How to make a capacitor?
An inventor lives in the soul of each of us, and therefore amateur radio is a fairly popular hobby. Self-manufacturing radio components - one of the most interesting components of this hobby. In this article we will talk about how to make a capacitor with your own hands at home.
materials
To make a capacitor, we need:
- foil,
- iron,
- papyrus paper,
- paraffin,
- lighter.
The foil does not need additional preparation, but with the help of the last three components we have to make waxed paper.
Manufacturing
So, the materials are prepared, let's get to work:
- We heat the paraffin and carefully process the papyrus paper.
- We fold it into an "accordion", the width of each section of which is about 30 mm. The number of harmonica layers determines the capacitance of the capacitor, each layer corresponds to approximately 100 pF.
- In each section we put a piece of foil with an area of 30 by 45 mm.
- We fold the accordion and iron it with a warm iron.
- Everything, the capacitor is ready! The pieces of foil peeking out are the connecting contacts of our capacitor, through which it can be connected to the circuit.
We got the simplest household capacitor, while it is worth noting that the thicker and better the foil, the more high-voltage it will be. However, we draw your attention to the fact that it is better not to try to make a capacitor at home that can withstand more than 50 kV. "Amateur professionals" advise, if you want to get close to this value, use lamination bags as a dielectric, but you will need a laminator to heat them.
This element is rightfully considered to be super universal, since it can be simultaneously used in the manufacture and repair of a wide variety of devices. And even if it is not difficult to purchase it in a ready-made form, many amateur craftsmen are happy to experiment, trying or even successfully making a capacitor with their own hands. Everything that is needed to create a home-made capacitor is described in detail above and, in principle, there should not be any difficulties with any of the necessary elements, since they can be found on the farm or, at worst, on free sale. The only exception, perhaps, can be paraffin paper, which is usually made independently using materials such as paraffin, papyrus and a disposable lighter (alternatively, you can use any other safe source of an open flame).
So, in order to process the paper properly, you should carefully heat the paraffin with a fire and walk its softened part over the entire surface of the papyrus on both sides. After the work is completed, and the material has properly set, the resulting paraffin paper must be folded with an accordion (meaning transverse advancement). The technique is common, but it involves maintaining a certain step (every three centimeters) and in order to make the fold line as accurate as possible, it is advisable to outline the first strip with a simple pencil even before initialing. You can continue in the same vein, completely drawing out the entire sheet, or you can act, focusing solely on the first segment (as convenient for you). As for the number of layers required, this indicator is determined solely by the capacity of the future product.
At this stage, the formed accordion should be put aside for a while in order to proceed with the preparation of rectangular pieces of foil, the dimensions of which should correspond in this case to the data 3 by 4.5 centimeters. These blanks are necessary to make the metal layer of the capacitor, therefore, at the end of the above work, the foil is inserted into all layers of the accordion, making sure that it fits evenly, after which they proceed to ironing the folded blank with a conventional iron. Paraffin and foil should do their job, providing a strong bond between themselves (other methods for soldering a capacitor at home are not practiced), after which the capacitor can be considered absolutely ready. As for the foil elements protruding beyond the former accordion, it should not give cause for concern, since they play the role of connecting contacts.
It is with the help of these small fragments that the with my own hands the capacitor can be fully used by connecting it to an electrical circuit. Naturally, we are talking about a primitive device, and in order to somehow increase its performance, it is necessary to use a higher quality foil with a high density, although here it is extremely important not to overdo it, since there are certain limits on the voltage used for crafts for adults of this kind. So, for example, it is better not to experiment, trying to make a capacitor with your own hands that can accept too high a voltage (more than 50 Volts), although some "homemade" ones manage to get around this side of the issue by using lamination bags instead of standard dielectrics, as well as a laminator for safe soldering.
There are several other methods for making a homemade capacitor, and one of them involves working with a higher voltage. The famous technique "Glass" can be attributed to it, the name of which came from the improvised tool used - a faceted glass. This element is necessary for wrapping with foil with an inner and outside, and this should be done in such a way that the fragments of the material used do not touch each other. The design itself in an already "assembled" form necessarily provides for the presence of supplies, after which it can be considered completely ready for use for its intended purpose. At the same time, during its inclusion in the circuit, it is necessary to carefully observe all the necessary safety measures in order to avoid possible negative consequences.
Alternatively, you can try to make your own hands and a more advanced design, using such improvised means as glass plates of the same size, all the same good old high-density foil and epoxy resins designed to securely connect the listed materials to each other. The undoubted advantage of such a homemade capacitor is that it is able to carry out more quality work, as they say, "without breakdown". However, as you know, a barrel of honey usually does not do without a fly in the ointment, and in this case it directly relates to one significant drawback of this invention, which lies in its more than impressive dimensions, which makes keeping such a "colossus" at home not very convenient and rational.
Requirements to reduce the size of radio components while increasing them technical specifications led to the emergence of a large number of devices that are used everywhere today. This fully affected the capacitors. The so-called ionisters or supercapacitors are elements with a large capacity (the range of this indicator is quite wide from 0.01 to 30 farads) with a charging voltage of 3 to 30 volts. However, their size is very small. And since the subject of our conversation is a do-it-yourself ionistr, it is necessary first of all to deal with the element itself, that is, what it is.
