First of all...sorry about the small image! Please right-click and save it separately. The JPEG has plenty of resolution to see everything
clearly if you view it in another program.
This circuit uses two FETs in a half-bridge configuration to drive the primary of a small Tesla coil, producing up to 9" sparks from 120 V.
The builder should be aware that, as I have built it, AC neutral and ground are the same. This means that one shouldn't operate the coil from
outlets protected with ground fault interrupters. Also note some other IMPORTANT features that are not in the diagram but which should not be
omitted: circuit breaker and a line filter. Also, I use an MOV and a series inrush current limiter where the power enters the circuit. The driver is
composed of three parts. On the left is a UC3825A high-frequency PWM chip that serves as a double-output square-wave oscillator. Its output
is buffered by a UC3708 FET driver chip in the middle of the diagram. This driver chip has its outputs periodically enabled and disabled by the
square wave from a standard 555 timer chip oscillator, which works to modulate the Tesla coil output with a range of "staccato"-type effects
and keep its duty cycle down. Finally, the two FETs are driven by the driver chip through an isolating gate-drive transformer, T1.
CRITICAL COMPONENTS: The FETs can be NTP12N50's or NTP15N40's or similar (click links to get datasheets and free
samples from ON Semiconductor.) Click here for info about the MUR1560 fast rectifier. Both the UC3825A and the UC3708 come from
Texas Instruments. Click the links to get datasheets. The ferrite bead should be a RFI suppression bead for cables or similar- don't
bother to get expensive low-loss high-frequency beads. Wind T1 with 20 turns on each winding, preferably in a trifiliar fashion. The
1-microfarad 400 V caps should be metallized polypropylene or similar with low RF loss and inductance. Use caps that you would build an
MMC out of. The phase control is optional- light dimmers of the $6 variety work well, and work even better if the internal capacitors are
replaced with 0.15 microfarad / 200 V caps. The 10-uF electrolytics shown are tantalum. They seem not to mind the reversing polarity of the
signal in the T1 primary. To make the tesla coil itself, wind a 9" length of 4.25" OD PVC sewer / drain line pipe with #30 magnet wire (this
can be obtained at McMaster-Carr). The primary is two turns of heavy "zip cord" positioned no further than 1" up the secondary.
CONSTRUCTION INFO: See the image below for what my board looks like. Most importantly, the paths that are shown in bold in the
circuit diagram must be short, low inductance traces- especially from the FET sources to the zeners! This is because induced potentials can
cause excessive gate-source voltage if there is much path length there. Shielding of the driver circuitry is imperative, or the 555 timer
in particular will get "confused" by RF from the Tesla coil. Observe the correct phasing represented by dots for T1!
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