Pulse generators are used in many radio devices (electronic meters, time relays) and are used when setting up digital equipment. The frequency range of such generators can be from a few hertz to many megahertz. Here are simple generator circuits, including those based on digital “logic” elements, which are widely used in more complex circuits as frequency-setting units, switches, sources of reference signals and sounds.
In Fig. Figure 1 shows a diagram of a generator that generates single rectangular pulses when the S1 button is pressed (that is, it is not a self-oscillator, the diagrams of which are given below). An RS trigger is assembled on the logical elements DD1.1 and DD1.2, which prevents the penetration of bounce pulses from the button contacts to the recalculating device. In the position of the contacts of button S1, shown in the diagram, output 1 will have a high level voltage, output 2 will have a low level voltage; when the button is pressed - vice versa. This generator is convenient to use when checking the performance of various meters.
In Fig. Figure 2 shows a diagram of a simple pulse generator based on an electromagnetic relay. When power is applied, capacitor C1 is charged through resistor R1 and the relay is activated, turning off the power source with contacts K 1.1. But the relay does not release immediately, since for some time current will flow through its winding due to the energy accumulated by capacitor C1. When contacts K 1.1 close again, the capacitor begins to charge again - the cycle repeats.
The switching frequency of the electromagnetic relay depends on its parameters, as well as the values of capacitor C1 and resistor R1. When using the RES-15 relay (passport RS4.591.004), switching occurs approximately once per second. Such a generator can be used, for example, to switch garlands on a New Year tree or to obtain other lighting effects. Its disadvantage is the need to use a capacitor of significant capacity.
In Fig. Figure 3 shows a diagram of another generator based on an electromagnetic relay, the operating principle of which is similar to the previous generator, but provides a pulse frequency of 1 Hz with a capacitor capacity 10 times smaller. When power is applied, capacitor C1 is charged through resistor R1. After some time, the zener diode VD1 will open and relay K1 will operate. The capacitor will begin to discharge through resistor R2 and the input resistance of the composite transistor VT1VT2. Soon the relay will release and a new cycle of generator operation will begin. Switching on transistors VT1 and VT2 according to a composite transistor circuit increases the input impedance of the cascade. Relay K 1 can be the same as in the previous device. But you can use RES-9 (passport RS4.524.201) or any other relay that operates at a voltage of 15...17 V and a current of 20...50 mA.
In the pulse generator, the diagram of which is shown in Fig. 4, the logic elements of the DD1 microcircuit and the field-effect transistor VT1 are used. When changing the values of capacitor C1 and resistors R2 and R3, pulses are generated with a frequency from 0.1 Hz to 1 MHz. Such a wide range was obtained through the use of a field-effect transistor, which made it possible to use resistors R2 and R3 with a resistance of several megaohms. Using these resistors, you can change the duty cycle of the pulses: resistor R2 sets the duration of the high level voltage at the output of the generator, and resistor R3 sets the duration of the low level voltage. The maximum capacitance of capacitor C1 depends on its own leakage current. In this case it is 1...2 µF. The resistance of resistors R2, R3 is 10...15 MOhm. Transistor VT1 can be any of the KP302, KP303 series. The microcircuit is K155LA3, its power supply is 5V stabilized voltage. You can use CMOS microcircuits of the K561, K564, K176 series, the power supply of which lies within the range of 3 ... 12 V, the pinout of such microcircuits is different and is shown at the end of the article.
If you have a CMOS chip (K176, K561 series), you can assemble a wide-range pulse generator without using a field-effect transistor. The diagram is shown in Fig. 5. For the convenience of setting the frequency, the capacitance of the timing circuit capacitor is changed with switch S1. The frequency range generated by the generator is 1...10,000 Hz. Microcircuit - K561LN2.
If you need high stability of the generated frequency, then such a generator can be made “quartzized” - turn on the quartz resonator at the desired frequency. Below is an example of a quartz oscillator at a frequency of 4.3 MHz:
In Fig. Figure 6 shows a diagram of a pulse generator with adjustable duty cycle.
Duty cycle is the ratio of the pulse repetition period (T) to their duration (t):
The duty cycle of high-level pulses at the output of logic element DD1.3, resistor R1, can vary from 1 to several thousand. In this case, the pulse frequency also changes slightly. Transistor VT1, operating in key mode, amplifies the power pulses.
