Secondary coil design and construction for Tesla coils

Published on: Jun 3, 2015. Last updated: June 30, 2021.

This is chapter 9 of the DRSSTC design guide: Secondary coil


Building the secondary coil be a very time consuming and tedious task, but it is of paramount importance that everything is done properly and that time is spent on making a high quality coil in order to avoid catastrophic failure while running it under high power conditions.

Due to switching losses in IGBTs, the most common resonant frequencies is somewhere between 35 kHz to 500 kHz, with most in the lower end around 100 kHz.

Spark loading

Spark loading is not yet fully understood as the load is never the same and many factors influences on it; complex in its branching structure, involves plasma physics with the calculation of arc resistance depending on how many thousand degrees hot the arc channel is, repetition rate, voltage profile that the secondary system is being charge with and the list goes on. This description on spark loading will only take the simple models into account.

Impedance matching should not be a foreign matter for most people that have worked with or designed electronics, it is simply a matter of ensuring that f.ex. your power supply can supply enough current to make a light bulb glow at its power rating. The secondary coil and the rest of the resonant system behind it is also a power supply that has to supply current to light up the spark and keep it fed with enough current for it to be able to grow.

As the spark gets longer and branches out to get bigger in a 3 dimensional array, the load grows in a way that can be represented by a resistance and a capacitance. This load will effectively change the resonant frequency of the secondary coil and at some point the load have pulled the system so much out of tune that it can no longer keep the spark channel fed with enough power and it collapses again. It is not just the frequency that can change so much that not enough energy is transferred, during heavy ground strikes the load can be so heavy that it pulls the voltage down completely on the secondary system and again the spark channel collapses.

Spark impedance is highly dependent on the amount of power that being fed into it and the impedance will drop with higher power levels. Typical spark impedance values are maybe around a few hundred kΩ. The relation to secondary impedance and coupling factor will be discussed further down.

Many people in the Tesla coil hobby have worked on simulating spark loading and comparing it to real experimental values. I would like to thank Terry Fritz, Steve Conner, Steve Ward, Udo Lenz, Bart Anderson and many others that I do not know by their full name.

Spark loading during ground strikes

Measurements and investigations into secondary charging currents, discharge currents and the voltages led out from this have shown that during ground strikes the spark impedance drops suddenly way below f.ex. secondary impedance of 50 kΩ and thus is clamps the topload voltage to less than 100 kV, bear in mind that these are rough estimates and not exact measurements.

A ground strike is a shorted circuit and consists of the following impedances: Secondary impedance Zsecondary, spark channel impedance Zspark and ground impedance Zground. Combined in series these impedances make up a new scenario compared to sparks just flying out in the air. As the spark impedance during a ground strike can suddenly drop to near zero and ground impedance can vary from near zero to some 10’s kΩ we have a system that is highly unpredictable.

The smaller ground impedance we have, the higher currents would suddenly rise in a low impedance ground strike, this rise in current have to be fed through the secondary impedance and is what pulls the voltage down. As the current propagates through the secondary coil the voltage distribution can get very uneven and there are many observations on secondary destruction from what seems to be reflected voltage rise from a good ground and has been called whiplash-effect.

More on grounding in Grounding and EMI chapter.

How to calculate the impedance and Q factor of the secondary coil system [1] [2]

Udo Lenz’ thoughts on optimal secondary impedance:
The power transfer to the secondary depends on the arc (resistive) load. For a given primary current it has a max at a specific arc resistance. If primary and secondary resonate at the same frequency this max is reached approximately, when Qsec = 1/k. Qsec is the ratio of arc resistance and secondary impedance.

Arc resistance depends very much on how much power you put into the arc and will decrease with more power. Typical values are maybe around a few hundred kohms, so the optimal secondary impedance would be k times a few hundred kohms, i.e. maybe 50k. I tend to believe, that a lower value might work somewhat better particularly for high power coils.

Since the arc detunes the secondary to a varying extent, the initial assumption, that primary and secondary resonate at the same frequency cannot really be upheld. Optimally this condition should be reached at the end of primary rampup.

Calculating the impedance for your secondary coil system makes you able to get a hint of what kind of performance you can expect from the coil and to determine how the coil should be driven or coupled. Q factor and its related calculations are shown and explained here, for those interested in more secondary coil parameters for the use of spark load modelling. I made a spreadsheet with all these and more example calculations.

The resonant frequency of the secondary coil system with topload can be calculated with a regular expression without taking care to the influence of the surroundings. In the following equations inductance L is given in Henry, capacitance C in Farad and frequency F in Hertz.

F_{resonant}=\frac{1}{2\cdot \pi \cdot \sqrt{ L_{secondary} \cdot C_{topload} }}

The impedance of the secondary coil and topload is also calculated with a regular expression.


Calculating the unloaded Q factor requires some more calculations as we need to know the DC and AC resistance, and to find those we first need to calculate the skin depth δ of the current flowing in the secondary coil. Coppers specific resistance at 20 oC is given as ρ 20.

\rho_{20} = 0.0179 \cdot 10^{-6}

\delta = \sqrt{\frac{\rho }{4 \cdot \pi ^{2} \cdot 10^{-7}}} \cdot \frac{1}{\sqrt{F_{resonant}}}

Next we calculate the DC resistance of the secondary coil. Coil diameter and wire diameter is given in meters and the result is in Ω.

R_{DC} = \frac{4 \cdot coil_{diameter} \cdot turns \cdot \rho}{wire_{ diameter}^{2}}

If the wire used is larger than what the skin depth is, we need to take this into consideration for the calculation of the AC resistance. We can calculate a factor that we multiply onto the DC resistance to account for this. It is simply the area of a ring and the factor of the area of the wire used divided by the area of the ring that the skin effect would represent. r is radius of the wire.

