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Transformer
The high voltage transformer supplies electricity to a Tesla coil,
taking wall voltage (120 or 240 volts) and boosts it to kilovolt
levels. There are several
types favored by Tesla enthusiasts, neon sign transformer (NST), oil
burner ignition transformer (OBIT), microwave oven transformer (MOT),
power distribution transformer (pole pig) and the potential
transformer (also called a pole pig, somtimes).
Neon sign transformers are used to light neon signs (duh,,, but sometimes they are called "luminous tube transformers, which you might not have known!) they usually range from 7,000 to 15,000 volts, and from 20 to 130 mA. They are current limited, meaning the amperage put out is limited so you don't need a ballast to use them, this is done by inserting magnetic shunts between the primary and secondary coils to provide leakage inductance and absorb some of the power.
Oil burner ignition transformers are used to ignite oil furnaces, and are almost always 10,000V at 23 mA (I have seen one of different ratings, but it was a small solid state transformer which would not have worked on a TC.) They are also current limited and are much like the neon sign transformer in most aspects, and are frequently used in parallel to increase amperage (I use them, and have had good success, but note, with current limited transformers you must put both the primaries and secondaries in parallel, or else they won't work). Please note, many NSTs and OBITs have two secondary windings, half the potential to the ground. It is important for safety and longevity reasons to ground transformers with current limiting, because if you don't and all is not well inside the transformer, the case of the transformer will float at several thousand volts, I have been zapped before, it is not fun, attach a wire from the case to the mains ground (usually there is a bolt or a screw for this purpose). Some people falsely believe that grounding your transformer will reduce it's output, don't listen to them, they are stupid and are going off a gut feeling and have never done the math (reduced output from grounding is a sign of a bad transformer, if this happens, fix or chuck the unit and get a different one, and make sure the new one is grounded so it doesn't go bad like the last one! Or perhaps your phasing is incorrect, switch ground all the units together, then wire thme up with the secondarys that don't arc together connected, that is the proper way.
Microwave oven transformers are sometimes used, they rate from 1,500 to 3,000 volts (usually 2000) at high milliamperage depending largly on input, but 120volt 20amp breaker usually gives about 200 mA (the voltage output is to low for normal usage, but you can stack them with primaries in parallel and secondaries in series to obtain higher voltages, which makes them highly formidible because of their high amperage) they are not well current limited (they do have magnetic shunts, but they do a lousey job of reducing current), and require a ballast as well as a corona suppressant, as they are not potted (some people submerge their stacks in mineral oil. On more than a four transformer stack this is almost always necessary unless you use very fancy wiring which floats the outer primaries at a higher potential, but this requires about double the MOTs you would normally use). One note, on MOTs the high voltage ground is usually attached to the core, for best results, disconnect it, or you will have problems with shorting to the core.
The power distribution transformer (pole pig), is a gargantuan transformer (the holy grail of Tesla coil tranformers). They are those large cylinders on power lines that reduce high voltage from the line, to send it into your house, to use them for coiling, you run it backwards, putting 240 in, and getting 14.4kV out. A potential transformer does the same thing but it is for testing instruments on the line, not your house. Either one of these puppies will give you 10-23 kV (14.4 is common in the US) at hundreds of milliamps (200 and well beyond, depending on input, some potentials go beyond a few amps) they are rated by the kVA, so a 10 kVA can put out 14.4 kV at 694 ma without exceeding the ratings. They are of course not current limited so a ballast is an absolute must, and a power factor correction capacitor really helps as well, but if used properly, they are virtually indestructible. I want one of these so bad, but they are incredibly hard to get a hold of (legally, it would be easy to steal one, But I can't do that!)
Often it is possible to repair a "dead" NST or OBIT transformer, depending on what is wrong with it, if it is just something not connected, it is a simple matter of de-potting the thing by soaking it in a solvent, and re-connecting the loose or fried wire. If a secondary has a short, occasionally only a few windings need to be removed to bridge the short, so not much voltage is lost, and worse case scenario, you can use windings from two identical KIA donors to make one living Frankenstein's monster. I have attemped to rebuild some dead OBITs, but while I was "in there" I thought about to cutting down on primary turns and current limiting, so as to get higher kilovolts, and milliamperage. Or maybe I could have attached additional secondaries pirated from another transformer in series to double the voltage, and keep the amperage, 20KV at 115 or 120 ma is astronomical, and rivals some of the potentials or MOT stacks available, but the experiment was not a success because of time constraints. It was not likely to last very long anyway, so it will inevitably be a failure (the primary and secondary wire is to thin, and the insulation is to weak, to make a practical transformer). Only if I wind a transformer completely from scratch, using sufficiently thick wire, will I be able to build a super transformer, that will last for a decent amount of time, and this is not worth the effort as it will take centuries to complete, I will spend a fortune on wire, and it may or may not work; so for now, recessitated OBITs and MOTs will do.
Somtimes OBITs and NSTs are solid state transformers, they basically work like a miniature tesla coil themselves, they operate at a high frequency to get higher voltage, so they cannot charge capacitors correctly (unless they are rectified and used in a DC coil, but that is a "whole 'nother ball game"). The rule of thumb is if it is really light and small and encased in plastic, it will not work. A regular transformer takes up a lot of space, and is alost always in a metal case, and will have a ground screw attached to it somwhere. If it is one of these seen below, you are good to go.
Her are some OBITs and MOTs.