The generator, the diagram of which is shown in the figure below, produces pulses of both rectangular and sawtooth shapes. The master oscillator is made on logical elements DD 1.1-DD1.3. A differentiating circuit is assembled on capacitor C2 and resistor R2, thanks to which short positive pulses (about 1 μs in duration) are formed at the output of the logical element DD1.5. An adjustable current stabilizer is made on field-effect transistor VT2 and variable resistor R4. This current charges the capacitor C3, and the voltage across it increases linearly. When a short positive pulse arrives at the base of transistor VT1, transistor VT1 opens, discharging capacitor S3. A sawtooth voltage is thus formed on its plates. Resistor R4 regulates the charging current of the capacitor and, consequently, the steepness of the increase in the sawtooth voltage and its amplitude. Capacitors C1 and SZ are selected based on the required pulse frequency. Microcircuit - K561LN2.
Digital microcircuits in generators are interchangeable in most cases and can be used in the same circuit as microcircuits with “NAND” and “NOR” elements, or simply inverters. A variant of such replacements is shown in the example of Figure 5, where a microcircuit with K561LN2 inverters was used. Exactly such a circuit, preserving all parameters, can be assembled on both K561LA7 and K561LE5 (or K176, K564, K164 series), as shown below. You just need to observe the pinout of the microcircuits, which in many cases even coincides.
The generator, depending on the voltage of the power source, produces high-voltage pulses with an amplitude of up to 25 kV. It can be powered by a 6V galvanic battery (four A-type cells), a 6...12V battery, a car's on-board power supply, or a laboratory power supply up to 15V. The range of applications is quite wide: electric fences on an animal farm, a gas lighter, an electroshock protective device, etc. In the manufacture of such devices, the greatest difficulties are caused by a high-voltage transformer.
Even if successfully manufactured, it is not reliable and often fails due to dampness or due to breakdown of the insulation between the coils. An attempt to make a high-voltage generator based on a diode voltage multiplier also does not always give a positive result.
The easiest way is to use a ready-made high-voltage transformer - a car ignition coil from a car with a classic ignition system. This transformer is highly reliable and can operate even in the most unfavorable field conditions. The ignition coil design is designed for tough operation in all weather conditions.
The schematic diagram of the generator is shown in the figure. An asymmetrical multivibrator is made on transistors VT1 and VT2; it produces pulses with a frequency of about 500 Hz. These pulses flow through the collector load of transistor VT2 - the primary winding of the ignition coil. As a result, an alternating pulsed high-voltage voltage is induced in its secondary winding, which has a significantly larger number of turns.
This voltage is supplied to the spark gap, if it is a self-defense device or a gas lighter, or to an electric fence. In this case, voltage is supplied to the fence from the central terminal of the ignition coil (from the terminal from which voltage is supplied to the distributor and spark plugs), and the common plus of the circuit must be grounded.
If the generator will be used as a means of self-defense, it is most convenient to make it in the form of a stick. Take a plastic or metal tube of such a diameter that the ignition coil with its metal body is tightly inserted into it. In the remaining space of the pipe, place batteries and transistors. S1 in this case is the instrument button. The upper part of the reel body will have to be redone.
It is most convenient to take an old-style plug for a 220V network, with screw-out contacts. The hole for the wire in it must be drilled so that the part of the ignition coil with a high-voltage contact fits tightly into it. Then you need to remove the mounting wires from this contact and from the general plus of the circuit and, along the very edges of the plug, bring them to the pin contacts of the plug.
Then this plug must be coated with epoxy glue in the drilled hole for the wire and pressed tightly onto the plastic body of the high-voltage contact of the coil. You need to screw discharge petals under the pin contacts of the plug, the distance between which should be about 15 mm.
The ignition coil can be anything from a contact ignition system (not suitable for electronic ignition), preferably imported - it is smaller in size and operating.
The setting consists of selecting the value of R1 so that there is a reliable electrical discharge between the discharge petals.
The pulse current generator (PGG) is designed for the primary conversion of electrical energy. Includes an AC electrical network with a frequency of 50 Hz, a high-voltage transformer, a rectifier, a current-limiting device, and protection equipment. In the GIT, charging and discharging circuits are distinguished, which are interconnected by a bank of capacitors. The GIT, which is a power source, is connected to the technological unit through a discharge circuit.