\Xi_{factor} = \frac{\pi \cdot r^{2}} {\pi \left ( r^{2} \cdot \left ( r - \delta \right )^{2} \right )}

The proximity effect is the greatest contributor to the AC losses in a secondary coil, adjacent turns on a coil form simply induce losses in each other. The complex task of calculating the AC losses in a coil was simplified by Medhurst’ paper from 1947 where he based his work on Butterworth’s work. This table can predict the AC resistance within 3% if the frequency the coil is driven at is below the self resonant frequency of the coil, which it is when a topload is added to lower the frequency. The table also only holds true for coils with more than 30 windings, again not a problem with Tesla coils.

spacing →  


Ratio ↓
Table 1

Spacing → is the ratio of distance between center point of turns divided by the wire diameter. So a closely wound coil using 0.25 mm wire would have approximately 10% wider diameter from varnish, that makes it 0.275 mm / 0.25 mm = 1.1 spacing ratio.

Ratio ↓ is the height/width ratio of the secondary coil. A coil that is 100 cm tall and 25 cm wide would have a ratio of 4.

If the above table is not precise enough or you need another H/W ratio, Mike derived the following formula to approximate proximity factor from plotting numbers calculated in Ed Sonderman’s excel spreadsheet VLOOKUP and getting a formula that is linearly for only ratios above 2.

\Phi_{factor} = 0.100976 \cdot \frac{height}{diameter} + 0.309630

Calculating the AC resistance is now as simple as to multiply the DC resistance with the skin effect factor and proximity factor.


Calculating the Q factor is done of an inductor is the ratio of its inductive reactance to its resistance at a given frequency, and is a measure of its efficiency.

Q=\frac{2\cdot\pi\cdot f\cdot L}{R_{AC}}

Steve Conner recommended the 50 kΩ impedance of the secondary coil system after working it out from Terry Fritz’s spark loading models and a assumption of a loaded secondary Q of 10. Steve Conner found that this “magic 50k”, as it has later been known as, agreed with  the better performing Tesla coils at that time, Tesla coils that mainly consisted of spark gap and off-line Tesla coils (the ancestor of DRSSTCs).

Udo Lenz describes the spark loaded secondary Q as it should remain equal for different sizes of coils. With spark load increasing with more power and bigger sparks, the impedance of the secondary should decrease slightly to compensate for the heavier load.

A higher impedance coil could however perform the same, just with longer on-time and more losses in the primary circuit as a result.

How to choose coupling coefficient between primary and secondary coils

The coupling coefficient, or just talked about as coupling, is denominated “k” and represents the ratio of the electromagnetic field between the primary coil and secondary coil. Generally k is in the low end somewhere between 0.12 to 0.2, compared to normal transformers that are often considered to be 1.

Low coupling coefficients are needed to avoid too high voltage stress from the fast energy transfer, with higher coupling comes faster energy transfer. We do however still want to get the coupling as high as possible as this is one of the limiting factors in how much energy we can transfer from the primary coil to the secondary coil and thus how long sparks we can produce.

With a optimal coupling and depending on the secondary coil being driven at the lower or upper resonant frequency, the secondary coil would have a either convex or concave voltage gradient. With a convex voltage profile the potential is high at the bottom of the secondary near the primary coil and at the top, whereas the middle of the coil is the valley of the convex curve. With a concave voltage profile the potential is close to ground at the grounded bottom to a high voltage at the middle with is the top of the concave curve and again a lower voltage at the top terminal. If the coupling is too high then the voltage gradient is no longer within these two defined distributions and there can be several places along the secondary coil with a high voltage present, this can cause flash over sparks going up and down the secondary coil, this phenomena is known as racing spark and could ruin the secondary coil if the sparks can punch through the insulation of the varnish and wire.

Poor tuning or high power runs can also cause racing sparks, but more on these issues in the chapter on tuning and testing.

Low coupling coefficient

  • Longer energy transfer time
  • More primary cycles needed
  • Tuning point is less critical
  • High power Tesla coils with low tank impedance

High coupling coefficient

  • Faster energy transfer time
  • High voltage stress
  • Risk of racing sparks
  • Tuning point is more critical
  • Higher secondary peak currents
  • Possible longer sparks
  • Fewer primary cycles needed
  • High power Tesla coils with high tank impedance

The above list gives a overview of some of the fall pits and advantages, it does however also fail to give any finite answer to the coupling. A thumb of rule have been developed by Bart Anderson to start with 0.117 for a 1 inch diameter secondary and as diameter increases, so does the “recommended first try” coupling coefficient k.

There is absolutely no fixed result to the coupling coefficient value and experimentation is encouraged, but the above can give a good starting point.

How to find the right secondary coil size and ratio

As a well designed DRSSTC is easily capable of producing sparks that are 2-3 times the length of the secondary coil, I will advice on using a width:height aspect ratio of 1:4 to 1:6, personally I prefer 1:5.

I will look at five different sizes of coil systems with a rough power input estimate:

  • Micro, less than 0.5 kW
  • Mini, less than 2 kW
  • Medium, less than 10 kW
  • Large, less than 20 kW
  • Very large, more than 20 kW
 Coil dia.Coil lengthRatioCoupling
Micro40 mm  


50 mm

160-200 mm  


200-250 mm

1:4 – 1:50.120  



Mini75 mm300-375 mm1:4 – 1:50.128
Medium110 mm  


160 mm

440-550 mm  


640-800 mm

1:4 – 1:50.130  



Large200 mm  


250 mm

800-1000 mm  


1000-1250 mm

1:5 – 1:60.140  



Very large315 mm  


400 mm

1250-1575 mm  


1600-2000 mm

1:5 – 1:60.154  



Table 2: Tesla coil secondary height width ratio

How to find the right secondary coil wire size and number of turns

The number of turns on the secondary coil is not directly comparable to the primary:secondary ratio known from normal power transformers. It is not important with fewer or more turns, but more to create a secondary circuit between coil and topload that has a impedance that matches the load presented by the sparks that we want to create.

