Coil
1:Coil 2:
Primary:
Secondary:
Spark Gap:
Tank Capacitor:
Transformer:
Top Load:
Input Voltage Control:
Ballast:
Power Factor Correction Capacitors:
Filters:
Safety:
Equations:
DC, Solid State, Three Phase, Miscellaneous:
Links:
By definition, a Tesla coil is a high voltage, resonant air core transformer. There are several components, here is a simplified schematic of a typical AC Tesla coil.
The
Basics:
Here is a basic rundown of what is going on. Firstly, you see there is a 120V AC power source. This is electricity that comes out of your wall. No, I am not trying to insult your intelligence, the reason I am taking time to write about this is that this can vary depending on where you live, and what kind of power you choose to use, which is dictated also by the transformer you employ. Pigs, PT's, and other such transformers generally run off of 240V, and everybody in Europe uses 240 as well. Next we have a switch, you know what it is for. Next we have a transformer, the purpose of the transformer, is to step up the line voltage to give a higher voltage output (which meas higher output of the secondary, as explained in the equations page) and to ensure the efficient functionality of the spark gap (it is easier to build a spark gap for a higher voltage, untill you get above about 20 kV). Next we have a tank oscillator (hold on, it's Ok, I know it is not on the schematic, let me explain). The tank oscillator is actually three components; the spark gap, the tank capacitor, and the primary coil. I am going to shy away from numbers as much as I can, until you have a theoretical grip on what is happening, so, here is what happens. Any time you have a capacitor and an inductor in a series circuit, a condition know as "resonance" occurs, I will explane resonance in the next paragraph in the more complicated rundown of whats going on. For now, just know this, resonance means a transfer of energy quickly between the cap and primary coil at a frequency X. This frequency is the same (when the coil is "in tune"), as the secondary's "resonant frequency". When resonance happens energy is not only tranferred back and forth quickly, but with a very high voltage, that is why the secondary makes high voltages. To understand the nuts and bolts of what is physically happening, read on.
Capacitors:
To understand resonance, we need to know what exactly capacitors and inductors are, and how they work, and what they do when they are "working". A capacitor, is basically two plate shaped conductors each with a surface area X, wires are attached to each plate, but the plates are not physically connected, they are separated by an insulator. As voltage is applied to one plate, a charge is developed, from electrons being pushed to the plate, and as they have nowhere else to go, they are stuck there. This means we have 1 charged plate. Once the first plate is charged, it drives the electrons out of the plate nearest it, for the same reason a balloon sticking to the ceiling from static electricity, or your hair standing on end, etc (opposite charges attract, and like ones repel, the strong potential of one plate repels the electrons of the other plate, and since they have somewhere to go unlike the first plate, they leave). Notice I never once mentioned current.
Inductors:
Now for the inductor. When you have electric current passing through a wire or wire coil, it creates a magnetic field. This magnetic field aligns the atoms of the wire and the metal around it, so if we have a wire, wrapped around a metal core the core will become magnetized along with the wire. Now once the current stops we have energy stored in the form of rotated atoms, which is released as the atoms shift back into their original positions. Electrons are driven away as the magnetic field "collapses", this is called "inductive kick". In theory, if we disconnected a charged inductor, it would store energy forever, but in reality we could never hope to disconnect it fast enough to do that, as as soon as one side was unhooked, the current would stop, the field would collapse and electrons would drain through the other side. Notice I mentioned that a capacitor works on voltage, and an inductor works on current, the reason for this, is that a capacitor cannot have current, as their is no direct path for electron flow, and an inductor cannot see voltage change as there is no physical resistance (remember ohms law, I=V/R?). Now when applied to an AC current (which is described in degrees over time, as it is constantly changing), a capacitor shifts the voltage one way, and the inducter shifts it the other way (both shifts are 90 degrees) which means they cancel each other, and all of the sudden there is no voltage, hence there is no current!
