Friday, May 31, 2013

Minuet 1.x

Time to go solid state!

For my first solid state coil, I decided to go with something on the moderate side of complexity, basing it off of Steve Ward's mini SSTC schematic: http://www.stevehv.4hv.org/SSTC5.htm

Operating Principles

Tesla coils are resonant air-cored transformers.

A primary inductor is coupled to a secondary inductor, resulting in a voltage step-up that allows for the breakdown of air after the secondary exceeds air's dielectric voltage (~3 × 106 V/m). 

By using an air cored transformer, Tesla coils avoid the pesky core saturation that would otherwise limit the magnetic field and the resulting high voltage. 

The simplest variation of a Tesla coil, a spark gap coil, consists of the following:


SGTC Schematic
Source: http://enterpriseprogrammer.files.wordpress.com/2012/07/tesla_coil_circuit_diagram.png

When the primary tank capacitor reaches the voltage neccesary to arc across the spark gap (literally two electrodes separated by air), an LC circuit is formed that exhibits a resonant frequency.

Spark gap Tesla coils (SGTCs) and Dual resonant solid state Tesla coils (DRSSTCs) use two resonant tuned circuits (The primary and secondary) to more effectively transfer energy from the primary to the secondary coils. This is achieved by the phenomenon in which at resonance, impedances and reactances effectively cancel each other out.  

Given the resistive nature of the primary and secondary, the oscillations at the resonant frequency in both sides of the transformer will undergo decay and display waveform damping. This damping effect produces distinct RF "cycles" that are offset by 90 degrees. It's during each cycle that energy is exchanged through the changing magnetic field.

Simple solid state Tesla coils only have a secondary which is coupled to a primary inductor. The primary inductor is then driven at the resonant frequency of the secondary. 


Being driven at resonance requires current to flow back and forth in the inductor at a resonance frequency (simulating AC). In a solid state coil, this is achieved through transistors assembled in either a half-bridge or a full bridge (H-bridge) with the primary inductor acting as the load. The resonant frequency of most secondaries is around a few hundred kHz and bus currents and voltages linger around in the tens of amps and 240V (bus voltages will vary from topology to topology), this puts quite a bit strain on the transistors, limiting the field of options down to two types of transistors: MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors); the transistor strain is limited by the fact that most of the oscillations are induced in pulses i.e. the coil is kept in operation in short bursts to prevent overheating. Spark production is also noticeably different when driven in an "interrupted" mode as opposed to continuous duty. 

More often, IGBTs are favored over MOSFETs because they exhibit diode-like voltage drop while MOSFETs exhibit a resistive drop that increases with current.

Dual resonant variations of solid state coils include a tank capacitor in the primary side circuit, creating, like a spark gap coil, an LC circuit while in operation. The addition of a capacitive element to the circuit also cancels out the inductive reactance of the primary coil, thus allowing for currents orders of magnitude greater than regular SSTCs to propagate within the primary.

Tuning

Spark gap coils are tuned via physical parameters: the inductance of the primary, the capacitance of the tank capacitor, etc. 

In solid state coils, the resonant frequency is maintained by some sort of feedback mechanism. SSTCs generally use antenna based feedback (literally reaching into the electric field produced by the secondary), which induces a changing voltage as the field changes, while DRSSTCs required primary feedback as the secondary is out of phase with the primary. SSTCs are unable to use primary feedback because the current isn't effectively measurable due to the lower primary currents found in SSTCs. 

Construction:

Minuet 1.x was intended to be a single-boarded audio modulated SSTC; unfortunately, it's had a hard and very confusing life.

Evolution (to be added when I bother to take some pictures)

At one point, it did produce sparks, albeit only in continuous wave operation:


video

6'' sparks run continuous wave
The board since exploded after the addition of a fiber-in jack to more reliably interrupt operation. Maybe duty cycle issues?

If any good came out of this project, it's the list of things you shouldn't do

The project was abandoned in lieu of -oneTesla (negative oneTesla) 5/29/13. 














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