Pulse generators are characterized by the following main parameters: voltage across the capacitor bank U, electric capacity of the battery C, energy accumulated in capacitors W n, energy in impulse W 0 pulse repetition rate υ.
The purpose of the charging circuit is to charge a bank of capacitors to a given voltage. The circuit includes a current-limiting device, a step-up transformer and a high-voltage rectifier. Selenium or silicon pillars are used to rectify the charging current. Using a high-voltage transformer, the initial voltage of the 380/220 V supply network is increased to (2-70) 10 3 V.
In the scheme L - C – D we have ή 3 > 50%.
When using pulsed current generators, energy losses are significant at the stage of discharge formation. The common system that combines pulse current and voltage generators does not have this drawback (Fig. 30). In this system, the breakdown of the forming gap is produced by the energy of the capacitor bank of the voltage generator, which creates a current-carrying channel in the main working gap and ensures the release of the main discharge energy in the discharge gap of the pulse current generator.
The characteristic ratio of electrical voltages and capacitances for such a system is: » where index 1 corresponds to the voltage generator, and index 2 to the current generator. So, for example
The energy and weight-size parameters of the generator significantly depend on the high-voltage transformer and rectifier. The efficiency of the charging-rectifying device increases when using high-voltage silicon pillars. Rectifiers have high characteristic values - specific
volume from 0.03 to 0.28 m 3 /kW and specific gravity 25-151 kg/kW.
In electric pulse installations, single units are also used, including a transformer and a rectifier, which reduces the main dimensions and simplifies the switching network.
Pulse capacitors are designed to store electrical energy. High-voltage pulse capacitors must have increased specific energy capacity, low internal inductance and low resistance at high discharge currents, and the ability to withstand multiple charge-discharge cycles. The main technical data of pulse capacitors are given below.
Voltage (nominal), kV................................5-50
Capacitance (nominal), µF. . ....................................0.5-800
Discharge frequency, number of pulses/min.................................1-780
Discharge current, kA................................................... .............0.5-300
Energy intensity, J/kg.................................................... .......4.3-30
Resource, number of pulses................................................... .10 e - 3 10 7
One of the main characteristics of pulse capacitors, which affects the size of the battery and the electric pulse installation as a whole, is the indicator of specific volumetric energy intensity
(3.23)
Where E n- accumulated energy; V to- capacitor volume.
For existing capacitors ω s= 20 -g 70 kJ/m 3, which determines the increased dimensions of the storage devices. So the battery capacity for E n= 100 kJ is 1.5-5.0 m 3. In storage devices, capacitors are connected into batteries, which ensures the summation of their electrical capacity, which is equal to 100-8000 μF.
High-voltage switches are used to instantly release electrical energy accumulated in a capacitor bank in a process unit. High-voltage switches (discharge arresters) perform two functions: they disconnect the discharge circuit
from the storage device when charging it; instantly connect the drive to the load circuit.
Various design schemes of arresters and types of switches corresponding to these schemes are possible: air, vacuum, gas-filled, contact disc, ignitron and trigatron, with a solid dielectric.
The basic requirements for switches are as follows: withstand high-voltage operating voltage without breakdown, have low inductance and low resistance, and provide a given current pulse repetition rate.
In laboratory electric pulse installations, mainly air-type spark gaps are used, which provide switching of high energies over a long service life and have a relatively simple design (Fig. 31).
Dischargers of this type have a number of significant disadvantages that limit their use: the influence of the surface condition and the state of the atmospheric air (dust, humidity, pressure) on the stability of the reproduced pulse; nitrogen oxides are formed, which have an effect on humans; powerful high-frequency sound pressure is generated.
In industrial mobile installations, mechanical disc switches have become widespread (see Fig. 31, A). Dischargers of this type are simple in electrical circuit and design, reliable during transportation and operation in areas with rough terrain, but require regular cleaning of the surface of the disc elements. I
The electric pulse installation also includes control units for the pulse generator and the technological process, protection and interlock systems, and auxiliary systems that provide mechanization and automation of processes in the technological unit.
The control unit includes electrical circuits for starting, blocking and a synchronization pulse generation circuit.
The interlock system serves to “instantly switch off the high voltage voltage. The control system consists of a voltmeter and a kipovoltmeter, indicating the mains voltage and the capacitor bank voltage, respectively, indicator lamps, sound signals, and a frequency meter.
Technological node
The technological unit is designed to convert electrical energy into other types of energy and to transfer the converted energy to the processing object.