Resonance:
This cannot be correct, the energy has to go somewhere does it not? Indeed it does! What basically happens is that voltage charges one capacitor plate, and this charge drives electrons from the second capacitor plate to ground (the ground completes the circuit) though the inductor (in this case a primary coil), this "current" passing through the capacitor is called transient current, and the math to calculate it is far too complex for me to get into here. Once the capacitor reaches a full charge, it stops passing more electrons through the inductor, so there is no more current, and the inductors magnetic field collapses. When the field collapses, electrons are sent to the capacitor (more transient current, but from inductive kick this time) reversing the process. The two components then swap energy back and forth until traces of DC resistance in the inductor and connection wires eat it up and turn it into heat (or in the case of a Tesla coil, it is transmitted to the secondary by the primary coil). The rate at which energy is transferred back and forth is called the "resonant frequency", and as stated the voltage from rapid oscillating can get mcuh higher than what is put in.
Spark Gap:
Now, if you are a clever monkey, you will notice, in the schematic, that there is a connection to the transformer secondary (another inductor, with a high inductance and DC resistance) which messes up our ideal world. That is where the spark gap comes in, when the capacitor is charged to a sufficient voltage, it fires allowing an extremely low resistance path from cap to primary, allowing the two to "ring" freely at their resonant frequency. This also prevents the voltage gain from tank resonance from exceeding the insulation ratings on the transformer or capacitor and frying them, as the voltage is clamped by the size of the gap. Once the spark gap fires, the transfer of energy to the secondary starts, and it can only decrease as energy is being used much faster that it is being supplied. If you open your gap too wide, you will get better sparks, as you are feeding the secondary with 35+kV insted of 10kV, but you are killing your transformer and cap. That is the tank oscillator, which gives us high frequency, high voltage AC, now what?
Notches and Ringdown:
When the tank cap and primary coil are alternating, they start transferring energy to the secondary coil, as this happens energy is drained out of the tank circuit. Eventually the energy in the tank circuit reaches 0 and the spark gap shuts off (this is called "primary ringdown and first notch"). Ideally this would be the end of things, all the energy would be trapped in the secondary but in reality, once the spark gap turns off, the reverse process usually occurs, energy (oscillating between the secondary "inductor" and "capacitor" top load/ground) begins to transfer back into the primary tank, re-igniting the spark gap (this is called first secondary ringdown). Once the energy in the secondary reaches 0 (first secondary notch) the tank circuit starts ringing down again. These cycles repeat until there is not enough energy in either system to reignite the spark gap, and all of this takes place in a nano-seconds worth of time. At that point the energy left in the secondary-top load system rings for a while until it eventually reaches 0 on its own. As I said, ideally you would want only one notch in the primary and as much energy trapped in the secondary as possible, but this rarely happens (although that is what good "quenching" hopes to achive, it cools the gap preventing too many cycles of energy transfer). Another thing to note, is that if enough energy is transferred to the secondary (assuming they are tuned to the same frequency) sparks will leap from the top load (using energy, and decreasing the amount of energy transferred back to the primary, and giving us what we wanted in the first place).
Coupling:
Another important term dealing with Tesla coiling is coupling. Coupling refers to the proximity of the primary coil to the secondary. In a standard transformer we shoot for a coupling coefficient of 1, that means all the energy from the primary winding to the secondary is transferred in real time. If we did this wiht a tesal coil, the secondary would probably melt, we need to transfer it slowly. We do this wiht two mechanisms; firstly a tesla coil has no magnetic core, so it cannot saturate, secondly the windings are far apart, so only a fraction of hte energy gets in every cycle (hence the reason we have notches and ringdown). With that said, typical values for the coupling coefficient of a Tesla coil are about 0.1 - 0.2. Solid state coils can go a little higher, and magnifier coils approach 0.6.
So there you have it, I have explained everything except for the RF ground, which is basically just a return path for the secondary resonant "circuit". I will soon draw up some graphs and such that will make everything much clearer to understand, so check back soon. Also a quick note on tuning, more top load means you need more primary coil, and more capacitance means you need less, so a balance must be achieved to have a good functioning system.
