As for the science museum, we made the largest Tesla coil in Ukraine

In 2016, I was offered to take part in a project to create a powerful Tesla coil for a science museum in my hometown. To give up would be to betray your ideals and your own children who adore scientific experiences. Under the cut, not only the description of the design of the resulting device, but also a little about the male love for science, enthusiasts and how social projects are born and implemented.

Prehistory

This is a story about people. Whoever doesn’t like stories about people can safely skip this section.

First, a little about yourself. My name is Artem. Now I work as a designer in a private firm, but by education I am a physicist, and the method of teaching physics to children is my hobby. On this topic, I have already published posts on Geektimes ( one and two ). Four years ago, I was completely immersed in science, worked in the Laboratory of Physics Teaching Methods of my university, conducted advanced training courses for teachers from all over the region, was a co-host of the Young Physicists circle for children from 5 years old, and also worked as a physics teacher and developed my own site.
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It was then that Scientific picnics were first held in Ukraine. This is an excellent event for the popularization of science, when universities, laboratories, research centers go to the square with physical exhibits, liquid nitrogen, skeletons and mannequins, microscopes, telescopes, rare insects and bats. In general, each exposes what he does and what will be interesting for the rest. Of course, both the laboratory, the school, and the circle where I worked took an active part in the Science Picnic.

There I met one of the organizers of the picnic, Sergey. This guy dreamed of a science museum for our city and convinced me that if you do not give up and be active, then everything will definitely work out.

And so it happened, the Science Picnic was a smashing success; on the basis of such a positive result, the university decided to give the green light to Sergey’s team and generously allocated some of the impressive space for the museum.

It was probably the most romantic time. Almost without financing, an attempt to make a science museum out of the destroyed premises for a month or two looked insane. But Sergei was not discouraged, he rushed like a meteorite, patching holes in the floor, covered the walls without repair with cloth, made partitions and supports for future exhibits. Sergey showed amazing commitment and will, as well as remarkable organizational skills. Our laboratory, like many others, helped the project with demonstration devices and ideas of budgetary, but fascinating exhibits.

The museum opened safely, and I went with my wife and children to Montenegro. There I also organized a circle on astronomy and physics, but the design department became the main place of work.

Last year I returned to Ukraine. Sergey made me a tour of the updated museum. For two years, the museum has made repairs, several times increased the exhibit base and significantly expanded over the occupied area. It was incredibly pleasant to find working one of the demonstration devices - the “optical table”, which he collected with his own hands at the dawn of the project.

Sergey told about the desire to get a “Tesla coil” for the museum, he not only found a seller, but agreed with a wonderful company “X” about sponsoring the project and even managed to get money. However, immediately before the sale, the seller decided to double the price. Of course, communication on this was completed. And Sergey was in the unenviable position.

After some time, Sergey called me and said that he had found some guy who had already assembled several small Tesla coils, and was ready to try to assemble a larger coil. But he needs help with the mechanics. Unobtrusively I was offered to join the project without the right to refuse.

There was a meeting with this “slasher”, who, by the way, was also called Sergei. It is quite obvious that he was some kind of freak, who has a craze for Tesla coils, and the fix idea is a big Coil. But as soon as we started discussing the project, the inconsistencies with the present one went (it seems to me that I need to insert “me”). The interviewee began to consistently present the design of the device, did not repeat, did not say anything extra. All physical terms were used correctly, and physical phenomena were given the correct interpretation. All questions sounded intelligible and logical answers. It turned out that I communicate with an adequate teacher of a well-known technical college, which is really keen on Tesla coils.

Sergey turned out to be the owner of such a valuable quality as scrupulousness. For that he immediately won deep respect. He prepared his workplace for a long time and carefully, picked up the tool in advance and settled in; he always, absolutely always, thoroughly deduced all contacts. If in a hurry to perform not a high-quality connection, then Sergey will keep silent, and then quietly take it and fix it. It may seem that such scrupulousness is excessive, but in fact it saves a huge amount of time. After all, for the entire period of work, we have never encountered an error of improper assembly. In order not to be confused among Sergeyev, we will further call him Sergey A., and the first one - Sergey V.

