QCW-like sword sparks from a Ramped Solid State Tesla
Coil
Introduction
After seeing how successful my weekend
SSTC 2 project went, I though I'd give another go
at making my third solid state tesla coil (hence SSTC 3). You
might have noticed that I have called it a RSSTC, or Ramped SSTC,
because of the straight sword-like sparks I wanted to create with his
coil, to mimic VTTC and QCW DRSSTC sparks. The 'ramped' part describes
the ramped voltage input into the inverter.
In addition, I wanted to use this opportunity to
develop a simple, easy and compact General Purpose SSTC platform
where I could use as a base to build various sorts of SSTCs and DRSSTCs
very quickly! My previous
SSTC 1, SSTC 2 as well as
other DRSSTC project pages provide a more detailed description of the
workings of an SSTC, so be sure to check them out if you're not familiar
with general SSTC operation.
RSSTC Project Status: Tested at 120VAC, 208VAC! (June 2014)..
and 240VAC! (July 2014)
*Boards for Sale*: I had a bunch of leftover boards
left from the development of this project - While stocks last! Check it
out here.
Page Contents:
1. Project Outline
2. Design & Construction with Schematics
3. Results and Media
4. Links and References
R-SSTC 3 in action at 120VAC input. Two control knobs control spark
repetition frequency and pulse width respectively.
RSSTC 3 Specifications (July 2014):
- 240VAC input
- Half bridge of FGH40N60SMD TO-247 IGBTs
- 450 - 420kHz Secondary Frequency (with and without loading)
- 5" x 2.875" (2.5" clear PVC Pipe) Secondary
- ~680 turns of AWG34 magnet wire
- 3.51" (3.5" PVC pipe) Primary AWG 18
- Two 1.5" x 6.0" Spun Aluminium Toroids (to be replaced with
one 1.75 x 7" toroid)
- Staccato Interrupter 0 to ~5ms (just a bit more than 1/4 mains period)
- Secondary current feedback
- Performance: 12" at 208V input (4 turn primary), ~15" at
240V input (4 turn primary, 2cm total length)
06 May 2014
Project Outline
Background
Before I begin, you should have a general idea of how an
SSTC works. I hope that my SSTC 2 page serves as
a reasonably comprehensive introduction. Check it out if you are not
familiar with SSTCs.
I haven't heard of the term RSSTC used before, but I
wanted to distinguish this coil from a regular SSTC due to the
specificity of how this SSTC is run. Just as how a continually running SSTC is
often referred to as a CWSSTC and the similar variant in the DRSSTC is
known as the QCW-DRSSTC, I came up with RSSTC to describe its main
characteristic features:
1. Instead of a rectified DC (smoothed or otherwise)
to the inverter, the RSSTC takes in a ramped DC input
2. Resonant frequency higher than 300+kHz
3. The coil runs in sync with the ramped DC input
So what's so different about an RSSTC compared to a
conventional SSTC? - It is designed to produce straight, sword-like
sparks, versus the characteristically bushy and branched sparks of
conventional coils. In a way, it mimics closely the output behaviour of
many Vacuum Tube Tesla Coils (VTTCs).
Till date, there has been very little academic research
on the physics of spark formation, especially for those produced in
Tesla Coils. Tesla Coils generally produce very branched 'tree-like'
sparks, often resembling real lighting, taking the shape of plant roots.
As amateurs began building more and more coils, they started to modify
various coil running parameters to improve performance.
Creating sword-like sparks can be traced back to the
early days of VTTCs, and was born out of result of trying to reduce the
input power of a VTTC while maintaining a spark output length. Back in
1993,
John Freau came up with the idea of achieving this in VTTCs by what
he called a staccato operation - in his own words, "(running) the VTTC
for a full AC half cycle, then (disabling) the VTTC for a selectable
number of AC half cycles." This has since become a staple design in
VTTCs and helps significantly in preventing tube overheating. Today, the
design is easily implemented as a simple zero voltage detector,
triggering two 555 timers, one dictating the pulse duration and the
other, the repetition frequency, among other designs.
When people started experimenting with this, they
realized that the VTTC not only made characteristically straight,
sword-like sparks, but it also seemed to do this because of the shape
of the input voltage. One more thing - coilers also noticed that as the
frequency went up, the straighter the sparks got in their VTTCs. A
conventional SSTC has an inverter which runs off a (usually) filtered DC
power supply, so the voltage output is essentially constant. This
produces the characteristic bushy spark appearance, but hardly
ever straight sparks of a VTTC.