Design features of the ionistr
In fact, this is an ordinary capacitor with a large capacitance. But ionistors have high resistance, because the element is based on an electrolyte. This is the first. The second is a small charging voltage. The thing is that in this supercapacitor, the plates are located very close to each other. This is precisely the reason for the reduced voltage, but it is for this reason that the capacitance of the capacitor increases.
Factory ionistry are made of different materials. Linings are usually made of foil, which delimits the dry substance of the separating action. For example, activated carbon (for large plates), metal oxides, polymeric substances that have high electrical conductivity.
We collect the ionistr with our own hands
Assembling an ionistr with your own hands is not the easiest thing, but you can still do it at home. There are several designs where there are different materials. We offer one of them. For this you will need:
- metal coffee jar (50 g);
- activated carbon, which is sold in pharmacies, can be replaced with crushed carbon electrodes;
- two circles of copper plate;
- cotton wool
The first step is to prepare the electrolyte. To do this, you first need to grind activated carbon into powder. Then make a saline solution, for which you need to add 25 g of salt to 100 g of water, and mix it all well. Further, activated carbon powder is gradually added to the solution. Its quantity determines the consistency of the electrolyte, it should be as dense as putty.
After that, the finished electrolyte is applied to copper circles (on one side). Please note that the thicker the electrolyte layer, the greater the capacity of the ionistr. And one more thing, the thickness of the applied electrolyte on two circles should be the same. So, the electrodes are ready, now they need to be demarcated with a material that would allow electric current to pass through, but not let the coal powder through. For this, ordinary cotton wool is used, although there are many options here. The thickness of the cotton layer determines the diameter of the metal coffee jar, that is, this entire electrode structure should fit comfortably into it. Hence, in principle, it is necessary to select the dimensions of the electrodes themselves (copper circles).
It remains only to connect the electrodes themselves to the terminals. Everything, a do-it-yourself ionistr, and even at home, is ready. This design does not have a very large capacity - no higher than 0.3 farads, and the charging voltage is only one volt, but this is a real ionistr. 
Conclusion on the topic
What else can be said in addition about this element. If we compare it, for example, with a nickel-metal hydride type battery, then the ionistr can easily hold a supply of electricity up to 10% of the battery power. In addition, the voltage drop in it occurs linearly, and not abruptly. But the level of charge of the element depends on its technological purpose.
Structurally, this is a "sandwich" of two conductors and a dielectric, which can be vacuum, gas, liquid, organic or inorganic solid. The first domestic capacitors (glass jars with shot, glued with foil) were made in 1752 by M. Lomonosov and G. Richter.
What can be interesting in a capacitor? Getting started on this article, I thought that I could collect and summarize everything about this primitive detail. But as I got to know the capacitor, I was surprised to understand that it would not be possible to tell even a hundredth of all the secrets and miracles hidden in it ...
The capacitor is already over 250 years old, but it does not even think of becoming obsolete .. In addition, 1 kg of “ordinary simple capacitors” stores less energy than a kilogram of batteries or fuel cells, but is able to give it out faster than they, while developing more power. - With the rapid discharge of the capacitor, a high-power pulse can be obtained, for example, in photo flashes, pulsed lasers with optical pumping and colliders. There are capacitors in almost any device, so if you do not have new capacitors, you can drop them out for experiments.
Capacitor charge is the absolute value of the charge of one of its plates. It is measured in pendants and is proportional to the number of extra (-) or missing (+) electrons. To collect a charge of 1 pendant, you need 6241509647120420000 electrons. In a hydrogen bubble, the size of a match head, there are about the same number of them.
Since the ability to accumulate charges at the electrode is limited by their mutual repulsion, their transfer to the electrode cannot be infinite. Like any storage, a capacitor has a well-defined capacity. That's what it's called - electrical capacitance. It is measured in farads and for a flat capacitor with plates with an area S(each) located at a distance d, the capacitance is Sε 0 ε/d(at S>> d), where ε is the relative permittivity, and ε 0 =8,85418781762039 * 10 -12 .
The capacitance of the capacitor is also q/U, where q is the positive charge, U is the tension between the 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 are collected opposite each other. Therefore, almost the entire electric field of the capacitor is concentrated between its plates.
On each plate, the charges are distributed so as to be away from the neighbors. And they are quite spacious: 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 there is only one extra (or missing) electron for millions of surface atoms.
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 densely. Capacity will increase capacitor. And so did the unsuspecting professor at the University of Leiden van Muschenbroek. He replaced the thick-walled bottle of the world's first condenser (invented by the German priest von Kleist in 1745) with a thin glass jar. He charged it and touched it, and waking up two days later he said that he would not agree to repeat the experiment, even if the French kingdom had been promised for this.