I will use the five different coil system sizes for a table on wire size and turns to achieve a circuit impedance as described above. This table assumes using a toroid that follow the rule of thump found in the topload part of the DRSSTC design guide. Major diameter as secondary coil winding length and minor as secondary coil diameter.

I will start the table at 0.1 mm wire, as it is my own experience that anything thinner than this is practically impossible to wind a coil from, even with 0.1 mm the risk of wire snapping or breaking is very high.

This table represents a optimal range of values for designing a classic low impedance Dual Resonant Solid State Tesla Coil. Secondary coil systems with a higher impedance can achieve the same results by longer on-time with lower peak currents, but however with more losses in the system.

The color coding of the secondary system impedance should be understood as a guide line towards the magic 50k which was explained further up. I made a spreadsheet with all these and more example calculations. The magic 50k is thought to be too high for large or very large coils that will experience serious spark loading, here it properly is more suitable with 40k or even as low as 30k. Experimentation is needed to confirm these assumptions.

 Wire dia.
Secondary_ (turns)Frequency


40 mm

1:4 – 1:5























50 mm

1:4 – 1:5























75 mm

1:4 – 1:5























110 mm

1:4 – 1:5





























160 mm

1:4 – 1:5



























 Wire dia.
Secondary_ (turns)Frequency


200 mm

1:4 – 1:6





























250 mm

1:4 – 1:6



























V. large  


315 mm

1:4 – 1:6



























V. large  


400 mm

1:4 – 1:6



























Table 3: Tesla Coil secondary impedance

How to find the right wire for the secondary coil

Most magnet wire is advertised and listed with its temperature classes as that is a major design impact for relay, motor or transformer winders. Breakdown voltages are less important here as it is standardized with a minimum set of requirements to all different wire diameters. Breakdown voltage depends on the thickness of the insulation, which can be of 3 types: Grade 1, Grade 2 and Grade 3. Higher grades have thicker insulation and thus higher breakdown voltages.

 Wire sizeGrade 1Grade 2Grade 3
0.090 mm500 V900 V1300 V
0.100 mm500 V950 V1400 V
0.127 mm1500 V2800 V4100 V
0.143 mm1600 V3000 V4200 V
0.160 mm1700 V3300 V4700 V
0.200 mm1800 V3500 V5100 V
0.250 mm2100 V3900 V5500 V
0.300 mm2200 V4050 V5950 V
0.350 mm2300 V4300 V6400 V
0.400 mm2300 V4400 V6600 V
0.450 mm2300 V4400 V6800 V
0.500 mm2400 V4600 V7000 V
0.600 mm2600 V4800 V
0.700 mm2700 V4800 V
0.750 mm2700 V4800 V
0.800 mm2700 V4900 V
0.900 mm2700 V4900 V
1.000 mm2700 V5000 V
1.200 mm2700 V5000 V
1.400 mm2700 V5000 V
Table 4: Copper magnet wire specifications

Although most insulated copper wire is described as “enameled”, it is not enameled wire, which would be coated with either a layer of enamel paint or with vitreous enamel made of fused glass powder.

Modern magnet wire typically uses one to four layers of different polymer film insulation, most likely a combination of two different compositions, to ensure mechanical strength and a high electrical insulating layer. Magnet wire insulating films are made from the following, in order of increasing temperature class: polyvinyl formal, polyurethane, polyamide, polyester, polyester-polyimide, polyamide-polyimide and polyimide. These insulation polymer films are rated from 90 up to 250 °C.

Most varnishes used for magnet wire has a Volt per micron insulation rating of about 200 V/µm.

 Wire sizeGrade 1Grade 2Grade 3
0.090 mm0.008-0.017 mm0.016-0.023 mm0.024-0.030 mm
0.100 mm0.008-0.017 mm0.018-0.025 mm0.026-0.032 mm
0.127 mm0.010-0.019 mm0.020-0.029 mm0.030-0.038 mm
0.143 mm0.011-0.020 mm0.021-0.031 mm0.032-0.041 mm
0.160 mm0.012-0.022 mm0.023-0.034 mm0.035-0.045 mm
0.200 mm0.014-0.026 mm0.027-0.039 mm0.040-0.052 mm
0.250 mm0.017-0.032 mm0.033-0.048 mm0.049-0.063 mm
0.300 mm0.019-0.034 mm0.035-0.052 mm0.053-0.069 mm
0.350 mm0.020-0.037 mm0.038-0.056 mm0.057-0.073 mm
0.400 mm0.021-0.039 mm0.040-0.059 mm0.060-0.078 mm
0.450 mm0.022-0.041 mm0.042-0.063 mm0.064-0.083 mm
0.500 mm0.024-0.044 mm0.045-0.066 mm0.067-0.087 mm
0.600 mm0.027-0.049 mm0.050-0.074 mm
0.700 mm0.028-0.052 mm0.053-0.079 mm
0.750 mm0.030-0.055 mm0.056-0.084 mm
0.800 mm0.030-0.055 mm0.056-0.084 mm
0.900 mm0.032-0.059 mm0.060-0.089 mm
1.000 mm0.034-0.062 mm0.063-0.094 mm
1.200 mm0.035-0.066 mm0.067-0.099 mm
1.400 mm0.036-0.068 mm0.069-0.102 mm
Table 5: Copper magnet wire enamel thickness

It is important to use magnet wire that is in a good shape without damage to the varnish or have had kinks, this could results in cracked insulation which means the insulation voltage is down to zero.

