QCW 2.0 -

QCW 2 - tinyQCW

Over the past two years or so, I successfully built a working QCW Tesla Coil (see QCW 1.0 and 1.5). After a lot of work, experimentation and rebuilds, the eventual coil performed very well and I was very impressed by how large I could make the sparks relative to the size of the coil via the QCW topology. This made me realize that the QCW is a fantastic candidate for a small table-top coil capable of producing fairly impressive sparks. This motivated me to build a QCW coil as small as I can, with the goal of developing an ultra-portable tiny QCW coil.

My QCW 1 employed a buck-converter front end with a max primary current of around 160A. For this new coil, the goal is to create a smaller coil with a peak current closer to 60A, and driven by a completely differently drive technique known as Phase-Shifted modulation, essentially eliminating the bulky buck-converter front end though at the cost of increased drive complexity and hard switching. Drive is implemented via a newly developed driver with a micro + CPLD-like implementation. The result should be a significantly smaller coil capable of being run on a small table and easily transportable in a small bag / box.

[This page is still under construction!]

For more information about basic QCW operation, please see my previous QCW pages :).
Project Status - Project Completed with >2' sparks.

Thanks for visiting my page! If you have any questions, wish to share your projects, or feel that my projects have inspired you in one way or another, feel free to email me at loneoceans[at]gmail(dot)com. I'd love to hear about your projects too. :-)

Specifications (Oct 2015)

- FGA60N65SMD Full Bridge Inverter '80mm Bridge', 15R gate-resistor
- 5.875nF MKP 3.2kVAC/8kVDC MMC
- 17 turn 1.2" dia  18AWG "half-donut" primary coil
- 60Apk Max Primary Current
- Secondary coil 2.41" x 3.52" 38.5AWG x ~610 Secondary
- 8 x 2.09" Spun Toroid
- ~308 kHz unloaded secondary frequency
- Run from 90-260VAC input
Size Goal (Oct 2015): Fit inside a USPS Medium-sized box
Performance Goal (Oct 2015): 10x secondary length = 2 feet

For much more videos and images of the coil in action, please scroll down.

 Sept 2015 

Design and Construction

The newest frontier of Tesla Coils in the modern age is the QCW DRSSTC capable of generating long, fat sparks with clever drive techniques. If you are not familiar with QCW operation, please read my QCW 1 and QCW 1.5 pages to learn about basic QCW principles otherwise the following may not make too much sense.

Overall Project Goals

In this project, I came up with a few concrete goals I could work towards. Having a clearly defined set of goals will help serve as constraints and aid in any engineering decisions and design I have to make along the way.

Project Goals

- Produce sparks >10x the secondary coil length
- Demonstrate reliable operation
- Planned running BPS of 0.5Hz to 5Hz
- Operate at ~300V ramp with 240VAC input
- Implement a new phase-shift drive technique
- Be compact enough to fit inside a medium-sized post box

Improved Drive Techniques

Phase Shift Drive Technique

Phase shift control is a method where a 50% duty cycle is maintained on each half bridge, but the relative phase of one pair is shifted relative to the other. By adjusting the phase angle, one can easily control the effective duty cycle. This eliminates the bus modulator front end.

In a bus-modulated QCW, C_BUS is replaced with a high current buck converter or similar modulated supply. This allows resonant switching operation as V_BUS increases in a ramp, which each half of the bridge is switched pi out of phase. In phase shifted drive, the idea is to eliminate the bus modulator, but doing some trick to mimic the resultant behavior. To achieve this, we allow one half of the bridge (Q1 Q2 or Q3 Q4) to switch normally in resonant switching, but shift the phase of the other bridge relative to the first half. Lets take a look more closely how this works.

Above shows the relative phase of the bridges along with the primary current. In normal switching operation, the green and red show the switching action of each half bridge (remember that the two switches in each half must be out of phase always, so we simplify it here by just showing say the top  switches). The orange and yellow traces show switching of the 2nd half of the bridge as the phase changes from 180 to 0. At 0-deg shift, there is always 0 potential difference across the primary, whereas at 180 deg, there is always the bus voltage across the primary.