If a dielectric is placed between the plates, then they polarize it, that is, they will attract opposite charges of which it consists. In this case, there will be the same effect as if the plates were closer. A dielectric with a high relative permittivity can be considered as a good electric field transporter. But no transporter 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 even better - 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 the crystal lattice, or in molecules - polar (representing dipoles) or not. If there is no external field, the dielectric is unpolarized, the dipoles and free charges are scattered randomly, and the dielectric does not have its own field. 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 pluses and minuses of neighboring dipoles inside the dielectric compensate each other. Only surface charges remain uncompensated - on one surface - one, on the other - the other. Free charges in an external field also drift and separate.
In this case, different polarization processes proceed at different rates. 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 are much faster in liquids than in solids. If the dielectric is heated, the rotations of the dipoles and the migration of charges 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 the thermal motion scatters the molecules into their original chaotic state. Therefore, for capacitors where the polarity switches with a high frequency, 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 the capacitor in a disassembled state. It is understandable - during disassembly, the charge from the plates did not go anywhere, and the voltage even increased, since the capacitance decreased and now the plates are bursting with charges. Wait, how did this tension grow, because then the energy will also grow? Indeed, we gave the system mechanical energy, 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 a few volts (and such is the voltage in chemical bonds) to tens and hundreds of thousands of volts. Now it’s clear why a synthetic jacket is not shocked when you wear it, but only when you take it off? Stop, why not up to 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 very E weakens quadratically with distance. And if on the whole 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 - the increase in voltage when the capacitor is stretched - experimentally. I wrote a simple Visual Basic program to receive data from our PMK018 controller and display it on the screen. In general, we take two 200x150 mm textolite plates coated on one side with foil and solder the wires going to the measuring module. Then we put a dielectric on one of them - a sheet of paper - and cover it with a second plate. The plates do not fit tightly, so we 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: the potentiometer R1 sets the voltage (in our case it is 3 volts) supplied to the capacitor, and the button S1 serves to apply it to the capacitor, or not to apply it.
So, press and release the button - we will see the graph shown on the left. The capacitor quickly discharges through the input of the oscilloscope. Now let's try to relieve the pressure on the plates during the discharge - we will see the voltage peak on the graph (on the right). This is just the desired effect. In this case, the distance between the plates of the capacitor increases, the capacitance drops and therefore the capacitor begins to discharge even faster.
Here I seriously thought about it .. It seems that we are on the verge of a great invention ... After all, if the voltage increases on them when the plates are moved apart, and the charge remains the same, then you can take two capacitors, push the plates on one of them, and at the point of maximum expansion transfer charge to a fixed capacitor. Then return the plates to their place and repeat the same thing in reverse, pushing 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 the generator! It will be possible to create new designs of 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 electrophore machine! 🙁
It did not take root as a generator, since it is inconvenient to deal with such voltages. But at the nanoscale, things can change. Magnetic phenomena in nanostructures are many times weaker than electric ones, and the electric fields there, as we have already seen, are huge, so the molecular electrophore 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 is fine, but if the rated voltage of the capacitor allows, it is better to take a larger one). The experience is to charge the capacitor, and then connect an LED to it (do not forget about the polarity), and watch how it blinks. V dark room a flash is visible even from capacitors of tens of picofarads. It's about a hundred million electrons emitting a hundred million photons. However, this is not the limit, because the human eye can notice much weaker light. I just did not find even less capacious capacitors. If the bill went to thousands of microfarads, take pity on 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 raised to a height of a load or a water tank (and the energy of an inductor, on the contrary, is similar to kinetic energy). The ability of a capacitor to accumulate energy has long been used to ensure the continuous operation of devices during short-term drops in the supply voltage - from clocks to trams.
The capacitor is also used to store "near-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 a long period of time allows wireless sensors and other electronic devices to work for some time. This principle is based on the eternal "finger" battery for devices with modest power consumption (like TV remotes). In its case there is a capacitor with a capacity of 500 millifarads and a generator that feeds it during oscillations with a frequency of 4–8 hertz with a 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, hitting the soles of shoes on the ground, and vibrations of technical equipment into the capacitor.
Another source of free energy is braking. Usually, when a vehicle decelerates, energy is converted 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 pollution of the atmosphere with exhaust emissions. In the Saratov region in 2010, the company "Elton" created "Ecobus" - an experimental minibus with unusual "motor-wheel" electric motors and supercapacitors - brake energy storage devices that reduce energy consumption by 40%. They used 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's products have been exported abroad since 1998, and recently the production of these products began in the USA under the license of 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 Coulomb!
They are used, as a rule, in car audio systems in order to reduce the peak load on the car's electrical wiring (during moments of powerful bass beats) 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 it can withstand a voltage of 40,000 volts (pay attention to the porcelain cups that protect all these volts from breakdown to the capacitor case). This is very convenient for the "electromagnetic bomb", in which the capacitor is discharged onto a copper tube, which at the same moment is compressed from the outside by the explosion. It turns out a very powerful electromagnetic pulse that disables radio equipment. By the way, in a nuclear explosion, unlike a conventional one, an electromagnetic pulse is also released, which once again emphasizes the similarity of the uranium nucleus with 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 the sad experience of van Muschenbroek in a very aggravated version.
