PCB TeslaCoil, USB powered

A new and improved PCB spiral Teslacoil. This Teslacoil has etched windings on a print circuit board. It  has a USB interface which also powers the coil. The resonance frequency is about 4MHz. It has a turns ratio of 1:160 with 6mil tracks for the secondary. The total trace length of the secondary is 25m.

What most Tesla coils have in common is that they are quite big and dangerous. You do not just put them on your desk to play with. There is a lot of energy in these devices, so the chance of a shock or burn is present. To make a (relatively) harmless Teslacoil the most important point is: limit the energy!

The big advantage of the Spiral MicroTesla is that it is powered by USB. With 5V 1A at your disposal you can not do much damage. The sparks of this Teslacoil can be touched with bare hands. Do not aim too much at the same spot, because it does get hot. With a voltage level of approximately 30kV you would expect to get a big shock, but the current is so low that you hardly feel it. When you touch the protective earth(PE) with one hand and with the other the spark you can feel a little tingling sensation.

Usually the most dangerous part of a Teslacoil is the primary circuit. Due to the series resonance, the primary voltage also spikes. But the big difference is that this circuit has much more power available. In this case the primary operating voltage is 32V with currents over 20A.

This Teslacoil has all windings etched onto the PCB. The coupling between the primary and secondary circuits is very good, but the secondary circuit has a less steep Q factor due to the parasitic capacitance between the turns and the lack of a toroid. In electrical terms, this construction is less ideal than the classic helicoil and toroid construction. However, this does not weigh against the simple production. The induction and distances between the windings are constant and do not require any manual work. Therefore, the characteristics are always identical. The PCB uses standard manufacturing specifications. The trace is drawn as a 6 mil spiral coil and has 6 mil clearance. The total trace length is about 25 meters, divided over 160 turns. One primary winding is on the bottom layer under the outer ring of the secondary.

At the end of the secondary winding is a spring pressure pin of an IC socket. Here you can attach a pin or a needle. A pointed tip causes a much higher local field strength, which makes it easier for the spark to get started. This is also called a breakout point. This allows you to ´aim´ the direction of the sparks with a conventional Teslacoil.

Most small Tesla coils do not work well enough to break out by themselves. This is because constructing them is more accurate work, the details become more critical and the switching frequency is very high. This version can break out without holding a ground wire close to it.

The PCB is designed in the program Pads layout of Mentorgraphics. To draw the coil, I have used a tool that is normally used to draw planar PCB transformers. With a few minor adjustments, this program could also generate a single spiral (special thanks to Paul Hoogeveen for the program).

The Spiral MicroTesla is set up as a Dual Resonant Solid State TeslaCoil. Both the primary and secondary circuitry use serial resonance of an LC circuit to maximize the voltage. The primary consists of a few film capacitors and one windings on the bottom of the PCB. The secondary consists of 160 turns on the top layer and the capacity to the environment. The resonance frequencies must be exactly the same for optimal power transfer. This is about 4 MHz for the Spiral MicroTesla.

The H bridge must be driven at this frequency. But what is the best way to do this? If you use a fixed frequency (for example, function generator), it is difficult to get the frequency exactly right. Also, the tolerance in components is too large to be reliable. It is much easier to use a feedback with the current in the LC circuit.

Precisely at the resonance point, the voltage and current are in phase. When a voltage step is applied to an LC circuit, it causes an oscillation in the current at the base frequency.

We can use this signal to drive the H bridge. When the current becomes negative, we reverse the output. The voltage then becomes negative and amplifies the negative current. If the current is positive again, the bridge switches again. Thus, the circuit continues to boost this oscillation. This is called a self-resonating circuit.

This trick is applicable in both the primary and secondary circuits. You can measure the current direction in the primary circuit with a current transformer with a winding ratio of for example 1 to 100. In the secondary curcuit flows much less current. Here we can measure the voltage directly across a resistor (R28). This signal only needs to pass an inverter to make the right polarity and to clean it up to a nice square wave. To prevent the voltage on the input of the inverter to become too high, it must be clamped (D3 and D4) to VCC. A 2-stage clamp provides extra protection.

The secondary resonant frequency is not fixed. As the spark is growing (a conductive plasma channel has capacitance to the environment) and the circuit is already less ideal due to its construction, the frequency changes significantly during operation. Secondary feedback therefore produces the best output. In practice it has been found that the secondary frequency changes so drastically that it is more favorable to lower the primary resonance point significantly. It takes a few periods before it really starts resonating.