Turn to turn voltage can not exceed the voltage rating of magnet wire. But even a relatively short coil with 1000 turns, stressed to 500 kV would only have 500 V between turns.

How to find the right secondary coil form material

Plastic materials all react differently in a high frequency electromagnetic field and thus some plastics are more suitable to use for a secondary form than others. Let us first take a look at the different dissipation factors of the following plastic types.

NameLoss tangent / dissipation factorMaximum temperature
Polyacrylonitrilebutadiene-styrene (ABS), molded0.005 – 0.019 @ 1 MHz– 0 + 105
Polyethylene (LDPE/HDPE)0.0001 – 0.001 @ 1 MHz– 70 + 80
Polycarbonate (PC / Lexan)0.00066 – 0.01– 0 + 135
Polyethylene terephthalate (PET, PETP)0.002 @ 1 kHz – 0 + 120
Polymethylmethacrylate (PMMA / Acrylic)0.014 @ 1 MHz – 70 + 65
Polyvinylchloride soft (PVC-P)0.12 @ 1 MHz– 50 + 105
Polyvinylchloride hard (PVC-U)0.015 @ 1 MHz– 50 + 105
Polyvinylidenfluorid (PVDF)0.17 @ 1 MHz 
Polypropylene (PP)0.0003 – 0.0005 @ 1 MHz– 10 + 90
Polystyrene (PS)0.0002 @ 1 kHz  


0.0001 @ 100 MHz

– 0 + 65
Teflon (PTFE)0.00028 @ 3 GHz– 0 + 250
Table 6: Electrical properties of plastic

High frequency heating: tan δ > 0,1 easily, tan δ > 0,01 possible and tan δ < 0,001 impossible.

A good secondary coil form material needs to have a low high frequency dissipation factor, have a wide temperature range to avoid thermal expansion / contraction ruining the copper winding and be mechanically stiff enough to be handled and support the weight of the topload assembly.

ABS (black), PMMA(Acrylic), PC(Lexan), PP and hard PVC (white/orange) pipes easily fulfill these requirements whereas PTFE is too soft and slippery, despite its superior high frequency properties.

ABS (black) sewer piping can contain conductive components, it is important to check if it has 10, 20 or 30% carbon fiber filling. Higher carbon fiber filling can result in higher losses or higher risk of flashover.

HDPE, PET and PS only comes on soft and flexible pipes and tubes that are impractical to use.

From a cost perspective I have compared prices as of 2016 from different retailers and using hard PVC as 100% starting point the relative costs of other materials are listed for the same size pipe.

3 meters of 160 mm diameter pipe would cost:

  • 100% PVC (white/orange)
  • 166% PP
  • 175% PMMA (acrylic)
  • 285% ABS (black) and PC (lexan).

PVC (white/orange) is by far the most cost effective material and PMMA (Acrylic) is still a good option if you want to pay extra for the looks.

PVC(white/orange) sewer piping usually comes in grey and orange. The grey being a little softer than the more rigid orange. Grey is used for indoor piping and orange is used for outdoor installation in gravel and thus their differences in rigidness.

Typical composition of orange PVC-U piping: 70-90% polyvinylchloride, 3-16% calcium carbonate filler, 3-16% modifiers (chlorinated PE, & 0 ‐ 5% acrylics). 1-6% stabilizer & lubricants and 1-4% titanium dioxide.

Calcium carbonate is used for higher mechanical strength and stiffness of the pipe, also higher electrical resistance. Titanium dioxide is used for UV resistance.

Used PVC(white/orange) pipes found on construction sites have a tendency so have absorbed some water when it has been lying unprotected in all kinds of weather. Avoid the use of old PVC(white/orange) pipes.

As pipe diameter goes up, so does the wall thickness for giving the wider pipe mechanical strength to be dug into the ground, thicker walls does however means more material affected by the electromagnetic field and thus there  are greater RF losses in thicker materials, so choose as thin walled pipes as possible.

How to wind the secondary coil

Before winding the secondary coil it is important to prepare and clean the secondary form. A good and easy way is to sand it down to get a as even as possible surface free of dirt, scratches and possible metal fillings that are stuck in the plastic.

Some people who can not source a PVC pipe or other plastic form have been forced into using card board tubes, here it is important to dry it in a oven at 100 oC for many hours to drive out any moisture that is trapped inside the card board. When it is baked it needs to be varnished to prevent it from absorbing new moisture from the air.

In the illustration below I have three different kind of secondary winding methods.

Method A is the most common, magnet wire is wound on the secondary form and the orange color shows how the varnish is lying on top of the windings, but there is still air underneath and between the windings.

The following pictures show how we used a variable frequency drive to control a small motor with a gear on, to turn the secondary coil. The magnet wire was being kept at a good tension just by the weight of the reel alone, which was 10 kg. The VFD was controlled by a foot switch, ramp up time set to 15 seconds and at maximum speed it would turn 30 turns a minute.

The following pictures show how I used a broom handle and just turned the secondary form by hand while keeping the magnet wire in tension with the other hand. A tiresome work but it will get you a perfectly fine secondary coil without having to build a rig. As shown further down, I was however forced to make a rotating rig to varnish the coil.