What is a Tesla coil?

I asked Sergey A. (the author of the coil) to tell my own about the coil. All the text in this section belongs to him:

In the late 19th and early 20th centuries, Nikola Tesla conducted experiments with high-voltage high-frequency resonant transformers without a ferromagnetic core (air transformer). This type of transformer was later named a Tesla transformer or a Tesla coil.

At present, the Tesla transformer is more of a toy for those who are fond of DIY and want to have something in their collection than a device with a certain scope of application. Infrequently, in one simple construction, so many physical phenomena can be combined. In addition, there is no such person whom the Tesla coil cannot surprise by the appearance of its electrical discharges.

Why do we need Tesla coils? First of all, for educational purposes. After all, many topics are covered here:

• in statics - studying the theory, familiarity with electrical components, reading circuits, learning how to use a multimeter and using electrical circuit simulators, finally, understand the difference between inductance and capacitance;
• in dynamics - electrical oscillations, voltage resonance, energy transfer, ionization, the nature of the plasma, the properties of electromagnetic radiation, the effect of current on a living person.

If you want to charge devices without wires, impress random strangers and you like the smell of burning electronics or ozone, then this is a reason to assemble a Tesla table coil, especially since it is cheaper than going on dates.

“To build a Tesla coil is something that everyone should do at least once” © - some guy.

The first resonant transformer was built by Nikola Tesla in 1891 and, in fact, was a wireless telegraph. The principle of operation is straightforward: it is necessary to charge a large capacitor with high voltage, and then discharge it through an inductance coil, the role of which is performed by the primary winding of the transformer, while it is necessary to close the energy in the resulting oscillatory circuit.

Due to the resonance of the voltages in the primary circuit, the amplitude of the oscillations will increase, with some of the energy being transferred to the secondary winding of the transformer (which is characterized by the coupling coefficient between the windings), the latter, together with the metallic toroid, also form an oscillating circuit.

Then the process can be described similarly to swinging a swing: if you push the swing at the right moment, they will soon begin to fly very high, this will be the voltage at the outlet of the Tesla coil. When the voltage becomes so high that the air for it ceases to be a dielectric, all the accumulated energy forms an electrical discharge, or streamer.

Over the years, changes have been made to the design of Tesla's transformers, due to which today there are several device topologies with a similar principle of operation, and at the same time differing on the component level.

SGTC (Spark Gap Tesla Coil) - Tesla transformer on the spark gap. It is a classic performance used by Nikola Tesla himself. As a key element, two contacts of arbitrary shape are used, between which an electrical discharge occurs. Structurally it consists of only 6 elements and often works the first time. True, in low-power performances it is not necessary to wait for beautiful discharges, but please light up the bulbs at a distance or enjoy the coronary glow.

A derivative of this topology is the ARSGTC (Asynchronous Rotary Spark Gap Tesla Coil) type, where a rotating electric motor is used as a switching key, closing the contacts and quickly tearing the resulting electric arc, which makes it possible to obtain the greatest length of discharges with a relatively small input power. This type was taken by us for repetition.

VTTC (Vacuum Tube Tesla Coil) - Tesla transformer on radio tubes. Requires work with high anode voltages and is not recommended for beginners.

SSTC (Solid State Tesla Coil) is a Tesla transformer, which uses semiconductor components as keys. The most common use of MOSFET or IGBT transistors (not to be confused with LGBT). Thanks to the use of a modern component base, coils of this type can be easily controlled by modulating the frequency of lightning with an external signal source, such as music.

DRSSTC (Dual Resonant SSTC) - Tesla coil with double resonant circuit. The topology is much more difficult to manufacture than its predecessor, but this is offset by an improved ratio of the discharge length to the length of the secondary winding.