Other projects:
Initially, I started this project simply to learn about another one, plasma balls. They are basically small solid state (transistorized) Tesla coils with the arcs contained within a glass sphere enclosing low pressure gases. After much work and expense, I have finally made a good looking, quiet, and safe prototype (it self-destructs if it is left on to long, but better gasses = less power = no overheating, so we are working on it!). I still need a reliable sealing method, but I am working on that as well. These are early prototypes I made, but you can learn much more about these on my plasma ball page.
Here is a basic rundown of what is going on. Firstly, you see there is a 120V AC power source. This is electricity that comes out of your wall. No, I am not trying to insult your intelligence, the reason I am taking time to write about this is that this can vary depending on where you live, and what kind of power you choose to use, which is dictated also by the transformer you employ. Pigs, PT's, and other such transformers generally run off of 240V, and everybody in Europe uses 240 as well. Next we have a switch, you know what it is for. Next we have a transformer, the purpose of the transformer, is to step up the line voltage to give a higher voltage output (which meas higher output of the secondary, as explained in the equations page) and to ensure the efficient functionality of the spark gap (it is easier to build a spark gap for a higher voltage, untill you get above about 20 kV). Next we have a tank oscillator (hold on, it's Ok, I know it is not on the schematic, let me explain). The tank oscillator is actually three components; the spark gap, the tank capacitor, and the primary coil. I am going to shy away from numbers as much as I can, until you have a theoretical grip on what is happening, so, here is what happens. Any time you have a capacitor and an inductor in a series circuit, a condition know as "resonance" occurs, I will explane resonance in the next paragraph in the more complicated rundown of whats going on. For now, just know this, resonance means a transfer of energy quickly between the cap and primary coil at a frequency X. This frequency is the same (when the coil is "in tune"), as the secondary's "resonant frequency". When resonance happens energy is not only tranferred back and forth quickly, but with a very high voltage, that is why the secondary makes high voltages. To understand the nuts and bolts of what is physically happening, read on.
Capacitors:
To understand resonance, we need to know what exactly capacitors and inductors are, and how they work, and what they do when they are "working". A capacitor, is basically two plate shaped conductors each with a surface area X, wires are attached to each plate, but the plates are not physically connected, they are separated by an insulator. As voltage is applied to one plate, a charge is developed, from electrons being pushed to the plate, and as they have nowhere else to go, they are stuck there. This means we have 1 charged plate. Once the first plate is charged, it drives the electrons out of the plate nearest it, for the same reason a balloon sticking to the ceiling from static electricity, or your hair standing on end, etc (opposite charges attract, and like ones repel, the strong potential of one plate repels the electrons of the other plate, and since they have somewhere to go unlike the first plate, they leave). Notice I never once mentioned current.
Inductors:
Now for the inductor. When you have electric current passing through a wire or wire coil, it creates a magnetic field. This magnetic field aligns the atoms of the wire and the metal around it, so if we have a wire, wrapped around a metal core the core will become magnetized along with the wire. Now once the current stops we have energy stored in the form of rotated atoms, which is released as the atoms shift back into their original positions. Electrons are driven away as the magnetic field "collapses", this is called "inductive kick". In theory, if we disconnected a charged inductor, it would store energy forever, but in reality we could never hope to disconnect it fast enough to do that, as as soon as one side was unhooked, the current would stop, the field would collapse and electrons would drain through the other side. Notice I mentioned that a capacitor works on voltage, and an inductor works on current, the reason for this, is that a capacitor cannot have current, as their is no direct path for electron flow, and an inductor cannot see voltage change as there is no physical resistance (remember ohms law, I=V/R?). Now when applied to an AC current (which is described in degrees over time, as it is constantly changing), a capacitor shifts the voltage one way, and the inducter shifts it the other way (both shifts are 90 degrees) which means they cancel each other, and all of the sudden there is no voltage, hence there is no current!
Resonance:
This cannot be correct, the energy has to go somewhere does it not? Indeed it does! What basically happens is that voltage charges one capacitor plate, and this charge drives electrons from the second capacitor plate to ground (the ground completes the circuit) though the inductor (in this case a primary coil), this "current" passing through the capacitor is called transient current, and the math to calculate it is far too complex for me to get into here. Once the capacitor reaches a full charge, it stops passing more electrons through the inductor, so there is no more current, and the inductors magnetic field collapses. When the field collapses, electrons are sent to the capacitor (more transient current, but from inductive kick this time) reversing the process. The two components then swap energy back and forth until traces of DC resistance in the inductor and connection wires eat it up and turn it into heat (or in the case of a Tesla coil, it is transmitted to the secondary by the primary coil). The rate at which energy is transferred back and forth is called the "resonant frequency", and as stated the voltage from rapid oscillating can get mcuh higher than what is put in.