Method B is similar to method A, a single layer coil is made with turn against turn, but here the secondary form is varnished and the coil is wound upon the wet and not yet dried varnish in order for it to fill out the gaps and get coil to better stick to the secondary form.

Method C can be done in two different ways, the main concept is that there is much more distance between turns than just the layer of varnish on the magnet wire. It can be done by winding nylon fishing line unto the secondary form along with the magnet wire, side by side, so that the nylon wire can afterwards be removed and left is a space wound coil. It can also be done by carving a small slit into the secondary form, but this will require building a lathe type of rig to do this cutting.

The turns does not need to be aligned and close together 100%, a coil with gaps and uneven winding will still work fine, but overlapping turns is a absolutely guarantee of failure,  the overlapping wire will represent a higher point on the coil and will become subject to corona discharge and possible sparks breaking out from it. Turn to turn voltage would also be higher with overlapping and that will result in insulation break down and a short circuited secondary coil.

While winding the coil you will quickly find that you want to take some breaks, either from arms, hands or fingers going fatigue or you are starting to getting cross-eyed from looking at the wire. Electrical tape is perfect for holding the wire in place and I just leave the pieces on as I go, should I get unlucky and loose the grip of the wire, much less of the coil would unwind itself and get loose.

Space wound coils are especially useful in high power coils where the losses in the secondary wire is governed by the proximity losses of the nearby turns.

Space wound coils are also believed to solve the whiplash problem as the greater distance between turns provide enough additional insulation so that the reflected voltage can not break down the turn to turn insulation.

If you want to seal the ends of the secondary coil with large caps or disks, it is important that you still have a small hole or other way of venting out fumes from varnish or glue that is used to put them in place, if there is flammable fumes or gasses trapped inside a secondary coil that fails, it might fail exploding instead of just smoking!

How to insulate the secondary coil with varnish

Corona glow can happen at spots with imperfections in the varnish on the secondary coil, it could be air bubbles, hairs from the brush or insects. UV light from the sparks can even help to make this become a fatal breakdown of the air and the corona spot will turn into a burning spot that quickly can damage the secondary for good. It is therefore important to put a great deal of work into insulating the secondary coil.

Three commonly used varnishes, resins or what other names it has among people is listed here with their respective break down voltages and loss tangent.

 Break down voltage (rms)Loss tangent 1 kHz
Polyurethane 13 – 25 kV/mm0.01 – 0.06
Polyester (unfilled)20 kV/mm0.006 – 0.04
Epoxy (unfilled)
bisphenol type
16 – 22 kV/mm0.002 – 0.02
Epoxy (unfilled)
cycloaliphatic type
20 – 21 kV/mm0.035 – 0.039
Table 7: Electrical properties of varnish

If you already have a winding rig you also have a varnishing rig. You will get the best and most even surface by having the secondary coil rotate while you apply the varnish and while it dries up.

Spray on varnish from cans leaves a very thin layer and it will literally take hundreds of layers before it reaches a good enough thickness.

Brushed on varnish is easier to control and you can even apply large amounts at a time, as the rotating movement will even it out.

Poured on epoxy is preferred by those that can buy it, with epoxy it is possible to achieve a layer as thick as 40 layers of polyurethane varnish, but it can done in one setting.

Between layers it is a good idea to wait for it to harden and using very fine sand paper, remove any air bubbles that might have been trapped in the varnish. Some people also use a hot air gun to quickly pop the air bubbles without overheating the varnish.

In the following pictures I am using a polyurethane varnish and apply it in plenty amounts, the brush is used to even out the varnish, burst air bubbles and almost shovel more varnish on. A large amount can be applied at one time when using a rotating rig to help it even out and not run or drip while drying

In the following pictures I also used a polyurethane varnish and that was more in the league of smearing it on, I applied as much as possible and when the rotation could no longer keep it from dripping I stopped. To keep things as simple as possible, I used my accu drill to turn the secondary coil around.

Termination of the wire ends of secondary coils

There are a few, but important, rules to keep in mind when terminating the ends of the wire on the secondary coil. It is less important with small systems, here you can get away with breaking the rules, but in high power systems it will result in damaged secondary coils.

  • Never drill holes in the side wall of the secondary coil form, even the slightest gap is enough for high voltage to pass through.
  • Never have any conductor inside of the secondary coil form, the risk of internal sparks are just greater.
  • Never use metal to pass through a hole in the side wall of the secondary coil form.

The following examples are all from secondary coils that I have made myself and I have not yet experienced a secondary flash over, despite that some of the coils does not obey the above rules, but this is due to them being relatively “low” voltage systems. Some of these coils would not work in a DRSSTC, but I am showing all these examples for inspiration on terminations.

The first example is from my second DRSSTC, here I have the wire going through a hole inside the top of the secondary coil form and at the bottom there is a brass screw through for the earth connection. I broke all three rules but got away with it because it is a small system.

The second example is from the first Tesla coil I built, a dual 811A VTTC. At the top there is again a hole in the side and the wire goes through and at the bottom there is a solid brass connector glued to the side wall. I broke two rules but still got away with it because a VTTC has a much lower RMS current and voltage than a high power DRSSTC have.

The third example is from my second SSTC, here I have the wire going through a hole inside the top of the secondary coil form and at the bottom there is solid brass connector. I broke two rules but have had no problems despite this coil have pushed 2 kW through and in peaks up to 4 kW.

The fourth example is of my third  SSTC and is a design with broken rules, but measures are taken to make up for it. The wire at the top is inside of the secondary form through a hole, but the top have been filled out with epoxy casting. The bottom has a hole through, despite I used a nylon screw there is still a hole making it easier for flash over to find a way. This is however not a big problem with a low power SSTC.

The fifth example is of my first DRSSTC and here I did a proper job on the terminations to ensure the best possible insulation, mechanical strength and for it to be practical. The top wire is led through a slit in the secondary form to the top across a end cap of acrylic plastic. It is soldered to a rounded brass connector that has a banana male connector that fits into a female connector on the topload. The wire at the bottom is soldered to a piece of copper strip with brass connector soldered on to. This is a optimal design with end cap, no broken rules, rounded surfaces and good mechanical strength and considerations to make it easier to assemble and disassemble.

The sixth example is of my third DRSSTC which is a large and high power coil. The top wire is guided to the top in a deep and very rounded curve slit in the secondary coil form and end cap material, it ends up screwed into the large aluminium topload holder. At the bottom there is a copper strip with a large brass connector soldered to it. Only thing missing here is a closed end cap at the top, but we did not have any material large enough to close the hole.

Failure modes of secondary coils

This example is from Michael, a user on 4hv called crashstudio, and he describes the what lead up to this failure, but the main culprit here could just aswell be a damaged secondary form or a spark that stroke through the secondary varnish layer, which seems to be thin.

There were 2 factors that contributed to the failure.
1- I ran the secondary winding leads through the inside of the coil form, so the connections to toroid and ground were both inside.
2- I literally forgot to complete the insulation of the 2 connections.
So with the added power of the industrial capacitors, the spark length was far greater than the 23 inches it needed to arc inside the coil form.

Here is some other secondary coil failures where the high voltage have punched through the secondary coil form or arced on the inside.

A faint flash-over on the inside.

A huge and brutal flash-over on the inside.

A huge and brutal flash-over from a internal ground connection has left carbon tracks all over the inside of the secondary form.

All of the above, small as large carbon tracks through the secondary coil form is just as bad, it is ruined and needs extensive repairs or just has to be discarded.

Life cycle of secondary coils

A secondary coil feels like a living organism as the plastic form changes shape and stiffness by temperature and so does the varnished windings. When lifting a heavy secondary coil it can be felt that the wire is not sticking to the secondary form or you can hear it expand and contract if moved between places of different temperature.

Transportation is a major culprit in secondary failures as here you get bumps and scratches in the varnish and these weak points could be the start of the end.

Some people have hundreds of run hours on their secondaries, but eventually they will fail and mostly around the bottom or near the ground connection.



[1] Coil calc with Medhurst & G3YNH, 04.11.2017,

[2] “An introduction to the art of Solenoid Inductance and Impedance Calculation”, “Part II Solenoid Impedance and Q”, David W Knight, Jan 4th 2016.

[3] “H. F. Resistance and Self-Capacitance of Single-Layer Solenoids”, R G Medhurst (GEC Research Labs.). Wireless Engineer, Feb. 1947, p35-43; Mar. 1947, p80-92. + corresp.; June 1947, p185; Sept. 1947, p281.

[4] IEC 60317-0-1:2013, “Specifications for particular types of winding wires – Part 0-1: General requirements – Enamelled round copper wire”

[5] Global Polymer Industries Inc. Technical information: and document:

[6] Electrical Engineer’s Reference Book, Sep 27, 2002, by M. A. Laughton and D.F. Warne. Table 7.13.

40 thoughts on “Secondary coil design and construction for Tesla coils”

  1. Yes I’ve already read ^^
    I just want to know if this resin especially Could work ? ( resin of inclusion gts polyester)

  2. Hi Max

    It will properly work just fine, but bear in mind that it is only rated for temperatures up to 55 °C. So be sure to check for secondary coil heating and inspect the coil for damages if you have heavy arcing to/from it.

    Kind regards

  3. Hello!
    Could you tell your opinion about a medium size secondary(110mm dia.,500mm length)with 2000-2500 turns of 32awg(0.2mm)wire?Would it give better resuslts with less turns and larger dia. wire?

  4. Hi Laci

    It depends on which kind of Tesla coil topology you using it for, is it a regular SGTC or a DRSSTC?

    You would end up around 100 kHz or so, with a topload the same dimensions as the winding. As so many turns gives a higher impedance coil, it would make sense if it was a DRSSTC with a small primary capacitance.

    Kind regards

  5. Thanks for the reply!
    I’m experimenting with a simple IR2110,IRFP460 halfbridge(upgrading to fullbridge)without gate drive transformer,since I can’t make that without an oscilloscope, and a 0.0027 MKP capacitor in series with the primary.The secondary is made out of two tesla coils in series,but both have a big dia.-length ratio.Do you recommend to make a coil that I wrote in the previuos message,or upgrade the driver circuit?

  6. Hi Laci

    You should see if you can find a old and cheap oscilloscope, that is a vital part of making and troubleshooting. Maybe you can even do with those real cheap DSO138 on ebay (max 100 kHz , so need to make a bigger coil to use that)

    The secondary you described in the previous post, with a ratio around 1:5, is rather large for a SSTC, something in the range of 1:2 to 1:3 is more suitable to the spark length that a SSTC can produce.

    Your bridge sounds fine with IRFP460 MOSFETs, I would however advise you to use a GDT, in case of a failure this will protect your driver circuit. Failures are not that uncommon in the high voltage hobby 🙂

    If you want to, sign up at and come show off your work 🙂

    Kind regards

  7. Thank you very much for the help!
    I already killed a lots of mosfets,the driver circuit however always survived.
    The new components will come in the next week,so I can try a full bridge with audio modulation.Also making the described coil.Hopefully I will get a scope,so I can upgrade everything…
    I will sine up to the forum,and maybe upload my noobie design. 🙂

    Best regards,

  8. Hello Mads,
    We have now finished building our coil and tested it as SGTC. That worked quite well. The sparks could still be a bit bigger (we had about 1.5m at a 3m high coil), but that may come about when we run the coil as DRSSTC. However, we had a problem. We have increased the voltage to about 20 kV, while we had a primary side of a current of 1.6 kA. Then there was damage in the lower third of the secondary coil. We are not sure if the primary to secondary coil flashover occurred, or if the damage was due to an increased electric field due to trapped air in the secondary coil’s epoxy resin. As a solution to this, we want to use a hot air blower to remove the air bubbles from the epoxy resin. On the other hand, we want to attach a acrylic glas about 2-3 cm above the primary coil, which should protect against flashovers between secondary and primary coil. Do you think these measures are useful? I wrote a similar text yesterday, but apparently there was something wrong with the post.

    Kind regards,

  9. Here is a picture of our damaged secondary coil.
    Meanwhile, we have replaced the broken turns and coated again with epoxy resin.

  10. Hi Michael

    A shield between primary and secondary will only help a little, the true key is adjusting coupling correct to avoid flash-overs. Also a very thick layer of varnish/epoxy on the secondary will protect it the best.

    I hope you got all the charred copper and tubing removed, as if any dark spots are left behind, the carbon content will make it easier for another failure to happen at this place once again.

    Your coil sounds very interesting, I would love to see a thread about it in all detail at 🙂

    Kind regards

  11. Here’s a crazy question:
    If you place a thick insulator around the secondary coil (i.e. PVC pipe), and then surround the insulator with a metal pipe not quite as tall as the secondary (not grounded, or close enough to arc to the top of the sec), can you amplify the secondary voltage even more? Of course this will act as capacitance…
    (no I’m not going to try this first with a big TC, but maybe a small 6″ secondary)

  12. Hi 200V

    A metal pipe around the secondary coil will act as 1 shorted turn in regard to both the secondary and primary coils and would likely result in melting stuff 🙂

    In order to push the secondary even further, you could do the same trick with another pipe outside of the secondary coil, pot it totally with epoxy and be sure its a space wound coil. Now you can push much higher peak voltages through it without flash-overs. But this is expensive, heavy and might just fail if there is even a slight amount of air somewhere in the construction or dissipation losses in the thicker form materials could be a heat problem.

    You could instead experiment with highly insulated ferrite rods to get a bigger coupling / energy transfer, but this is still a tough area to improve anything in, as the relative low coupling in a DRSSTC is enough to transfer the energy in the time window available.

    Kind regards

  13. Nguyen Thanh Chinh

    Hi Max,
    Thank you for your hard work on this topic. Please review the following information to determine whether or not to upgrade this article.
    “The length of the…coil in each transformer should be approximately one quarter of the wave length of the electric disturbance in the circuit, this estimate being based on the velocity of propagation of the disturbaiice through the coil itself…” US Patent No. 645576, Nikola Tesla, System of transmission of electrical energy, filed September 2, 1897; granted March 20, 1900
    History of the Tesla coil wikipedia (25th references)
    Kind regards

  14. Hi Nguyen Thanh Chinh

    Many older coilers and experimenters have not been able to prove that quarter-wave specified secondary coils perform any better than going for the impedance matched designs, so that is why I do not detail out these old Tesla patents.

    Kind regards

  15. Nguyen Thanh Chinh

    I like your sound. I agree that, matching impedance is an importance reason will cause the highest power output.
    Over 3 month ago I has been saw on my first little SRSSTC 2.5’x 15′. The longest arc appears at the half of the height of the coil. Almost nothing electrical voltage on the top of the coil.
    My scope shows it run at 1 MHz, i like to make it work at 500 KHz but I can’t do that, because at that time I have not any experience and not enough knowledge about Tesla coil. So i throw away it by side.
    By now, my first DRSSTC just discharge 20’ spark length and I am going to get the as best as posible it’s performance . . .
    I would like to discuss with you some matter about the way to get the optimal of Tesla coil performance by email, may I ?
    Kind regards
    Chính Nguyễn.

  16. Nguyen Thanh Chinh

    Please allow me to summarize your subject with the following post:
    ●Zsecondary ; Tesla coil’s internal output impedance = (Secondary impedance)
    ●Zspark : spark channel impedance
    ●Zground : ground impedance Zground
    Z total : Zsecondary + Zspark + Zground
    V : Discharge voltage
    Pd = V* V/Z total : Discharge power (spark length)

    Pd Is highest when Zsecondary = Zspark + Zground (impedance match)
    This problem base on the discharge voltage is constant.
    We need one more problem to find the way to get highest discharge voltage bae on a hard ware given first.
    It depends on square role so it is also importance?
    I think that.
    Kind regards
    Chính Nguyễn.

  17. Nguyen Thanh Chinh

    Every body! please give me some comment about this matter:
    In the topic: DRSSTC-1 update 12/15/05
    Steve ward wrote “I tested these IGBTs to over 1000A at 95khz so this should be no problem at all.”
    I’m confused and do not believe that component can withstand that level of current.( He said about H-bridge of 40N60 mini-brick IGBTs).
    Thank so much.

  18. Hi Nguyen Thanh Chinh

    It is true that the old versions of the 40N60 mini-bricks were over-engineered and could withstand much higher peak currents than the newer “upgrade” 60N60 that superseded it.

    So the 40N60 could take 1000 A peak, but I would not push the 60N60 over 600 A peak.

    Kind regards

  19. Andrew Villalpando

    Hello Mads, I’m about to build the SSTC II of Loneoceans, but one of my concerns about the secondary coil is the winding direction, clockwise or counter clockwise. I read differents opinions about the topic, some says that it doesn’t matter, other says that the phasing could be incorrect. And what about the primary coil, does it have to match with the direction of the secondary?. I come to you because I know you have a lot of experience in the field and you can clarify the things for me.

    Thanks in advance

  20. Hi Andrew

    It is a good question as phasing can be confusing when starting with Tesla coils, I had the same doubts in the start 🙂

    The winding direction of both primary and secondary does not matter in the making proces. It only matter in the operation and here you can switch the phasing of the primary coil if its wrong. Its that easy, no worries and if it only gives a very little sparkoutput, switch the primary connections around.

    Kind regards

  21. Andrew Villalpando

    Hi Mads, thank you for the response, that’s what I thought but I was uncertain. So, now it is clarified 🙂

    Merry Christmas and Happy New year.

  22. Antony McDonagh

    Hi Mads,

    Thanks for the article, it’s hard to find this info elsewhere. I did notice a couple things with the formula for the 𝛯 factor though.

    It’s given as
    𝛯 = (π * R^2) / π ( R^2 * (R-ρ)^2)

    Which I believe should actually be
    𝛯 = (π * R^2) / π ( R^2 – (R-𝛿)^2)

    Since the area of the ring is the difference between the area of the inner and outer circles, and 𝛿 is skin depth, not ρ. Also I think it would be nicer to use little r for radius since R is already used for resistance.

    But again many thanks for the info, I’ll let you know how my build goes!

  23. Hi Antony McDonagh

    Thank you for the correction, you are right that I used a wrong symbol (*cough* copy/paste) and I also correct R to r for better distinguishing.

    Kind regards

  24. Antony McDonagh

    Yooo Mads,

    Thanks for the speedy response, however what I was trying to point out was the product in the denominator, which I believe should actually be a subtraction. Since the area represented by the skin depth is the area of the whole wire minus the area of the centre part without the skin depth.

    Again, props for keeping this thread active by the way, it’s super helpful.


  25. Antony McDonagh

    Hi again Mads,

    As I mentioned I’ve been having a little trouble getting my calculations correct for my secondary coil impedance. I’m not sure as to what range the DC resistance should come out at, mine is in the order of 10s of Ohms. With the 𝛯 and Φ factors in the order of 1s, the AC resistance also comes out in the order of 10s of Ohms (about 1000 times smaller than the 50kOhms it should be).

    If you’re interested the build I’m planning is a spark gap tesla coil, and I was originally following the plans from this vintage article
    (around page 39).

    This is obviously a little dated, and I’m planning on using a 0-50V DC power supply hooked up to a flyback driver which steps up the voltage by around 1000, rather than the ford ignition coil. Also I’m planning on using insulated wire for my priary winding as described in this article rather than large copper tubing, since I’m only aiming for around 2-5inch sparking.

    Let me know what you think.


  26. Hi Tony

    I always use JavaTC on as a sanity check, try to run the same numbers on that calculator and see what you get.

    Kind regards

  27. Some people say that using black plastic is not a good idea… because it can contain carbon as a filler or colorant which is conductive.
    I once noticed that hockey pucks conduct surprisingly well when tested with ignition coil, 10 to 50 kV.

  28. great guide! But I think this needs more insight:

    Zsecondary = sqrt(Lsecondary/Ctopload)

    this is confusing to many who haven’t worked with resonators because in a steady state the reactance of a series resonance circuit is zero and of a parallel resonance circuit is infinite.

    (And because the wavelengths at typical teslacoil frequencies are still large compared to the coil we aren’t dealing with transmission line stuff either)

    So it would be cool to have some insight on 1) how this impedance is derived and 2) how it affects the current/voltage ratio and the rising rate of current and voltage. (in both primary, and secondary, maybe).

  29. Hi bobodemon

    It is a extremely difficult topic and not something that can be done in a few sentences. The purpose of this guide is to stay very practical with hints on the theory used. Instead I would refer to read something like this thread: where the current/voltage ratio and the rising rate of current and voltage is discussed from actual measurements.

    Kind regards

  30. Above your skin depth factor calculation you mention “r is diameter.” I suspect this is meant to say radius.

  31. Hi Mads – thanks for a great article. I have built a few old-school spark-gap TC and TM in the 2kW range, magnetic coupled and electric coupled (with YT videos) and I am interested in eliminating the problems of the air-cored EHV transformer (getting high k while maintaining good HV insulation). I am looking for information about studies of ‘best k’ for getting longest streamers or most powerful arcs. I saw a formula- 1/k = sqrt(Qp*Qs) and I note that Qp and Qs can both be of order ten when a power arc is formed. That would put k quite low at 0.1 ?? I don’t have any further information about it – can you recommend any authors or papers about that topic? Regards

  32. Hi Ian

    JAVATC is properly one of the best resources with it being a simulator with good documentation. Perhaps also Richie Burnetts site.

    Kind regards

  33. Have you thought about making a vacuüm pipe of a larger diameter to get all of the air out this wil make sure there are no more bubbels or gaps inbetween and under the wires … i think they do this on HV transformers and large electromotor s … this amateur just started his first ( big for me ) SSTC build the seize like your SSTC III but as i read trough your website … i have a lot to learn … thanks for al the sharing , there is a lot of good advice here ..

    Grtz and spark on

  34. Love your article, but no mention of direction of windings ? Haven’t found much on the web either, would like you opinion on this if possible. Thank you, Kirk

  35. Hi Kirk

    Direction does not matter. You can reverse the polarity of your primary coil or feedback current transformers to get it all to match up.

    Kind regards

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