If you simply rub a fountain pen (comb, balloon, synthetic underwear, etc.) on your hair, then the LED from it will not light up. This is because the excess electrons (taken from the hair) are each trapped 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 glow of the LED to be noticeable to the naked eye. Another thing is if you transfer charges from a fountain pen to a capacitor. To do this, take the capacitor for one output and rub the fountain pen in turn on the hair, then on the free output of the capacitor. Why rub? To maximize the harvest of electrons from the entire surface of the pen! We repeat this cycle several times and connect the 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 a low-voltage one, but it turned out that this was an unnecessary precaution. With a limited supply of charge, the voltage across the capacitor can be much less than the voltage of the power supply. A capacitor can convert a large voltage into a small one. For example, static high-voltage electricity - in the usual. Indeed, is there any difference: charge the 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 single-volt power source (i.e. its capacitance is higher than 1 μF), then there is no difference. It's just that if pendants are not forcibly limited, then more will want to come running from a high-voltage source. Yes, and the thermal power released at the terminals of the capacitor will be greater (and the amount of heat is the same, it will simply 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 it will take a long time to charge it to get enough voltage for the LED. On the other hand, if the leakage currents in the capacitor are small, the LED will burn longer. You can think about creating on this principle a device for recharging a cell phone from rubbing it against your hair during a conversation 🙂
An excellent high-voltage capacitor is a screwdriver. At the same time, its handle serves as a dielectric, and the metal rod and the 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, then nothing will come of it - the metal does not have the ability to take electrons from proteins - it did not attract papers, 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, is quickly charged to a high voltage and the papers begin to be attracted to it.
Glowing from a screwdriver and LED. In the photo it is unrealistic to catch a brief moment of his flash. But - let's remember the properties of the exponent - the fading of the flash lasts a long time (by the standards of the camera shutter). And now we have become witnesses of a unique linguistic-optical-mathematical phenomenon: the exhibitor exposed the camera's matrix!
However, why such difficulties - there is video filming. It shows that the LED flashes quite brightly:
When capacitors are charged to high voltages, the edge effect begins to play its role, which consists in 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, which is detected by the characteristic noise and glow in the dark. The magnitude of the critical voltage depends on the thickness of the lining, 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 the dielectric, the lower it is. To reduce the edge effect, the edges of the plates 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 breaking it into several connected in series.
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 section on the surface of the ball is further reduced, then the curvature of neighboring sections will inevitably increase. And here, apparently, in our electrostatic cases, it is not the average but the maximum curvature of the surface that is important, which is minimal, of course, for the ball.
Hmm .. but if the capacity of the body is the ability to accumulate a 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 bottom of our hearts, not sparing power plants and gigawatt-hours (that’s what a thought experiment is good for!)… but at some point there will be so many excess electrons on this ball that they will simply start to scatter around throughout the vacuum, just not to be in such electronegative crowding. But this will not happen with a positive charge - electrons, no matter how few of them are left, will not fly anywhere from the crystal lattice of the capacitor.
What happens if the positive capacitance is obviously much larger than the negative capacitance? Not! 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 will instantly blow the most armored capacitor to dust 🙂
In fact, without a secondary lining, the capacitance of the “solitary halves” of the capacitor is very small: the electric capacitance of a solitary 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 microfarads.
It is possible to build an absolute capacitance standard by calculating its capacitance using physical formulas based on accurate measurements of the dimensions of the plates. This is how the most accurate capacitors in our country are made, which are located in two places. The state standard GET 107-77 is located at FSUE SNIIM and consists of 4 unsupported coaxial-cylindrical capacitors, the capacitance of which is calculated with high accuracy in terms of the speed of light and units of length and frequency, as well as a high-frequency capacitive comparator that 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 of 1-100 MHz (photo on the left).
Standard GET 25-79 (photo on the right), located in the Federal State Unitary Enterprise VNIIM. DI. Mendeleev contains a design capacitor and an interferometer in a vacuum unit, 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 calculated capacitor when the length of the electrodes changes by a given number of wavelengths of highly stable light radiation. This ensures that an accurate capacitance value of 0.2 pF is maintained with an accuracy better than 0.00005%
But on the radio market in Mitino, I found it difficult to find a capacitor with an accuracy of more 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 the capacity in two ways. The first method is based on the properties of the exponent and the ratio of the voltages on the capacitor, measured at different moments of the discharge. The second - on the measurement of the charge given off by the capacitor during discharge, it is obtained by integrating the current over time. The area bounded by the current graph and the coordinate axes is numerically equal to the charge given off 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 kΩ precision resistor from an electronic designer.
And here are the results of the experiment. Pay attention to how beautiful and smooth the exhibitor turned out. After all, 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 exponent is exactly observed - to decrease by an equal number of times at regular intervals (I even measured it with a ruler on the screen 🙂 Thus, we see that the 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 is a pity that there is no standard to determine which of them is still true! If anyone knows a capacitance standard that is inexpensive or available at home, be sure to write about it here in the comments.
Pavel Nikolaevich Yablochkov was the first in the world to use a capacitor in power electrical engineering in 1877. He simplified and at the same time improved Lomonosov capacitors, replacing fraction and foil with liquid, and connecting banks in parallel. He owns not only the invention of 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 conductive liquid, and a glass jar of vegetables as a jar. The result was a capacitance of 0.442 nF. If we replace the jar with a plastic bag, which has a large area and many times less thickness, the capacitance will increase to 85.7 nF. (First, let's 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 to include it in the generator instead of a conventional capacitor - everything works).
Water here plays a very modest role as a conductor, and if there is foil, then you can do without it. We will do the same, following Yablochkov. 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 the introduction of an adhesive between the plate and the dielectric should be avoided, which will cause additional losses on alternating current. Therefore, now, as the plates, mainly metal is used, chemically or mechanically deposited on the dielectric (glass) or tightly pressed to it (mica).
You can use a bunch of different dielectrics instead of mica, whatever you like. Measurements (for dielectrics of equal thickness) showed that air has ε the smallest, fluoroplast has more, silicone has even more, and mica has even more, and lead zirconate titanate has it just huge. According to science, this is exactly how it should be - after all, in a fluoroplastic, electrons, one might say, are tightly chained by fluorocarbon chains and can only slightly deviate - there is nowhere even for an electron to jump from atom to atom.
You can conduct such experiments yourself with substances having different dielectric constants. Which do you think has the highest dielectric constant, distilled water or oil? Salt or sugar? Paraffin or soap? Why? The permittivity depends on a lot of things… one could write a whole book about it.
That's all? 🙁
No, not all! There will be a sequel next week! 🙂
Capacitor - not a toy for children
(Archive of Pioneer Wisdom)
Scary story from non-horror movie
“A charged high-voltage capacitor can be attributed to a direct current source. It is believed that direct current is less dangerous than alternating current. Based on my experience, I can disagree. If you "plug in" to a household power outlet, you will twitch. Although the frequency of the current in the socket is 50 Hz, and the person will not have time to respond to such a quick event, nevertheless, you will have a chance to free yourself from the action of the electric current during the convulsions. After all, the voltage in the outlet is zero 50 times per second. If you are "connecting" to a powerful DC source, then there are no options. Your muscles will contract strongly, and no amount of willpower will be enough to relax them. You will be glued to a DC source. At the same time, your carcass will heat up, slowly turning into coal. Horror!
The damaging effect of a charged high-voltage capacitor is somewhat different and depends on specific conditions. However, in any case, you will definitely not have pleasant sensations from touching the electrodes of a charged capacitor. Definitely! You won’t have time to char, but the balls will climb on your forehead. Chick ... and you're already in heaven! In especially severe cases, with a monstrously large charge (let's not talk about numbers), the capacitor will tear you apart like a heating pad. The balls will be in one corner of the room and the forehead will be in the other corner of the room.
Shortly speaking, be vigilant! When working with high-voltage equipment, it is better to overdo it than underdo it.”
The capacitor is one of the main elements in the power supply of pulsed lasers. A high-voltage capacitor is used to power flash lamps, as well as to pump pulsed gas-discharge lasers. Capacitor parameters are selected depending on the specific type of laser. The determining factors are such quantities as capacitance, operating voltage, wave resistance and self-inductance of the capacitor. The pump energy depends on the capacitance and operating voltage of the capacitor. The energy of a capacitor is calculated using a simple formula
E \u003d CU 2 / 2, where E is the energy of the capacitor
C - capacitor capacitance
U - capacitor charging voltage
The magnitude of the current that will pass when the capacitor is discharged through a small load depends on the wave resistance. The lower the impedance of the capacitor, the higher the current. Into the wave resistance is calculated by the formula
ρ to = √(L to /C to), where ρ to -vcapacitor impedance
L to - inductance capacitor
C to - capacitor capacitance
The rate of energy transfer of the capacitor to the load depends on the self-inductance of the capacitor. The lower the inductance of the capacitor, the higher the steepness of the front of the pump pulse. Where does inductance come from in a capacitor? The fact is that the capacitor plates are a current conductor, and the conductor through which the current flows has an inductance. Even if the capacitor consists of only two plates, real circuit capacitor as shown in the figure below.
This is a classic oscillatory circuit with active resistance R, which depends on the dielectric between the capacitor plates and the specific resistance of all current-carrying elements of the capacitor. Thus, the charge and discharge of the capacitor does not occur instantly, but has an oscillatory character. The oscillation frequency is determined by the Thompson formula, from which the self-inductance of the capacitor is calculated.
Where L to - own inductance capacitor
C to - capacitor capacitance
F p - fundamental resonant frequency
Of course, the higher the energy of the capacitor, the greater the pump power. However, with an increase in the capacitance of the capacitor, the time of the pump pulse also increases. If the duration of pumping is not of fundamental importance, then high-voltage electrolytic capacitors are suitable for laser operation. Such capacitors can be used, for example, to pump a ruby or neodymium laser. Of course, it is problematic to get a conder that has 1000 microfarads at an operating voltage of 3 kV. But this problem is easily solved by using a bank of capacitors. When individual capacitors are connected in series, the total charging voltage increases, and the capacity can be increased by connecting capacitors in parallel. In radio engineering stores, you can buy electrolytic capacitors having, for example, 150 microfarads x 450 V.
