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Spiral MicroTesla, USB powered [160498]

Status: Abgeschlossen
7. August 2017

MIDI Synthesizer Firmware Update

New firmware is available below that adds a MIDI interface and some synthesizer features to the MicroTesla transforming it into a musical instrument. MIDI data can be sent over USB or over a MIDI input connected to the serial port. A detailed description of the new firmware and how to connect a MIDI input can be found in the January/February 2018 issue (article ID 160592). Also see schematics below.

The video below shows what is possible with the new firmware. Please note that it is important to shield the electronics to avoid crashing the USB bus (and your computer).

MicroTesla Version 1.0 

Many have seen Tesla transformers in action, often shooting meters long electric arcs. This can be dangerous. But it doesn’t have to be big to be appealing. Why not small and something to play with? Yes, here we present a circuit where you can touch the electric arc with your bare fingers. No harm done. Maybe a little tingling sensation. That’s it. High voltage, about 30 kV here, isn’t dangerous as long as the current (energy) is kept to a minimum. Everything is on a single PCB and no coils have to be wound. The transformer is completely made by spiral tracks on the PCB. The circuit is powered by 5 V (mini-USB connector). All you need is an AC adapter that can deliver 5 V at 1.5 A preferably 2 A. In high power mode the circuit will use a little over 1 A maximum.