Around 2004, Steve Ward wanted to re-create the straight
sparks produced by VTTCs in his solid state coils. Comparing with VTTCs,
it was clear that one main difference was the input voltage. While SSTCs
conventionally run from a filtered DC source, VTTCs usually run from an
unfiltered source. The goal was then to mimic this rising action seen by
VTTCs in staccato operation. This has led to newer designs such as the
QCW DRSSTC, or the SSVC (Solid State Valve Coil), created by Philip
Slawinski. Both of these coils are conventional DRSSTCs and VTTCs but
running with a ramped voltage input, usually from a buck converter.
Newer designs such as the phase shifted QCW also mimic this effect
across the bridge.
After many experiments by the Tesla coil hobbyist
community, it seems that higher frequency leads to straighter sparks,
but at the expense of silicon switching capability. It seems that the
sweet spot of compromise is around 380-420kHz, with a lower bound of
about 320 to 350kHz. Any lower and the sparks begin to take on the usual
branched / fractal form.
Project Goals
Back to RSSTC 3 - I wanted a simple platform to study
these effects, but without the complexity of a buck converter to create
the voltage ramp. So my plan is very straightforward: to run a
conventional SSTC from a non-filtered main source using staccato
operation to turn on the coil only during the rising voltage wave (so
the first 1/4 wave) of the AC cycle - this makes for a easy
ramped input.
The second goal for this project was to work on a
prototype for a single-pcb generic PCB design which I could mass produce
in larger quantities, which I may or may not offer for sale in the
future. This will
allow me to realize the 'Tesla Coil on a Board', so I can basically wake
up in the morning, decide I'll like a Tesla Coil, a get one built before
dinner! I also wanted to design the board to be general purpose, so I
could use it at not only different input voltages (120 / 240) but also
allow for DRSSTC operation and quick swappable controllers /
interrupters.
I also just shifted to a new apartment, and therefore lack a lot
of the tools in the workshop I used to build my things at. So
another goal would be to see if I can streamline my process and make the
whole coil as simple and elegant as possible with my limited tools and
materials.
One of the characteristics of such a RSSTC design is to
create long - straight sparks, but without the complexity of QCW DRSSTC
work. So a performance goal would be to produce impressive looking
sparks for a given coil size. I also happen to have a 1.5 x 6" toroid
on hand which I bought a while ago on ebay. Therefore, I plan for the coil to be my most compact SSTC to
date, being easily transportable in a small bag, robust enough to be
thrown around in luggage, and produce sparks at least 2x the secondary
length 5"), with a 2.5x stretch goal (12.5" or ~32cm sparks from a <13cm
secondary). [Update - this goal was easily reached with final 240VAC
operation and two stacked toroids - might replace with one bigger one].
The next section will discuss each component in detail
as I document the design process and construction.
Design and Construction
Circuit Design and Schematics
The first thing I had to do was to design a general purpose
circuit which I could then design and route into a PCB. After a while of
tinkering, I came up with the following design, which builds upon
on what I've learned from my previous SSTC and DRSSTC projects.
Some particular features / design decisions are:
- Integrated 12VAC transformer on board so I wouldn't need an
external power supply for the logic
- Full bridge rectifier with no doubler so I can use it with a staccato controller
- Dual UCC gate driver instead of the smaller single UCC I used
in my SSTC 2, for more power
- Eight-pin breakout jack for swappable interrupters just like a
graphics card on a motherboard!
- General purpose feedback input (works with antennas, primary
or secondary CTs)
- Switch for interrupter signal
- A flip-flop so it can be used as a DRSSTC with
addition of an external resonant primary capacitor
- Fuse and Undervoltage lockout circuitry, and bleeder resistors
for safety
- Optional 680uF 350V bus capacitor
- Finally I tried to use as few components as possible
This is the result - note that I decided to use a transformer for which allows
both 120V or 240V operation :-)
Here's a quick description of the circuit. The top right
describes a general low voltage power supply providing 12V for
the gate drivers and a fan, and 5V for everything else including
a power indicator LED. Power to the bridge is simply full wave
rectified. The bridge accepts any sort of TO-247 IGBTs and is
wired up in a conventional half bridge configuration with 680nF
DC blocking capacitors. The bridge was also designed to be
laminated and of very low inductance to reduce switching spikes.
The gates of the IGBTs are turned on via 5.6R resistors and
shorted by a diode to allow fast turn-off - this creates a
little bit of dead-time to prevent any shoot-through.
When a 5V 'on' signal is supplied by the interrupter (pin 2 of
the breakout), it sends a signal to the D-flip flop. The flip
flop ensures that the Enable signal to the gate driver chips
terminates at a rising zero crossing. This is especially important for
DRSSTC use. At startup, a short pulse is sent through the two
UCCs, through the GDT, and one IGBT is turned on. This sets up
oscillations in the secondary coil at its resonant frequency.
This signal is picked up either by a secondary feedback coil
(which I'm using - read below for more detail), or just simply
an antenna. A primary CT can be used instead if I decide to run
this as a DRSSTC. This signal is clamped by the two diodes from
the feedback input and cleaned up by the 7414 inverters, which
sends a clean square switching signal to the gate drivers. In
all, quite straightforward and no different from the standard
SSTC design used by hobbyists around the world.
For more explanation how the D-flip flop works, notice Pre' and
D are pulled high always. When the interrupter turns on (H), it
is inverted and fed into CLR' (L). Output Q' is H and drives the
enable turning the UCCs on. When my interrupter pulse ends (L),
H is fed into CLR'. However nothing happens, only until the next
rising clock edge from the feedback (and with D at H), Q'(n+1)
then transitions to L turning off the UCCs, so this way the flip
flop terminates the pulse only at rising crossings. By doing so,
it prevents the bridge from turning off half-way through a
cycle, which is hard on IGBTs in DRSSTC operation where currents
can be very high. This is not so crucial for normal SSTC
operation.
More design explanations are outlined in the following sections
below.
With the schematic done, it was time to lay out the board! I
grabbed a cold drink from the fridge and routed the traces.
The board layout builds upon my SSTC 2 design, but this was
designed specifically to fit nicely into an ATX power supply
box!
While almost all ATX boxes are the same size, it turns out that the
PCB
dimensions inside differ slightly, as do their mounting points. I managed
to find a bunch of different ATX power supplies and disassembled
them. I found that they are (the PCB inside) generally 145mm long
with a 134mm hole spacing. The widths differ depending on the
design, but 110mm seems to be as wide as it can go since many
cases include an 80mm fan. Hole spacing on the short side ranges from 95mm, to
99mm and 103mm.
Based on these measurements, I designed the board to fit the
most common configurations and added some clearance around the
holes to allowing making slots if required. The final v1.0 result is
shown above! Note the
ground/power planes on the logic side, along with the laminated
bus on the power-electronics half. The laminated design is
crucial for clean switching :-).
As for height, there
are ATX PSUs with a 120mm fan at the top of the case (25mm
thick), or those with
none at the top but with an 80mm fan at the side. To accommodate
all cases with any sort of fans, the height limit for components on the board
was designed to be 70mm - 25mm = 45mm. This formed the size
limit for my heatsinks, bus capacitor and low voltage
transformer. I carefully chose my components to fit.
Due to differing voltages across the world, I decided to design
the board for 240VAC (hence no
voltage doubler on the power bus). If desired, a voltage doubler
can also be added if required, separately out of the board. The
logic power supply will use a dual voltage input 12VAC signal
transformer for use on both 120 and 240V lines, providing power for the logic, drive, and fan.
For safety, an under-voltage lockout was also added.
I decided to bite the bullet and send the boards for fabrication
instead of etching one myself. I used OSHpark which was somewhat
pricey, but I thought it was a good choice for a low-volume test
run. Less than two weeks later, I got the boards. Look how
beautiful it turned out in purple solder mask and gold plated
surfaces! Note that I left some of the power traces on the
bottom unmasked so I could thicken them with solder for greater
current carrying capacity / thermal performance.
Staccato Interrupter Design
Next I designed a few swappable interrupters for the board. The
most important was of course the staccato interrupter to make my
sword-like sparks.
The design is simply made from two 555 timers and a zero
crossing detector. Pins 3 and 4 of
my breakout connect to 12VAC from the transformer. My schematic
is shown above. Note the
jumpers to allow selection of the correct direction of the AC
signal. The pulse-width and pulses per second are controlled by
a 20k variable resistor (and the 200nF C4 capacitor) as well as
a 100k variable resistor (together with a 10uF capacitor). I did
a simulation in LTspice to make sure I had set up the circuit
right.
The result is a frequency range from 60Hz to 0.9Hz with a
pulse width ranging from 900us to 4.4ms for a maximum
duty cycle of about 27%.
With that done, I proceeded to route the board to be as small as
I could make it while still using parts I already had. The
result is a small cute board measuring less than 1 by 2 inches!
While at it, I also designed another small board which uses an
ATtiny interrupter I designed for SSTC 2, as well as a fiber
optic input for use with my musical DRSSTC controllers!
I was glad how beautiful they both came out. The 8 pin socket
for the interrupter boards also worked flawlessly. These 8 pins
provide 12VDC, 5VDC, 12VAC, interrupter signal and 4 grounds.
Note that I made sure to orientate and route the 3-pin
potentiometers correctly for both interrupters so I can simply
unplug and plug them in easily when swapping interrupters cards.
Housing
I designed the Driver board to be exactly the same
dimensions as the PCB board found in most standard ATX power supplies.
This allows me to mass produce these boards in the future, and it'll fit
perfectly into the screw-mount holes of any almost all standard ATX power supply
cases!
For this project, I used a nice used black ATX power
supply box just like my SSTC 2. It contains a switch which I will wire
to the bus (so the logic is on the moment the coil is plugged in). Unfortunately the power supply came with a jammed
fan, so I replaced it with a thin-profile 12mm thick 120mm fan. Other
connections / attachments include two holes drilled into the case to
accept two potentiometers. Extra holes were also drilled for a signal
indicator LED and a signal switch which enables the interrupter signal
to the driver.
But will the board I designed fit?
It does!
The beauty of using an ATX power supply means that not only will
a standard IEC jack be provided in box, but also a switch, EMI
filter, as well as a fan, grounding cable, and even a cut-out
slot for the primary cables.
Coil Geometry
The coil's geometry was mostly determined by an existing
toroid I had on hand - a very nice 1.5 x 6.0" spun aluminum
toroid I got on eBay for $20. Therefore I used this as a basis for designing the coil.
I wanted the coil to be as compact as possible, whilst still
looking reasonable. To allow using as thick a secondary wire as
possible, I had to choose the diameter of the secondary coil to
be as large as possible - 3". However, nobody makes a pipe
this exact outer diameter size, so I'm decided to use a 2.5"
Sch40 PVC pipe (OD of 2.875). Using just about 5" winding length
gives me just about 400kHz with 34AWG.
Using a 3" PVC pipe as a core for the primary (3.5" OD), and
a winding length of 0.6" with 5 turns
(arbitrary number) of AWG 14, we arrive at a secondary Fres of about 410kHz,
a primary inductance of 3.7uH and a
coupling of 0.345k. Here is a more or less to-scale mock up of
the coil on the right using JavaTC. I've also added other
components. Note that it does look similar to my SSTC 2, but is
smaller and more compact.
[Update - it turns out that I once again forgot to take into
account the thickness of the insulation. I found that for most
34 or 36 AWG wires, the enamel thickness is about 0.4mil on each
side. This means that I will have to multiply the number of
windings by 0.887. This gives me closer to 680
turns, and a resulting new resonant frequency closer to 490kHz.
I later added a second toroid to bring the new resonant
frequency down to 452kHz. The calculations below are updated.
With a primary spaced 0.8" long, 4 turns in total, and a new
452kHz frequency, the reactance of the primary (2.12uH) can be
calculated to be X_L = 2pi * f * L = 6.02 ohms. Since the DC
blocking cap presents a low impedance (we can usually ignore it,
but since I'm using a small 340nF cap equivalent, it has an impedance of X_c
= 1 / 2pifC = 1.035 ohms or 7.06 ohms total). Assuming no
primary resistance and a 208*Sqrt2 peak to peak square wave across the primary at the end
of the ramp (RMS of 147V across the bridge at 208VAC input in
the full wave rectifier), we should see a peak
current of about 20.8 Amps. The current can be increased by
reducing the primary impedance - i.e. using fewer number of
turns, or spacing the turns further apart.
Note that the addition of a primary series capacitor to make the total tank
capacitance to 58nF will bring the coil into resonance (reactance
cancels) and thus become a DRSSTC! This is easily done by adding
a 68nF capacitor in series with the primary for this geometry.
Note that I used 208VAC input above in my calculation for the
following reasons:
- The coil is designed to be run off 240VAC eventually, but I
currently only have access to a 120V and 208VAC line.
- Due to the nature of the ramped-voltage input, I can only use
full wave rectification with no smoothing for the bus input.
Running the coil on 240VAC instead of 208VAC will also increase
primary current. The goal is to try to run my transistors more
or less within specifications for reliable operation.
Coil Construction
I managed to find a nice source for the hard-to-find 2.5" PVC
pipe. There were clear PVC pipes available so I bought a small
length of 2.5" Sch 40 PVC (2.875" OD) and a thin-wall 3" PVC
(3.5" OD) pipe and cut them to length. I also made a simple
wooden jig with a 3/16" steel rod as my 'winding platform'. To
support the secondary, I cut out two acrylic discs and threaded
them through the thickness to accept three small screws each to
secure them to the pipe as end-caps.
Next up was winding the 34AWG enameled copper wire. I usually
wind the coils by hand and every winding was a long, slow and
tedious process. This time, I found that there was sufficient
friction between the end-caps and the metal rod for me to attach
a hand-drill to the rod and use the drill to spin the secondary
pipe! This allowed me to wind the wire on very very quickly, and
much more evenly than before. I found that there was an easy way
to make sure the wire tracked in place perfectly - this was done
by using my finger to 'guide' the wire into position, while the
wire came in at a slight angle to the perpendicular of the
secondary coil form. After some practice, I got the hang of it
and wound the entire coil (700 turns or so) in less than
ten minutes.
Once the coil was wound, I secured the ends with vinyl tape, and
then gave the coil five thin coats of oil-based clear gloss
finish Minwax polyurethane varnish. This gave it a beautiful
finish, like glass! I again use the drill to spin the coil while
applying the varnish, allowing for a smooth finish with no
drips.
Finally, I threaded two 1/4-20 threads at the end-caps to accept
nylon bolts. The magnet wire was terminated to thin copper tape
at each end of the coil and soldered in place.
For the primary, I used the 3" pipe and glued it to another
acrylic base. The base also has a copper strip with a wire lug
termination for connecting to the ground-end of the secondary
coil. The entire assembly is then easily screwed together with
nylon screws, though this is not strictly necessary (I've used
metal bolts before).
I also wanted to make the coil as easy to disassemble as
possible, so I simply glued four magnets to the base of the
acrylic coil-stand. This allows the coilform to stick easily
onto the metal box of the coil. The only thing remains is to
connect the two primary wires and the secondary ground!
Inverter
For simplicity and lower part count/cost, I decided to use a
conventional half-bridge of FGH40N60SMD TO-247 IGBTs. These are
rated 600V 80A, and I chose them for their low price (~$3 a
piece) and surprisingly fast switching speeds. The data-sheet also lists their peak current at
120A at 25C. But how fast can we actually run them?
Summing their total switching delays, we get 137ns typical
(compared with the 219ns from the 60n65 IGBTs I used in some
previous projects). The general rule of thumb is to keep the
switching time no more than 10% of each cycle. Since the
transistors need to switch twice a cycle, we end up with a
maximum frequency of about 365kHz.
This is great because we want to keep the switching frequency as
high as possible to produce straight sparks. Hopefully, the
IGBTs will stand up to being switched at 400kHz at <40A or so,
which is the maximum current I plan to run the primary at.
In fact, comparing specifications to a well known high-voltage
20A 500V MOSFET (the IRFP460), you'll notice that not only does
the the IGBT have faster switching characteristics, but is also
comparable to drive. I have seen the IRFP460 running at about
30-35A at around 500-600kHz, so we'll see if these 40n60smd IGBTs
hold up!
For gate driver, I used a simple gate-drive-transformer with a
10:13 primary : secondary step up. with +-12V input in the
primary, I get a +-15.6V gate drive signal to the IGBTs. For
more information, see my SSTC 2 write-up on GDTs. The only thing
to take note of is the phasing of the gate drive (so the IGBTs
are in opposition) and the use of a suitable ferrite toroid.
Power Source
Long story short - I designed the coil for 240VAC operation but
was limited to 120VAC in my apartment. For more power and bigger
sparks, I used 2
phases off 3 phase for a 208VAC. This section might be helpful
for those wanting more voltage than 120VAC.
In order to create as big sparks as possible, we want to
maximize the primary current as well as the primary voltage, but
not so much that we destroy our transistors. We also want our
input voltage ramp to be as large as possible. However, being in the US at
the moment, I am limited to a 120VAC source, which gives me at
best a 0 to 170V ramp along the first quarter of the sine wave
in a period.