In relation to the specifics of discharge-pulse technology for rock destruction, the technological unit includes: a working discharge chamber, a working element in the form of an electrode system or an electrohydraulic fuse, a device for inlet and outlet of working fluid and a device for moving electrodes or an exploding conductor (Fig. 32). The working discharge chamber is filled with a working liquid or a special dielectric compound.
Discharge (working) chambers are divided into open and closed, buried and surface, stationary, mixed and remote. Cameras can be disposable or reusable; vertical, horizontal and inclined. The type and shape of the working chamber must ensure maximum release of accumulated electrical energy, maximum hp. converting this energy into mechanical energy, transferring this energy to the processing object or to its specified zone.
The working technological element is designed to directly convert electrical energy into mechanical energy and to input this energy into the working environment, and through it to the processing object. The type of working element depends on the type of electrical discharge in the liquid used in a given technological process - with free formation of the discharge, electrode systems are rational (Fig. 33, A); with an initiated discharge - an electro-hydraulic fuse with an exploding conductor (Fig. 33.6).
The working body experiences dynamic loads, the action of an electromagnetic field and ultraviolet radiation, as well as the influence of the working fluid.
The electrode system is used with free discharge formation. According to the design factor, rod linear and coaxial systems are distinguished. The simplest in design are linear (opposing or parallel) systems with combinations of electrode shapes: tip - tip and tip - plane. The disadvantages of linear systems are their significant inductance (1-10 µH) and non-directional action.
Coaxial systems are more advanced, having low self-inductance and high efficiency. converting accumulated electrical energy into plasma energy. The disadvantage of coaxial systems is their low reliability and fragility. The electrode system is technologically advanced and highly productive due to the high frequency of the process of creating mechanical loading forces.
Based on the number of repeated discharges, single-acting and multiple-acting systems are distinguished. Reusable systems are more economical and productive. The amount of energy converted by the electrode system also affects design and durability.
In the mining industry, electrode systems designed for pulse repetition rates of 1-12 per minute have become more widely used. During an electrical discharge, due to thermal processes, erosion of the electrodes occurs, the intensity of which depends on the material of the electrodes and the working fluid, as well as on the amount of energy released in
discharge channel. The working part of the electrodes is made of steel St3 or St45; the diameter of the protruding part must be more than 8 mm with a length of at least 12 mm. In the electrode zone, the melting temperature of iron is reached in 10 -6 s, and the boiling point in 5 10 -6 s.
The resulting intense destruction of the electrode is accompanied by the formation of plasma jets (vapors and liquid drops of metal). The weakened zone of the electrode is the insulating layer at the boundary between the output of the rod - the current conductor and water.
The main requirements for the electrode system are: high electrical energy conversion coefficient, high
operational and technological indicators, economically feasible durability. Electrodes made of an alloy of copper, tungsten carbide and nickel have the greatest erosion resistance.
The surface area of the cathode should exceed the area of the anode by 60-100 times, which, combined with the application of a positive voltage pulse to the anode, will reduce energy losses at the stage of discharge formation and increase efficiency. systems. Rational insulation materials are fiberglass, vacuum rubber, polyethylene.
An electrohydraulic fuse is used in an initiated discharge; it absorbs dynamic loads, the effects of high-current fields and working fluid, which leads to the destruction of the housing, insulation and electrode.
In an electrohydraulic fuse, the positive electrode is isolated from the body; an exploding conductor is installed between the electrode and a grounded body, which acts as a negative electrode.
Depending on the technological problems being solved, conductors made of copper, aluminum, and tungsten are used; Conductor dimensions range from diameter 0.25-2 mm, length 60-300 mm. The design of the electrohydraulic fuse must ensure the concentration of energy in the required direction and the formation of a cylindrical shock wave front, as well as the manufacturability of operations for installing and replacing the exploding conductor.
To fulfill part of these requirements, it is necessary that the body of the electrohydraulic fuse serves as a rigid barrier for the propagating wave front.
This is ensured by the use of special cumulative recesses in the fuse body and a certain combination of linear dimensions of the body and conductor. Thus, the diameter of the fuse body should be 60 times or more the diameter of the exploding conductor.
In recent years, new design schemes and special devices have been developed that increase the efficiency of the working bodies, ensuring that the action is directed towards the processing object of the generated waves and hydraulic flow.
