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Transcript
ECE791/792 First Semester Progress Report
Project Title: Singing Tesla coil
Project Team: Peter Lorenz, Ryan Lashin
ECE Faculty Advisor: Dr. Kent Chamberlin
Current Date: December 5, 2009
Project Completion Date: May, 2010
General Problem Definition
The objective for this project is to use a fly-back transformer to create rapidly pulsing electrical
arcs that reproduce desired sounds. By using an input signal and control circuitry, the fly-back
transformer will pulse these arcs at its fundamental frequency and will be amplitude modulated by the
input audio signal. To the human ear the sound created should be recognizable as a distorted version of
the original input signal.
Progress
During this semester we have focused primarily on the design aspect of our project. Through
research, testing and input from our advisor we have updated our design numerous times. These
revisions have led us to a refined and finalized version of our project, which we are ready to implement
and test next semester. Below follows a list of the steps we have taken on this project at the time of this
writing.
Step I: Original design
Figure 1; 1st Design
Figure 1 shows our original design which features an H-bridge type driver powered by a DC
supply. The driver provides current to the Tesla coil’s primary coil in both the positive and negative
directions. The Tesla coil in this design is the basic Tesla coil with a torroidal capacitor at the top, arcing
to a grounded copper rod. On the front end of this design is the very basic version of our audio
detecting setup, which controls the oscillator that runs the driver circuit. This design calls for the
oscillator to be switched on and off as the audio signal traverses between positive and negative values,
which is used to create the “envelope” effect that is common to all of our designs.
The operation of this design is intended to go like this: The H-bridge driver delivers a highvoltage square wave through the primary coil, which causes the secondary coil to activate. The driver
circuit is controlled by the oscillator, which is in turn controlled by the input audio signal. This will result
in the Tesla coil firing continuously at high frequency whenever the audio signal is high, thus creating the
desired “envelope”.
The above design has several faults, and the more significant of those problems is listed below:
1. Through testing and research we realized that we did not need to have the driver circuit
sending current through the coil in both directions. When the current was flowing in the positive
direction, the coil operated as we intended it to. When it flowed in the negative direction, however, the
transformed current would simply flow into the secondary coil’s ground and be lost.
2. This design had no specific provision for how a control signal would be obtained from the
incoming audio signal. At the time that this design was created we were still researching methods of
creating the necessary control signal, so we had not decided which method to employ.
3. This design called for us to build our own oscillator. This was an unnecessarily complicated
and time consuming component to build, and could easily have been replaced with an IC circuit.
4. This design required a very large amount of power, possibly more than could be drawn
reliably and safely from a wall socket. Testing also showed that this circuit would have been very
inefficient, largely due to problem number 1.
5. This version also called for us to design our own rectifying power supply, another
unnecessarily complicated and time consuming component.
6. This version included a large number of power MOSFETs, but had no circuitry in place to
protect them from back-emf generated by the coil’s collapsing magnetic field.
Due to these problems, we spent a large amount of time researching alternatives to this design.
In the end we came up with a whole new revision, as shown below.
Step II: Second design
Figure 2; 2nd Design
This revised design is specifically designed to overcome the drawbacks of the original. It
features a 75% reduction in the number of power transistors needed, and it has significantly greater
efficiency because it no longer loses half of its power to ground. This design retains both the DC power
supply and the oscillator, but these two components are now replaced by off-the-shelf components.
This design also includes a set of diodes protecting the power MOSFET from back-emf, and it now
employs heat dissipation techniques. The basic idea behind this design is to have it operate in a similar
manner to the original, but without the bidirectional H-bridge driver. In its place we use a single-power
MOSFET, which uses the primary coil as an inductive load across its drain, and ground at its source.
When the MOSFET switched, it will cause a rapid transition from zero voltage to high voltage across the
primary coil (essentially a square pulse), causing a “spike” or impulse of power to run through the
transformer and fire the secondary coil. The oscillator is placed to switch the power MOSFET, and the
oscillator is switched by the audio source. This means that when the audio signal is high the oscillator
will turn on and switch the MOSFET continually, which will in turn cause the Tesla coil to arc continually.