Fortunately, I discovered that my apartment had a NEMA 14-50R
receptacle used for my oven/stove. I was initially excited
because this is the same socket use commonly in Recreational
Vehicles (RV), which is wired up as described
here.
RVs in the US have a standardized 4-pin 14-50R receptacle, with
G (green, ground), W (white, neutral), and a X and Y, usually
black and red, and rated 50A. The configuration allows for three
circuits. W to X or Y gives two 125VAC services. Ideally they
should be balanced. Any imbalanced current flows through the
neutral wire, or none if both loads are balanced. X and Y
provide a 250VAC service for larger appliances like a stove top.
Therefore, I thought that my had what is known as a
High-leg
delta configuration, which gives 240/250V between two phases
and one 208VAC high leg. You can also read about
Split-phase electric power, which also gives 240/120VAC,
common in USA residential homes. However, this was not the case!
As per the above 14-50R labeling, I measured 120V between X or Y
and W, and 208V between X and Y. This suggests that my apartment
uses a standard 3-phase Y-configuration 120V which each phase
is 120 degrees apart, give 208V between two phases. This makes
sense for apartments since lift motors and large motors for HVAC
run well on these conventional 120/208 3 phase power.
As the above image from
here shows, my apartment is wired with the left
configuration (with 208V instead of 400V and 120 instead of
230). Regardless, I'm still glad to have a 208V 50A power source
for this and future projects, which gives me an ample 10.4kW of
power at my disposal!
Specifically for this project, this means that I will be able to
achieve a ramp from 0 to 294V, a big jump over 170V.
Additionally, I plan to make this coil suitable for running on a
240VAC line. My driver electronics running from a 12VAC
transformer should still perform well with 208V input, since
this will still give me 14.7VDC after rectification. For a quick
and easy connection, I bought a NEMA 14-50P plug from ebay and
spliced a standard IEC cable to it for easy connectivity. Note
that this coil will not use anywhere near 50A RMS, which justifies
my use in the low-current-carrying IEC cable.
Driver
Gate Driver
One consideration I had in mind was whether to use a 4A single
chip driver for my IGBT gates or a double chip 9A driver. To
find out, lets look at the datasheet. The power of the drive
circuit can be calculated as a function of the switching
frequency and the energy required to charge/discharge the gates.
That is, Power = Freq x Energy. We can easily find that Energy =
Q x (Vgate_on - Vgate_off), and thus arrive at:
Drive Power = Q_gate x f_switching x ( Vgate_on - Vgate_off )
From the FGA60N65SMD IGBT (many others of the same class have
similar characteristics; I used this instead of the 40N60 I'm
actually using since it's slightly more difficult to drive), we find Q_g = 189nC with 284nC max.
With f = 400kHz, and assuming we use a +-18V gate drive with the
same configuration as my SSTC 2 for a half bridge, the total
drive power will be 2.72W per IGBT, 5.44W total. Given that the
UCC27425 handles an average current of 0.2A while the
UCC27322/21 pair handles 0.6A (7.2W at 12V), we can see that
using the higher power capability of the 27322/1 pair will help
out, especially since we'll be driving this coil at a high
frequency. Using the pair leads to higher cost but I believe
it'll be worth the extra cost for increased robustness.
After I assembled the driver, I tested the basic functionality
with an oscilloscope.
Feedback
One thing I had to think about when making this design was
whether to use primary current or secondary current feedback. In
a DRSSTC, primary current makes more sense because we want to
get the IGBTs to switch as nicely as possible - ZVS (resonant
switching), and primary feedback allows this. However, this is
not possible in a SSTC because conventional SSTCs do not have
tuned primaries.
Fortunately, either variation can be done with
the same feedback input, so I have decided to go with the easier
secondary feedback for the driver. This not only allows a simple
antenna to be attached to the driver, but also an off-board CT for secondary
or primary feedback. I made a secondary feedback transformer
using a ferrite core with 50 turns of wire.
Interrupter (Staccato / ATtiny / Optical)
As part of my general purpose design philosophy for this
project, I made the main driver and inverter board have a
general purpose 8-pin jack for plugging in any sort of
interrupter. The 8 pins provision for +12V, +5V, 12VAC, Signal
In, and 4 grounds. The core of this project was to produce
sword-like sparks, which is achieved via a standard staccato
interrupter, as discussed previously in the background section.
The controller makes use of the 12VAC input from the transformer
(which is more or less in phase with the 120/240V primary) and
creates the appropriate pulses triggered at the zero voltage
crossing of the 12VAC wave.
Putting it all together
With all the components soldered up and the coil wound, it was
time to put the Tesla Coil together.
Everything came together perfectly and I must say the clear
primary really completes the clean look of the coil. Note the
copper tape which allows easy connection to the base of the
secondary coil. I glued four magnets to the base of the acrylic
support so it can easily attach to its power supply.
The photos above show how the coil goes together. It fits
perfectly and all that needed to be done were to solder the
power cables to the switch and plug the potentiometers /
switches to the board. Only three wire come out - the two
primary cables and the secondary ground. Note the installed GDT
and the current transformer.
And here's how it compares in
size with my previous SSTC 2!
The coil is now ready for
testing!
Results & Media
First light at 120VAC
11 June 2014
After checking that the driver was working at 400kHz (I tested
the driver by sending a signal from a function generator into
the feedback and scoping the GDT outputs, and making sure it was
responding to the interrupter), it was time to run the coil!
First light was achieved at 120V input and everything worked
perfectly on the first try!
I used a 4-turn primary coil made of thin wire. With the primary coil
bunched together, it easily produced about 5 to 6 inches of
sword spark. This increased to something like 8 to 9" after
widening the primary coil to about 2+cm wide. This is still sub 15A peak current.
With testing having gone well, I went ahead to get some photos
of it in action!
The coil performed exactly as expected, making curiously long,
straight sparks with no branching. In addition, the smooth spark
growth leads to a nice side-effect where the spark is surprisingly
quiet compared to my other coils. It's beautiful and quite
mesmerizing to watch. Running the coil out-of-sync with the
rising ramp leads to the usual branched / forked sparks, so it
is indeed the case that a nice ramped voltage input results in
straight spark growth and quiet operation.
And finally here's a comparison of my SSTC 2 and 3 running
together. Note the big differences in spark characteristics!
It's nice to see how the smaller coil is making sparks about the
same length as the larger one despite running at half the bus
voltage (SSTC 2 has a voltage doubler on the bus). Now the next
thing to do is to prep RSSTC 3 for a 208V run, and see if I can
reach the goal of 30cm of sword spark from a small coil!
208V Run
17 June 2014
The coil works great at 120V, but with a small
input ramp (only +- 85V at the end of the ramp), the coil is
really running at a much lower voltage than say SSTC 2 (which
does +- 170V), it was time to test it at a higher voltage. As
described above, I managed to get a cable to get 208VAC from my
stove socket. I also had to make sure that this was sufficient
for the logic transformer. It outputs 12VAC when wired for
230VAC in, this means I get 10.85V out, FWRed to get 15.3V. That's
sufficient for my 12V regulator, and everything checks out well.
I made sure the phasing for the staccato controller was
correct, and plugged the coil right in. The pulse width control makes it
easy to adjust the ramp magnitude. Then I plugged it in!
It works great, almost like a small QCW coil! The longest
spark I got was just about 12" from the photographs, which I think is
brilliant for a 5" secondary with a normal SSTC with no doubler and a half
bridge! It's doing really well with just 208V to the primary and I'm
confident it'll hold up at 240V making even bigger sparks if I want to
push it (and the extra ramp time I get with 50Hz instead of 60Hz in the
US).
So how does the primary current look like in practice?
Well it looks exactly as we'd expect it to be! I measured it on my scope
via a home-made current probe.
Other than the spike at the start, we can see how the
primary current grows from the start of the pulse till 3.75ms. The
current slowly ramps up, ending at a bit less than 1.27V? or so,
corresponding to 31.75A on my 0.04V/A current probe. A FFT on the
primary current also shows what looks like the FFT of the driving square
wave fed into the resonant secondary.
Links
Here are some useful links on SSTCs and related
information!
- To come soon! -
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(c) Gao Guangyan 2024
Contact: loneoceans [at] gmail [dot] com
Loneoceans Laboratories. Copyright (c) 2003 - 2024 Gao Guangyan, All
Rights Reserved. Design 3.
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Disclaimer: Projects and experiments listed here are dangerous and should
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Before I begin, you should have a general idea of how an
SSTC works. I hope that my 
Coil Geometry