Such devices include passive reflective surfaces, electrodes with complex geometries, and divergent wave generators. There are also devices for drawing the exploding conductor, which complicates the design of the fuse, but increases the manufacturability of the process.
To directly convert the energy of an electric discharge into the energy of a compression pulse, special electric explosive cartridges are used (Fig. 34).
The working fluid filling the technological unit plays a very significant role in the process of electrical discharge. It is in the liquid that the discharge is reproduced with the direct conversion of electrical energy into mechanical energy.
Ionization is observed in the liquid, as well as gas release of unreacted oxygen and hydrogen (up to 0.5 10 -6 m 3 / kJ), the liquid is drawn into motion by the propagating wave front, which forms a hydraulic flow in the technological unit, capable of performing mechanical work.
Water (technical, sea, distilled) and aqueous electrolytes are used as the working fluid; hydrocarbon (kerosene, glycerin, transformer oil) and silicone (polymethylsiloxane) liquids, as well as special dielectric, liquid and solid compositions. Process water, whose specific electrical conductivity is (1-10) S/m, has become more widely used.
The electrical conductivity of the liquid significantly affects the amount of energy required to form a discharge, since it determines the magnitude of the breakdown voltage and the speed of movement of the streamers. The minimum voltage at which streamers appear is estimated at 3.6 10 3 V/mm.
The specific electrical conductivity values (S/m) of some liquids used to fill the technological unit are given below.
Process water (tap)................................................... ............(1-10) 10 -2
Sea water................................................ ........................................1-10
Distilled water................................................ ........................4.3 -10 -4
Glycerol................................................. ........................................................ ..6.4 10 -6
It can be seen that dielectric liquids have low ionic conductivity. The specific electrical resistance of the liquid (r l) also determines the value of the electrical efficiency. and depends on the amount of energy introduced per unit volume of working fluid. Thus, for water, the parameter rj decreases with increase to values of 500-1000 kJ/; with a further increase in W 0, the parameter rz stabilizes within the range of 10-25 Ohm-m.
The electric discharge in a liquid also depends on the density of the working liquid - with increasing density, the peak of overvoltages and the steepness of the current decline decrease. To increase the voltage of the discharge circuit, and, accordingly, the value of the breakdown voltage, working fluids with low specific conductivity (for example, industrial water) should be used.
The use of liquids with higher conductivity facilitates the formation of sliding discharges; increases energy losses at the stage of channel formation and reduces the amplitude of the shock wave.
Viscous compositions are also used as a working fluid (spindle oil - 70%, aluminum powder - 20%, chalk - 10%), which increases the amplitude of the shock wave by 20-25% and reduces energy losses.
Metallized dielectric thread and paper tapes impregnated with electrolyte are also used as a dielectric. The introduction of a solid dielectric reduces the total energy consumption for breakdown (4-5 times), reduces the required number of streamers (4-6 times), reduces thermal radiation and ultraviolet radiation. The introduction of solid particles of conductive additives into the working fluid flow is used instead of exploding conductors.
Pulse generators are designed to produce pulses of a certain shape and duration. They are used in many circuits and devices. They are also used in measuring technology for setting up and repairing various digital devices. Rectangular pulses are great for testing the functionality of digital circuits, while triangular pulses can be useful for sweep or sweep generators.
The generator generates a single rectangular pulse by pressing a button. The circuit is assembled on logical elements based on a regular RS trigger, which also eliminates the possibility of bouncing pulses from the button contacts reaching the counter.
In the position of the button contacts, as shown in the diagram, a high level voltage will be present at the first output, and at the second output a low level or logical zero, when the button is pressed, the state of the trigger will change to the opposite. This generator is perfect for testing the operation of various meters
In this circuit, a single pulse is generated, the duration of which does not depend on the duration of the input pulse. Such a generator is used in a wide variety of options: to simulate input signals of digital devices, when testing the functionality of circuits based on digital microcircuits, the need to supply a certain number of pulses to some device under test with visual control of processes, etc.
As soon as the power supply to the circuit is turned on, capacitor C1 begins to charge and the relay is activated, opening the power supply circuit with its front contacts, but the relay will not turn off immediately, but with a delay, since the discharge current of capacitor C1 will flow through its winding. When the rear contacts of the relay are closed again, a new cycle will begin. The switching frequency of the electromagnetic relay depends on the capacitance of capacitor C1 and resistor R1.
