McTesla – A Strange Mini Tesla Coil

The McTesla is small 1000 watt half bridge Tesla coil with 3D printed forms for the terminal, secondary and the enclosure. The plastic used was PETG, which has low dielectric loss. The primary is 4 turns of 16 gauge automotive hookup wire(thick jacket) separated from the secondary about 1/8″ by means of 3D printed spacers. The power is provided directly from the mains through a half wave rectifier. The actual foot print is for a bridge rectifier but with a jumper wire you can fit the selected single diode on the board. Either will work fine. I get a little more crackle out of the half wave version so I just left it in place.

The secondary is 5″ tall and 3′ in diameter at the bottom and 5″ in diameter at the top. This alters the distribution of inductance of the coil and all but requires you have a substantial top load to suppress corona on the upper part of the winding. I get larger arcs with the conical secondary than an equivalent cylindrical secondary…maybe an inch or so more spark. I need to analyze why this is the case in more detail. The math shows a drum coil with equal diameter and height produces optimal Q. However, I think I am getting better primary coupling and impedance match allowing more energy couple to the secondary. It is wound with 700 turns of 33 gauge wire and resonates at 350 kHz. I tried 27 gauge wire and it worked fine. It resonated at 600kHz and streamer length was a couple inches less. Anything between 33-28 gauge will work well.

I have tried all sorts of different finishes for the secondary, including: polyurethane, and two part epoxy, ultimately, I ended up using 3 coats of spray shellac. It dries very quickly and you can apply 3 coats within a couple of hours. I found that when the coil is properly tuned and primary coupling is correct, a heavy insulative finish is not required.

The half bridge architecture I used for this is described in more detail in a previous blog post: https://wordpress.com/post/circuitsalad.com/1875 . The main feature is the inclusion of a simple passive adjustable phase shift network in the feedback path. This allow one to minimize switching losses by minimizing current and voltage overlap in the mosfets. The end result is the FETS are less stressed and need less heat sinking. In this design, I have made some minor changes from the previous circuit. I am now using the IXTQ52N30P 300V@52 amp mosfet instead of the IRFP4229. The first IRFP4229’s I got worked great but then the next two I ordered failed quickly. As well, they seem scarce and hard to source so I thought it best to try another. The IXQ52N30P is near indestructible and I have yet to blow one. I simplified the modulation input circuit and decided to remove the diodes I was using across the gate drives. The diodes were in place to ensure the mosfets were not both conducting at the same time but I found when properly tuned, this not needed. I also used terminal blocks for the mosfet connections for easy experimental swapping or replacement . You can see this on the picture of the circuit board below. The driver board uses a 200:1 current sensing transformer for magnetic feedback to generate self excitation. This feedback signal is then phase shifted with a two stages of RC low pass filtering. One stage is fixed and the other adjustable. The adjustment range is from about 1 MHz to 200 kHz and is very forgiving. Optimal adjustment can be achieved by use of an oscilloscope, but simply tuning for the largest spark and then reducing the resistance of the POT slightly works well too.

Here is a link to all the tech data and files: https://www.adrive.com/public/5RrD3r/McTesla%20Tech%20data.zip

It includes native expresspcb cad files, schematics, gerbers, 3D print files and software development files

If anyone is interested in building this device everything you need is provided in the download. I am sure there will be questions which I am happy to answer. The interrupter is not required and can be replaced with a momentary switch or any other modulated switch closure. Some of the component choices are non-critical; such as 1N5819 diodes. The UCC27322 can be used instead of the UCC27321. The gate drive transformer can be any small 73 type ferrite material balun core or toroid. Other current sensing transformers will work but the CS1200L is a good choice and available. Certain IGBT’s will work well as well as other mosfets. You may need more heat sinking depending on the device selected. The AC to DC 12V module is a cheapo pcb module from Amazon. https://www.amazon.com/dp/B07SJRX9R6?psc=1&ref=ppx_yo2_dt_b_product_details

McTesla Schematic
10″ Streamer with the McTesla
View of the McTesla circuit board

The Interrupter circuit is based on a PIC microcontroller and uses a simple opto- isolator as a transistor switching output. The OLED display used is a common 128×64 SSD1306 type. Ironically, I didn’t use the PWM output on the chip and created my own in software….but you easily could. I have it setup to provide 4 different duty cycles up to 50% , which can be adjusted by means of the two buttons. A rotary encoder sets frequency from 1hz -220Hz. I have driven the coil with 20Khz and it works. The momentary switch on the encoder is used to turn on and off the device. The software was written in C with the MikroC pro compiler. Next, I will make a PWM modulator to play music!

IGBT’s In My Optimized Half Bridge Tesla Coil

I decided to try using IGBT’s in my optimized half bridge Tesla Coil driver circuit. The results were good but the IGBT’s did have more heating than the very low RDS on mosfets such as the IRFP4229. I was able to phase shift the drive feedback such that zero crossing switching was achieved, but the Collector to Emitter voltage of approximately 1 volt @ 10 amp current produces 10 watts of dissipation, whereas the IRFP4229 RDS of. 04 ohm yields only 4 watts of dissipation @ 10 amps. So for a low power coil the FET is a better performer. At much larger currents, the IR dissipation the Mosfet grows exponentially, so in DRSSTC designs the IGBT would be a better choice as its dissipation grows linearly. My test coil operates at nearly 400KHz. This is a very high frequency for IGBT’s. The main issue with IGBTs is that they are slow to turn off when driven to saturation(switching). I chose the STGW35HF60W 600V, 35 amp ultra fast IGBT for my experiments. It is optimized to minimize turn off delay and residual tail current. It worked well at this frequency but the price you pay is a higher Collector to Emitter saturation voltage than you get with slower IGBTs. So again this is a trade off. Over all these IGBTs work well in the circuit and while they do get warmer than the Mosfets, they do not overheat with the small heat sinks and switching losses are low. They seem tolerant of abuse and I have not blown any up yet!