But don’t be surprised by the current flowing in the primary circuit. The driver of the primary coil has a 32 V power supply and peak currents of 20 A flowing. The primary is just one full winding. It uses series resonance to increase the voltage across the winding. The secondary spiral is a 6 mil track of 160 windings and has 6 mil clearance. It has a length of approximately 25 m. For the output a sharp pointed piece of wire is all you need. On the PCB a single socket from a modular IC socket strip with gold plated contacts is used to easily replace the wire. It will wear down.
The Spiral MicroTesla is configured as a Dual Resonant Solid State TeslaCoil. Both the primary and secondary circuit use series resonance to increase the voltage. The primary uses three polypropylene film capacitors in parallel and the secondary has a stray capacitance to the environment. Both resonance frequencies should be closely tuned for best power transfer. In this case the frequency is roughly 4 MHz.
A H-bridge consisting of bipolar transistors (T4…T7) is used to drive the primary circuit. At 4 MHz these are preferable to mosfets. The 32 V power supply is produced by a 3A DC/DC Converter with Dual Rail-to-Rail Current Sense, a LT3477 from Linear Technology. Current limiting is used to avoid overloading the AC adapter. Current sense levels can be set by adjusting the voltage on the Iadj pins. When T3 is activated the maximum current drawn from the 5 V power supply is limited to about 0.5 A (sense level is reduced to 50 mV). If the Iadj1 voltage is higher than 625 mV the sense level is 100 mV and the maximum current is 1 A (R24 is 0.1 Ω). The output of this 32 V power supply is heavily buffered with 8 low-esr electrolytic capacitors and additional ceramic capacitors in parallel (C4/C5,C19/C21/C23…C31) to be able to supply the high peak currents at this high frequency. R26 limits the inrush current after power-up caused by the decoupling of the 32 V boost converter (about 1800 µF). T1 connects the boost converter fully with the 5 V power supply after 1 second.
A fixed frequency would be apparent since the frequency seems to be fixed by design. But then there’s tolerance in components  and the resonance of the secondary dependents on the capacitance of the conducting plasma channel and causes the resonance frequency to change considerably. That’s why secondary feedback is used to maintain the highest output voltage. Because of the secondary resonance frequency change it proved best to set the cut-off frequency of the primary circuit much lower. It takes a few periods before the resonance sets in properly.
In resonance voltage and current are in phase. Placing a voltage step on the LC circuit causes a current oscillation at the resonance frequency. This signal is used to drive the H-bridge. When the current gets negative the drive of H-bridge is inverted to amplify this negative current and so on. This is self-resonance and causes amplification of the current and thus the voltage across the primary winding increases. But since the secondary resonance frequency isn’t constant it’s best to measure the current in the secondary winding (R28). D3 and D4 protect the input of the inverter used for this purpose (IC4). To keep the propagation delay to a minimum the H-bridge is made by using high power mosfet drivers (IC2/IC3) followed by emitter followers (T4…T7).
It’s quite a challenge to make a good functioning Tesla coil. You like the spark to be big but without stressing the circuit too much. Also power consumption should be minimal. Turning it on for a short period of time and then off again is a possible solution. The power consumption is directly proportional to the duty cycle. A power supply of 5 V/1 A will permit a maximum duty cycle of only 1.5 %. 100 % would mean more than 300 W, something this circuit isn’t designed for. The duty cycle can be varied so bigger pulses can be generated, but less frequent (<10 Hz). Smaller but more frequent pulses (>20 Hz) is another option. A melody can be produced this way.
No heatsinks are required. The small power transistors of the H-bridge are cooled by copper surfaces on the PCB. Extra vias are added where possible to also use copper planes on the bottom side to increase the cooling surface.
A tesla coil needs a RF ground. The cable from the 5 V power supply is part of the grounding. In most AC adapters there’s a capacitance from output to the mains voltage. At 4 MHz the extra voltage drop across it will be minimal due to the very low current flowing. So because a ground reference is needed battery power won’t work or very poorly. You would still need to connect an extra ground wire.
A PIC microcontroller controls the pre-charge circuit (T1), current setting, PWM and the different modes. Different modes can be set by pushing S1:
- Press S1 briefly to step through: 5 Hz (turn on), 10 Hz or 20 Hz pulses, music scale, turn off
- Press S1 1 sec. to jump to beginning
- Press S1 3 sec. to switch to high power mode (1 A), red led (LED1) blinks, press 3 sec. again for normal power (0.5 A)
- Press S1 8 sec. to switch off the blue leds (to take photographs in the dark)
4 blue leds  (LED2…LED5) are blinking to heighten the experience.
We strongly point out that the use of this circuit is at your own risk. Connect the PCB  to a descent AC adapter (there are a  lot out there that are not fully compliant with regulations). Do not connect the circuit to the USB port of a computer or any other device. Keep electronic devices at a safe distance from the discharges.
Bill of materials (PCB 160498-1 v1.1)
R1,R2,R3,R4 = 10 kΩ, 125 mW, 1 %, SMD 0805
R5 = 47 kΩ, 125 mW, 1 %, SMD 0805
R6,R7 = 2k2, 125 mW, 1 %, SMD 0805
R8 = 150 kΩ, 125 mW, 1 %, SMD 0805
R9, R11,R12,R13 = 33 kΩ, 125 mW, 1 %, SMD 0805
R10,R18,R19,R20,R21,R22 = 120 Ω, 125 mW, 1 %, SMD 0805
R14,R15,R16,R17,R23 = 330 Ω. 125 mW, 1 %, SMD 0805
R24 = 0R1, 250 mW, 1 %, SMD 1206
R25 = 1k5, 125 mW, 1 %, SMD 0805
R26 = 68 Ω, 250 mW, 5 %, SMD 1206
R27 = 10 Ω, 125 mW, 1 %, SMD 0805
R28,R29 = 100 kΩ, 125 mW, 1 %, SMD 0805
C1,C2,C3 = 47 nF, 400 VDC, 5 %,  PP, lead spacing 15 mm, depth 5 mm, R76MI24704030J Kemet
C4,C5,C19,C21,C23 = 10 µF, 35 V, 10 %, SMD 1210, X7R
C6,C7,C8,C9,C10,C11,C12,C32 = 220 nF, 50 V, 20 %, SMD 0805, Y5V
C13,C14,C15,C16,C17,C18 = 10 nF, 50 V,10 %, SMD 0805, X7R
C20 = 2µ2, 50 V, 10 %, SMD 0805, X5R
C22 = 220 pF, 50V, 5 %, SMD 0805, C0G/NP0
C24,C25,C26,C27,C28,C29,C30,C31 = 220 µF, 35 V, 20 %, diam. 8 mm, lead spacing 3.5 mm
EEUFC1V221L Panasonic
C33,C34,C35 = 4µ7, 16 V, 10 %, SMD 0805, X7R
L1 = 6.8 µH, 3.04 A, 0.0498 Ω, SMD, MCSDRH73B-6R8MHF Multicomp
D1 = SP0504SHTG, SMD SOT-23-6
D2 = VSSA310S-M3/61T, 100V/3A, SMD SMA
D3 = BAT54S, SMD SOT-23
D4 = BAV99, SMD SOT-23
LED1 = Led, red, low power, SMD, KPTL-3216EC (Kingbright)
LED2,LED3,LED4,LED5 = Led, blue, low power, SMD, KPTL-3216QBC-D (Kingbright)
T1 = DMP1045U, SMD SOT-23
T4,T6 = FZT851, SMD SOT-223
T5,T7 = FZT951, SMD SOT223
IC2 = UCC27537DBVT, SMD SOT-23-5
IC3 = UCC27536DBVT, SMD SOT-23-5
IC5 = PIC18F14K50-I/SS, SMD SSOP-20
K1 = USB-B mini PCB horizontal, SMD, 65100516121 Würth Elektronik
K2 = single pin socket (cut from IC socket)
K3 = leave open
K4 = Pin header, 1x6, vertical, pitch 2.54 mm
S1 = Tactile switch, horizontal, SKHHLQA010 Alps
X1 = Resonator 12.0MHz, 0.5 %, 5pF w/built in Cap, AWSCR-12.00CV-T Abracon
PCB 160498-1 v1.1
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Top view of Spiral MicroTesla (PCB 160498-1 v1.1)
Side view 1 (PCB 160498-1 v1.1)
Side view 2 (PCB 160498-1 v1.1)
Side view 3 (PCB 160498-1 v1.1)
High voltage arcs (exposure time 6 seconds)

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