Home, Hobbies, Tesla Coil.
Neon sign transformers are used to light neon signs (duh,,, but sometimes they are called "luminous tube transformers, which you might not have known!) they usually range from 7,000 to 15,000 volts, and from 20 to 130 mA. They are current limited, meaning the amperage put out is limited so you don't need a ballast to use them, this is done by inserting magnetic shunts between the primary and secondary coils to provide leakage inductance and absorb some of the power.
Oil burner ignition transformers are used to ignite oil furnaces, and are almost always 10,000V at 23 mA (I have seen one of different ratings, but it was a small solid state transformer which would not have worked on a TC.) They are also current limited and are much like the neon sign transformer in most aspects, and are frequently used in parallel to increase amperage (I use them, and have had good success, but note, with current limited transformers you must put both the primaries and secondaries in parallel, or else they won't work). Please note, many NSTs and OBITs have two secondary windings, half the potential to the ground. It is important for safety and longevity reasons to ground transformers with current limiting, because if you don't and all is not well inside the transformer, the case of the transformer will float at several thousand volts, I have been zapped before, it is not fun, attach a wire from the case to the mains ground (usually there is a bolt or a screw for this purpose). Some people falsely believe that grounding your transformer will reduce it's output, don't listen to them, they are stupid and are going off a gut feeling and have never done the math (reduced output from grounding is a sign of a bad transformer, if this happens, fix or chuck the unit and get a different one, and make sure the new one is grounded so it doesn't go bad like the last one! Or perhaps your phasing is incorrect, switch ground all the units together, then wire thme up with the secondarys that don't arc together connected, that is the proper way.
Microwave oven transformers are sometimes used, they rate from 1,500 to 3,000 volts (usually 2000) at high milliamperage depending largly on input, but 120volt 20amp breaker usually gives about 200 mA (the voltage output is to low for normal usage, but you can stack them with primaries in parallel and secondaries in series to obtain higher voltages, which makes them highly formidible because of their high amperage) they are not well current limited (they do have magnetic shunts, but they do a lousey job of reducing current), and require a ballast as well as a corona suppressant, as they are not potted (some people submerge their stacks in mineral oil. On more than a four transformer stack this is almost always necessary unless you use very fancy wiring which floats the outer primaries at a higher potential, but this requires about double the MOTs you would normally use). One note, on MOTs the high voltage ground is usually attached to the core, for best results, disconnect it, or you will have problems with shorting to the core.
The power distribution transformer (pole pig), is a gargantuan transformer (the holy grail of Tesla coil tranformers). They are those large cylinders on power lines that reduce high voltage from the line, to send it into your house, to use them for coiling, you run it backwards, putting 240 in, and getting 14.4kV out. A potential transformer does the same thing but it is for testing instruments on the line, not your house. Either one of these puppies will give you 10-23 kV (14.4 is common in the US) at hundreds of milliamps (200 and well beyond, depending on input, some potentials go beyond a few amps) they are rated by the kVA, so a 10 kVA can put out 14.4 kV at 694 ma without exceeding the ratings. They are of course not current limited so a ballast is an absolute must, and a power factor correction capacitor really helps as well, but if used properly, they are virtually indestructible. I want one of these so bad, but they are incredibly hard to get a hold of (legally, it would be easy to steal one, But I can't do that!)
Often it is possible to repair a "dead" NST or OBIT transformer, depending on what is wrong with it, if it is just something not connected, it is a simple matter of de-potting the thing by soaking it in a solvent, and re-connecting the loose or fried wire. If a secondary has a short, occasionally only a few windings need to be removed to bridge the short, so not much voltage is lost, and worse case scenario, you can use windings from two identical KIA donors to make one living Frankenstein's monster. I have attemped to rebuild some dead OBITs, but while I was "in there" I thought about to cutting down on primary turns and current limiting, so as to get higher kilovolts, and milliamperage. Or maybe I could have attached additional secondaries pirated from another transformer in series to double the voltage, and keep the amperage, 20KV at 115 or 120 ma is astronomical, and rivals some of the potentials or MOT stacks available, but the experiment was not a success because of time constraints. It was not likely to last very long anyway, so it will inevitably be a failure (the primary and secondary wire is to thin, and the insulation is to weak, to make a practical transformer). Only if I wind a transformer completely from scratch, using sufficiently thick wire, will I be able to build a super transformer, that will last for a decent amount of time, and this is not worth the effort as it will take centuries to complete, I will spend a fortune on wire, and it may or may not work; so for now, recessitated OBITs and MOTs will do.
Somtimes OBITs and NSTs are solid state transformers, they basically work like a miniature tesla coil themselves, they operate at a high frequency to get higher voltage, so they cannot charge capacitors correctly (unless they are rectified and used in a DC coil, but that is a "whole 'nother ball game"). The rule of thumb is if it is really light and small and encased in plastic, it will not work. A regular transformer takes up a lot of space, and is alost always in a metal case, and will have a ground screw attached to it somwhere. If it is one of these seen below, you are good to go.
Her are some OBITs and MOTs.
For new
Tesla coil, I am useing a four or six MOT stack as my power supply,
they are powerful, but somwhat unreliable buggers. One has to
wire them all in phase, or they will fight, instead of cooperate.
Also they have a tendancy to burn out, I have blown about 6 of them,
but I may be doing something wrong, more on that later.
Home, Hobbies, Tesla Coil.
Scott Bogard. 2007