Summary: This
review has important information for anyone who is thinking about building the
oneTeslaTS Tesla coil kit. It can
be done, but it is a very challenging project. Helpful work-arounds for difficult steps in the project
assembly are described, and omissions or confusing parts of the manual are
explained.
When a company markets a product kit for the
"experienced" do-it-yourselfer, you can expect a few bumps along the
road. After all, meeting
challenges and overcoming them is part of what doing it yourself is all
about. The oneTeslaTS (the big
kit—not the tinyTesla, which is a separate product) promises to make sparks up
to 20 inches (50 cm) long. At a
cost of $400, this is a serious commitment, but it is much less expensive than
other commercially built Tesla coils, which cost in the multi-kilobuck
range. Partly for research
purposes, and partly out of curiosity to see how the oneTesla folks had pulled
this off, this reviewer ordered the kit (plus a set of spare parts), and it
arrived a few days after Christmas.
First, a brief summary of my qualifications to critique a
design like this. I teach
electrical engineering at Texas State University, San Marcos, Texas. I have a BS from CalTech, an M. Eng.
from Cornell, and a Ph. D. from the University of Texas. I built my first Tesla coil when I was
about 12 (with a college-age neighbor) back in the 1960s. About a decade ago, a colleague of mine teamed with me on building
a second Tesla coil from scratch, a unit I still use in
research. Last year I published a
textbook, Analog and Mixed-Signal
Electronics (Wiley, 2015). So
I think I am qualified to render an opinion on the quality of the circuit
design, the manual, and the experience of building the kit.
To cut to the chase:
My oneTeslaTS works, but I ended up having to modify certain steps in
the assembly and do some rewiring to get it to work at all. And my particular unit had a recurrent
problem that was remedied only by replacing the main board. First, a word about the good features
of the design.
Good News: Small and Straightforward
The oneTeslaTS is specifically a double-resonant solid-state
Tesla coil (DRSSTC). Solid-state
simply means that the high-frequency AC power that is transformed up by the
transformer at the heart of the unit, is produced by transistors instead of by
old-fashioned (noisy, inefficient) spark gaps, which is what my first 1960s
Tesla coil used, and is what some of the true monsters of the Tesla-coil world
still use. Double-resonant means
that there are two tuned circuits, one involving the primary coil and one
involving the secondary coil, and the thing works by putting energy into the
primary and then letting the mutual coupling between the coils transfer the
energy to the secondary at a much higher voltage. For further explanations of any technical stuff here, see
the Legacy Gen1 manual on the oneTesla website at http://onetesla.com/oneTesla_User_Manual_v1.3.6.pdf. That manual is for an earlier
discontinued model, but does a much better job of explaining how a Tesla coil
works than the newer model's manual (http://www.onetesla.com/oneTeslaTS_User_Manual.pdf).
One of many challenges faced by the oneTeslaTS designers was
how to squeeze a high-performance Tesla coil, able to put out in excess of
200,000 volts (200 kV), into a small package, less than a foot (30.5 cm)
tall. Most DRSSTCs are at
least twice that tall to allow room for the thousands of turns of fine wire
wound in a single layer on the long high-voltage secondary coil. More turns means more voltage, and so
obtaining 200 kV from a primary coil excited by 340 VDC (which is all you can
get from a full-wave rectifier operated directly from the 120-VAC mains supply
in the U. S.) requires lots of turns of wire.
The way the oneTesla people squeezed all those turns into a
coil less than 8 inches (20 cm) long is by using fine wire. Very
fine wire. No. 38.5, according
to the oneTesla manual. That's
about 0.1 mm, less than the diameter of a human hair.
The only places such fine wire is used routinely that I'm
familiar with is in earphones and the old-fashioned "D'Arsonval"
analog meter movements. But such
things require close to watchmaker precision and steadiness of hand to
build. Can an average kit-builder
deal with wire that fine? We'll
talk about that later. Suffice it
to say that with such fine wire, the designers got enough turns in their
secondary to make sparks that rival other units that are twice or three times
the physical size of this one.
Another clever trick was to make the primary coil a
circuit-board pattern, etched directly into a two-sided board on which the
secondary is mounted. This
eliminates any need to hand-wind or insulate a primary coil, which the oneTesla's
earlier (Gen1) model required. It's
also probably cheaper than a long hunk of solid wire.
The driver electronics also has some nice features. It's controlled (or supposed to be
controlled—see below) by an "interrupter board" which is at the other
end of a ten-foot fiber-optic cable.
This cable is just a piece of plastic and will not interfere with or
pick up the strong electric field coming from the operating Tesla coil. I learned about this the hard way when
I tried to control my previous research Tesla coil with a coaxial copper cable. The first time I tried it, induced
currents fried the electronics, and after that I switched to fiber-optic
control, which has caused no problems.
The other problem solved by the driver electronics is
tuning. Like two tuning forks set
at the same pitch, the primary coil and the secondary coil have to resonate
("ring") at or near the same frequency for good energy transfer. And the driver electronics has to send
that frequency to the primary or else the driver can't work well and may burn
up. The oneTeslaTS circuit is
basically a feedback loop that senses the current in the primary and
automatically tunes its frequency to the resonance frequency of the
system. This is a clever solution
that avoids a lot of fussy manual tuning that other systems require.
Small, efficient, easy to build—what could go wrong? Well, quite a few things,
actually. But not until I got past
the circuit-board assembly steps, which are by and large straightforward,
accurate, and not that hard to do if you know how to solder, how to read
resistor color codes, and how to follow instructions. The interrupter went together without a hitch, but I was
left clueless by the manual as to how to operate it.
The main circuit board passed its low-power test, making a satisfying
buzz when the interrupter was connected to it. The next step was completing the secondary coil. Fortunately, the kit comes with a
completely wound secondary, but it's just a coil: a plastic tube with several thousand turns of wire on
it. It's your job to insulate it
better and put end caps on it.
Not So Good News
Five Ways To Break No.
38.5 Wire
I was well aware that the quality of the secondary coil
would determine the quality of the unit, so I was careful to follow the
instructions about how to treat it.
The secondary is supplied "bare" (only the
insulated wire on it) and it's the kit-builder's job to apply enough insulation
on the winding itself to keep it from breaking down under the extreme stress of
holding off some 200 kV across only a few inches of space. The manual says to "varnish"
the secondary, and shows a picture of one next to spray cans of Krylon Crystal
Clear acrylic coating, and Minwax Helmsman spar urethane. There was some comment in the manual about
substituting packing tape, but the important thing is to surround each turn of
wire with a dielectric (insulator) that works a lot better than plain air. I decided to set up a rig (Fig. 1)
Fig. 1. Coil-varnishing rig.
on my workbench that would slowly rotate the coil about a
horizontal axis at a few RPM. I
had a geared DC motor lying around, but an old AC motor from a rotisserie
barbecue set would work. I cut a
couple of pieces of cardboard out to make rings that would just fit inside the
coil and around the broomstick axle to support the coil on the axle. The idea is to keep the spray from
sliding to the lowest part of the coil and beading up due to gravity. The manual said to put at least five
coats on, and I used five coats of the Minwax Helmsman brand spar urethane, moving the spray
evenly back and forth as the coil turned and spacing the coats apart by about
an hour. After letting it dry
overnight, the resulting finish was very smooth and thick enough that I have no
hesitation in grabbing the middle of the coil and picking the unit up that
way. Don't try this with the bare
coil—you'll probably break something and have to order a new one.
The next major operation on the secondary was to assemble
the end caps. The whole point of
the end caps is to insulate the two ends of the coil from each other as much as
possible, because those two ends are where the 200 kV shows up. My coil was supplied with several
inches of wire at each end, and nothing specific was said in the manual about
how much of this wire to use or how long to make it before you assemble the end
caps.
HERE IS THE FIRST SUPPLEMENT I recommend to the manual. Cut off all the wire from both ends
except what you HAVE to use to make the end caps fit. That is probably about 40 or 50 mm, depending on how you
assembled your end caps.
The end caps are stacks of clear insulating plastic
laser-cut in various patterns to form a totally enclosed cavity where a ring
terminal makes a connection between a bolt going through the middle, and the
end of the wire adjacent to the cap.
The manual recommends assembling the caps using HOT GLUE, NOT EPOXY. I TOTALLY AGREE, because it
took me five tries to get the bottom end cap together. I broke the wire the first four times. If the layers of the end caps are not
almost EXACTLY coaxial (all their outside edges perfectly in line), the end cap
will not fit into the inner diameter of the secondary tube and you will have to
pry the thing apart. My wife (who
is familiar with hot glue) recommended putting the stack in the freezer for 15
minutes or so to make the hot glue more brittle and easily broken up. This worked great, but I still had to
take a sharp razor or Xacto knife and scrape all the old glue off before lining
things up for the new glue.
As a participant in the oneTesla forum noted, the existing
end cap design sort of makes a pair of scissors that tend to cut the wire right
where it leaves the top of the tube and runs down between the inside of the
tube and the end cap layers to get to the ring terminal, where you have
carefully soldered it. Fig. 2 is a
picture (adapted from the oneTesla v. 1.4 manual) of how they want you to
assemble an end cap:
Fig. 2. Manufacturer's recommended way of
connecting secondary wire to end cap.
In order to do this successfully, you must (a) solder the
fine wire from the coil to the "ring terminal" (I call it a solder
lug) that goes around the bolt, (b) while holding the entire assembly out of
the top of the coil, but near enough not to stretch or break the wire, finish
assembling the top pieces of the end cap together, and (c) slide the entire end
cap assembly into the coil WITH THE WIRE ATTACHED, WITHOUT BREAKING THE WIRE.
This is very hard.
After five tries, I managed this on the bottom terminal, but
one failure on the top terminal convinced me to come up with a better way,
which leads to this: HERE IS THE
SECOND SUPPLEMENT I recommend to the manual.
Instead of
trying to connect the wire to the ring terminal, either save it out before you
assemble the end cap, or make your own out of 5-1000ths of an inch thick solderable brass or copper "foil"
(available at hobby shops like Hobby Lobby), as shown in Fig. 3:
Fig. 3. My way of connecting secondary wire to
end cap.
I simply soldered the wire at the top end to my hand-made
brass ring terminal (cut out with a pair of scissors), and screwed the toroid
and the retaining bolt down on top of it all. (If you save out the ring terminal when you assemble the end
cap, you won't have to roll your own.)
This is vastly easier than first connecting the wire to the terminal
inside the end cap, and then trying to fit the whole thing, with the wire
attached, into the coil. My way,
you can put the end cap on first, and then lay the terminal on top of the bolt
and put the toroid on it.
While the original end cap design does a good job of
insulating the wire and the ring terminal all around, there is no need to
insulate them from the toroid (on the top) or the ground plane (on the bottom),
because the wire is directly connected to those points anyway. My point is that placing the ring
terminal (or a substitute) directly on the end cap surface should not cause any
insulation worries, and so far my modifications in this department have not
caused any issues. There is still
plenty of end-cap insulation where it counts (between the top and bottom
wires), and as long as you keep the wires well away from the hollow part of the
inside of the secondary tube, you should not have a problem in this department,
as some unfortunate kit-builders
have run into. But maybe they
didn't know what to do with all that extra wire, so they left it dangling
inside the secondary tube. No, no,
no.
Blowing the First Fuse
One of the tie points for the power connector is very close
to one of the tie points for R5, one of the high-voltage power supply bleeder
resistors that discharge the high-voltage filter capacitors to keep you from
killing yourself when you take your kit apart to fix it (as you surely
will—take it apart, I mean, not kill yourself). So leaving that resistor off is not an option. But I was not the only person
accidentally to leave a solder bridge between the resistor and one of the AC
terminals of the bridge rectifier.
The result was that when I followed the instructions for the first
startup and plugged it in, I saw a bright green flash from somewhere on the
circuit board and that was that.
The solder bridge had not only blown the fuse, but had vaporized a
section of circuit-board trace (vaporized copper glows green, in case you're
wondering). Subsequent waiting and
inspection revealed the problem, and a trip to Radio Shack (yes, there's a few
left) laid in a supply of 10-amp fast-blow fuses of the right physical
dimensions. Be sure and buy or
order at least a dozen of those before you start running your unit. It's also a good idea to order at least
one set of four replacement IGBTs, which oneTesla offers as an option.
This was the last major problem that I can say was
definitely attributable to me. The
following issues have more subtle causes that took a few more weeks to resolve.
Eating IGBTs
Here is the way this device is supposed to work. The interrupter board generates light
pulses that occur at a frequency (number of pulses per second) determined by
the "frequency" setting of the board. Just for the record, and for the convenience of everybody
who wants to use the "SD" interrupter board, here is THE THIRD
SUPPLEMENT I recommend to the manual:
a flow chart for the interrupter board controls.
Controls: main
power (slide switch on left side of enclosure), LCD display illumination (slide
switch above LCD display), "select" (*) (middle pushbutton),
"up" (^) (top pushbutton), and "down" (v) (bottom
pushbutton). User operations are
*, ^, or v.
Condition: No
SD card inserted.
Main power ON (LCD display on or off, doesn't matter):
>"Checking SD Card"
>"Fixed Mode"
*
>"[Freq:
20Hz ]"
The unit is now producing a 13.6-microsecond pulse at a
frequency of 20 Hz. The pulse
frequency can be adjusted upward to a maximum of 1000 Hz (1 kHz) by pressing
and holding the ^ button, and lowered down to 1 Hz (the minimum) with the v
button.
At any given frequency, the duration ("dwell time") of the pulse can be lengthened by
pressing * again:
This moves the pair of brackets [] from the top display line
to the bottom display line. Now,
the user can increase the pulse duration ("power") by pressing the ^
or v buttons. The longer durations
are indicated by dark rectangles extending across the bottom of the screen
display, up to a maximum of 14 rectangles.
By direct measurement of the pulse duration in my lab, the
minimum pulse duration (lowest "power") at all frequencies is 13.6
microseconds. The maximum pulse
duration varies from 48.6 microseconds at 1 Hz to 25.6 microseconds at 1
kHz. The longer pulses are needed
at lower frequencies, apparently to even out the loudness at different
rates. The fact that the maximum
pulse duration occurs at the lowest pulse frequency probably explains the
rather cryptic statement in the manual that "higher frequencies draw less
power from the coil." This is
true if you have run your interrupter power level higher than the minimum.
If you haven't figured it out by now, the longer the
interrupter is HI (on), the more cycles of 260-or-so kHz power the driver will
pump into the primary, and the more energy will come out the top of the
secondary. At some point the
voltage on the secondary coil will be high enough at the tip of the breakout
point (the brass wire supplied with the kit that is shown sticking out
horizontally from the toroid), and the energy will start to dissipate into the
air. The discharge at the tip of
the breakout point starts as what plasma physicists call
"corona": a burst of
tiny fast cold spark-like feathery discharges that absorb some energy and turn
it into heat, light, and some chemical reactions (this is where the ozone smell
comes from).
Fig. 4. Video of first test of oneTeslaTS.
Although the interrupter was set to low power
(short-duration pulse), what I saw in that video did not look like any low
power to me. After unplugging the
unit and waiting a suitable time (at least five minutes) for the power
capacitors to discharge, I disassembled it and checked the IGBTs. They had failed, and so had the 10-amp
fuse.
How did I know
they failed? Using the "diode
check" function of a DVM, a good IGBT will show about 0.4 V when the
collector is negative and the emitter is positive, and open with the opposite
polarity. (See the IGBT data
sheet, available online at https://www.fairchildsemi.com/products/discretes/igbts/discrete-igbts/FGA60N65SMD.html for pinouts). When the device fails, usually a short
circuit appears between these two terminals, with either polarity.
I highly recommend (as the manual does)
leaving the full lead length of the IGBTs sticking up from the board when you
replace them, because these leads form convenient test points to contact with
DVM leads without taking the board stack apart. Be very careful to place the sil-pads correctly, because any
electrical contact between the IGBT's
metal case and the heat sink will in all
likelihood blow a hole in your heat sink and
possibly turn the IGBT into a small bomb,
too. (This is on the user's forum
as well, once
you register.
You can't see everything until you register.)
I was somewhat mystified by the pretty catastrophic failure
of both transistors and the fuse, despite my precautions of starting with the
lowest power setting on the interrupter. The way the oneTeslaTS is designed, it
is not possible to operate the IGBTs at any voltage lower than 320 V or
whatever the voltage-doubler power supply creates from your local power
line. Even with the
lowest-duration pulse, this is asking the IGBTs to do a lot if something
happens to make the pulse a lot longer than minimum. And it turns out, that was exactly what was happening.
In one chapter of my book, I discuss a fairly obscure
topic: EMI, which stands for
"electromagnetic interference."
EMI is what happens when somebody's cellphone starts bothering an
auditorium's PA system, for example.
It is unintentional coupling of a signal to a circuit by means of
electric, magnetic, or electromagnetic fields.