Once we had this design solidified, we built a small version of it on a breadboard. After
obtaining a few outputs from this setup, we realized that impulses were being created both on the rising
edge (low to high) and falling edge (high to low) of the pulse. The result was a train of alternating
positive and negative impulses, which was undesirable because it was an inefficient use of power. After
experimenting with various different waveforms, we discovered that a sawtooth waveform would
reliably create impulses in only one direction. This caused us to make another alteration to our design,
which is discussed subsequently.
Design 2 suffered from some of the same problems as the original design. It still required a
prohibitively large amount of power and a variety of expensive high-power components. It made
provisions for the power supply and audio detector, but still lacked a specific implementation plan for
them. Despite the changes made from the original, there were still some problems that had not been
entirely solved.
Step III: Third design
Figure 3; 3rd Design
This design retains many of the changes made in the second design, and solves some of the
problems left over from the original. The decision was made in this revision to drop the DC source
altogether in favor of an AC sawtooth power supply. This allows us to forego the oscillator and by using
the AC source to drive the coil directly. The control signal from the audio source is applied directly to
the MOSFET, which is used to switch the AC source to the primary coil.
This setup had some distinct advantages over the second design, particularly in the combination
of the oscillator and source. The transformer we were using to test our design responded better to an
AC waveform than it had to our DC square wave, and our overall efficiency was greater.
Unfortunately this design still suffered from a very high power requirement, and obtaining the
necessary high voltage AC signal generator proved to be expensive and difficult. Ultimately, however,
this design was the setup we intended to use as we started to build our full-sized coil.
Before we started the physical construction of our coil, our advisor Prof Chamberlin requested
that we downsize our project to a proof-of-concept to reduce the power requirements and personal
safety risks. At his suggestion we redesigned our project again to replace the Tesla coil with a flyback
transformer. This led to our current design, discussed below.
Step IV: Final design
Figure 4; Final Design
Our final design involves using a DC source to power a flyback transformer. The transformer is
capable of generating its own fundamental frequency and arcing by itself, allowing us to create our
envelope by controlling its power supply directly. The entire circuit now operates at a fraction of the
original power requirements, and thus no longer requires many of its high-power components and
safety measures. The power supply will be a modified computer power supply, which should greatly
simplify the process of obtaining sufficient power from the wall socket. Additionally, the audio source
has been simplified to a square wave input which represents a single tone. This proof-of-concept design
should allow us much greater latitude in construction and implementation, and open up the possibility
of adding features.
At the time of this writing, most of the components for the above design have been acquired.
We are currently in the process of researching the necessary control circuitry for the flyback transformer
we have obtained, and we are testing various methods of modifying our computer power supply to
meet our needs. We have everything we need to start implementation and testing immediately next
semester, with the goal of completing a working model as soon as possible.
Progress and Timeline Comparison
Table 1; 1st Semester Timeline
Our first objective over the past semester was to finalize the digital control circuitry by the end
of October. We accomplished this goal by reducing the number of transistors in the control circuitry to
just one MOSFET. The very first design included multiple transistors but through simulations and testing
we concluded that one transistor will drive the fly-back transformer properly with the input signal.
We succeeded in simulating the third design with one transistor and a transformer. We also
created a low powered circuit using a transistor and a transformer that were available to us. We used a
signal generator to test the design under different conditions. We concluded that a sawtooth wave
provided the best results (see page 4).
We no longer needed to construct a Tesla coil since our design was scaled back to a proof-ofconcept instead of a full size construction. This means we do not need to physically construct the coil,
design and build a torroid, or build an analog version of the circuit.
By the end of this semester we have managed to gather most of the parts required for the final
design. We have procured a flyback transformer along with a computer power supply. We have not
found power transistors but since our design has been downsized, we may no longer require power
transistors. If this is the case, than we have access to numerous types and models of MOSFET’s in the lab.