on the generation frequency. They are calculated based on the parameters of the selected transistors. Thus, first we calculate the collector resistors, then the base resistors, then the capacitors, and then the buffer stage.
Procedure and example of calculating a transistor symmetrical multivibrator
Initial data:
Supply voltage Ui.p. = 12 V.
Required multivibrator frequency F = 0.2 Hz (T = 5 seconds), and the pulse duration is equal to 1 (one) second.
A car incandescent light bulb is used as a load. 12 volts, 15 watts.
As you guessed, we will calculate a “flashing light” that will blink once every five seconds, and the duration of the glow will be 1 second.
Selecting transistors for the multivibrator. For example, we have the most common transistors in Soviet times KT315G.
For them: Pmax=150 mW; Imax=150 mA; h21>50.
Transistors for the buffer stage are selected based on the load current.
In order not to depict the diagram twice, I have already signed the values of the elements on the diagram. Their calculation is given further in the Decision.
Solution:
1. First of all, you need to understand that operating a transistor at high currents in switching mode is safer for the transistor itself than operating in amplification mode. Therefore, there is no need to calculate the power for the transition state at the moments of passage of an alternating signal through the operating point “B” of the static mode of the transistor - the transition from the open state to the closed state and back. For pulse circuits built on bipolar transistors, the power is usually calculated for the transistors in the open state.
First, we determine the maximum power dissipation of the transistors, which should be a value 20 percent less (factor 0.8) than the maximum power of the transistor indicated in the reference book. But why do we need to drive the multivibrator into the rigid framework of high currents? And even with increased power, energy consumption from the power source will be large, but there will be little benefit. Therefore, having determined the maximum power dissipation of transistors, we will reduce it by 3 times. A further reduction in power dissipation is undesirable because the operation of a multivibrator based on bipolar transistors in low current mode is an “unstable” phenomenon. If the power source is used not only for the multivibrator, or it is not entirely stable, the frequency of the multivibrator will also “float”.
We determine the maximum power dissipation: Pdis.max = 0.8 * Pmax = 0.8 * 150 mW = 120 mW
We determine the rated dissipated power: Pdis.nom. = 120 / 3 = 40mW
2. Determine the collector current in the open state: Ik0 = Pdis.nom. / Ui.p. = 40mW / 12V = 3.3mA
Let's take it as the maximum collector current.
3. Let’s find the value of the resistance and power of the collector load: Rk.total = Ui.p./Ik0 = 12V/3.3mA = 3.6 kOhm
We select resistors from the existing nominal range that are as close as possible to 3.6 kOhm. The nominal series of resistors has a nominal value of 3.6 kOhm, so we first calculate the value of the collector resistors R1 and R4 of the multivibrator: Rк = R1 = R4 = 3.6 kOhm.
The power of the collector resistors R1 and R4 is equal to the rated power dissipation of the transistors Pras.nom. = 40 mW. We use resistors with a power exceeding the specified Pras.nom. - type MLT-0.125.
4. Let's move on to calculating the basic resistors R2 and R3. Their rating is determined based on the gain of transistors h21. At the same time, for reliable operation of the multivibrator, the resistance value must be within the range: 5 times greater than the resistance of the collector resistors, and less than the product Rк * h21. In our case Rmin = 3.6 * 5 = 18 kOhm, and Rmax = 3.6 * 50 = 180 kOhm
Thus, the values of resistance Rb (R2 and R3) can be in the range of 18...180 kOhm. We first select the average value = 100 kOhm. But it is not final, since we need to provide the required frequency of the multivibrator, and as I wrote earlier, the frequency of the multivibrator directly depends on the base resistors R2 and R3, as well as on the capacitance of the capacitors.
5. Calculate the capacitances of capacitors C1 and C2 and, if necessary, recalculate the values of R2 and R3.
The values of the capacitance of capacitor C1 and the resistance of resistor R2 determine the duration of the output pulse on the collector VT2. It is during this impulse that our light bulb should light up. And in the condition the pulse duration was set to 1 second.
Let's determine the capacitance of the capacitor: C1 = 1 sec / 100 kOhm = 10 µF
A capacitor with a capacity of 10 μF is included in the nominal range, so it suits us.
The values of the capacitance of capacitor C2 and the resistance of resistor R3 determine the duration of the output pulse on the collector VT1. It is during this pulse that there is a “pause” on the VT2 collector and our light bulb should not light up. And in the condition, a full period of 5 seconds with a pulse duration of 1 second was specified. Therefore, the duration of the pause is 5 seconds – 1 second = 4 seconds.
Having transformed the recharge duration formula, we Let's determine the capacitance of the capacitor: C2 = 4 sec / 100 kOhm = 40 µF
A capacitor with a capacity of 40 μF is not included in the nominal range, so it does not suit us, and we will take the capacitor with a capacity of 47 μF that is as close as possible to it. But as you understand, the “pause” time will also change. To prevent this from happening, we Let's recalculate the resistance of resistor R3 based on the duration of the pause and the capacitance of capacitor C2: R3 = 4sec / 47 µF = 85 kOhm
According to the nominal series, the closest value of the resistor resistance is 82 kOhm.