By ramping the shift from a low number to 180, the result is a variable duty cycle. Because of this, note that half the bridge is always switched in resonance and half doing hard switching with the hardest switching at 90 degrees. In order to share the load of half switching, the driver goes one step further to alternate hard switching between the two halves every cycle.

[More scope shots and images to come]


In usual Tesla Coil operation, the inverter drive is shut down when the primary current peaks at some over-current threshold. This immediately stops power being pumped into the resonant and the coil stops making sparks. However, it is possible to 'pause' the inverter for a short while, allow current to flow back to the bus via the reverse diodes as the current rings down, then occasionally driving the primary again. This is known as free-wheeling and allows long pulse durations without exceeding the current limit.

[More scope shots and images to come]

Selectable Pole Operation

Any coupled system will have two resonant poles, moving apart as the coupling between the two systems increase. The driver is capable of driving the system at a desired frequency - once the coil 'locks on' that resonant, regular feedback takes over the resonant drive.

With the main plans set, it is time to work on the build.

Coil Geometry

One of the driving design constraints of this coil was to make a working QCW be as small as possible yet still practical to construct. The coil was first designed and modeled via FEMM to determine its inductance.

In this case, the coil inductance is simply the flux linkage (Wb) divided by the coil current (A). L = λ/I = 28.9mH.

To get a better and very rough understanding of how the shape affects the magnetic distribution, I did 3 simple plots in FEMM. This allowed me to determine the desired geometry of the coil from a magnetics perspective. After crunching some numbers, I had to design the primary circuit with f_pri = 273kHz.

With the primary capacitor and operating frequency decided, we can now calculate the desired primary inductance where f = 1 / 2pi(LC)^0.5, L_pri = 57.9uH. From this I designed a half doughnut primary coil. With 18 turns, FEMM gives me about 61.58uH and a coupling of k=0.365. This gives some leeway for tuning later on.

Finally I did a very rough electrostatics simulation - these plots clearly show the benefits of this geometry. Now, it's time to build the coil and see if the simulations are right! 

On to the power electronics. 


Coil Construction

Design of the coils was done in CAD and a laser cutter was used to cut the primary supports out of acrylic. The primary coil was then constructed carefully by gluing the primary coil to the supports.


Next was to put together the power electronics and to build a box to house them. This was fairly straightforward.

Once everything came together, I put the coil on the bench for tuning and adjustments.

The coil is ready to run! 


 08 Nov 2015 

I was finally able to put the coil together - the project turned out well in the end, with the coil making ~2 feet + sparks from a 2.4" secondary coil, so far running with about 340VDC on the bus. Enjoy the photos / videos below!



And below is the coil in action with about 235VAC input to the bus. with various pulse widths and ramp configurations.


The super high impedance primary circuit self-limits current to around 50A. A long high-duty-cycle run was also conducted with the coil drawing some 4.5A at 235VAC (though with crappy power factor), and thermals analyzed using a FLIR thermal camera. The warmest parts of the coil was the secondary coil getting up to over 60C, primary ~40C and the power electronics remaining pretty cold. Looks like I can push the coil a bit more :-).


Above is another video of the QCW 2 in action running similarly with a variety of pulse levels.

Finally, we answer the question - can we make a cake make sparks?


Who knew that the answer is yes! Well technically the QCW 2 is creating the spark - there is a wire which connects the metal toroid to the cake toroid. The cake toroid is a regular moist cake covered with silver fondant. The 'secondary coil' here is just a plastic tube, sitting atop another 'base-box' cake. At the top of the cake toroid, another wire was inserted in which acts as the breakout point. The two wires were not connected. This cake was made by my talented friend Wendy. The QCW 2 was then operated at regular mode. Notice how much smaller the sparks are - this is due to massive de-tuning where the resonant frequency was significantly lowered due to the extra 'toroid' and capacitance provided by the cake.

In the end, I found that the area around where the two wires were had some severe 'cooking' of the batter, but the rest of the cake turned out just fine! Perhaps the next step is to adjust the driving frequency to lower it, and this may produce better spark length!

As usual, more to come soon!

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(c) Gao Guangyan 2024
Contact: loneoceans [at] gmail [dot] com

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