Of these capacitors, you can make a bank for any capacity and operating voltage.
The figure below shows an example of a bank of capacitors equivalent to one 30uF x 2kV capacitor.
If the pump duration should be as short as possible, then electrolytic capacitors are no longer suitable for laser operation, and pulse capacitors must be purchased. Unfortunately, high-voltage pulse capacitors are a rare commodity in radio engineering stores. In the Chip and Dip store, you can stock up on high-voltage capacitors from the company MURATA».
However, the maximum voltage of such capacitors is limited to 15 kV with a capacitance of 1 nF. Such capacitors can be used to pump homemade nitrogen lasers or metal vapor lasers.
For pumping dye lasers, 100 - 1000 pieces of such capacitors connected in parallel are required. Considering the cost of one such conder at the level of ~ 80 rubles / piece, all the pleasure will cost the amateur at least 8,000 rubles. So you still need to solder a single bank from a bunch of capacitors.
You can buy capacitors of the KVI-3 type via the Internet, which are also suitable for pumping lasers, but their price will be even more expensive (~ 200 rubles / piece).
Also, capacitors of the KPIM type are purchased via the Internet, which are quite suitable for pumping a dye laser.
These capacitors have impressive performance. The operating voltage can be in the range of 5 - 100 kV with a capacitor capacitance of 0.1 - 240 microfarads. But the pulse frequency will be< 1Гц. По стоимости эти конденсаторы самые дорогие. Их цена за штуку начинается от 20 000 руб. За такие деньги можно купить готовый лазер, причем нехилой мощности и не заниматься творческим онанизмом.
If there is no money, but you really want to, then we proceed to masturbation, namely, the manufacture of a home-made high-voltage capacitor.
Homemade high voltage capacitor
The capacitor circuit is simple, but here are the difficulties of implementing this circuit in the form finished construction increase with increasing operating voltage of the capacitor. To begin with, we will analyze the possible options for a simple capacitor from two plates separated by air. Figure 1 shows the plates of a charged capacitor. If you need to make a capacitor with a low inductance, then you should strive to shorten all current-carrying elements. Moreover, the direction of the currents in the capacitor plates during discharge should be opposite in order to reduce the magnetic field. The direction of the currents depends on the place where the electrodes of the capacitor are connected. The inductance of the capacitor will be the smallest if the electrodes of the capacitor are connected to the plates in the center, as shown in Figure 2.
Actually, according to this scheme, commercial ceramic capacitors are manufactured. Only for high voltage capacitors, the plates are in the form of a circle in order to avoid the occurrence of corona discharges. Possible options the connection of the electrodes to the capacitor plates, as well as the directions of the currents during discharge, are shown in the figure below.
The circuit in Figure 3 corresponds to the minimum inductance of the capacitor. According to this scheme, it is necessary to manufacture a capacitor if a short pump pulse is required.
The capacitance of a flat capacitor is calculated by the formula:
liningscapacitor
S is the area of the capacitor plates
D is the thickness of the dielectric between the plates of the capacitor
As can be seen from the formula, to increase the capacitance of the capacitor, it is necessary to reduce the thickness of the dielectric and increase the area of the capacitor plates. It is possible to reduce the thickness of the dielectric up to a certain limit, which depends on the dielectric strength of the dielectric material. Below this limit, dielectric breakdown will occur and the capacitor can be discarded. An increase in the area of the plates leads to an increase in the size of the capacitor. For the compactness of the capacitor, its plates are either rolled up (roll technology) or assembled into a package (package technology).
Roll technology
The roll technology for manufacturing a capacitor is understood as a method of arranging capacitor plates, when long strips of plates are rolled up, thereby reducing the size of the capacitor. Schematically, such a capacitor is a strip line, shown in the figure below.
To make a capacitor, you will need plastic wrap, food-grade aluminum foil, tin strips from a can (for example, “condensed milk”), and scotch tape. Polyethylene film can be bought at the construction market or at the Household Goods store. It is better to take the thickest film (~200 microns), although a film of 100 microns will also work. Just the film consumption will be more. The main thing is that the surface of the film should not have scratches and punctures. The polyethylene film will serve as a dielectric separating the capacitor plates, and the reliability of the capacitor depends on the quality of the film surface. Any speck or hair on the surface of the film will be a source of corona discharge, which will eventually break through the film.
First of all, you need to determine the operating voltage of the capacitor. The choice of the thickness of the polyethylene film depends on this. The dielectric strength of polyethylene is in the range of 40 - 60 kV/mm. This means that with a film thickness of 100 μm, the limiting operating voltage of the capacitor will be ~ 5 kV.
With a film thickness of 200 μm, the limiting operating voltage of the capacitor will be ~ 10 kV. To increase the operating voltage, you simply need to use several layers of film superimposed one on top of the other.
We will manufacture the capacitor according to the scheme of Figure 3 (see above).
