A Midsummer Night's Blog Post

WHERE DID ALL THE TIME GO!?

No matter. 

IGBTs! IGBTs! IGBTs!

LOOK AT THESE IGBTs!



Current limited by the package leads, among other things, these are the perfect candidate for some pulsed, hard-switched, power dense goodness.

And so the journey to building a QCW bus modulator begins.

In essence, the modulator portion of your QCW coil is just a buck converter capable of producing arbitrary waveforms across the bus of your DRSSTC.

The tried and true control scheme for these guys is hysteresis (bang-bang) control, which involves turning on the high side of your buck converter when the sense voltage falls below your hysteresis band and turning the high side on when your sense falls below the hysteresis band.

In lieu of a microcontroller, I opted to make my control loop out of op-amps for noise immunity and beardy-weirdy cred.

Troller Schematic. Forgive me, for I have text overlap up the wazoo. 

The first two op amps are set up as an adder and differentiator, which produce the hysteresis band given the input waveform. These signals are then passed to the latter two, which acts as a window comparator.

The outputs of the comparators are then fed to an SR latch so that the hysteresis thing happens.

Squiggles of Science

The control loop can then be tuned using the width of your hysteresis band or some fancier processing downstream.

Eagle'd:



Special notes: the input signal is actually taken from an opto, which is filtered by an LC to reproduce the desired waveform. It's like L-C filterception. Additionally, this allows me to send square wave pulses optically, instead of an analog signal through a long length of coax, which is bound to cause massive damage pick up noise.

Here it is in board form:

Note: not fully routed not the actual board

After doing some math and running the simulation for the bus at full load in spice, I concluded that I simply couldn't find bus caps large enough and in the desired package, so, the some revisions were made to the design.

The boards are now split up into three sections: a DC-chopper motherboard, controller daughterboard, and DRSSTC with filter LC.

Can't have too much bus cap (a work in progress)

By moving the filter LC to the DRSSTC bridge, I could then populate the DC-Chopper with as much bus cap as my heart desired (and keep the ripple voltage below 5%).

The addition of the driver daughterboard would allow me to send out for plenty of driver revisions without having to spend a fortune on the whole 5.5" x 5.5" board. Oh, and did I mention how small everything is? :3

Current limiting is implemented on the driver side as desaturation detection: in an over-current state, the voltage drop across the switches increases to well above the typical ~2.0V, which is detected by the drivers, which do some fancy soft-turn off to prevent voltage spikes induced by the typically high dI/dt. The gate drivers also conveniently have built-in opto-isolated inputs.

IN OTHER NEWS:

Bluescooter got an overhaul to make it more of a reliable commuter vehicle than junkyard scrapper on the verge of collapse.

After eating through a current modded controller, a few things were changed:

1) Motor upgrade!

Sweet Bajeesus

After trolling around the internet for a sufficiently large motor, I had at some point chosen this guy:

http://www.hobbyking.com/hobbyking/store/__18179__Turnigy_Aerodrive_SK3_6354_215kv_Brushless_Outrunner_Motor.html

An especially squat, low kv motor, perfect for squeezing into the tiny 4" u-channel frame. 

But everything changed when the fire nation attacked the motor went on backorder.

I was forced to look beyond Ye' Old Hobbyking, and found a cheaper, dare I say, better option:

http://www.sdshobby.com/emp-n-series-outrunner-brushless-motor-n6354200kv-p-2486.html

Even with expedited shipping, the SDSHobby motor lends itself to be a good $20 cheaper than the Hobbyking equivalent, the only downside from ordering from SDSHobby being their rather small selection of EV-sized motors.

2) The controller with the infinite heatsink

After eating through a controller, and being too lazy to replace the dead FETs, I opted to avoid yet another headache by heatsinking the controller to the chassis.




Some fresh thermal grease, three holes, and some cap screws later, a happier 'troller was born.
Even on especially hot days, I've yet to find the area around the FETs go more than 5°C above ambient. 