Propagation delay
For optimal performance, there must be as little delay as possible between the current measurement and the reversal of the bridge. An H-bridge at 4 MHz requires very fast components. That's why I chose the FZTX51 power transistors. An emitter follower behind a high power MOSFET driver (UCC2753X) has very little delay and can be used at very high frequencies. The maximum voltage that these drivers can handle is 35V. With a safety margin, the operating voltage can be 32V.

It is quite a challenge to make a well-functioning Teslacoil. You want to make the greatest possible arcs, but the circuit should also be able to handle the currents. In addition, the power consumption should be minimal. One of the things you can do to limit the power, is to turn it on very shortly and turn it off again (this can be done multiple times a second). The duty cycle is directly proportional to the power consumption. With 1A 5V at your disposal you get the most out of your power supply with a duty cycle of approximately 1.5%. If you turn on the Teslacoil 100% of the time, this circuit would consume over 300W. The dissipation would also be proportionally higher, which these components could never handle. The base frequency for this duty cycle can be varied to make slow heavy pulses (<10Hz) or faster small pulses, which fall into the audible range (> 20Hz). In this way it is also possible to make the Teslacoil  'sing' a melody.
Virtually all power consumed is converted into heat. Most of the losses are dissipated in the power stage. The PCB must be designed so that this heat can be drained from the components, so that they do not become too hot. This can be done by placing large copper surfaces under the power components and by adding thermal via´s.

The image below shows a thermal image of the Spiral microTesla after 2 hours of operation at 5W. 38 °C increase above ambient temperature is acceptable in power electronics. This kind of boar will probably reach the final temperature after about 30 minutes, due to the small thermal mass.

Because the Teslacoil is USB powered, there are special requirements to the circuit. For example, the maximum capacitance that is connected directly at 5V has a maximum of 10μF. There is much more capacity (~ 1800μF) required for the peak power of the coil. Therefore, this capacitance is charged with a 68Ω pre-charge resistor. This resistor is bypassed by a P-FET after 1 second.

A USB charger has an internal current limitation. If you pull more than that current, then the voltage starts dropping. Part of the circuit is a boost converter to go to a higher operating voltage of 32V. A switched power supply always tries to keep its output constant. If the source voltage drops, then it will pull more current, thus reducing the voltage even further. This is a situation that cannot restores itself. To counteract this effect, the LT3477 has been used, which has a rail to rail current measurement. This allows him to control the input current, so the USB adapter will never be overloaded.

To make it suitable for connecting directly to a PC, the input current must be further limited to 500mA. This can be done by pulling down IADJ1 from the LT3477. Doing this with a mosfet gives you an adjustable limit of 0.5A and 1A. However, USB communication is not yet supported.
A Teslacoil needs a RF-ground. You can see this as a kind of reference point to push against. Without stable reference, performance is much less. In this case, the cable shield and ground of the 5V are used as a ground source. The USB adapter itself always has some parasitic capacitance between the mains and the output. At 100pF, the resistance at 4MHz is approximately 332Ω. There is hardly any voltage across with this low current. This is also the reason why a battery / battery powered Teslacoil is not practical. You still need to connect a ground wire for the RF ground.

In order to control the signals of the pre-charge circuit, power setting and PWM, a simple PIC microcontroller (PIC18F14K50) has been used. This controller also has the ability to do more with USB communication in the future, but for now it is easy that all timers and pulses are adjustable. And because we have a microcontroller, we can also put in a few fancy leds.

The firmware is default set to 500mA (2.5W) for a PC connection. Operation by push button S1:

- Short press: Step through the different states (5,10,20Hz, Musical scale, off)
- Hold for 1 sec: jumps to first state (off)
- Hold for 3 sec: Jump to 'high power' mode (1A) (red light flashes), and back to low power mode if pressed again for 3 sec. This can be done in any state (also in 'off').
- Hold for 8 sec: turn off blue LEDs (for making photos in the dark) and on again if you press 8 sec.

Firmware (special thanks to Jorrit Zuiker)
Schematic (only non-commercial use)

The design has been published in Elektor magazine nov/dec 2017 and made available as a construction kit. Read the article at: www.elektormagazine.com/160498

Sneak preview:
The same circuit can also be used with a traditional Teslacoil structure with a vertical coil and toroid. This is the perfect high voltage gadget. This kit will soon be offered by Elektor magazine. Keep an eye on the site.

For liability it is important to state that the use of this circuit is at your own risk. Only connect the PCB to an AC adapter and not to a PC or other device. Also, do not hold any sensitive electronics against / near the discharges.

Original article in dutch, translation by me and google translate.

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