You can use almost any relay, I took . Such a generator can be used, for example, to switch Christmas tree lights and other effects. The disadvantage of this scheme is the use of a large capacitor.
Another generator circuit based on a relay, with an operating principle similar to the previous circuit, but unlike it, the repetition frequency is 1 Hz with a smaller capacitor capacitance. When the generator is turned on, capacitor C1 begins to charge, then the zener diode opens and relay K1 operates. The capacitor begins to discharge through the resistor and the composite transistor. After a short period of time, the relay turns off and a new generator cycle begins.
The pulse generator, in Figure A, uses three AND-NOT logic elements and a unipolar transistor VT1. Depending on the values of capacitor C1 and resistors R2 and R3, pulses with a frequency of 0.1 - up to 1 MHz are generated at output 8. Such a huge range is explained by the use of a field-effect transistor in the circuit, which made it possible to use megaohm resistors R2 and R3. Using them, you can also change the duty cycle of the pulses: resistor R2 sets the duration of the high level, and R3 sets the duration of the low level voltage. VT1 can be taken from any of the KP302, KP303 series. - K155LA3.
If you use CMOS microcircuits, for example K561LN2, instead of K155LA3, you can make a wide-range pulse generator without using a field-effect transistor in the circuit. The circuit of this generator is shown in Figure B. To expand the number of generated frequencies, the capacitance of the timing circuit capacitor is selected by switch S1. The frequency range of this generator is 1 Hz to 10 kHz.
The last figure shows the circuit of the pulse generator, which includes the ability to adjust the duty cycle. For those who have forgotten, let us remind you. The duty cycle of pulses is the ratio of the repetition period (T) to the duration (t):
The duty cycle at the output of the circuit can be set from 1 to several thousand using resistor R1. The transistor operating in switching mode is designed to amplify power pulses
If there is a need for a highly stable pulse generator, then it is necessary to use quartz at the appropriate frequency.
The generator circuit shown in the figure is capable of generating rectangular and sawtooth pulses. The master oscillator is made on logic elements DD 1.1-DD1.3 of the K561LN2 digital microcircuit. Resistor R2 paired with capacitor C2 form a differentiating circuit, which generates short pulses with a duration of 1 μs at the output of DD1.5. An adjustable current stabilizer is assembled on a field-effect transistor and resistor R4. Current flows from its output to charging capacitor C3 and the voltage across it increases linearly. When a short positive pulse arrives, transistor VT1 opens and capacitor SZ discharges. Thereby forming a sawtooth voltage on its plates. Using a variable resistor, you can regulate the capacitor charge current and the steepness of the sawtooth voltage pulse, as well as its amplitude.
Variant of an oscillator circuit using two operational amplifiersThe circuit is built using two LM741 type op-amps. The first op amp is used to generate a rectangular shape, and the second one generates a triangular shape. The generator circuit is constructed as follows:
In the first LM741, feedback (FE) is connected to the inverting input from the output of the amplifier, made using resistor R1 and capacitor C2, and feedback is also connected to the non-inverting input, but through a voltage divider based on resistors R2 and R5. The output of the first op-amp is directly connected to the inverting input of the second LM741 through resistance R4. This second op amp, together with R4 and C1, form an integrator circuit. Its non-inverting input is grounded. Supply voltages +Vcc and –Vee are supplied to both op-amps, as usual to the seventh and fourth pins.
The scheme works as follows. Suppose that initially there is +Vcc at the output of U1. Then capacitance C2 begins to charge through resistor R1. At a certain point in time, the voltage at C2 will exceed the level at the non-inverting input, which is calculated using the formula below:
V 1 = (R 2 / (R 2 +R 5)) × V o = (10 / 20) × V o = 0.5 × V o
The output of V 1 will become –Vee. So, the capacitor begins to discharge through resistor R1. When the voltage across the capacitance becomes less than the voltage determined by the formula, the output signal will again be + Vcc. Thus, the cycle is repeated, and due to this, rectangular pulses are generated with a time period determined by the RC circuit consisting of resistance R1 and capacitor C2. These rectangular shapes are also input signals to the integrator circuit, which converts them into a triangular shape. When the output of op amp U1 is +Vcc, capacitance C1 is charged to its maximum level and produces a positive, upward slope of the triangle at the output of op amp U2. And, accordingly, if there is –Vee at the output of the first op-amp, then a negative, downward slope will be formed. That is, we get a triangular wave at the output of the second op-amp.