3) It has a new caddy. 

Structural Hot Glue

Made entirely out of mystery plastic polypropylene and a bit of polycarb from MITERS. The key switch was also moved to the outer face of the port panel to accommodate for the larger motor. 

Fin.

One Bit ADC and a (Not) Wheelbarrow Shaped Object

With the semester winding down, and final project(s) nearing completion, the coil was revamped and the project part of my final project was made.

Mmm remounted IGBTs

The goal was to have a working analog interrupter which would take an analog signal, such as the one coming out of an mp3 player, and then be able to play it through a Tesla coil.

The block diagram is the following:

Signal -> Amplification -> Triggering -> Poopy sparks

This led me to use an LM358 and 555 timer due to their availability. A 74HC14 was added to buffer the output of the 555 to preserve waveform integrity.

Both channels of the audio signal are sent through an inverting adder and then to a monostable 555. The trigger voltage is set using R5 and the gain of the opamp can be adjusted using R3. The output of the 555 is then buffered by the inverter and sent to the opto-out. The interrupter is optically coupled to the coil to avoid the potential ground loop. Unused inputs are grounded and decoupling caps are added appropriately. 

At this point, I was running out of time, so I quickly breadboarded the circuit which also made the system incredibly noise prone. Oops :P

Input and Output Waveforms Lookin' Legit.

Lulz.

It even did the interrupting thing!

Further testing involved hooking up a dinky little speaker to the output to test if audio came out intelligibly. 

The results:

It's important to note that because this is, for all intents and purposes, a one-bit ADC, the audio will be pretty terrible.

Nevertheless, it seemed to work on the revamped coil.

Modifications: new secondary, primary,heatsinking, and rectifier diodes.


One concern was that the bridge was blowing prematurely due to the rectifier diodes failing - perfectly valid given that they were rated for only 4A (MUR460). They were then replaced with complete overkill: some minibrick diodes Bayley bought a while ago.

Rate for 96A at 600V. Yummy. 

The assumption was that the diodes were failing short due to transients, sending X amps of 60Hz AC to V+ and V- of the inverter. Eww. Another useful feature of these diodes is that when your bridge blows, the diodes won't.

Power testing also revealed that the new bridge is now capable of doing up to 75% duty cycle.

Heatsinking was improved by bottom mounting the IGBTs and increasing thermal mass substantially.

A squashier primary was wound using the old chassis as a coil winding jig. A power drill was used to speed up the unbearable process of winding 1330 turns of 36 gauge wire.

Only took five tries...

The final secondary dimensions came out to 7" long, 3.5" OD compared to the old 10", 2.4"OD. The resonant frequency also sank from 300kHz to ~150kHz making it suitable for brick coil use. (Whether it'll stay a coil is another question).

Finished!

Unfortunately, the demo involved swapping out the secondary for a smaller one to reduce coupling - in this configuration, the coil had a tendency to be quite hot and burn-y.

For more details on the driver, you can view the project proposal here (Dropbox link).

Other news:

IT FINALLY HAS ANOTHER WHEEL

After relentlessly avoiding HSMXpress, I finally got around to generating the G-code to mill out the fork of my electric scooter on the MITERS CNC mill. 

And while HSMXpress seems to be incapable of informing the user why it failed to generate a toolpath, it can do this:

Totally worth it. So, sit back, relax, and listen to the elevator music courtesy of youtube's audio edit function.

And IRL:

Front assembly sans mounting screws, rear plate and brake. 

I opted to use a caliper brake as it requires a single mounting point on the fork and a shorter pull in terms of brake lever travel.

No longer a sad wheelbarrow!

Derpy photo shoot

Now that DOHLAC (Derpy/-oneTesla/Hot Long/Ass Coil) lives at home, I finally decided to take pictures.

Bad lighting is bad >__<.

Specs:

Half bridge of 60N65s kept well within spec to avoid damn-I-blew-the-bridge-again syndrome (up to 75% duty cycle at 340VDC)

Gate drive provided by a pair of UCCs and feedback courtesy of question-mark antenna.