After about four days of trying to get my oneTeslaTS to work
at home, I went through two sets of IGBTs, an equal number of fuses, and was no
closer to a solution. I could not
understand why even though I set the interrupter to a specific frequency, the
sparks would emerge from the breakout point in loud random pops until something
busted. Finally, after modifying the unit to apply less than full
voltage to the IGBTs and observing its output with an oscilloscope, I
determined the problem: loss of
control by the interrupter.
Loss of Interrupter
Control
By using a variable AC supply device called a variable
autotransformer (tradename "variac"), I slowly increased the voltage
on the IGBTs, slow enough so that when the bad thing happened, they would not
be working under the full stress of 340 V and might not immediately fail. (This was after I connected the driver
power supply to a separate constant 120VAC source so that it could operate
normally regardless of the IGBT supply voltage.) Up to about half voltage, I saw that the system was behaving
properly. The interrupter was
signaling the driver to allow oscillation for a few cycles, and the secondary
was putting out high-voltage pulses exactly in time with the interrupter
pulses. Then at about 55% of full voltage, POP! I saw the scope screen fill with a constant oscillation,
much longer than the interrupter would allow it to do if it was still
controlling the system. Somehow,
the high electric field from the secondary was getting back into the
electronics and keeping the oscillations going even AFTER the interrupter
"told" it to quit.
Here in Fig. 5 is a captured pop from a Tektronix TDS2014B
scope with its probe picking up an arbitrary-scale electric field (vertical
axis) and the timebase in milliseconds (horizontal axis):
Fig. 5. Electric field (arbitrary units) from
Tesla coil during a "pop."
Time axis units are milliseconds.
This explained my problem. The IGBTs are not capable of operating continuously at 340 V
supplying full power to the secondary.
After a few milliseconds of what amounts to a VERY high-power pulse,
they heat up so much that they cease to switch efficiently, the field falls, and
the EMI loop is broken. The pulse
quits, but the IGBTs are so hot that the system can no longer oscillate
normally for a short time. Then it
recovers, runs under interrupter control for a time, the field builds up again,
and POP! the cycle repeats. This is somewhat speculative, but explains the data.
If I had increased the IGBT voltage to full (100%, 340 V)
they would undoubtedly have failed again the next time the interrupter lost
control.
How is this thing to be prevented? Commercial and hand-crafted Tesla coils usually have
elaborate electrostatic metal shielding around the control electronics. There is no cheap shortcut to good
shielding, so this is why they cost so much. My first thought was to put on the side panels in hopes that
they would provide enough shielding to stop the loss of interrupter control
(POPS for short). But alas! the
manual's statement that "installing the cosmetic side panels is
optional" turned out to be true.
They are largely cosmetic and did not provide enough shielding to fix my
POP problem.
At this point, I turned to the user forum hosted by oneTesla
on their website. I wrote a
detailed post describing my problem, and after a few days received a short
reply from
Bayley (the "lord protector"), saying it sounded
like a grounding problem. I
rechecked grounds and found nothing obviously wrong, so eventually I resolved
to replace the entire main board.
This costs about $80.
Fortunately, exchanging main boards is pretty straightforward, mostly a
matter of lining up a new set of IGBTs and disassembling and reassembling the
board stack.
Fig. 6.
Electric field (arbitrary units) during one interrupter pulse from
proper operation of oneTeslaTS.
Time axis units are microseconds.
Parting Words
If you are careful to avoid solder bridges, do a good job of
varnishing the secondary, and make connections to the secondary on the outside
of the end caps instead of the way recommended by the manual, you could
conceivably have the oneTeslaTS up and running in a week of working on it a
couple of hours an evening. I was
not so lucky.
Although I haven't
used the MIDI feature, the interrupter takes a memory card with a MIDI file on
it, and so the tunes you can play on the thing are limited only by your stock
of MIDI-file tunes (there are five on the oneTesla site to get you going). Especially if you haven't done this
kind of thing before, building this kit will be a learning experience. But don't start unless you're prepared
to devote a lot of time and thought to this project, and a fair amount of filling
in the blanks left by the overly brief instruction manual.
REVIEW POLICY NOTICE: I paid for the
above-described kit myself and have no connection with oneTesla or its
employees. I will not review other
kits by request, and will turn down any such requests.
Karl Stephan
Feb. 3, 2016
See my blog on
engineering in the news at