Each of the capacitor plates will be placed in its own polyethylene film envelope. The envelope is a strip of polyethylene film of arbitrary size folded in half. The longer the strip, the higher the possible capacitance of the capacitor. The width of the strip is made somewhat larger than the width of the capacitor plates in order to prevent the occurrence of an air discharge between the capacitor plates.
Capacitor electrodes are cut out of canning tin in the form of a rectangular strip ~ 1 cm wide. The length of the tin strip is arbitrary, but not less than the width of the polyethylene film. To prevent corona discharges, the ends of the tin strip are rounded with a file (Fig. 7 below). To reduce active resistance, the tin strip is wrapped with several layers of aluminum foil (Fig. 8 below).
To prevent the occurrence of a spark discharge between the electrodes of the capacitor, a strip of tin is wrapped at one end with several layers of polyethylene film, which is fixed with adhesive tape (Fig. 9 below).
The capacitor plates are cut out in the form of a rectangular strip of aluminum foil. The dimensions of the lining are made such that it is somewhat smaller sizes polyethylene envelope. The ends of the aluminum strip are rounded with scissors to prevent corona discharge.
The electrode is fixed on the lining with adhesive tape as shown in the figure below.
The capacitor plate is placed on a polyethylene film as shown in the figure below.
Then the plastic film is folded in half as shown in the figure below.
The second capacitor plate is prepared in the same way.
Now you can roll the strips into a roll. If the polyethylene strips are very long, then it is easier to roll the roll on the floor of the room.
One envelope of a polyethylene film with a capacitor plate is spread on the floor and a second envelope with a capacitor plate is placed on top of it so that both plates are parallel to each other (Figure below).
The roll is rolled up starting from the electrodes, as shown in the figure below.
Since the aluminum foil in the polyethylene envelope is not fixed, when rolling the roll, care must be taken that the capacitor plates remain parallel to each other and do not crawl out of the polyethylene film. The folded roll is pulled together as tightly as possible with adhesive tape, which serves not only as a tie, but also fixes the roll, preventing the polyethylene film from unwinding.
The manufactured capacitor is shown in the figure below.
To prevent breakdown through air, the capacitor electrodes are slightly bent from each other. But it is better to install a Plexiglas plate 3–4 mm thick between the capacitor electrodes at capacitor operating voltages of more than 10 kV. The dimensions of the plate are selected based on the operating voltage of the capacitor. The purpose of the plexiglass plate is to reduce the electric field strength between the electrodes of the capacitor and thereby prevent interelectrode breakdown through air.
The capacitance of the manufactured capacitor can be measured with a digital LC meter.
Batch technology
The batch manufacturing technology of a capacitor is understood as a method of arranging capacitor plates, when short strips of plates are superimposed on each other, forming a package.
Schematically, such a capacitor is shown in the figure below.
by the most in a simple way manufacturing a capacitor using batch technology will use a double-sided foil getinax, which can be bought at the radio market or in a store (for example, Chip and Dip). Double-sided foiled getinaks is an almost finished capacitor (Fig. 1 below). It remains only to remove a strip of copper around the perimeter of the sheet from both sides (Fig. 2 below) to prevent interelectrode breakdown in air and connect the electrodes to both surfaces of the sheet (Fig. 3 below).
Everything! The capacitor is ready!
Of course, the capacitance of such a capacitor will be small. But if you put several sheets on top of each other, connecting plus to plus, and minus to minus, you can get a significant capacity. Unfortunately, getinaks, as well as textolite, is not the most best material for high voltage technology. The dielectric strength of these materials is ~ 18 kV/mm. This means that the most common 1.5 mm thick foil sheet of getinax on sale can be charged up to ~ 20 kV. With a higher charging voltage, the probability of breakdown of the getinax increases. In addition, the cost of manufacturing such a homemade capacitor will be very high if a large capacity is needed.
Cheaper, but laborious, will be the manufacture of a high-voltage capacitor using plastic film and food-grade aluminum foil. Below is a variant of the technique for manufacturing a capacitor using batch technology.
First of all, we determine the operating voltage of the capacitor, which determines the choice of the thickness of the plastic film. Let me remind you once again that the dielectric strength of polyethylene is in the range of 40 - 60 kV / mm. For the manufacture of a large capacitor, a significant amount of both aluminum foil and polyethylene film will be required. In addition, you will need two thick (4 - 5 mm) dielectric sheets (plexiglass is used in my homemade products) to tie the capacitor package.
Each capacitor plate is a strip of aluminum foil, the ends of which are rounded with scissors to prevent corona discharges. Each plate is connected to other plates of the same polarity through a contact strip, which is cut out of aluminum foil and fixed with adhesive tape on the plate (figure below).
A strip is cut from a polyethylene film, the dimensions of which are several more sizes capacitor plates. A strip of aluminum foil is fixed on the film using adhesive tape (figure below).
Then the film is folded in half, forming a dielectric layer on both sides of the capacitor plate (figure below).
A capacitor plate of opposite polarity is also made. Then the plates are superimposed on each other (figure below).
In principle, the capacitor is ready. It is only necessary to press the plates against each other with the help of dielectric plates and pull off the entire package. However, the capacitance of the capacitor will be negligible. To increase the capacitance, you need to increase the number of capacitor plates. A cross section of a capacitor with several plates is shown in the figure below.