Secondary:

2.5" x 10" PVC, 32AWG wire

fRES @ 316kHz

I can probably push the coil to give more spark, but due to my unwillingness to replace the bridge, it'll be kept at a lofty 7.5". 

Long pulse widths for fire-y streamer goodness.

Longest spark on this run from a nice straight streamer. Top-mounted breakout point seems to distribute the E-field in a nice symmetric way that promotes longer sparks. 

Special thanks to Bayley and Kramnik for helping me troubleshoot this thing at one point or another.

Project Dump

Working from nine to five doesn't seem to be helping my projects.

Part 1: The still imaginary scooter 

After learning how2mill, I set out on machining one of the few components that didn't require the motor or battery (which I have yet to order): the fork!

I should learn how2CAD at some point too.

After rough sketches and some trigy math in the comp book, I sadly discovered that If I were to set the axle in line with the handlebar shaft, I'd end up with a scooter nose-up. Not to worry though, as forward offset fork is here to the rescue! Combined with some derpy angle finagling by cutting away part of the  rubber 'shock absorber,' I'd achieve a scooter that is both aesthetically and structurally sound. Well, aesthetically sound at least. 

The resulting hunk of 1/4" thick aluminum resembled something slightly too angular for my taste, but as the first thing I've ever milled, I think it turned out ok. 

Part 2: Derpy Coil lives!

Also known as "Long Hot," Derpy coil has finally come to life after scraping the secondary base feedback scheme. 

Here's how the troubleshooting adventure went:

1. Coil is completed, run off a variac at low power. Nothing

2. Scoped across primary: looks legit. The primary waveform only goes to shit when it's running off of its own feedback. 

3. Add turns to secondary base feedback CT and hope it doesn't reach saturation. Nothing.

4. Remove high-pass filter. Poopy sparks appear. 

5. Feed the coil a 316kHz offset sine wave. Success! Kind of out of tune 2" long sparks appear.

6. Abort secondary base feedback, go to antenna: nothing.

7. Give up on this driver board and repopulate another one (without secondary base feedback junk attached).

8. ???

9. Success!

Here it is playing Solfegietto by CPE Bach. Spark performance isn't quite impressive as duty cycle was turned down to maintain note clarity. Modified oneTesla interrupter pulse widths were overlapping each other, which you'll notice when the really low notes start playing.

After Bayley reflashed the interrupter for true continuous wave output, several things happened:

Hot Long finally got hot and long.

Then the bridge blew after getting to ~140VDC. 

The number of primary turns then grew to 27.

Then the bridge died again, but at ~200VDC.

Failure is probably due to transients - further testing will resume after TVS is added and primary current is scoped.

UPDATE: 8/18/2013

Long Hot will no longer endure CW abuse and will continue to live life happy at 75% duty cycle, occasionally pushing out the odd midi file. 

-oneTesla Lives!

After weeks hours of deliberation, I sent out -oneTesla to OSH Park, hoping to receive a functioning board.

But, oh, I got so much more.

Dat gold plating.

It might have been the fact that this was my first fabhouse board, but, damn. That looks sexy. The board features silkscreen, gold through-hole plating, and purple solder mask, making it quite the specimen.

A fully populated "Derpy Coil" with secondary base feedback tentacle.

Look! The inverter is inverting...!

It even does that inverting thing.

Power testing to come soon...

Maker Faire Detroit!

My first Maker Faire, and in the Motor City, no less.



July 26th marked the first day of Maker Faire Detroit and all of its insanity: lots of fire, homemade jet engines, (one)Tesla coils, giant dragon-sculpture-things, and of course, the wonderful community of makers to name a few.

I was there on oneTesla business, showing off demo coils and whatnot, but for me at least, the excitement lie in the other exhibitions.

(side note: oneTesla won editor's choice!)

The oneTesla team.