QCWDRSSTC (Quasi-Continuous-Wave DRSSTC) is a variant of Tesla coils with a double resonant circuit, but capable of producing long straight lightnings characteristic only for this topology, sometimes 10 times longer than the secondary winding.

And there are Tesla bipolar coils, but more about that another time.

As you can see, Tesla coils come in any “taste and color”, and anyone can choose for themselves the one they want. In recent years, the market has tried to meet the increased demand of amateurs and experimenters for Tesla transformers, and now even on AliExpress there are kits from which you can assemble a working device at home.

Not as an advertisement, but it is worth mentioning a group of enthusiasts who launched the company at Kickstarter in early 2013 (which was covered at one time) and, having successfully collected the necessary amount, continue to work on the project called OneTesla. What distinguishes it qualitatively from other projects is the open documentation and schemes, where a person can, following the instructions, assemble a music coil on the DRSSTC topology at home. Guided by instructions from the OneTesla website, I collected the first work coil for myself.

Finally, I want to note that it is very easy to make a Tesla coil, but to make it so that it is not a shame to show it, it’s still very expensive and time-consuming.

Our choice of "Old school"

For lovers of tsiferok - a brief technical description of our exhibit from Sergey A.

To operate the Tesla coil, a high-voltage power supply is needed, and to save time on winding its own transformer, it was decided to use the ready one. Thus, the power supply circuit involves 4 transformers from microwave ovens in series with a midpoint, which convert the 220 volt mains into 8600 volts.

To ensure their cooling and to avoid interturn breakdowns, all 4 transformers were immersed in synthetic motor oil.

As a working capacity, a battery of 5 branches of 9 series-connected capacitors (MMC) was used, that is, 45 capacitors with a capacity of 0.1 µF and a voltage of 2 kV gave a capacity of 55 nF with a voltage of 18 kV. Also used were 8 AWG switching wires, a 6.35 mm diameter copper tube as the primary winding with 6 ½ turns. 1000 turns of copper wire with a cross section of 0.5 mm on a ventilation pipe with a diameter of 10 cm form a secondary coil with a winding length of 56 cm. The toroid is made of an aluminum ventilation pipe with a diameter of 11 cm.



The calculated resonant frequency of the primary circuit, and then confirmed with a pulse generator and frequency meter, was 220 kHz. The resonant frequency of the secondary circuit is about 250 kHz in terms of calculation and measurement, but at the time of the streamer, due to the electrical capacitance of the discharge itself, the oscillation frequency decreases, and resonance occurs between the primary and secondary circuits, which leads to an increase in the length of lightning.

And now, about the mechanical component

The existing budget did not allow much to roam. The number of milling and turning works had to be minimized. All irresponsible parts were made by us personally.

The breaker disc was made of PCB with a thickness of 8 mm. There are 6 contacts on the disk, which close the arrester. The disk is located on the axis of a 0.37 kW dual-pole asynchronous motor. The engine speed is 2800 rpm. Considering that 6 closures occur in one revolution, we get 280 closures per second.



On the Internet, you can find the works of real maniacs who put a discharger disk on the shaft of an angle grinder:



Such solutions are captivating for their simplicity, but not at all compatible with such a concept as safety technology. The grinder is loosely attached to the base with a clamp around the handle. The electrodes on the disk are overheated, as they have a small time and cooling area. The disk rotates at a frequency of 4000-6000 rpm! It is easy to break this delicate balance, and then hot electrodes, like shrapnel, will scatter in arbitrary directions. Please use the angle grinder only for its intended purpose!



The primary coil was made in the form of a spiral of a copper tube with a diameter of ¼ ”, freon is usually circulated through the freon between the units of the air conditioner. The geometry of the coil is very important, the inductance depends on it, and as a result - the frequency of the oscillating circuit. So that the form strictly corresponded to the calculated one, a simple fastening of four supports was developed. The supports were made of sheet PVC using CNC milling.

Good exhibit - safe exhibit

Of course, the Tesla coil is a dangerous device, more correctly even say deadly. Consider all the factors as they are important.

• If you thought that you need to be afraid of two-meter lightning that fly in all directions, then you should know that these are only flowers. The real danger is the primary winding, such a nondescript coil, which is under voltage of 8600 V and is ready to share its current with a force of hundreds of milliamperes. Accidentally touch it - the right way to heaven. Lightning can also cause significant harm to humans, but their danger is already an order of magnitude less.
• It is also worth mentioning that in the process of operation the Tesla coil generates a powerful electromagnetic field. This field is capable of igniting gas-discharge lamps at a distance of up to several meters, and also in the radius of more than ten meters disabling electrical devices that are sensitive to interference. Thus, even at considerable distances, the device carries a real threat to people with pacemakers.
• In addition, in the process of work, a huge amount of ozone is released, Wikipedia says that ozone is a highly toxic gas with oxidizing properties, which can lead to premature death. The good news is that ozone effectively kills mold and bacteria.
• Do not forget that lightning shines not only with visible light, but also with ultraviolet light, which is not particularly useful for the eyes.
• It is also necessary to take into account that the installation generates noise (about 100 dB at a distance of 3 meters). Noise causes discomfort and can also frighten a child.

We faced a difficult task - to turn a deadly installation into a safe and exciting exhibit for children. The following remedies have been identified.

• The work of the exhibit is only in the presence of a person familiar with the TB (master).
• Multiple circuit breakers, breakers and magnetic starter. In order for the coil to be energized, a number of conditions must be met, in particular, a signal is given from the panel, to which only the master has the key. But even the visitor can de-energize the installation. In addition, the installation is turned off by the time relay itself after 40 s.
• The installation was placed in a Faraday cage. The cage has its own grounding and is locked. This tool protects against lightning, shields the electromagnetic field, and also performs the functions of a reliable fence.
• 10 mm thick tempered glass partition. The glass absorbs ultraviolet light and, in combination with forced air extraction, prevents ozone from entering the hall with visitors.
• Glass acts as a sound insulation wall, it significantly (-10 dB) reduces the sound pressure level. However, noise-canceling headphones were additionally ordered for younger visitors.

I would like to note that Sergey A. interpreted in his own way the purpose of the Faraday cage and the glass partition:

“In the work of Tesla's transformer, some kind of elegance, magical attraction was traced, so much that he wanted to show everyone and not show anyone at the same time, therefore, following his instinctive desire to protect the coil from the external world, it was decided to place it in a room without windows, inside a metal cage with a door on the lock and behind a wall of thick glass ”.

How we got into UMBRELLA INC.

When the coil is turned on, a certain sequence must be performed. At first, the electric motor should turn on and go into operating mode, and only after that can the power be supplied to the primary winding. We needed a control unit that would provide a simple start-up of the installation, as well as perform a number of other algorithms. Collect the device "on the knee" is not even considered. They formulated the TZ and asked for help in one design bureau “Y”. Our order for them was small and not at all interesting, but, given the social orientation of the project and the small amount of work, they agreed to fulfill it free of charge. We only paid for the components and the board fabrication at the factory.

When the work was completed, the electronics engineer, smiling sweetly, handed me boxes with cards. Instead of the company logo, we found a modest inscription “UMBRELLA”. The company refused to PR, and the electronics engineer from our thanks.

Start and adjustment

Everything went according to plan. All parts were manufactured and placed on their intended places without surprises. Well, almost no surprises. The fact is that we had a desire to regulate the frequency of interruptions for arresters. To this end, for the electric motor was purchased chastotnik national production. This chastotnik took away from us several days of work, we were drawn into a funny, sad and instructive history at the same time. The moral was that it was better not to use frequency regulators of national production. If there are requests in the comments, a separate post will be devoted to the chastotnik with an overview and photos of the autopsy.