According to this scheme, you can make a capacitor for any capacity and operating voltage. At least 1,000,000 V. The fundamental limitation is the size of the room where the capacitor will be located. As the capacitance increases, so does the size of the capacitor. Even if the operating voltage is 20 kV, increasing the capacitance will cause the capacitor to turn ...
... the capacitor turns ...
... into an elegant nightstand for the interior of the room.
And the thicker the package of capacitor plates, the more effort is needed to pull it off. Thick dielectric plates will help facilitate the contraction of the package, between which the entire package of plates is placed.
As an option, the figure below shows two 5 mm thick Plexiglas plates, which will serve as both a capacitor case and compress the plate pack. On the upper plate, an interelectrode separating partition with grooves for plastic ties is glued along the entire length.
The entire package of plates is placed on the bottom dielectric plate, and the top plate is superimposed on the package. Then, as much as possible, the upper plate is pressed (with arms, legs, abs, etc.) to the lower one. The tightened plates are fixed with plastic ties.
The finished tightened package of capacitor plates is shown in the figure below.
After tightening and fixing the package, you can fix the contact strips of the capacitor plates. The scheme for fastening the contact strips is shown in the figure below.
The advantage of a "dry" capacitor, made according to the rolled or package technology described above, is the small amount of electric charge leakage, which is important when the capacitor operates in high-frequency circuits. However, such a capacitor also has a significant drawback, namely the presence of air between the plates. No matter how strong the compression of the plates is, there will always be air between them. By itself, the presence of air in no way affects the energy characteristics of the capacitor. "Dry" capacitors may well be used as storage capacitors, which serve to smooth out ripples of the rectified voltage up to 1 kV. However, with an increase in the charging voltage, the air begins to ionize, which manifests itself in the characteristic hiss of the capacitor when it is connected to a voltage source > 10 kV. The hiss is caused by the occurrence of corona discharges, which eventually lead to a breakdown of the dielectric between the capacitor plates. And if you use the capacitor in the short circuit mode, which is typical for the operation of a pulsed capacitor, then the manifestation of corona discharges will be maximum. Even with an ideal film surface between the plates of the capacitor, corona discharges will occur along the perimeter of the edge of the aluminum foil at the moment of rapid discharge of the capacitor, as shown in the figure below.
The glow of corona discharges in a homemade capacitor can be seen in a darkened room.
Due to the occurrence of corona discharges, commercial high-voltage capacitors are always immersed in a liquid dielectric, which, firstly, has a greater dielectric strength than air, and secondly, increases the capacitance of the capacitor, since the dielectric constant of any liquid dielectric is higher than that of air. Moreover, high-voltage capacitors with an operating voltage of tens of kilovolts are never made in the form of a single roll or a separate package. If it is required to make a high-voltage capacitor, then it is assembled from several sections (rolls or packages), which are interconnected in parallel to increase the capacitance and in series to increase the operating voltage. Moreover, the operating voltage of each section does not exceed 10 kV. All sections of the assembled capacitor are placed in a robust case and filled with a liquid dielectric.
Oil is used as a liquid dielectric, which can be either mineral (petroleum), or vegetable (castor), or synthetic (for example, silicone). Each of the oils has its pros and cons, which are not of particular importance for improvised designs. If there is a desire to immerse your homemade capacitor in oil, then it is not at all necessary to stock up, for example, with castor oil, which can be bought at a pharmacy. Edible vegetable oil such as Oleina, Milora, etc., which will be cheaper, is quite suitable. For example, a roll capacitor can be put into a glass jar and filled with oil (figure below).
It is tempting to use glycerol (ε ≈ 40) or distilled water (ε ≈ 80) as a liquid dielectric. These liquids increase the capacitance of the capacitor by an order of magnitude. Unfortunately, both glycerin and water have relatively low resistivity, which will shunt a high-voltage source having a high-resistance output (eg, a diode-capacitor voltage multiplier). Simply put, the capacitor will close the power supply, and there will be no high voltage. However, glycerin and water are successfully used in pulsed high-voltage capacitors. The trick is that the capacitor is charged not from a constant voltage source, but from a pulsed voltage generator (GVP).
The design of the pulse capacitor is a coaxial line made up of two duralumin tubes, between which either glycerin or distilled water is poured.
1 - outer and inner metal tubes
2 - liquid dielectric (glycerin or water)
3 - contact of the inner metal tube
4 - dielectric tube
5 - hole for dielectric filling
Liquid dielectric is poured into the capacitor through a hole made at the end of the outer tube.
The ratio of the diameters of the duralumin tubes will determine the capacitance of the capacitor in accordance with the formula for the capacitance of a cylindrical capacitor:
Where C is the capacitance of the capacitor
ε is the relative permittivity of the dielectric between
liningscapacitor
ε 0 - absolute permittivity equal to 8.85x10 -12 F / m
L - length of condenser tubes
r 2 - radius of the outer tube of the condenser
R 1 - radius of the inner tube of the condenser
The connection diagram of a pulsed coaxial capacitor is shown in the figure below.