 I also happened across Paul Kidwell from the Geekgroup: their youtube series on SGTCs is part of the reason I'm coiling today, and as you can imagine, I was screaming like a soldier at a KPOP concert.

My expression does not do justice to the fangirl trapped inside omgomgomgomgogmomg

Enough words. Photodump!

-oneTesla

Pronunciation: "ne-guh-tive wan-tess-luh"

This summer, I'm working as a slave for oneTesla, a start-up founded by three MIT students: Bayley, Heidi, and Kramnik.

The recent bout of employment happened to coincide with my recent quest to get a "singing" SSTC working, and so -oneTesla was born.

Conceived from its namesake, oneTesla, -oneTesla is the SSTC derivative of the DRSSTC. The conversion process involved, to name a few things, removing the tank cap, fiddling with the feedback, and some trace rerouting.

DISCLAIMER: I do not recommend that you undertake this path as an alternative to purchasing an actual oneTesla kit. Trust me. The amount of pure butthurt incurred in troubleshooting alone is not worth the minuscule monetary sum that you'd save.

I started out with this very humble chassis and fan assembly, orphaned from Minuet 1.x:

4020, some lasercut acrylic, and HDPE (currently smothered in electrical tape) machined on a lathe. obligatory zip-tied fan included.

The original plan involved simply etching a oneTesla. It went less than swimmingly. For the longest time, I was under the impression that the optical receiver on the board was dead. Four hours later, I tried the interrupter on a oneTesla unit and it also didn't seem to work, and it turned out that the interrupter was just running out of batteries and the LED on the output just wasn't bright enough. Oh, and this was after I accidentally shorted +12 to ground, frying a few traces.

Important lessons learned:
1. Humans are not manufacturing houses.
2. Traces are not magical.
3. Always check the battery to your opto-coupled interrupter.

Version 1.1(6/10/13)

After receiving the board file for one of the original oneTeslas (meant to be etched), some more finagling occurred, which lead to the creation of another functional board.

Results: 3" of spark run at ~8% duty cycle.

Not bad considering the low duty cycle. Too bad that the bridge fried itself a few times before I could reflash the interrupter to increase the pulse width.

To be continued...

UPDATE (7/22/13)

New and improved -oneTeslas with secondary base feedback have been sent to the fabhouse. ETA July 29th.

UPDATE (7/30/13)

ERMAHGERD ETCHED BOARDS

Minuet 1.x

Time to go solid state!

For my first solid state coil, I decided to go with something on the moderate side of complexity, basing it off of Steve Ward's mini SSTC schematic: http://www.stevehv.4hv.org/SSTC5.htm

Operating Principles

Tesla coils are resonant air-cored transformers.

A primary inductor is coupled to a secondary inductor, resulting in a voltage step-up that allows for the breakdown of air after the secondary exceeds air's dielectric voltage (~3 × 10⁶ V/m). 

By using an air cored transformer, Tesla coils avoid the pesky core saturation that would otherwise limit the magnetic field and the resulting high voltage. 

The simplest variation of a Tesla coil, a spark gap coil, consists of the following:

SGTC Schematic
Source: http://enterpriseprogrammer.files.wordpress.com/2012/07/tesla_coil_circuit_diagram.png

When the primary tank capacitor reaches the voltage neccesary to arc across the spark gap (literally two electrodes separated by air), an LC circuit is formed that exhibits a resonant frequency.

Spark gap Tesla coils (SGTCs) and Dual resonant solid state Tesla coils (DRSSTCs) use two resonant tuned circuits (The primary and secondary) to more effectively transfer energy from the primary to the secondary coils. This is achieved by the phenomenon in which at resonance, impedances and reactances effectively cancel each other out.  

Given the resistive nature of the primary and secondary, the oscillations at the resonant frequency in both sides of the transformer will undergo decay and display waveform damping. This damping effect produces distinct RF "cycles" that are offset by 90 degrees. It's during each cycle that energy is exchanged through the changing magnetic field.