Optimized ZVS switching with IGBT’s

Optimised Half Bridge Tesla Coil Driver

Lately, I have been experimenting with solid state Tesla Coil design ideas and after many destroyed mosfets, I have developed a reliable and efficient conventional solid state half bridge Tesla Coil driver. The topology is a basic half bridge so the primary is not resonant. This choice was made so I could operate it in continuous mode when desired. It is designed to operate with 120 VAC. The circuit is self exciting, utilizing current feedback from the secondary by means of a small 200:1 coilcraft current sensing transformer. The driver is efficient requiring minimal heat sinking of the mosfet switches. As such, the driver can be run for long intervals. The limitation on run time is constrained by the coils(secondary and primary ) and the electrolytic power supply capacitors heating up. I can generate 11 inch streamers on my 5″ diameter x 7″ tall secondary with a 6″ toroid terminal. I can just change out the different secondary and primary combinations I want to test, as the driver will easily operate from 100Khz to over 1 MHz. Depending on the frequency, you may need to change one resistor. The driver board is only 3.6″x 3.6″ and including simple stick on heatsinks for the mosfets.

Complete Half Bridge driver board with opto-isolator input
Demo of Driver Circuit with Small Coil

Features that improve efficiency and reduce mosfet heating:

Properly designed gate drive: I used an appropriately sized and correct ferrite material xfmr core for the drive transformer. The core I used is very small (.3 inches) and was cannibalized from a small pulse xformer obtained from Mouser(photo below). There were no cheap small cores available on Mouser so i just found a small pulse isolation transformer that was inexpensive and I could get the core from. If you prefer, you can purchase stand alone core. Type 73 material is a good choice and a core no bigger than a .5 inch will be fine for this circuit. I cannibalized the core out of the housing and re-wound it with a trifilar 28 gauge winding of 5 turns. I also placed schottky diodes across the 10 ohm gate resistor providing for slightly faster off time than on time, This ensures minimal shoot through(both mosfets on at the same time). The drive is clean with no ringing and fast rise and fall times(pictures below). This reduces switching losses.

Drive signal as seen from mosfet gates(no ringing)
Output switch waveform as seen from drain of mosfets(very little ringing)

Choice of mosfet: I wanted to select a low cost and high performance mosfet for the circuit. There are two I found that work well. The basic criteria are : > 250V breakdown rating @ > 20 amp current capacity. Low output capacitance, gate capacitance and on resistance were also important. The IRFP4229 is the ideal candidate. It has a breakdown of 250V. It can handle 40 amps of current, and has total switch delay of of 100ns(very fast). The on resistance is .04 ohm; so IR losses are low. A similar device, only slightly slower and with a slightly greater on resistance is the STW46NF30. Both were evaluated and worked well. The cost of both of these mosfets is around $3 each.

Adjustable phase shift control in feedback loop: A substantial improvement in output and efficiency was obtained by placing a simple low pass RC phase shift network in series with the secondary feedback path. It was observed that when my test coil was driven from an external source, the input drive was approximately 90 deg out of phase with the secondary output. This was the case when the coil was tuned for maximum output. This amount of phase shift will not support oscillation properly. Forcing the coil to self excite will not produce optimal output unless some sort of phase shifting is employed. The notion that a self excited tesla coil will inherently tune itself to best performance is incorrect. Adjusting the input /output the phase relationship maximizes coil output but also lets one adjust the drain voltage/current relationship such that zero crossing switching can be obtained. This is when the current and voltage across the switches do not overlap. This greatly reduces power losses across the switch. The phase shift network consists of two simple RC low pass filters in series. One with a fixed resistor and the other is a variable resistor to allow adjustment. The low Q nature of the RC filter provides optimal tuning over a wide range and is easy to adjust and is stable. The values for the filter as shown in the schematic are easy to calculate. Simply pick a resistor value(10k for example), then use the approximate value of operating frequency(+-20% is fine) to determine the capacitors required. The value of capacitance is correct when the reactance of the capacitor at the given frequency is the same as the resistor value(10k). Or chose a capacitor value such as 47pf. With this value, the reactance at 650KHz is 1/2Pi(650KHz)(47pF), which is 5212 ohms. So you could use a 4.7k resistor for one RC stage and a 10k to 50k pot for the second stage. In my example coil, the frequency is around 400KHz and so I used 10k with 47pF and a 25K Pot with a 47 pF for the second stage. The low pass characteristic of the phase shifting also has the benefit of reducing higher frequency noise and parasitics in the feedback loop. The result of this simple addition to the circuit is substantial and when you tune the variable resistor you can see the output significantly increase and decrease. You can monitor the drain voltage and current to adjust phase for optimal performance. Alternatively, I have found that you can simply adjust for longest streamer length and then add just a tiny bit more phase shift to optimize switching efficiency.

Input to output relationship before phase shift added
Input to output phase relationship after phase adjustment
Drain voltage and current(purple) when poorly tuned
Drain Voltage and Current when phase adjusted for maximum output and lowest switch loss