| Darrens Tesla Coil Site |
| Why build a Tesla Coil? Well, the answer is simple. It is a challenge, A Tesla Coil produces the highest voltage you can create (In your shed). Why do people fly model planes or race model cars? Besides the challenge, there are other reasons, Electricity is quite amazing to watch and the general concensus among coilers is that once you get your coil working, you will end up making it bigger and better. Like most hobbies, it is born from fascination. The coil is the biggest and most complex project I have ever undertaken. The design had to be created from scratch and based around the materials I had available. There is a hell of a lot of improvisation involved in the design and the circuit diagram has evolved into a very full A4 page. So what does a Tesla Coil do? Tesla coils actually work more like a radio transciever than a transformer in that the power is actually transmitted from the primary to the secondary. The primary circuit begins with a High Tension Power Supply that usually takes one of the following forms: OBITs MOTs NSTs Pole Pigs You have probably seen these terms before and may not know what they are, so I will give a brief description of each: 1) The OBIT. Obits are popular in colder countries where Oil Burners are used for central heating. Obit is the abreviated name for Oil Burner Ignition Transformer. Usually small banks of these are used to power smaller coils 2) The MOT. Mots are Microwave Oven Transformers, as with Obits, usually small banks of these are used to create the required power. Mot's are becoming very common in coils now due to the ease with obtaining them and their low cost. The down side is that they produce a lot of heat due to the spark gap short circuiting the output and unlike NST's, they aren't designed to self limit the current output (To my knowledge). This means that they need a fair amout of cooling or they may fail prematurely. The cooling usually is achieved by immersing the transformer in electrical oil. 3) The NST. Nsts are Neon Sign Transformers. They are my favorite and the Power Source of my coil. They come in several voltages (6kV, 12kV and 15kV mainly) and several different current ratings (30mA, 60mA and 120mA mainly). They can be connected in parallel to increase the output current. When parallelling them, care should be taken to match them closely in terms of voltage and current ratings. Having said that, I have broken that rule by using a 30mA and 60mA transformer in parallel and it seems to work properly (Touch Wood). They are relatively cheap as the demand for second-hand units is very low (Why pay a fortune for a Neon sign for your company then risk having it stop working due to a failed transformer). Due to businesses opening and closing, they tend to turn up all around the place so keep an eye out. I paid $60 Australian for my two units which I purchased from an anonymous sign company. The trick is, convincing someone to sell you one and you may need to phone around a fair bit. 4)Pole Pigs. These suckers are the holy grail for serious coilers. Pole Pigs are utility transformers like the ones found hanging off power poles. Usually they convert 13.8kV down to 240V (In Australia), but if you wire the output to 240V, the input will produce 13.8kV. Actually that isn't true because you will blow fuses if you don't use an appropriate Ballast to control its power draw. They are the most dangerous option but the results of these can be quite amazing. They are the most difficult to work with due to the requirement of ballasting. Generally, this is done by wiring a load in series which limits the current draw of the transformer. These loads are usually very heavy and often, banks of bar heaters or welders with the electrodes short circuited are used. For serious control, variacs (autotransformers) are used to allow the voltage and current to be controlled How Tesla Coils Work: The output of the power supply is fed to a capacitor through the primary winding. At 50hz, the power goes through the winding as if it was just a wire and charges the capacitor. A Spark Gap is connected across the power supply. As the capacitor charges, the power drawn by it will cause a substantial voltage drop from the transformers output. As the capacitor approaches full charge, its power draw decreases and the voltage from the transformer will increase gradually. When it gets close to full voltage, the spark gap will 'fire'. Since the distance electricity will arc depends mostly on voltage, the distance the gap is set to will select the voltage which it 'fires' at. The spark gap becomes like a switch that activates at a preset voltage. It is also worth noting that when an Arc breaks out, it becomes a short circuit. When the gap fires, it not only short circuits the transformer, but also reconfigures the primary circuit. The transformer obviously should be able to handle being short circuited. The arc forms a 'link' between the capacitor and the primary winding. Keeping in mind that the other end of the primary winding and capacitor are connected together anyway, it forms a tuned circuit. The tuned circuit will resonate using the power stored in the capacitor. The resonant frequency depends largely on the design of the coil and can vary from hundreds of Kilohertz up into the Gigahertz range. Since the capacitor stores a large amount of power and the primary is usually Very heavy wire (in fact most coils, like mine, use copper pipe), the power level produced is huge. The secondary winding is in parallel to a capacitor aswell (although it is not evident) which forms a tuned circuit matched to the frequency of the primary circuit. The secondary Capacitor is made using the Earth as one of the plates and the Toroid as the other. The result is the power is 'recieved' by the tuned circuit and stored on one of the plates (the toroid). The power builds up in this capacitor, stored on the toroid until the voltage breaks down the dielectric of the capacitor (the air between the toroid and the nearest ground) causing the corona's and sparks that we all want to see. Each time the gap fires, the power is transferred to the secondary until the voltage in the capacitor is too low and the spark quenches. The transformer starts charging the capacitor through the primary and the cycle begins again. This information is very basic and probably not completely accurate but is the best I can come up with. The basic CCT is very simple although by the time safety switching is added, it can become quite complex. I strongly recommend safety switches and a key to prevent "unauthorized" use. |
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