Simple solid state Tesla coils only have a secondary which is coupled to a primary inductor. The primary inductor is then driven at the resonant frequency of the secondary. 

Being driven at resonance requires current to flow back and forth in the inductor at a resonance frequency (simulating AC). In a solid state coil, this is achieved through transistors assembled in either a half-bridge or a full bridge (H-bridge) with the primary inductor acting as the load. The resonant frequency of most secondaries is around a few hundred kHz and bus currents and voltages linger around in the tens of amps and 240V (bus voltages will vary from topology to topology), this puts quite a bit strain on the transistors, limiting the field of options down to two types of transistors: MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors); the transistor strain is limited by the fact that most of the oscillations are induced in pulses i.e. the coil is kept in operation in short bursts to prevent overheating. Spark production is also noticeably different when driven in an "interrupted" mode as opposed to continuous duty. 

More often, IGBTs are favored over MOSFETs because they exhibit diode-like voltage drop while MOSFETs exhibit a resistive drop that increases with current.

Dual resonant variations of solid state coils include a tank capacitor in the primary side circuit, creating, like a spark gap coil, an LC circuit while in operation. The addition of a capacitive element to the circuit also cancels out the inductive reactance of the primary coil, thus allowing for currents orders of magnitude greater than regular SSTCs to propagate within the primary.

Tuning

Spark gap coils are tuned via physical parameters: the inductance of the primary, the capacitance of the tank capacitor, etc. 

In solid state coils, the resonant frequency is maintained by some sort of feedback mechanism. SSTCs generally use antenna based feedback (literally reaching into the electric field produced by the secondary), which induces a changing voltage as the field changes, while DRSSTCs required primary feedback as the secondary is out of phase with the primary. SSTCs are unable to use primary feedback because the current isn't effectively measurable due to the lower primary currents found in SSTCs. 

Construction:

Minuet 1.x was intended to be a single-boarded audio modulated SSTC; unfortunately, it's had a hard and very confusing life.

Evolution (to be added when I bother to take some pictures)
At one point, it did produce sparks, albeit only in continuous wave operation:

6'' sparks run continuous wave

The board since exploded after the addition of a fiber-in jack to more reliably interrupt operation. Maybe duty cycle issues?

If any good came out of this project, it's the list of things you shouldn't do

The project was abandoned in lieu of -oneTesla (negative oneTesla) 5/29/13. 

Spark Gap Tesla Coil 1.0

Conceived during a summer that probably could've gone to better use (Junction, whyyyyyyy did I attend you???), SGTC 1.0 was what first got me into the whole business of shooting lightning.

Long exposure is best exposure.

Design Considerations:

Modularity

- At some point, I want this thing to shoot monster streamers, so being able to swap components would be nice.

Reliability

- Demos are far more fun when things work.

Sexiness

-Hnngggghhhh *ahem*

Construction breakdown:

Chassis

-Two sheets of HDPE held up by threaded rod columns

- Fan bolted onto a piece of plexi that was subsequently ziptied onto the threaded rod (farming hard)

Secondary

- Donated by the Daniel Kramnik

- 4" diameter, 2.5'~ tall

- 24 AWG???

- Resonant frequency ~555kHz

- Added toilet flange for increased mounting stability to chassis

Primary

- 1/4" copper refrigeration tubing wound spiral-style

- Added supports for sexiness

Topload

-  Some very sad aluminum ducting wrapped in even sadder aluminum tape. Not exactly my best handiwork.

MMC

- Scavenged Panasonic metal film caps

- Rated for ~8kV 

-Capacitance: ??? (something in the 0.005uF range)

NST

- 9kV, 30mA (problem, MMC?)

Spark Gap

- Zinc-plated hardware (a bad idea)

- Cooled via muffin fan (a yummy idea)

Here, have some pictures media!

Note: bleed resistors help
 prevent accidental electrocution

Protip: when laying the primary,
 go from the center outward 

mmmm, HDPE

Results:

14"~ streamers