So, we got the values of the multivibrator elements:
R1 = 3.6 kOhm, R2 = 100 kOhm, R3 = 82 kOhm, R4 = 3.6 kOhm, C1 = 10 µF, C2 = 47 µF.
6. Calculate the value of resistor R5 of the buffer stage.
To eliminate the influence on the multivibrator, the resistance of the additional limiting resistor R5 is selected to be at least 2 times greater than the resistance of the collector resistor R4 (and in some cases more). Its resistance, together with the resistance of the emitter-base junctions VT3 and VT4, in this case will not affect the parameters of the multivibrator.
R5 = R4 * 2 = 3.6 * 2 = 7.2 kOhm
According to the nominal series, the nearest resistor is 7.5 kOhm.
With a resistor value of R5 = 7.5 kOhm, the buffer stage control current will be equal to:
Icontrol = (Ui.p. - Ube) / R5 = (12v - 1.2v) / 7.5 kOhm = 1.44 mA
In addition, as I wrote earlier, the collector load rating of the multivibrator transistors does not affect its frequency, so if you do not have such a resistor, then you can replace it with another “close” rating (5 ... 9 kOhm). It is better if this is in the direction of decrease, so that there is no drop in the control current in the buffer stage. But keep in mind that the additional resistor is an additional load for transistor VT2 of the multivibrator, so the current flowing through this resistor adds up to the current of collector resistor R4 and is a load for transistor VT2: Itotal = Ik + Icontrol. = 3.3mA + 1.44mA = 4.74mA
The total load on the collector of transistor VT2 is within normal limits. If it exceeds the maximum collector current specified in the reference book and multiplied by a factor of 0.8, increase resistance R4 until the load current is sufficiently reduced, or use a more powerful transistor.
7. We need to provide current to the light bulb In = Рн / Ui.p. = 15W / 12V = 1.25 A
But the control current of the buffer stage is 1.44 mA. The multivibrator current must be increased by a value equal to the ratio:
In / Icontrol = 1.25A / 0.00144A = 870 times.
How to do it? For significant output current amplification use transistor cascades built according to the “composite transistor” circuit. The first transistor is usually low-power (we will use KT361G), it has the highest gain, and the second must provide sufficient load current (let’s take the no less common KT814B). Then their transmission coefficients h21 are multiplied. So, for the KT361G transistor h21>50, and for the KT814B transistor h21=40. And the overall transmission coefficient of these transistors connected according to the “composite transistor” circuit: h21 = 50 * 40 = 2000. This figure is greater than 870, so these transistors are quite enough to control a light bulb.
Well, that's all!
The multivibrator circuit shown in Figure 1 is a cascade connection of transistor amplifiers where the output of the first stage is connected to the input of the second through a circuit containing a capacitor and the output of the second stage is connected to the input of the first through a circuit containing a capacitor. Multivibrator amplifiers are transistor switches that can be in two states. The multivibrator circuit in Figure 1 differs from the trigger circuit discussed in the article "". Because it has reactive elements in the feedback circuits, the circuit can therefore generate non-sinusoidal oscillations. You can find the resistance of resistors R1 and R4 from relations 1 and 2:
Where I KBO = 0.5 μA is the maximum reverse collector current of the KT315a transistor,
Ikmax=0.1A is the maximum collector current of the KT315a transistor, Up=3V is the supply voltage. Let's choose R1=R4=100Ohm. Capacitors C1 and C2 are selected depending on the required oscillation frequency of the multivibrator.
Figure 1 - Multivibrator based on KT315A transistors
You can relieve the voltage between points 2 and 3 or between points 2 and 1. The graphs below show how approximately the voltage will change between points 2 and 3 and between points 2 and 1.
T - oscillation period, t1 - time constant of the left arm of the multivibrator, t2 - time constant of the right arm of the multivibrator can be calculated using the formulas:
You can set the frequency and duty cycle of the pulses generated by the multivibrator by changing the resistance of trimming resistors R2 and R3. You can also replace capacitors C1 and C2 with variable (or trimmer) capacitors and, by changing their capacitance, set the frequency and duty cycle of the pulses generated by the multivibrator, this method is even more preferable, so if there are trimmer (or better variable) capacitors, then it is better to use them, and in place set variable resistors R2 and R3 to constant ones. The photo below shows the assembled multivibrator:
In order to make sure that the assembled multivibrator works, a piezodynamic speaker was connected to it (between points 2 and 3). After applying power to the circuit, the piezo speaker began to crackle. Changes in the resistance of the tuning resistors led to either an increase in the frequency of the sound emitted by the piezodynamics, or to its decrease, or to the fact that the multivibrator stopped generating.
A program for calculating the frequency, period and time constants, duty cycle of pulses taken from a multivibrator:
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Other multivibrators:
