Receiving electrical injuries from residual charge. Experiments with capacitors Charging a capacitor with a pulsed current

We recently dealt with , now let's get down to it capacitors.

Capacitor- is a device for storing charge and energy of an electric field. Structurally, it is a “sandwich” of two conductors and a dielectric, which can be a vacuum, gas, liquid, organic or inorganic solid. The first domestic capacitors (glass jars with shot, covered with foil) were made in 1752 by M. Lomonosov and G. Richman.

What could be interesting about a capacitor? When starting to work on this article, I thought that I could collect and briefly present everything about this primitive part. But as I got to know the capacitor, I was surprised to realize that I couldn’t tell even a hundredth part of all the secrets and wonders hidden in it...

The capacitor is already more than 250 years old, but it does not even think of becoming obsolete.. In addition, 1 kg of “ordinary just capacitors” stores less energy than a kilogram of batteries or fuel cells, but is capable of releasing it faster than they do, while developing more power. - When a capacitor is quickly discharged, a high-power pulse can be obtained, for example, in photoflashes, optically pumped pulsed lasers and colliders. There are capacitors in almost any device, so if you don’t have new capacitors, you can remove them from there for experiments.

Capacitor charge is the absolute value of the charge of one of its plates. It is measured in coulombs and is proportional to the number of extra (-) or missing (+) electrons. To collect a charge of 1 coulomb, you will need 6241509647120420000 electrons. There are about the same number of them in a hydrogen bubble the size of a match head.

Since the ability to accumulate charges at the electrode is limited by their mutual repulsion, their transfer to the electrode cannot be endless. Like any storage device, a capacitor has a very specific capacity. That's what it's called - electrical capacitance. It is measured in farads and for a flat capacitor with plates of area S(each), located at a distance d, the capacity isSε 0 ε / d (atS >> d), Where ε - relative dielectric constant, andε 0 =8,85418781762039 * 10 -12 .

The capacitance of the capacitor is also equal to q/U, Where q- charge of the positive plate, U- tension between plates. The capacitance depends on the geometry of the capacitor and the dielectric constant of the dielectric, and does not depend on the charge of the plates.

In a charged conductor, the charges try to scatter from each other as far as possible and therefore are not in the thickness of the capacitor, but in the surface layer of the metal, like a film of gasoline on the surface of water. If two conductors form a capacitor, then these excess charges collect opposite each other. Therefore, almost the entire electric field of the capacitor is concentrated between its plates.

On each plate, charges are distributed so as to be away from neighbors. And they are located quite spaciously: in an air capacitor with a distance between the plates of 1 mm, charged up to 120 V, the average distance between electrons is more than 400 nanometers, which is thousands of times greater than the distance between atoms (0.1-0.3 nm), and This means that for millions of surface atoms there is only one extra (or missing) electron.

If reduce the distance between the plates, then the attractive forces will increase, and at the same voltage the charges on the plates will be able to “get along” more closely. Capacity will increase capacitor. This is what the unsuspecting professor at Leiden University, van Musschenbroeck, did. He replaced the thick-walled bottle of the world's first condenser (created by the German priest von Kleist in 1745) with a thin glass jar. He charged it and touched it, and when he woke up two days later, he said that he would not agree to repeat the experiment, even if they promised the French kingdom for it.

If you place a dielectric between the plates, they will polarize it, that is, they will attract the opposite charges of which it consists. This will have the same effect as if the plates were brought closer. A dielectric with a high relative dielectric constant can be considered as a good transporter of the electric field. But no conveyor is perfect, so no matter what wonderful dielectric we add on top of the existing one, the capacitance of the capacitor will only decrease. You can increase the capacitance only if you add a dielectric (or better yet, a conductor) instead of already existing but having a smaller ε.

There are almost no free charges in dielectrics. All of them are fixed either in a crystal lattice or in molecules - polar (representing dipoles) or not. If there is no external field, the dielectric is unpolarized, dipoles and free charges are scattered chaotically and the dielectric has no field of its own. in an electric field it is polarized: the dipoles are oriented along the field. Since there are a lot of molecular dipoles, when they are oriented, the pros and cons of neighboring dipoles inside the dielectric compensate each other. Only surface charges remain uncompensated - on one surface - one, on the other - another. Free charges in the external field also drift and separate.

In this case, different polarization processes occur with at different speeds. One thing is the displacement of electron shells, which occurs almost instantly, another thing is the rotation of molecules, especially large ones, and the third is the migration of free charges. The last two processes obviously depend on temperature, and in liquids they occur much more quickly than in solids. If the dielectric is heated, dipole rotations and charge migration will accelerate. If the field is turned off, the depolarization of the dielectric does not occur instantly either. It remains polarized for some time until thermal motion scatters the molecules into their original chaotic state. Therefore, for capacitors where the polarity is switched at high frequencies, only non-polar dielectrics are suitable: fluoroplastic, polypropylene.

If you disassemble a charged capacitor and then reassemble it (with plastic tweezers), the energy will not go anywhere, and the LED will be able to blink. It will even blink if you connect it to a capacitor in a disassembled state. This is understandable - during disassembly, the charge did not disappear from the plates, and the voltage even increased, since the capacity decreased and now the plates are literally bursting with charges. Wait, how did this tension increase, because then the energy will also increase? That’s right, we imparted mechanical energy to the system, overcoming the Coulomb attraction of the plates. Actually, this is the trick of electrification by friction - to hook electrons at a distance of the order of the size of atoms and drag them to a macroscopic distance, thereby increasing the voltage from several volts (and this is the voltage in chemical bonds) to tens and hundreds of thousands of volts. Now it’s clear why a synthetic jacket does not generate electric shock when you wear it, but only when you take it off? Wait, why not billions? A decimeter is a billion times larger than an angstrom, on which we snatched electrons? Yes, because the work of moving a charge in an electric field is equal to the integral of Eq over d, and this same E weakens quadratically with distance. And if on the entire decimeter between the jacket and the nose there was the same field as inside the molecules, then a billion volts would click on the nose.

Let's check this phenomenon - an increase in voltage when the capacitor is stretched - experimentally. I wrote a simple program inVisual Basic to receive data from our PMK018 controllerand displaying them on the screen. In general, we take two 200x150 mm plates of textolite, covered on one side with foil, and solder the wires going to the measuring module. Then we place a dielectric - a sheet of paper - on one of them and cover it with a second plate. The plates do not fit tightly, so we will press them on top with the body of the pen (if you press with your hand, you can create interference).

The measurement circuit is simple: potentiometerR1 sets the voltage (in our case it is 3 volts) applied to the capacitor, and the buttonS1 serves to supply it to the capacitor, or not to supply it.

So, press and release the button - we will see the graph shown on the left. The capacitor quickly discharges through the oscilloscope input. Now let's try to relieve the pressure on the plates during the discharge - we will see a voltage peak on the graph (right). This is exactly the desired effect. At the same time, the distance between the capacitor plates increases, the capacitance decreases, and therefore the capacitor begins to discharge even faster.

Here I seriously thought... It seems that we are on the verge of a great invention... After all, if when moving the plates apart, the voltage on them increases, but the charge remains the same, then you can take two capacitors, on one you push the plates apart on them, and at the point of maximum expansion transfer charge to a stationary capacitor. Then return the plates to their place and repeat the same thing in reverse, moving the other capacitor apart. In theory, the voltage on both capacitors will increase with each cycle by a certain number of times. Great idea for a power generator! It will be possible to create new designs for windmills, turbines and all that! So, great... for convenience, you can place all this on two disks rotating in opposite directions.... oh, what is this... ugh, this is a school electric machine! :(

It did not take root as a generator, since it is inconvenient to deal with such voltages. But at the nanoscale, everything can change. Magnetic phenomena in nanostructures are many times weaker than electric ones, and the electric fields there, as we have already seen, are enormous, so a molecular electrophoric machine can become very popular.

Capacitor as an energy store

It is very easy to make sure that energy is stored in the smallest capacitor. To do this, we need a transparent red LED and a constant current source (a 9-volt battery will do, but if the rated voltage of the capacitor allows, it is better to take a larger one). The experiment consists of charging a capacitor, and then connecting an LED to it (don’t forget about the polarity), and watching it blink. IN dark room a flash is visible even from capacitors of tens of picofarads. Some hundred million electrons emit one hundred million photons. However, this is not the limit, because the human eye can notice much weaker light. I just haven’t found any less capacitive capacitors. If the count goes to thousands of microfarads, spare the LED, and instead short the capacitor to a metal object to see a spark - obvious evidence of the presence of energy in the capacitor.

The energy of a charged capacitor behaves in many ways like potential mechanical energy - the energy of a compressed spring, a weight raised to a height, or a water tank (and the energy of an inductor, on the contrary, is similar to kinetic energy). The ability of a capacitor to store energy has long been used to ensure continuous operation of devices during short-term drops in supply voltage - from watches to trams.

The capacitor is also used to store "almost eternal" energy generated by shaking, vibration, sound, detecting radio waves or power grid radiation. Little by little, the accumulated energy from such weak sources over time allows wireless sensors and other electronic devices to operate for some time. This principle is the basis of an eternal “finger-type” battery for devices with modest power consumption (like TV remote controls). Its body contains a capacitor with a capacity of 500 millifarads and a generator that feeds it with oscillations at a frequency of 4-8 hertz with free power from 10 to 180 milliwatts. Generators based on piezoelectric nanowires are being developed that are capable of directing the energy of such weak vibrations as heartbeats, shoe soles hitting the ground, and vibrations of technical equipment into a capacitor.

Another source of free energy is inhibition. Usually, when a vehicle brakes, energy turns into heat, but it can be stored and then used during acceleration. This problem is especially acute for public transport, which slows down and accelerates at every stop, which leads to significant fuel consumption and air pollution from exhaust emissions. In the Saratov region in 2010, the Elton company created the Ecobus - an experimental minibus with unusual motor-wheel electric motors and supercapacitors - braking energy storage devices, reducing energy consumption by 40%. It uses materials developed in the Energia-Buran project, in particular carbon foil. In general, thanks to the scientific school created back in the USSR, Russia is one of the world leaders in the development and production of electrochemical capacitors. For example, Elton products have been exported abroad since 1998, and recently the production of these products began in the USA under a license from a Russian company.

The capacity of one modern capacitor (2 farads, photo on the left) is thousands of times greater than the capacity of the entire globe. They are able to store electric charge at 40 Pendant!

They are used, as a rule, in car audio systems to reduce the peak load on the car's electrical wiring (at moments of powerful bass impacts) and, due to the huge capacitance of the capacitor, suppress all high-frequency interference in the on-board network.

But this Soviet “grandfather’s chest” for electrons (photo on the right) is not so capacious, but can withstand a voltage of 40,000 volts (note the porcelain cups that protect all these volts from breakdown on the capacitor body). This is very convenient for an “electromagnetic bomb”, in which a capacitor is discharged onto a copper tube, which at the same moment is compressed from the outside by an explosion. It turns out very powerful electromagnetic pulse, disabling radio equipment. By the way, during a nuclear explosion, unlike a normal one, an electromagnetic pulse is also released, which once again emphasizes the similarity of the uranium nucleus to a capacitor. By the way, such a capacitor can be directly charged with static electricity from a comb, but of course it will take a long time to charge to full voltage. But it will be possible to repeat van Musschenbroeck’s sad experience in a very aggravated version.

If you simply rub a pen (comb, balloon, synthetic underwear, etc.) on your hair, the LED will not light up. This is because the excess (taken from the hair) electrons are captive, each at their own point on the surface of the plastic. Therefore, even if we hit some electron with the output of the LED, others will not be able to rush after it and create the current necessary for the LED to glow noticeably to the naked eye. It’s another matter if you transfer charges from a pen to a capacitor. To do this, take the capacitor by one terminal and rub the pen in turn, first on your hair, then on the free terminal of the capacitor. Why rub? To maximize the harvest of electrons from the entire surface of the pen! Let's repeat this cycle several times and connect an LED to the capacitor. It will blink, and only if the polarity is observed. So the capacitor became a bridge between the worlds of “static” and “ordinary” electricity :)

I took a high-voltage capacitor for this experiment, fearing a breakdown of the low-voltage one, but it turned out that this was an unnecessary precaution. When the charge supply is limited, the voltage across the capacitor can be much less than the power supply voltage. A capacitor can convert high voltage to low voltage. For example, static high-voltage electricity - into ordinary electricity. In fact, is there a difference: charging a capacitor with one microcoulomb from a source with a voltage of 1 V or 1000 V? If this capacitor is so capacious that a charge of 1 µC on it does not increase the voltage above the voltage of a one-volt power source (i.e. its capacitance is higher than 1 µF), then there is no difference. It’s just that if you don’t forcefully limit the pendants, then more of them will want to come running from a high-willed source. And the thermal power released at the terminals of the capacitor will be greater (and the amount of heat is the same, it will just be released faster, which is why the power is greater).

In general, apparently, any capacitor with a capacity of no more than 100 nf is suitable for this experiment. You can do more, but you will need to charge it for a long time to get enough voltage for the LED. But if the leakage currents in the capacitor are small, the LED will burn longer. You might think about creating a charging device based on this principle. cell phone from rubbing it against your hair during a conversation :)

Excellent high voltage capacitor is a screwdriver. In this case, its handle serves as a dielectric, and the metal rod and human hand serve as plates. We know that a fountain pen rubbed on hair attracts scraps of paper. If you rub a screwdriver on your hair, nothing will come of it - metal does not have the ability to take away electrons from proteins - it did not attract pieces of paper, and it did not. But if, as in the previous experiment, you rub it with a charged fountain pen, the screwdriver, due to its low capacity, quickly charges to a high voltage and pieces of paper begin to be attracted to it.

The LED also lights up from the screwdriver. It is impossible to capture a brief moment of his flash in a photo. But - let's remember the properties of the exponential - the extinction of the flash lasts a long time (by the standards of a camera shutter). And so we witnessed a unique linguistic-optical-mathematical phenomenon: the exhibitor was exposing the camera’s matrix!

However, why such difficulties - there is video recording. It shows that the LED flashes quite brightly:

When capacitors are charged to high voltage, the edge effect begins to play its role, consisting of the following. If a dielectric is placed in air between the plates and a gradually increasing voltage is applied to them, then at a certain voltage value a quiet discharge occurs at the edge of the plate, detectable by characteristic noise and glow in the dark. The magnitude of the critical voltage depends on the thickness of the plate, the sharpness of the edge, the type and thickness of the dielectric, etc. The thicker the dielectric, the higher the cr. For example, the higher the dielectric constant of a dielectric, the lower it is. To reduce the edge effect, the edges of the plate are embedded in a dielectric with high electrical strength, the dielectric gasket is thickened at the edges, the edges of the plates are rounded, and a zone with a gradually decreasing voltage is created at the edge of the plates by making the edges of the plates from a material with high resistance, reducing the voltage per one capacitor by dividing it into several series-connected ones.

That's why the founding fathers of electrostatics liked to have balls at the end of the electrodes. This, it turns out, is not a design feature, but a way to minimize the flow of charge into the air. There is nowhere else to go. If the curvature of some area on the surface of the ball is further reduced, then the curvature of neighboring areas will inevitably increase. And here, apparently, in our electrostatic affairs, it is not the average but the maximum curvature of the surface that is important, which is minimal, of course, for a ball.

What happens, the positive capacitance is obviously much larger than the negative one? No! Because the electrons were actually there not for our pampering, but for connecting atoms, and without any noticeable share of them, the Coulomb repulsion of the positive ions of the crystal lattice would instantly smash the most armored capacitor into dust :)

In fact, without a secondary plate, the capacitance of the “solitary halves” of the capacitor is very small: the electrical capacitance of a single piece of wire with a diameter of 2 mm and a length of 1 m is approximately 10 pF, and the entire globe is 700 μF.

In power electrical engineering, the first in the world to use a capacitor was Pavel Nikolaevich Yablochkov in 1877. He simplified and at the same time improved the Lomonosov capacitors, replacing shot and foil with liquid, and connecting the banks in parallel. He is responsible not only for the invention of innovative arc lamps, which conquered Europe, but also a number of patents related to capacitors. Let's try to assemble a Yablochkov capacitor using salted water as a conducting liquid, and a glass jar of vegetables as a jar. The resulting capacity was 0.442 nf. If we replace the jar with a plastic bag, which has a larger area and many times less thickness, the capacity will increase to 85.7 nf. (First, fill the bag with water and check for leakage currents!) The capacitor works - it even allows you to blink the LED! It also successfully performs its functions in electronic circuits

Metal plates should fit as tightly as possible to the dielectric, and it is necessary to avoid introducing an adhesive between the plate and the dielectric, which will cause additional losses on alternating current. Therefore, now mainly metal is used as plating, chemically or mechanically deposited on a dielectric (glass) or tightly pressed to it (mica).

Instead of mica, you can use a bunch of different dielectrics, whatever you like. Measurements (for dielectrics of equal thickness) showed that airε the smallest, for fluoroplastic it is larger, for silicone it is even larger, and for mica it is even larger, and in lead zirconate titanate it is simply huge. This is exactly how it should be according to science - after all, in fluoroplastic, electrons, one might say, are tightly chained to fluorocarbon chains and can only deviate slightly - there is nowhere for an electron to jump from atom to atom.

65 nanometers is the next goal of the Zelenograd plant Angstrem-T, which will cost 300-350 million euros. The company has already submitted an application for a preferential loan for the modernization of production technologies to Vnesheconombank (VEB), Vedomosti reported this week with reference to the chairman of the board of directors of the plant, Leonid Reiman. Now Angstrem-T is preparing to launch a production line for microcircuits with a 90nm topology. Payments on the previous VEB loan, for which it was purchased, will begin in mid-2017.

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Charging current at 100J and ~1 sec. when starting cold capacitors (first turn on) up to 10 amperes at peak, during operation up to 6A, and at the moment of switching on it’s absolutely terrible - 100A. If you successfully hit the voltage peak 310V / 3 Ohm = 103A.

So, even based on 6A we get impulse load in the network equivalent to 1-1.5kW - 6A * 220V = 1320W !!

And this is 100 J, and if there were several flashes, if I were a machine gun, I would be offended by such an impulse and after the first good flash I would not give any more current.
If we take a circuit with a power supply without a doubler, then the initial current surge is even greater and there is a clear asymmetry - only one half-cycle is used.

On the other hand - 100J when charging for 1 second. equivalent to 100 watts, well, 130 with all sorts of losses - not terrible power at all.What if you charge the capacitor through something like a power factor corrector - a booster voltage converter without a capacitor at the input?

The shape of the current will be something like this:

It turns out a profile mains voltage, filled with high-frequency current pulses.If the control circuit operates in the mode of limiting the output current, and interrupts charging upon reaching the specified voltage, then we will get fast charging- for example at 350W - 300J/sec. and smooth power control.
And the machine is happy, and the charging circuits are relatively low-current, and there are no large hot resistors, and it can be powered with a constant voltage, and the energy supervision is happy - the power factor is like that of a samovar...

There's just one BUT!I was doing a flash ALMOST according to the above diagram by Waldemar Szymanski.Here is the diagram I used.

if you don’t go into details, only the quenching resistor was set to 5.1 ohms and the capacitors in the doubler are 22mF, so there the 1A fuse lives happily ever after if the circuit works correctly. And if not, then this same fuse is there for emergency shutdown.So, either something was wrong in the calculations, or theory and practice do not coincide.

Taking a microcircuit and design from a datasheet will not work - you need to adapt it and strange questions begin -for example, how will the circuit behave when really large capacitor? - it will heat up until it charges it to 310V, and only then it will start working...

Everything is fine in the calculations - firstly, I assumed a 100uF charging capacitor and a 3 Ohm resistor, secondly fuse the device is quite inertial and can easily withstand a short pulse several times larger than the nominal value, and the machine that I mentioned also responds to a pulse overload 5 - 15 times larger than the nominal value (depending on the class).
In real conditions, with such an impulse in the network, the light will only blink slightly. For example, I can clearly see how a kilowatt electric kettle in the kitchen turns on.Here you would rather get an elegant solution without overloads and heating.

Everything is the same as with a capacitor, IN PORTIONS.Only the PORTION accumulates per charge, AND THE MAGNETIC FIELD IS IN THE COIL.

No current limitation in case of emergency...
The only drawback of the solution, unlike capacitance, is that inductance itself cannot limit the current after electromagnetic energy has been collected and the current can flow in vain.
And the capacitor will not take more than it will fit into.And in the end, the current will stop.And the coil also needs to be turned off... This is dangerous and unreliable...

If without a multiplier, then I agree - even if the switch breaks down, the capacitor will survive, but at reasonable currents it will take too long to charge, but with a multiplier - if you don’t turn it off in time, it will bang.Switching power supplies are quite well designed, but when charging the capacitor, the unit will operate in a short circuit - you need to take this into account somehow.

So, what I have found so far is that the flyback circuit is most suitable

She has output voltage does not depend on the input and also depends little on the turns ratio and you can easily charge the capacitor to any voltage. It turns out that there is no need to install a capacitor after the rectifier and the main capacitor will be charged not only by the peaks of the sine wave, but almost the entire period.
We get complete galvanic isolation from the network, good power factor (if without an input capacitor). A power transistor is needed for a fairly small current - 100 J/sec, about 3A (IRF830-IRF840).Theoretically, you can make it work on 12V without modification.

Of the minuses, the circuit is clearly more difficult to calculate (and you can’t do it by eye) and setup than thyristor ones. You need a fairly high-voltage transistor - according to books - twice the amplitude of the mains voltage + reserve - about 800-900V, or more complex circuit with 2 transistors at 400V, but it is still cheaper than a powerful IGBT and comparable to a thyristor.
You NEED to wind the transformer
If you don’t set out to isolate from the network, then the buck converter looks beautiful,
but it is step-down and the question is still unclear to me - what is more convenient: 300V and a larger capacity, or for example 400V-500V with a series connection of capacitors?

The unit charges 1300 uF to 310V in 2.5-4 seconds, depending on the condition of the batteries! Flash capacitors are protected from overvoltage, threshold impulse protection power transistor for current and something else...

This is how the duty kit turned out. Yes, the ability to charge from a 220V network is retained. But, when powered from the unit, the flash energy is almost one and a half times greater...

The idea about network flyback is good, if not for:

1) IRF840, the voltage will be low. Need 1200v

2) Diode, if the voltage on the capacitors is 600--1200V diodemay not be enough.

3) ultrafasts at such voltages will have a drop of 2-3 volts. Efficiency 80-85 cannot be higher.

4) In order not to torment yourself, you can roughly evaluate all ideologies http://schmidt-walter.eit.h-da.de/smps_e/smps_e.html#Aww

5) About the charge of the capacitor from the network up to 300V, this is a pitchfork on the water, let’s say the top of the sine wave is cut off by 25-30 volts. And the Chinese tester will show 220V in the network, but you can charge the jar up to 300 volts.

6) Energy is calculated as voltage squared per capacitance; it is always more profitable to increase the voltage.

7) Reliable pulse block more complex and more expensive than a thyristor charger. It makes sense to use it only in a few cases:

Charging from batteries
--- high speed charging with small dimensions (meaning a speed of 600-1000 J/sec)
--- Galvanic isolation from the network (usually solved by competent designs)

You will be pleasantly surprised! The choke, with the same size, is one and a half times more powerful and there is no doubling of the voltage on the diode! But without galvanic isolation you will survive somehow! We lived without her...You work in the range of 240-410V (after a mains rectifier and smoothing. For an output voltage of 410V you don’t even need a boost winding.

in the Oblique Bridge they forgot one diode and an output choke; without a choke it would be very difficult for the keys.

In terms of simplicity, of course, the flyback is certainly better, there is a minimum of parts, it is not afraid of short circuits, etc.

What are we talking about? This and there is a flyback 2-key circuit.

But then the main advantage of flyback (simplicity) is lost; you need to install an upper-side driver, or a transformer driver.

So:Only a flyback circuit is suitable for charging the flash capacitor, because it is a source of current (all forward drives are voltage sources - and we already have a voltage source - a 220 volt network).

Let's look at some theory. I’m not giving the diagram, everyone knows it very well.

The maximum voltage on the transistor is determined by the sum of the rectified supply voltage and reverse voltage on primary winding. Everything is clear with the supply, it is 310 volts (plus, minus). The reverse voltage on the primary winding depends _only_ on the duty cycle of the pulse or duty cycle! Let me explain - in a steady state of operation, the energy stored in the forward motion must be completely transferred to the load in the reverse (if it is not all transferred, then it begins to accumulate in the core, we reach the current limit of the primary winding (and, possibly, saturation) and The PWM controller reduces the pulse duration). Let's remember the formula:

U = L(dI/dt)

those. if T of the reverse stroke is twice as large as that of the forward stroke, then U of the reverse stroke will be two times less. ABOUThere at D = 33% we get a reverse voltage of 155 volts. All. This is our calculated value, we rely on it. TThus, not counting the surge due to leakage inductance, there will be only 310 + 155 = 465 volts on the switch! At _any_ output voltage (output voltage is calculated as N2*155/N1, where N1 and N2 are the number of turns in the primary and secondary windings, respectively). N1 is selected based on T forward stroke and the energy that must be transferred in one pulse. N2 is selected to achieve the specified maximum output voltage. ABOUTThere was a problem of overshoot due to leakage inductance. Its amplitude is not limited by anything, and the power depends on the current through the primary winding and, in fact, the leakage inductance. You can follow the standard path and install a snubber, then all this energy will be released on its resistor (or zener diode). You don’t have to install a snubber, then the energy will be released on the switch (mosfets are quite resistant to avalanche processes and allow you to dissipate a fairly large emission power without failure or deterioration of parameters, which cannot be said about bipolars).
But, in our case, there is no need to decouple the flash from the network, so we can make a pulse transformer in the form of an autotransformer (or a choke with a tap) and... then we will not have leakage inductance at all! In this case, the voltage on the key will always be 465 volts! HAs for the reverse voltage on the output diode, then yes, it will be large and may well exceed a kilovolt (i.e., the voltage for which most modern diodes are designed). but here we can connect two diodes in series and get a 2 kilovolt rectifier.

So, we have calculated the circuit for the maximum output voltage. what will happen to it if we want to stop charging the capacitor at a voltage two (for example) times less? but nothing bad. the voltage amplitude on the key will not even reach 465 volts - it will be 310 + 155/2 volts.

The main problem in this circuit will be the manufacture of the transformer - it will have to store a sufficiently large amount of energy at each pulse in order to charge the output capacitor at the required speed. it can be made on a fairly large W-shaped core with a gap or on a throttle ring with low permeability. the parameters can be calculated and/or selected experimentally by winding a winding, passing current through it and watching the moment of saturation. Mthe maximum current through the switch will be more than modest - 4-6 amperes, depending on the circuit mode (discontinuous or continuous currents) and power (I calculated at about 300-320 watts).

I present a sketch of the scheme. The circuit is based on the UC3842 (or 3844) - an inexpensive PWM controller (in principle, the circuit can be adapted for any other).

I'll briefly tell you how everything works.

When you connect the power (I leave the input filter, rectifier and capacitor to your choice) through resistor R7, capacitor C3 is charged to a voltage of 16.5 volts, which is the threshold for starting the PWM controller. After this, power is taken from winding III of the transformer through a rectifier and filter R9, VD4, C8. Diode VD1 is necessary so that only capacitor C3, but not C8, is charged through resistor R7. It should be noted that winding III is connected in such a way that the voltage on it is taken in forward motion, and not in reverse, and thus does not depend on the output voltage of the unit, but depends only on the supply voltage. Winding IV is connected using the same principle, which provides power to the feedback circuit. Since the currents in these circuits are small (limited by resistors R8 and R9), their inclusion has virtually no effect on the operation of the circuit.

The frequency and maximum duty cycle of the PWM generator are set by capacitor C1 and resistor R1. I provide approximate data in the diagram; these elements may have to be selected (I planned a frequency of 100 KHz). The general principle of operation of a PWM generator is as follows: at the beginning, capacitor C1 is charged through resistor R1 from the reference voltage of the microcircuit (5 volts), then discharged through an internal current source. At the same time, during the process of discharging the capacitor, the output voltage of the microcircuit is always low (i.e., dead time).

Resistor R2 produces a voltage proportional to the current through the switch. When it reaches 4A (voltage 1V at the CS input), the PWM closes the transistor. The R3C6 filter is designed to suppress noise associated with transistor switching. Resistor R1 and diode VD2 are designed to open the key relatively slowly and close it as quickly as possible.

So, now let's look at getting the output voltage. While the key is open, current flows through winding I of the transformer. At the same time, the voltage on diodes VD5-VD6 is reversed and they are closed. When the switch is closed, the voltage on windings I and II sharply changes sign, the diodes open and begin to charge the capacitor with a linearly decreasing current. Due to the fact that in this case the voltage is also taken from the primary winding, we have no leakage inductance at all, and we do not need to install a snubber. The only drawback of this circuit is that the output voltage has a different “common” wire and is galvanically connected to the network. But for powering flashes this does not matter.

The TL431A and optocoupler 817C have an output voltage stabilizer, which is regulated by resistor R16 from approximately 150 to 350 volts. Resistor R13 is needed so that the capacitor is constantly discharged a little and the PWM controller does not turn off when the specified voltage is reached (since it powers itself and the feedback circuit). Although, I’m not entirely sure that such power supply will work reliably - it needs to be assembled and tested. Alternatively, you can power the controller and feedback from a separate power source on a transformer, but this will increase the dimensions of the structure.

As I said earlier, the approximate data of the transformer are windings I and II of 500 μH each, windings III and IV - such that the required voltages are generated on them during forward running (about 16 V and 12 V, respectively). The transformer must withstand 4A current in the primary winding without saturation. In principle, the current can be different - this will only change the power of the unit and the charging rate of the capacitor (only R2 must be selected for the maximum permissible winding current).

Structurally, it is a “sandwich” of two conductors and a dielectric, which can be a vacuum, gas, liquid, organic or inorganic solid. The first domestic capacitors (glass jars with shot, covered with foil) were made in 1752 by M. Lomonosov and G. Richter.

The capacitor is already more than 250 years old, but it does not even think of becoming obsolete.. In addition, 1 kg of “ordinary just capacitors” stores less energy than a kilogram of batteries or fuel cells, but is capable of releasing it faster than they do, while developing more power. — When a capacitor is quickly discharged, a high-power pulse can be obtained, for example, in photoflashes, optically pumped pulsed lasers and colliders. There are capacitors in almost any device, so if you don’t have new capacitors, you can remove them from there for experiments.

Since the ability to accumulate charges at the electrode is limited by their mutual repulsion, their transfer to the electrode cannot be endless. Like any storage device, a capacitor has a very specific capacity. That's what it's called - electrical capacitance. It is measured in farads and for a flat capacitor with plates of area S(each), located at a distance d, the capacity is Sε 0 ε/d(at S>> d), Where ε – relative dielectric constant, and ε 0 =8,85418781762039 * 10 -12 .

The capacitance of the capacitor is also equal to q/U, Where q– charge of the positive plate, U— tension between plates. The capacitance depends on the geometry of the capacitor and the dielectric constant of the dielectric, and does not depend on the charge of the plates.

In this case, different polarization processes occur at different speeds. One thing is the displacement of electron shells, which occurs almost instantly, another thing is the rotation of molecules, especially large ones, and the third is the migration of free charges. The last two processes obviously depend on temperature, and in liquids they occur much more quickly than in solids. If the dielectric is heated, dipole rotations and charge migration will accelerate. If the field is turned off, the depolarization of the dielectric does not occur instantly either. It remains polarized for some time until thermal motion scatters the molecules into their original chaotic state. Therefore, for capacitors where the polarity is switched at high frequencies, only non-polar dielectrics are suitable: fluoroplastic, polypropylene.

Let's check this phenomenon - an increase in voltage when the capacitor is stretched - experimentally. I wrote a simple program in Visual Basic to receive data from our PMK018 controller and display it on the screen. In general, we take two 200x150 mm plates of textolite, covered on one side with foil, and solder the wires going to the measuring module. Then we put a dielectric - a sheet of paper - on one of them and cover it with a second plate. The plates do not fit tightly, so we will press them on top with the body of the pen (if you press with your hand, you can create interference).

The measurement circuit is simple: potentiometer R1 sets the voltage (in our case it is 3 volts) supplied to the capacitor, and button S1 serves to supply it to the capacitor or not.

Capacitor as an energy store

It is very easy to make sure that energy is stored in the smallest capacitor. To do this, we need a transparent red LED and a constant current source (a 9-volt battery will do, but if the rated voltage of the capacitor allows, it is better to take a larger one). The experiment consists of charging a capacitor, and then connecting an LED to it (don’t forget about the polarity), and watching it blink. In a dark room, a flash is visible even from capacitors of tens of picofarads. Some hundred million electrons emit one hundred million photons. However, this is not the limit, because the human eye can notice much weaker light. I just haven’t found any less capacitive capacitors. If the count goes to thousands of microfarads, spare the LED, and instead short the capacitor to a metal object to see a spark - an obvious indication of the presence of energy in the capacitor.

The capacitor is also used to store "almost eternal" energy generated by shaking, vibration, sound, detecting radio waves or power grid radiation. Little by little, the accumulated energy from such weak sources over time allows wireless sensors and other electronic devices to operate for some time. This principle is the basis of an eternal “finger-type” battery for devices with modest power consumption (like TV remote controls). Its body contains a capacitor with a capacity of 500 millifarads and a generator that feeds it with oscillations at a frequency of 4–8 hertz with a free power of 10 to 180 milliwatts. Generators based on piezoelectric nanowires are being developed that are capable of directing the energy of such weak vibrations as heartbeats, shoe soles hitting the ground, and vibrations of technical equipment into a capacitor.

Another source of free energy is inhibition. Usually, when a vehicle brakes, energy turns into heat, but it can be stored and then used during acceleration. This problem is especially acute for public transport, which slows down and accelerates at every stop, which leads to significant fuel consumption and air pollution from exhaust emissions. In the Saratov region in 2010, the Elton company created the Ecobus - an experimental minibus with unusual motor-wheel electric motors and supercapacitors - braking energy storage devices that reduce energy consumption by 40%. It uses materials developed in the Energia-Buran project, in particular carbon foil. In general, thanks to the scientific school created back in the USSR, Russia is one of the world leaders in the development and production of electrochemical capacitors. For example, Elton products have been exported abroad since 1998, and recently the production of these products began in the USA under a license from a Russian company.

The capacity of one modern capacitor (2 farads, photo on the left) is thousands of times greater than the capacity of the entire globe. They are capable of storing an electrical charge of 40 Coulombs!

But this Soviet “grandfather’s chest” for electrons (photo on the right) is not so capacious, but can withstand a voltage of 40,000 volts (note the porcelain cups that protect all these volts from breakdown on the capacitor body). This is very convenient for an “electromagnetic bomb”, in which a capacitor is discharged onto a copper tube, which at the same moment is compressed from the outside by an explosion. The result is a very powerful electromagnetic pulse that disables radio equipment. By the way, during a nuclear explosion, unlike a normal one, an electromagnetic pulse is also released, which once again emphasizes the similarity of the uranium nucleus to a capacitor. By the way, such a capacitor can be directly charged with static electricity from a comb, but of course it will take a long time to charge to full voltage. But it will be possible to repeat van Musschenbroeck’s sad experience in a very aggravated version.

In general, apparently, any capacitor with a capacity of no more than 100 nf is suitable for this experiment. You can do more, but you will need to charge it for a long time to get enough voltage for the LED. But if the leakage currents in the capacitor are small, the LED will burn longer. You might think about using this principle to create a device for recharging a cell phone by rubbing it against your hair during a conversation :)

An excellent high voltage capacitor is a screwdriver. In this case, its handle serves as a dielectric, and the metal rod and human hand serve as plates. We know that a fountain pen rubbed on hair attracts scraps of paper. If you rub a screwdriver on your hair, nothing will come of it - metal does not have the ability to take away electrons from proteins - it did not attract the pieces of paper, and it did not. But if, as in the previous experiment, you rub it with a charged fountain pen, the screwdriver, due to its low capacity, quickly charges to a high voltage and pieces of paper begin to be attracted to it.

However, why such difficulties - there is video recording. It shows that the LED flashes quite brightly:

When capacitors are charged to high voltages, the edge effect begins to play a role, which consists of the following. If a dielectric is placed in air between the plates and a gradually increasing voltage is applied to them, then at a certain voltage value a quiet discharge occurs at the edge of the plate, detectable by characteristic noise and glow in the dark. The magnitude of the critical voltage depends on the thickness of the plate, the sharpness of the edge, the type and thickness of the dielectric, etc. The thicker the dielectric, the higher the cr. For example, the higher the dielectric constant of a dielectric, the lower it is. To reduce the edge effect, the edges of the plate are embedded in a dielectric with high electrical strength, the dielectric gasket is thickened at the edges, the edges of the plates are rounded, and a zone with a gradually decreasing voltage is created at the edge of the plates by making the edges of the plates from a material with high resistance, reducing the voltage per one capacitor by dividing it into several series-connected ones.

Hmm.. but if the capacity of a body is the ability to accumulate charge, then it is probably very different for positive and negative charges…. Let's imagine a spherical capacitor in a vacuum... Let's charge it negatively from the heart, not sparing power plants and gigawatt-hours (that's what's good about a thought experiment!)... but at some point there will be so many excess electrons on this ball that they will simply start scattering around the entire vacuum, just not to be in such electronegative tightness. But this will not happen with a positive charge - electrons, no matter how few of them remain, will not fly away from the crystal lattice of the capacitor.
What happens, the positive capacitance is obviously much larger than the negative one? No! Because the electrons were actually there not for our pampering, but for connecting atoms, and without any noticeable share of them, the Coulomb repulsion of the positive ions of the crystal lattice would instantly smash the most armored capacitor into dust :)

It is possible to construct an absolute standard of capacity by calculating its capacity using physical formulas based on accurate measurements of the dimensions of the plates. This is how the most precise capacitors in our country are made, which are located in two places. The state standard GET 107-77 is located in the Federal State Unitary Enterprise SNIIM and consists of 4 unsupported coaxial-cylindrical capacitors, the capacitance of which is calculated with high accuracy using the speed of light and units of length and frequency, as well as a high-frequency capacitive comparator, which allows you to compare the capacitances of capacitors brought for verification with a standard (10 pf) with an error of less than 0.01% in the frequency range 1-100 MHz (photo on the left).

Standard GET 25-79 (photo on the right), located at the Federal State Unitary Enterprise VNIIM named after. DI. Mendeleev contains a calculation capacitor and an interferometer in a vacuum block, a capacitive transformer bridge complete with capacitance measures and a thermostat, and radiation sources with a stabilized wavelength. The standard is based on a method for determining the increments in the capacitance of a system of cross electrodes of a design capacitor when the length of the electrodes changes by a given number of wavelengths of highly stable light radiation. This ensures that a precise 0.2 pF capacitance value is maintained with an accuracy of better than 0.00005%

But at the radio market in Mitino, I found it difficult to find a capacitor with an accuracy higher than 5% 🙁 Well, let's try to calculate the capacitance using formulas based on voltage and time measurements through our favorite PMK018. We will calculate capacity in two ways. The first method is based on the properties of the exponential and the ratio of voltages on the capacitor, measured at different moments of the discharge. The second is by measuring the charge given off by the capacitor during discharge; it is obtained by integrating the current over time. The area limited by the current graph and coordinate axes is numerically equal to the charge given by the capacitor. For these calculations, you need to know exactly the resistance of the circuit through which the capacitor is discharged. I set this resistance with a 10 kOhm precision resistor from an electronic kit.

And here are the results of the experiment. Pay attention to how beautiful and smooth the exhibitor turned out. It is not mathematically calculated by a computer, but directly measured from nature itself. Thanks to the coordinate grid on the screen, it is clear that the property of the exponential is precisely observed - at equal intervals of time it decreases by an equal number of times (I even measured it with a ruler on the screen :) Thus, we see that physical formulas quite adequately reflect the reality around us.

As you can see, the measured and calculated capacitance approximately coincides with the nominal one (and with the readings of Chinese multimeters), but not exactly. It’s a pity that there is no standard to determine which of them is true! If anyone knows a standard container that is inexpensive or available at home, be sure to write about it here in the comments.

In power electrical engineering, the first in the world to use a capacitor was Pavel Nikolaevich Yablochkov in 1877. He simplified and at the same time improved the Lomonosov capacitors, replacing shot and foil with liquid, and connecting the banks in parallel. He owns not only the invention of the innovative arc lamps that conquered Europe, but also a number of patents related to capacitors. Let's try to assemble a Yablochkov capacitor using salted water as a conducting liquid, and a glass jar of vegetables as a jar. The resulting capacity was 0.442 nf. If we replace the jar with a plastic bag, which has a larger area and many times less thickness, the capacity will increase to 85.7 nf. (First, fill the bag with water and check for leakage currents!) The capacitor works - it even allows you to blink the LED! It also successfully performs its functions in electronic circuits (I tried connecting it to a generator instead of a regular capacitor - everything works).

Water here plays a very modest role as a conductor, and if you have foil, you can do without it. Following Yablochkov, we will do the same. Here is a mica and copper foil capacitor with a capacity of 130 pf.

The metal plates should fit as closely as possible to the dielectric, and it is necessary to avoid introducing an adhesive between the plate and the dielectric, which will cause additional losses on alternating current. Therefore, now mainly metal is used as plating, chemically or mechanically deposited on a dielectric (glass) or tightly pressed to it (mica).

Instead of mica, you can use a bunch of different dielectrics, whatever you like. Measurements (for dielectrics of equal thickness) showed that air ε the smallest, for fluoroplastic it is larger, for silicone it is even larger, and for mica it is even larger, and in lead zirconate titanate it is simply huge. This is exactly how it should be according to science - after all, in fluoroplastic, electrons, one might say, are tightly chained to fluorocarbon chains and can only deviate slightly - there is nowhere for an electron to jump from atom to atom.

You can conduct such experiments yourself with substances that have different dielectric constants. What do you think has a higher dielectric constant, distilled water or oil? Salt or sugar? Paraffin or soap? Why? Dielectric constant depends on a lot of things... a whole book could be written about it.

That's all? 🙁

No, not all! There will be a continuation in a week! 🙂