Download Tuning your oneTesla

Tuning your oneTesla
Draft version 2013-11-19, Stefan Gustavson ([email protected]).
Published under CC BY-SA license (see creativecommons.org for details).
The oneTesla kit was designed to be a manageable project for relatively inexperienced hobbyists. If
assembled correctly, it should perform well and give you long, beautiful sparks without any extra
adjustments. However, the performance of a Tesla coil is very much dependent on its tuning, and
due to component tolerances and small details in your assembly of the kit, it is quite possible that
your particular coil is not as well tuned as it could be, and that it could perform even better. A tuned
coil gives longer sparks and a louder sound, draws less power and runs cooler, thereby reducing the
risk of component failures. A coil that is very badly out of tune will produce weak sparks and might
break down on higher power settings. This guide is meant to teach you how to detect tuning
problems in your oneTesla, and how to bring it into tune.
Some people have had problems with secondary coils that were accidentally wound with the
incorrect number of turns, which results in more severe tuning problems and component failures
even at modest power settings. This guide is meant to help also in those circumstances.
Tuning?
In any dual resonant Tesla coil, including the solid state variety of which the oneTesla is an
example, there are two LC resonant circuits involved in creating the high tension sparks: the
primary circuit and the secondary circuit. An LC resonant circuit consists of an inductor (L) and a
capacitor (C) connected either in series or in parallel. Its resonant frequency f is easily computed
from the inductance L and the capacitance C according to:
1
f=
2 π √( LC)
For a Tesla coil to be well tuned, the primary and secondary circuits should resonate at the same
frequency, or at least almost the same. This is paramount to performance.
How does it work?
In order to understand why tuning is so important, it is helpful to understand how a Tesla coil
works. (You do not need to understand this section, or even read it, to detect and fix tuning
problems, but we believe that the best toys are also educational.)
The oneTesla is an interrupted coil, which means that it is not creating a continuous spark, but rapid
successions of sparks in short bursts, controlled by the interrupter. This is what makes it able to emit
a tone of a specific pitch. During the “on” time for each pulse from the interrupter, the output
transistors (insulated gate bipolar transistors, or IGBTs for short) are driving the primary circuit by
means of a feedback circuit that adapts to the primary resonance frequency. With each cycle, more
electrical energy is “pumped” into the primary circuit, gradually increasing its resonance amplitude.
This energy will then transfer to the secondary, but the magnetic coupling between the primary and
secondary coils is weak by design, unlike a regular transformer, were you want strong coupling.
This means that the energy transfer will not happen instantaneously, but over time across several
cycles. In this manner, the secondary gets pumped by the primary, and increases its resonance
amplitude (“rings up”), while the primary is active. Some power is lost to the spark when it breaks
out, and there is a maximum voltage you can reach both in the primary and the secondary, partly
because of thermal and magnetic losses, but also because a pumped-up secondary will start
transferring some of its stored energy back to the primary. However, the main limitation of spark
power during normal operation of the oneTesla is the time you allow the primary to ring up. If you
make that time too long, the device will actually suffer a catastrophic breakdown. The primary
circuit, and the IGBT bridge in particular, was designed for an interrupted Tesla coil, and the
components are not rated to handle continuous operation at full mains voltage. If you were to try
continous operation, the current in the primary coil would become too high and fry the IGBTs
within milliseconds. Continuous operation is possible at a much reduced AC supply voltage from a
variac, like 30 volts or so, or by feeding the bus capacitors directly from a DC power supply, but
then the output voltage from the secondary will be reduced accordingly, and the spark will be short,
faint and not very loud.
The secondary oscillations will tend to follow the oscillations of the primary while it is being
pumped by it, but when left alone it will resonate at its own preferred frequency. If the two are
significantly different, the secondary will not respond well to being pumped by the primary
frequency, and the energy transfer will be inefficient. Perfect tuning is definitely best, but rather
hard to accomplish, because the secondary resonance frequency varies quite significantly with the
spark length. A slight detuning is OK and will only make a small difference. With stronger
detuning, the secondary will respond well to the first few cycles of the primary, but after a while,
when the secondary has had more energy transferred to it, the secondary self-resonance will drift
out of phase with the primary, and the energy transfer will stop. A serious detuning will make the
secondary run out of phase sooner, making it fight against the primary oscillations for some part of
the cycle and load the primary circuit down instead of sucking up the energy from it.
Conversely, if the secondary is well tuned, the energy transfer from the primary will be efficient for
the entire duration of the primary oscillations, making the secondary ring up to a high voltage while
only putting a minimal load on the primary. The maximum output voltage from a well tuned Tesla
coil is much higher than what you could get from an ordinary, non-resonant transformer circuit with
the same ratio of turns between the primary and secondary windings.
Deliberate detuning
You generally don't want to hit perfect tuning (exactly the same frequency for the primary and the
secondary) at lowest power, because then you will be out of tune at high power. This is because a
longer spark, or “streamer” in coiler-speak, adds some to the top load capacitance and influences the
resonance frequency. This is called “streamer loading”. Therefore, after you have found the perfect
tuning on low power, you will need to retune the coil some more to perform its best at higher power.
Experimental evidence from the oneTesla forum seems to indicate that the oneTesla, being a small
coil that throws big sparks, needs to have the primary frequency tuned significantly lower than the
secondary frequency for best performance. A good rule of thumb seems to be that you should aim
for a primary frequency that is 15% to 20% lower than secondary frequency. Popular online
resources for Tesla coils in general usually state this number as somewhere around 5% to 10%, but
the oneTesla is an unusually strong performer for its size, and needs a larger margin for the streamer
loading.
A strong, full length streamer from the oneTesla seems to reduce the secondary resonance frequency
by up to 20% from the frequency at lowest power. At half power setting on the interrupter, the
difference is more like 10% from the low power state. Thus, if you want your coil to give good
results at full power, you will run it slightly out of tune at half power, and vice versa.
Note that this 15-20% figure is only a rule of thumb. Depending on your particular setup, you might
get good performance with as little as 10% detuning, or your coil might work just fine with a
slightly larger detuning. However, if your secondary resonance frequency is significantly more than
20% higher than your primary frequency, you are bound to have problems. The symptoms of a
strong detuning are an underperforming coil with no good spark at lower power, and a strong
tendency to destroy the expensive insulated gate bipolar transistors (IGBTs) somewhere around the
half-way power setting on the interrupter. Often one or several of the power diodes are also
destroyed along with the IGBTs. Such failures are expensive and cumbersome to fix.
Measuring the resonance frequencies
There are many ways of measuring the resonance frequencies of your primary and secondary, some
direct and some indirect. The most convenient ways require you to have access to an oscilloscope.
You can manage without one, but let's start by explaining how to do it with an oscillosope.
Using an oscilloscope and a running coil
A correctly built oneTesla will create a small spark from the breakout point even on the lowest
power setting. If the coil is not well tuned, the spark might be weak, almost invisible and not very
loud, but there should definitely be a spark. Using an oscilloscope, you can measure the oscillating
electrical field from the top load by holding or hanging the probe somewhere in the air a few meters
from the running coil. Set the time base to 20 us or 10 us/div, and adjust the sensitivity of the scope
to have a trace that looks similar to the image below.
(image: a typical trace of a oneTesla with some detuning issues)
On this trace, you can notice detuning problems in several ways, the most direct being a difference
in frequency between the first and the second part of the waveform. A healthy, tuned coil will show
the same frequency across the entire trace.
(image: a trace from a oneTesla with good tuning)
When you have a trace like the ones shown above, you can count and measure the pulses in the
leading part and the trailing part of the waveform, and calculate the frequency for each part. Most
digital oscilloscopes have built-in tools in the user interface to measure a time interval. Set the
cursors to coincide with two peaks or two zero crossings in the ringing-up or ringing-down part of
the trace, respectively, read the time interval, and divide by the number of cycles between the peaks
to get the period T . Your corresponding resonance frequency is 1/ T .
If you have an analog oscilloscope, you can get a very reliable measurement by snapping a straight
head-on, high resolution picture of the trace with a digital camera and count pixel distances in an
image editing program, as shown in the image below. Measuring this directly on the display is also
possible, but it gives you a less accurate reading.
Using an oscilloscope and a signal generator without running the coil
You can also measure the primary and the secondary resonance without powering up your oneTesla,
but then you need both an oscilloscope and a signal generator capable of outputting a signal with a
variable frequency up to 1 MHz, or at least in the range 200 kHz to 350 kHz. The signal does not
have to be sinus shaped – a square wave from the digital output of a simple timer circuit will do just
fine.
TO DO: include or link to instructions on how to build a 555 timer circuit to test resonance.
If your signal generator does not have a built-in accurate reading of its currently set frequency, a
frequency counter or an oscilloscope with built-in frequency measurement is a big help. You can
count cycles and measure distances on an analog scope as shown in the previous section, but it's not
nearly as convenient.
Primary
To measure the primary resonance, disconnect your oneTesla from all power sources. Locate the
primary lead that is farthest from the tank capacitor and disconnect it from its screw terminal.
Connect a short and straight wire from the tank capacitor lead that is closest to the corner of the
board to the primary lead you just detached. Then, connect your oscilloscope across the tank
capacitor. Connect the signal generator to the same points, but through a 1k resistor. A schematic is
presented below.
Start the signal generator at around 250 kHz and adjust the oscilloscope until you see a clear trace.
Then, adjust the frequency until you find the maximum amplitude for the trace. This peak amplitude
happens at the resonance frequency. The signal is not very strong, and the peak is not very
pronounced, but with some care, you should be able to determine the resonance frequency with
good accuracy. A good digital oscilloscope with amplitude and frequency readout helps a lot here,
but an analog scope will do.
Secondary
There are several ways to measure the secondary resonance, but we recommend the following
method for its accuracy. Disconnect the primary coil from its terminals, and connect your signal
generator to the primary coil, possibly through a small resistor of about 100 ohms if your signal
generator is not comfortable with a short circuit across its output. Connect the bottom of the
secondary coil to a good, low impedance ground or to a counterpoise (a sheet of metal, like
aluminium foil, about 1 m square), and place the coil in the same kind of environment as during
operation, keeping it reasonably far away from the ceiling, walls and other objects. Hang the
oscilloscope probe still in a position a few feet away from the top load, and set the amplitude scale
of the oscilloscope to somewhere in the mV/div range to detect weak signals. Start your signal
generator and adjust its frequency. When you hit the secondary resonance frequency, you should see
a strong increase in the amplitude of the field that is emitted from the top load through the air to
your oscilloscope probe. This peak is a lot more pronounced than the peak you detected from the
primary circuit.
Measuring without an oscilloscope
Link to the online schematics and instructions for the circuit with the 555 timer and the LED
indicator. Does it work? I have no experience with it. Need to build it and test it, or insert a
disclaimer here saying ”untested”. Secondary measurement should work. Primary is more
doubtful.
Is it possible to measure the peak with a multimeter capable of measuring AC voltage? The AC
voltages involved are small, but at least for the secondary measurement it might be enough.
Build an instrument amplifier with a simple op-amp to measure small AC voltages? Would be
inexpensive, but not dead simple.
Hand tuning
If you do not have access to an oscilloscope and do not feel like assembling the home-brew signal
generator mentioned in the section above, you can tune your oneTesla using nothing but your eyes
and ears and some experimentation. You will not be running blind, but you will be running with a
sort of tunnel vision, so an oscilloscope really makes things a lot more convenient. However, it is
perfectly possible to tune a Tesla coil without any extra equipment.
Bad tuning manifests itself as poor performance of the coil: the sound is not as loud as expected,
and the sparks are shorter than expected. Use the gallery at the end of this document for reference.
Exactly how to tune your coil is presented in the next section. Below, we just give you a hint on
how to tune without access to measuring equipment. The adjustments and modifications you need
to make are the same as described in the section “Getting into tune”.
Without any frequency measurements for reference, it is impossible to know which way to tune
your coil, but try a small adjustment in one direction and see if the situation improves. If it doesn't,
try the other way. If things do not improve nor get any worse either way, try a somewhat larger
adjustment. Continue making small adjustments until you reach the optimal performance. It is wise
to do this on low power, around one-third setting on the power knob, or else you risk destroying
something. You can also try it on minimum power to be absolutely sure that nothing bad happens.
For reference, you should know that when the oneTesla is well tuned at lowest power (a situation
you generally don't want, see the section “Deliberate detuning” above), the sound is a little too loud
for comfort without ear protection when you are standing close to it.
Tuning with an AC current meter
As strange as it sounds, you can also tune a Tesla coil by hooking up an AC current meter to the
mains power line and adjust the tuning until the current draw is as small as possible for a certain
power setting on the interrupter. It may seem a bit counter-intuitive that a stronger spark is produced
when the power input is smaller, but that is how it works. The power losses to heat when the tuning
is bad have a stronger influence on the total mains current than the extra power that gets pushed into
the spark when you hit perfect tuning. Not all multimeters can measure AC current, but if you have
one at hand, you can put it to use. Use extreme caution if you are using test leads to connect the
AC current meter in series with your mains feed. A clamp AC meter is a lot more safe and not
prone to breaking or blowing an internal fuse if you happen to push too much current through it.
Getting into tune
Measure your resonance frequencies on minimum power, or on an unpowered coil using a signal
generator and an oscilloscope. If you are lucky, your primary frequency is about 15% lower than
your secondary frequency. In that case, you're basically fine. Your coil does not need any strong
tuning adjustments. The reason for having different frequencies for the primary and secondary is
that the secondary frequency will shift down somewhat during operation when there is a long spark
present. That spark is a conductor that will add to the capacitance of the top load and reduce the
resonance frequency by about 15%. See the section “Deliberate detuning” above.
If you want absolutely top performance from your coil, you should measure the frequencies during
operation at full power, and aim for perfect tuning when there is a strong spark present. To measure
the resonance frequency during operation at higher power, hang the oscilloscope probe somewhere
a few meters from your coil. Ground the ground lead but keep the probe tip unconnected, and you
should be able to get a trace of the radiated electrical field, similar to what you saw earlier in this
guide for measuring the secondary resonance, only you will see a much stronger signal.
On longer pulse widths (higher power settings on the interrupter), you will see slight detuning
issues as an uneven amplitude in the “on” part of the trace. This happens because the primary and
secondary circuits drift in and out of phase with each other, and when they are out of phase, some of
the energy in the secondary is transferred back into the primary. By seasoned coilers, this is referred
to as “bucking”. As long as the bucking is small and only creates small variations in amplitude, like
in the plot below, it's not a big issue, but if it gets stronger, it can make the primary coil run hot to
the point of smoking, and in extreme cases it will destroy the IGBTs.
(image: A trace where bucking is clearly visible, but not strong enough to be a big problem)
(possible additional image: A trace of a long pulse without any bucking – if I can make one)
(possible additional image: A trace where bucking is strong enough to be a big problem)
A relevant measure of your amount of detuning is the secondary frequency divided by the primary
frequency. You want this ratio to be somewhere in the range 1.1 to 1.2. If it is greater than 1.2 or
smaller than 1.1, you are out of the “comfort zone” and should consider improving the performance
and stability of your coil by tuning it. If the ratio is as high as 1.25 or 1.3 or more, you will destroy
your coil if you run it at full power, and in that case you must perform a tuning before you crank up
the power setting on your interrupter. If the ratio is lower than 1.1, you will see good performance at
lower power, but a bad performance at higher power, and even in this case there is a risk of
destroying the coil by pushing it to higher power.
Adjusting the primary resonance
From above, recall the formula for the resonance frequency of an LC circuit:
1
f=
2 π √( LC)
The primary resonance circuit can be tuned in two ways: change the capacitance, or change the
inductance. In the oneTesla design, the value of the tank capacitor cannot be varied easily and
freely. If you happen to have access to a large stash of such capacitors, you could probably fix slight
detuning issues by using a capacitance meter to find one that has just the right value, but that is out
of reach for most people. The primary coil, however, is relatively easy to change. If your primary
has a too low resonance frequency, you should decrease the primary inductance, and if the
frequency is too high, you should increase the inductance. Increasing the inductance can be
performed by adding more turns to the default six turns. The inductance of a short coil like the
oneTesla primary is proportional to the number of turns N squared, and inversely proportional to
a weighted sum of its length l and diameter d :
N2
L∝
l+0.9 d
There is also some stray inductance in the connecting wires leading up to the coil, but that is not a
big contribution to the overall inductance of the primary.
The inductance of the standard 6 turn primary with the wire wound tightly (minimum length of the
coil, about 14 mm) is approximately 7 µH. Spreading the windings out to twice that length reduces
the inductance to about 5 µH.
Reducing the coil to 5 turns over 12 mm length gives an inductance of about 4.8 µH, and spreading
that 5 turn coil out to 30 mm length reduces the inductance further to around 3.5 µH.
Conversely, adding one turn and packing the 7 turns tightly together increases the inductance to 9
µH, and spreading the 7 turns out over 30 mm reduces it to about 6 µH.
Because the resonance frequency is inversely proportional to the square root of the inductance, the
nominal resonance frequency for the 6 turn tight coil of around 236 kHz can be increased to 330
kHz for a widely spread 5 turn coil, and decreased to 200 kHz for a tightly wound 7 turn coil. This
should be a more than adequate tuning range to match even a very problematic secondary. The table
below summarizes this. Green cells represent a tightly wound coil. Red cells represent coils that
would require thinner wire than the default to fit across the specified length. Yellow cells represent a
more or less spread-out winding.
Coil length (mm)
Number of turns
5
6
7
10
12
14
16
18
20
24
28
32
4.97
4.80
6.91
4.64
6.68
9.09
4.49
6.46
8.80
4.35
6.26
8.52
4.21
6.07
8.26
3.97
5.72
7.78
3.76
5.41
7.36
3.56
5.13
6.98
Frequency with 68 nF
5 turns
6 turns
7 turns
40
3.23 uH
4.65
6.33
278.6 283.4 288.1 292.7 297.3 306.3 314.9 323.4 339.7 kHz
236.1 240.1 243.9 247.8 255.2 262.5 269.5 283.1
205.8 209.1 212.4 218.8 225.0 231.0 242.6
The process of changing the resonance frequency by expanding and contracting the winding is
called “scrunch tuning”. This is not quite as convenient as a commonly used method on larger Tesla
coils, where a movable tap on an uninsulated primary makes it possible to tap the primary coil at,
say 5.7 turns, but the insulated primary coil on the oneTesla is a simpler and more safe design, and
it makes the coil less likely to create arcs between the secondary and the primary.
Adjusting the secondary resonance
The secondary coil of a oneTesla kit comes pre-manufactured and is cumbersome to rewind, but it's
easier to change the capacitance of the top load. Slight adjustments can be made by making the
breakout rod a little shorter or longer. This will change the tuning only slightly, but it is a good way
to do fine tuning for absolutely maximum performance.
For stronger re-tuning, you should consider adjusting your primary. However, it is possible to adjust
the secondary by using a smaller or larger toroid for the top load, or adding a second toroid on top
of the first. The stamped toroid that comes with the oneTesla kit is a large part of its good looks, so
replacing it is not the recommended way of tuning a oneTesla, but it can be done. Any custom top
load you make should have a smooth, softly rounded surface, or else you will have sparks breaking
out all over the place. It should also be symmetrical about the central axis of the secondary. A
toroidal shape is best, for various reasons, but a sphere, or something reasonably close to a spherical
shape, will work too.
The capacitance of a toroid can be computed with this excellent online resource:
http://deepfriedneon.com/tesla_f_calctoroid.html
The formula is a little bit hairy, even though it's only an approximation, but it goes like this:
D
C=K 1 (1.2781− 2 )
D1
√
2 π2 (D1− D2)
4π
D2
2
(pF)
where D 1 and D2 are the major and minor diameters of the toroid, respectively. K 1 =7.11
if D1 and D2 are measured in centimeters, and K 1 =2.8 if measurements are in inches. The
stamped aluminum toroid has D1=200 mm (8") and D2=45 mm (1.8"), which gives a capacitance of
8.6 pF.
The capacitance of a sphere is a lot more simple to compute: C=K 2 R (pF), where R is the
radius of the sphere, and K 2=1.11 if R is in centimeters, and K 2=0.438 if R is in inches.
A combination of toroids and spheres is also fine. If you want to be creative, go ahead. Note,
however, that when using combinations of top loads, like two toroids or a toroid and a sphere, their
capacitances do not simply add up. Two shapes of equal size placed reasonably close together will
have significantly less than double the capacitance of one of the shapes alone. The resulting
capacitance for a complicated shape is very difficult to compute. Most capacitance meters will also
have problems measuring small capacitances in the single-digit pF range. Measuring the resulting
resonance frequency for the secondary coil with the top load, and calculating the capacitance from
that, is usually the best way to determine the capacitance of a top load. With resonance frequency
F (in Hz) and inductance L (in H), the capacitance C (in F) is:
1
C=
L(2 π F )2
Never run the oneTesla without a breakout point! The IGBTs are not specified to withstand the
high energy that might accumulate in the primary resonant circuit if a spark does not break out early
during the interrupter pulse. If you don't put a breakout point on a smooth, well rounded top load
that retains the charge well, the voltage buildup over longer interrupter pulses might be fatal to the
IGBTs. Also, the coil will remain silent if no spark breaks out, which is not what you want for a
device that is supposed to play music.
A gallery of sparks
Last, we present a visual gallery of sparks from a well tuned oneTesla coil. This is the kind of
performance you should expect from a device that is properly tuned. Tesla coils are individuals, and
your coil might not perform exactly like the one we used to make this gallery, but if the results from
your coil are significantly smaller than what you see below, if your coil draws an abnormal power
from AC mains, or if either the heatsink or the primary coil becomes hot to the touch during
operation, you need to tune your coil.
Above is the kind of image you get if you just point and shoot in dim ambient lighting using
automatic exposure mode with a typical camera: a long exposure, showing how the spark dances
around over the course of several seconds. This is not what it looks like in real life, but it's a nice
image. In the close-up, you can see that this is an interrupted Tesla coil. To the naked eye, it looks
like a continuous streamer licking the air, but a spark is actually forming and extinguishing in a
cycle that repeats hundreds of times per second.
These are shorter (1/30 second) exposures from the same run, showing the spark frozen in time.
This spark is 25-30 cm long (10-12 inches) and was obtained in fixed frequency mode, at medium
frequency and 50% power setting on the interrupter. The current drawn from 230V AC mains was
1.5A (RMS), and the primary coil stayed cool to the touch even when the coil was left running in
this mode for several minutes. This coil was not in perfect tune (the ratio between secondary and
primary was 1.1), so the spark length did not increase significantly when the power was increased
from 50% to 100%. It got brighter and louder, but not longer, and the coil ran hot.
A coil that is in perfect tune can throw some really impressive sparks up to 20 inches long at full
power, as can be seen from the images below. Note that these sparks hit the power cords, which is
definitely not a good thing. To avoid this, point your breakout point away from the cables.
(TODO: Some more images needed. Show various power settings from min to max, in tune at
that setting, and also show the spark at half power from a coil that is tuned at full power. Include
measurements of true RMS current drawn from mains for each image. Try to make current
measurements both for 110V and 220V, either by asking Bayley for help, or by temporarily
modifying my 220V coil and running it off a variac.)