Spark Gap:
Now, if you are a clever monkey, you will notice, in the schematic, that there is a connection to the transformer secondary (another inductor, with a high inductance and DC resistance) which messes up our ideal world. That is where the spark gap comes in, when the capacitor is charged to a sufficient voltage, it fires allowing an extremely low resistance path from cap to primary, allowing the two to "ring" freely at their resonant frequency. This also prevents the voltage gain from tank resonance from exceeding the insulation ratings on the transformer or capacitor and frying them, as the voltage is clamped by the size of the gap. Once the spark gap fires, the transfer of energy to the secondary starts, and it can only decrease as energy is being used much faster that it is being supplied. If you open your gap too wide, you will get better sparks, as you are feeding the secondary with 35+kV insted of 10kV, but you are killing your transformer and cap. That is the tank oscillator, which gives us high frequency, high voltage AC, now what?
Notches and Ringdown:
When the tank cap and primary coil are alternating, they start transferring energy to the secondary coil, as this happens energy is drained out of the tank circuit. Eventually the energy in the tank circuit reaches 0 and the spark gap shuts off (this is called "primary ringdown and first notch"). Ideally this would be the end of things, all the energy would be trapped in the secondary but in reality, once the spark gap turns off, the reverse process usually occurs, energy (oscillating between the secondary "inductor" and "capacitor" top load/ground) begins to transfer back into the primary tank, re-igniting the spark gap (this is called first secondary ringdown). Once the energy in the secondary reaches 0 (first secondary notch) the tank circuit starts ringing down again. These cycles repeat until there is not enough energy in either system to reignite the spark gap, and all of this takes place in a nano-seconds worth of time. At that point the energy left in the secondary-top load system rings for a while until it eventually reaches 0 on its own. As I said, ideally you would want only one notch in the primary and as much energy trapped in the secondary as possible, but this rarely happens (although that is what good "quenching" hopes to achive, it cools the gap preventing too many cycles of energy transfer). Another thing to note, is that if enough energy is transferred to the secondary (assuming they are tuned to the same frequency) sparks will leap from the top load (using energy, and decreasing the amount of energy transferred back to the primary, and giving us what we wanted in the first place).
Coupling:
Another important term dealing with Tesla coiling is coupling. Coupling refers to the proximity of the primary coil to the secondary. In a standard transformer we shoot for a coupling coefficient of 1, that means all the energy from the primary winding to the secondary is transferred in real time. If we did this wiht a tesal coil, the secondary would probably melt, we need to transfer it slowly. We do this wiht two mechanisms; firstly a tesla coil has no magnetic core, so it cannot saturate, secondly the windings are far apart, so only a fraction of hte energy gets in every cycle (hence the reason we have notches and ringdown). With that said, typical values for the coupling coefficient of a Tesla coil are about 0.1 - 0.2. Solid state coils can go a little higher, and magnifier coils approach 0.6.
So there you have it, I have explained everything except for the RF ground, which is basically just a return path for the secondary resonant "circuit". I will soon draw up some graphs and such that will make everything much clearer to understand, so check back soon. Also a quick note on tuning, more top load means you need more primary coil, and more capacitance means you need less, so a balance must be achieved to have a good functioning system.
Other projects:
Initially, I started this project simply to learn about another one, plasma balls. They are basically small solid state (transistorized) Tesla coils with the arcs contained within a glass sphere enclosing low pressure gases. After much work and expense, I have finally made a good looking, quiet, and safe prototype (it self-destructs if it is left on to long, but better gasses = less power = no overheating, so we are working on it!). I still need a reliable sealing method, but I am working on that as well. These are early prototypes I made, but you can learn much more about these on my plasma ball page.