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.