Transistor-Driven
Resonant Coupler
A working circuit. An open-ended coil. A question the initial model could not explain.
Project overview. For my thesis, I conducted independent electronics research. I built and tested a transistor-driven resonant coupler to investigate short range electromagnetic excitation. Observations of a fluorescent tube and a mismatch with an early PSpice model led to a deeper investigation of distributed capacitance and feedback.
Excitation without a wired connection.
Motivated by wireless energy transfer and sustainability applications, I investigated whether a compact resonant circuit could illuminate a fluorescent tube at close range without a direct electrical connection—and what physical constraints would limit the effect.
Build. Test. Reconsider.
I developed the prototype through hands-on iteration and later revisited its behavior through circuit modeling.
- 01Construction & troubleshooting
Tested components, practiced soldering, adjusted winding configurations, and assembled a working transistor-driven circuit.
- 02Experimental observation
Tested fluorescence near the secondary and recorded oscilloscope readings of approximately 9.3–9.6 MHz.
- 03Modeling & interpretation
Built an initial PSpice representation and examined why its simplified behavior differed from the physical prototype.
The physical circuit posed a better question.
A 9 V supply drove a transistor-switched primary coil, coupled to an approximately 120-turn secondary wound on a PVC former. The secondary’s high-voltage end remained open; its lower end participated in transistor-base feedback.
Construct
Refine coil geometry and connections after failed configurations and a series-capacitor experiment that did not produce the intended response.
Observe
Illuminate an unconnected fluorescent tube near the winding; note the reduction in visible response as separation increases.
Revisit the model
Compare measured behavior with idealized PSpice assumptions and investigate the role of parasitic capacitance, coupling, and feedback.
An open terminal isn’t the absence of capacitance.
The working circuit excited a nearby fluorescent tube and oscillated at approximately 9.3–9.6 MHz. Its open-ended secondary presented a question absent from the simplified model: how could the system resonate without an explicitly connected capacitor?
In subsequent analysis, I examined how distributed capacitance between windings and with the surroundings permits an oscillating electric potential at the open end. Voltage from the lower end of the secondary feeds back to the transistor base, influencing primary-current switching. With the correct feedback phase, energy supplied by the battery compensates for losses and sustains the oscillation.
Near-field fluorescent illumination, declining visible response with distance, and oscilloscope readings near 9.3–9.6 MHz.
Distributed capacitance, coil geometry, electromagnetic coupling, and base feedback help explain why the initial idealized representation was incomplete. Transferred power and efficiency were not measured.
Experiment and explanation.
Three reconstructions illustrate the observed behavior, physical arrangement, and later theoretical interpretation.
Research context & documentation.
A physical experiment can reveal what’s missing from a model—and become the starting point for a better question.