The pulse generator in the first circuit is built on the TL494 microcircuit, perfect for setting up any electronic circuits. The peculiarity of this circuit is that the amplitude of the output pulses can be equal to the supply voltage of the circuit, and the microcircuit is capable of operating up to 41 V, because it is not for nothing that it can be found in power supplies of personal computers.
You can download the PCB layout from the link above.
The pulse repetition rate can be changed with switch S2 and variable resistor RV1; resistor RV2 is used to adjust the duty cycle. Switch SA1 is designed to change the operating modes of the generator from in-phase to anti-phase. Resistor R3 must cover the frequency range, and the duty cycle adjustment range is regulated by selecting R1, R2
Capacitors C1-4 from 1000 pF to 10 µF. Any high-frequency transistors KT972
A selection of circuits and designs of rectangular pulse generators. The amplitude of the generated signal in such generators is very stable and close to the supply voltage. But the shape of the oscillations is very far from sinusoidal - the signal is pulsed, and the duration of the pulses and pauses between them is easily adjustable. Pulses can easily be given the appearance of a meander when the duration of the pulse is equal to the duration of the pause between them
Generates powerful short single pulses that set a logical level opposite to the existing one at the input or output of any digital element. The pulse duration is chosen so as not to damage the element whose output is connected to the input under test. This makes it possible not to disrupt the electrical connection of the element under test with the rest.
And finally, we got around to it. After assembling small coils, I decided to take a swing at a new circuit, more serious and complex to set up and operate. Let's move from words to action. The complete diagram looks like this:
It works on the principle of a self-generator. The breaker kicks the driver UCC27425 and the process begins. The driver supplies an impulse to the GDT (Gate Drive Transformator - literally: a transformer that controls the gates) with the GDT there are 2 secondary windings connected in antiphase. This connection ensures the alternating opening of the transistors. During opening, the transistor pumps current through itself and the 4.7 µF capacitor. At this moment, a discharge is formed on the coil, and the signal goes through the OS to the driver. The driver changes the direction of the current in the GDT and the transistors change (the one that was open closes, and the second one opens). And this process is repeated as long as there is a signal from the breaker.
GDT is best wound on an imported ring - Epcos N80. The windings are wound in a ratio of 1:1:1 or 1:2:2. On average, about 7-8 turns, you can calculate it if you wish. Let's consider an RD chain in the gates of power transistors. This chain provides Dead Time. This is the time when both transistors are closed. That is, one transistor has already closed, and the second has not yet had time to open. The principle is this: the transistor opens smoothly through a resistor and quickly discharges through a diode. The oscillogram looks something like this:
If you do not provide dead time, it may turn out that both transistors will be open and then a power explosion will occur.
Go ahead. OS (feedback) is made in this case in the form of a CT (current transformer). The CT is wound on an Epcos N80 ferrite ring with at least 50 turns. The lower end of the secondary winding is pulled through the ring and grounded. Thus, the high current from the secondary winding is converted into sufficient potential at the CT. Next, the current from the CT goes to the capacitor (smoothes out interference), Schottky diodes (pass only one half-cycle) and the LED (acts as a zener diode and visualizes generation). For generation to occur, the phrasing of the transformer must also be observed. If there is no generation or very weak, you just need to turn the CT over.
Let's look at the breaker separately. Of course I sweated with the breaker. I collected about 5 different ones... Some swell from HF current, others do not work as they should. Next I’ll tell you about all the breakers I made. I'll probably start from the very first - on TL494. The scheme is standard. Independent adjustment of frequency and duty cycle is possible. The circuit below can generate from 0 to 800-900 Hz if you replace the 1 uF capacitor with a 4.7 uF capacitor. Duty ratio from 0 to 50. Just what you need! However, there is one BUT. This PWM controller is very sensitive to RF current and various fields from the coil. In general, when connected to the coil, the breaker simply did not work, either everything was at 0 or CW mode. Shielding partially helped, but did not completely solve the problem.
The following breaker was assembled using UC3843 very often found in IIP, especially ATX, that’s where I actually took it from. The scheme is also not bad and is not inferior TL494 by parameters. Here it is possible to adjust the frequency from 0 to 1 kHz and the duty cycle from 0 to 100%. This suited me too. But again these pickups from the coil ruined everything. Even shielding didn't help here. I had to refuse, although I assembled it well on the board...