After abandoning the frequency regulation, the installation started working at the first start. Sergey's theoretical calculations have completely justified themselves. We cut off the floor of the extra coil of the primary coil and hit the resonance. All nodes worked normally.



Ultraviolet lightning caressed our faces, loud rumbles delighted the heart. The smell of ozone spread throughout the entire area of ​​150 m2. We realized that it was time to finish the work. It was decided to carry out the following launches already in the intended room, with all the specified means of protection. We waited only for the completion of construction work.

***

When the repair was completed, we had only two weeks to install the Tesla coil, make all the connections and test the system for reliability. It was a short time, considering that two of the three members of the team had a permanent job outside the museum of science, and it was impossible to perform any operations alone.

We moved the equipment, made all the connections, checked them all, energized the control boards, turned on all the machines, solemnly pressed the button and ...
... and nothing happened. Nothing at all. When searching for faults found that one of the phases has a voltage of 60 V instead of 220 V! Fortunately, we had access to all three phases and were able to quickly reconnect. For us, the issue was resolved quickly, but in a building where the science museum is located, a third of the equipment did not work for more than a week.

The real difficulties were waiting for us in the Tesla coil control unit. Despite the fact that all the inputs and outputs to the unit were galvanically isolated, and the power to the spark gap was supplied in a separate phase, as soon as the first spark slipped the first spark, the control block went crazy. He either turned off the coil immediately, or did not turn it off at all, or turned off the engine. The root of evil lay in the high-frequency noise that went from the coil through the wires back into the network. The pulses were so strong that they induced interference in all the adjacent wires. In previous tests, we laid the wires so that they went away from one another. Here, according to the design of designers, they walked along common pipes. We had to make changes to the project. We brought the control board from the common shield to a separate grounded cabinet. But that did not change anything. Here began the dance with a tambourine.

In search of solutions, we tried everything, even resorted to such homeopathic remedies as the installation of ferrite rings and suppressors. Time passed, and the tension increased.

Periodically, I took my eldest six-year-old son with me. While we were working, he was having fun at the museum. Often I did not come, and the guys did most of the work themselves. Everyone was sympathetic to my marital status.

At some point, we were joined by another enthusiast, who was also called Sergei. This guy was a university student, but very sensible. The appearance of the third Sergey unexpectedly positively affected the efficiency of our company. With his participation, we found the main cause of the malfunction. The weak link was the input from the start button, the only button that the visitor should press. We once again changed the project and ran the cable from the button to the control unit in a separate pipe. Additionally, we soldered several components on the board. Changed the ratings of the pull-up resistors, as well as a single resistor from the contact bounce block. This measure worked, and we solved the problem. With peace of mind, we celebrated New Year together at my home.

Result

The opening of the exhibit took place on January 4th, since then it has been working regularly and entertaining visitors. Pompous headlines appeared in the media: "Scientists restored the coil of Nikola Tesla himself", "Tesla's largest coil in Europe earned", "A high-energy room appeared in the center X", "Scientists invented a cure for cancer", etc. As a designer, I do not feel particularly proud of the project, I think that for many of the readers it is clear that from an engineering point of view this is a rather simple device, and, besides, it has been done with some flaws.

But it is very pleasant to remember the process itself: our meetings after work, the discussion of concepts behind cheap pizza pizza, the first launch, the tedious struggle with malfunctions and victories. For some reason, these memories intersect with different events from early childhood: as at 4, he made a short circuit in the apartment, stuffing the pin into the socket, as his brother's tape recorder “repaired”, as if he made a plane out of boards and nails, as for the first time picked up a soldering iron.Having implemented this small project, each of us managed to realize the dreams and aspirations of childhood. From this, I feel very warm and pleasant in my heart, and I want to share it with you.

UPD1

Added video from the first start.
frost273 - the same Sergey A.