I decided to return to oak and reliable, but low-functional 555 . I decided to start with burst interrupter. The essence of an interrupter is that it interrupts itself. One microcircuit (U1) sets the frequency, another (2) the duration, and the third (U3) sets the operating time of the first two. Everything would be fine if it were not for the short pulse duration with U2. This breaker is designed for DRSSTC and can work with SSTC, but I didn’t like it - the discharges are thin, but fluffy. Then there were several attempts to increase the duration, but they were unsuccessful.
Generator circuits for 555
Then I decided to fundamentally change the circuit and make independent duration on the capacitor, diode and resistor. Many may consider this scheme absurd and stupid, but it works. The principle is this: the signal goes to the driver until the capacitor is charged (I think no one will argue with this). NE555 generates a signal, it goes through a resistor and a capacitor, and if the resistance of the resistor is 0 Ohm, then it goes only through the capacitor and the duration is maximum (as long as the capacitance is enough) regardless of the duty cycle of the generator. The resistor limits the charging time, i.e. The greater the resistance, the shorter the pulse will take. The driver receives a signal of shorter duration, but of the same frequency. The capacitor discharges quickly through a resistor (which goes to ground 1k) and a diode.
Advantages and disadvantages
pros: frequency independent duty cycle adjustment, SSTC will never go into CW mode if the breaker burns out.
Minuses: the duty cycle cannot be increased “infinitely”, as for example on UC3843, it is limited by the capacitance of the capacitor and the duty cycle of the generator itself (it cannot be greater than the duty cycle of the generator). The current flows smoothly through the capacitor.
I don’t know how the driver reacts to the latter (smooth charging). On the one hand, the driver can also smoothly open the transistors and they will heat up more. On the other side UCC27425- digital microcircuit. For it there is only a log. 0 and log. 1. This means that as long as the voltage is above the threshold, the UCC works; as soon as it drops below the minimum, it does not work. In this case, everything works as normal, and the transistors open completely.
Let's move from theory to practice
I assembled a Tesla generator into an ATX housing. Power supply capacitor 1000 uF 400V. Diode bridge from the same ATX at 8A 600V. I placed a 10 W 4.7 Ohm resistor in front of the bridge. This ensures smooth charging of the capacitor. To power the driver, I installed a 220-12V transformer and a stabilizer with a 1800 uF capacitor.
I screwed the diode bridges onto the radiator for convenience and for heat removal, although they barely heat up.
The breaker was assembled almost like a canopy, took a piece of PCB and cut out the tracks with a utility knife.
The power unit was assembled on a small radiator with a fan; it later turned out that this radiator was quite sufficient for cooling. The driver was mounted above the power one through a thick piece of cardboard. Below is a photo of the almost assembled design of the Tesla generator, but it is being tested; I measured the power temperature in various modes (you can see an ordinary room thermometer attached to the power one on thermoplastic).
The coil toroid is assembled from a corrugated plastic pipe with a diameter of 50 mm and covered with aluminum tape. The secondary winding itself is wound on a 110 mm pipe 20 cm high with 0.22 mm wire about 1000 turns. The primary winding contains as many as 12 turns, made with a margin in order to reduce the current through the power section. I did it with 6 turns at the beginning, the result is almost the same, but I think it’s not worth risking transistors for the sake of a couple of extra centimeters of discharge. The frame of the primary is an ordinary flower pot. From the beginning I thought that it would not pierce if I wrapped the secondary with tape and the primary on top of the tape. But alas, it broke through... Of course, it also broke through in the pot, but here the tape helped solve the problem. In general, the finished design looks like this:
Well, a few photos with the discharge
Now everything seems to be done.
A few more tips: don’t try to plug a coil into the network right away, it’s not a fact that it will work right away. Constantly monitor the power temperature; if it overheats, it may boom. Do not wind too high-frequency secondary transistors 50b60 can operate at a maximum of 150 kHz according to the datasheet, in fact a little more. Check the breakers, the life of the coil depends on them. Find the maximum frequency and duty cycle at which the power temperature is stable for a long time. A toroid that is too large can also damage the power supply.
Video of SSTC operation
P.S. Power transistors used IRGP50B60PD1PBF. Project files. Good luck, I was with you [)eNiS!
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