Micro Tesla Coil makes a Perfect Stocking Stuffer

Tesla coils are always a hit around here at the office. Giant ones that play music with modern-day chain mail wearing DJ’s, ones thrown together in garages by self-proclaimed mad scientists… But have you ever seen one that can fit in the palm of your hand?

[Ludic Science] just released this tutorial video on how to make it. It’s a miniature diagram of slayer circuitsolid state Tesla coil that’s based on the ever popular Slayer Exciter circuit that was first developed by [GBluer]. The beauty is it’s a very simple circuit to build. It consists of one power transistor, a few diodes, some resistors, and the coil. That’s it!

He even repurposes the magnet wire from a small relay, it’s literally a project you can build from scrap parts around the shop. Awesome.

One of our favorite Tesla coil builds has gotta be [JJ Dasher’s] ridiculous Halloween setup. He goes all out. And if you do ever want to build one on a macro scale, you might need this auto-coil-winder.

[Thanks Andrew!]

39 thoughts on “Micro Tesla Coil makes a Perfect Stocking Stuffer

  1. It’s on things like this that my analog knowledge is lacking. Questions:

    1) Wouldn’t this still benefit from a little metallic sphere at the top, to act as a capacitor for the secondary?
    2) Is it possible that a salvaged ferrite of similar size, but otherwise unknown specs, may saturate? If this occurs does the transistor burn, or does this oscillator deal gracefully with this situation?
    3) How would one *safely* get an accurate measurement of the operating frequency with a frequency counter? (I’ve tried non-contact measurements on similar circuits, and it seems this sometimes results in reading a multiple of the true frequency.)

    1. By having a top load would help to lower the resonant frequency of the coil. Given the size of the coil, the resonant frequency is very high. Won’t be too surprise if it is in the MHz (or tens of MHz?).

      Tesla purists do not use magnetic core. It should be air cored or it should not be called a Telsa coil With a core, it is a flyback circuit.

      My 10x scope probe with nothing connected to it can pick up a very strong ac voltage at a feet from my (SSTC) coil. I would assume that you could do something similar. That waveform should be a sine wave at the fundamental frequency. Too strong a signal might saturate the input stage and may cause false triggers in your frequency counter?

      A different way to get a rough estimate is to excite the coil from a function generator to find the resonant point. Any changes to the circuits (include probes etc) would have parasitic capacitance thus alter the resonant frequency.

      I am tempted to get my first coil – a very tiny one that to work. Most of the designs I have seen relies on spark gaps for that size.

    2. Regarding question #2, the short answer is “no”.

      The magnetic circuit in the resonator goes through the ferrite, out one side, through the air to the other side, and back into the ferrite. This makes a “series circuit” for the magnetic field.

      The relative permeability of ferrite (relative to vacuum) is on the order of a few thousand, if you get one from a power circuit(*). Let’s take 1000 as a worst case scenario.

      Reluctance is inversely proportional to, so in our magnetic circuit the ferrite has a reluctance of 1/1000 and air has a reluctance of 1/1. This is a magnetic circuit with a magnetic “resistance” of 1/1000 in series with a “resistance” of 1.

      In effect the reluctance of the air is so large that it overshadows the reluctance of the ferrite, and as a first approximation we can consider the total reluctance to be that of air.

      Now “ohms law” for resistance is F = Phi*R, where “R” is reluctance, Phi is the magnetic field (in Webers), and F is the magneto-motive force in ampere-turns.

      The previous reluctances were relative (to vacuum). The absolute reluctance of vacuum is 4πe-7, so 1/(4π10-7) = 1.2e6 Ampere-Turns/Weber.

      Now Phi in the “Ohms Law” formula is magnetic field in Webers, we need to convert from field strength in Tesla. 1T = 1Wb/M, so we also need the area of the ferrite core. I’ve just now measured one (from a PC power supply) and it looks to be 8mm in diameter. The cross sectional area is therefore πR^2 = 3.14*(.004)^2 = 50.24e-6 square meters.

      Using the link below, we see that the saturation level for ferrite is about 360mT. Plugging that in, the saturation field strength of my measured ferrite is
      0.360 = X/(50.24e-6), X = 18e-6 Webers. It takes this many Webers in such a small area to saturate the core.

      The circuit above uses 3 turns on the primary.

      So, using our “Ohms Law” of magnetic field we get:

      Amps*(3 turns) = 18e-6*Webers*1.2e6 Ampere-Turns/Weber

      Amps = 7.2

      So it would take about 7 amps of current through 3 turns in an air-gapped magnetic circuit to reach the saturation field strength of the ferrite. This level of current can’t be supplied by a 9-volt battery.

      Chances are good that for any ferrite you use, if taken from a power supply, you won’t get close to saturation with a 9-volt battery.

      (And if you use a bigger battery, you’ll quickly burn out your wires.)

      (*) Using this link as reference: http://info.ee.surrey.ac.uk/Workshop/advice/coils/mu/index.html#mur

      (**) Using saturation field strength for ferrite from this link: http://info.ee.surrey.ac.uk/Workshop/advice/coils/terms.html#fdens

      N.B. – I’ve been trying to study magnetics for the past few months. Let me know if I’ve missed a step or made an error in this calculation!

      1. Some typos in the post above. I *could not* figure out how to edit the typos after pressing “post” but before Gravater. Going back to edit posts is apparently not possible on this site.

        1) Reluctance is inversely proportional to (missing word) permeability.

        2) “Ohms law for resistance” should read “Ohms law for magnetics”.

        3) Permeability (not reluctance) of vacuum is 4πe-7, inverse of that is reluctance.

        The misspellings don’t affect the analysis.

        1. I appreciate the time and effort, but you forgot that this is the hackaday comments section! Any errors in grammar or spelling completely nullify everything you say.

          No but really, I appreciate that you spent the time to actually inform people instead of the usual hackaday comments on spelling or the presence (or lack thereof) of an arduino.

  2. is it just me? Or did the video seem a little . . . off?

    the large florescent bulb was a lot brighter at the right hand side of the screen, like there was another coil lighting it up, but then we look at the far left side, and then back to left and the bulb as a whole is dimmer than before. I think he has another coil helping him because his original design didn’t hold up under filming conditions, does anyone else see that? or am I just nitpicking?

        1. Remember, the amount of light you see is affected by the auto-iris of the camera so you can’t judge anything by the video. Also as a tesla coil builder I can verify a fluorescent tube will light brightest near the coil and near the hand that holds the tube.

        2. no problem at all. Totally legit. I work with Neon and the low pressure in the tubes is what helps here. Its brightest near his hand because thats the earth return. The normal earth return is through the water molecules in the air. To the electricity we’re just giant bags of water. The high frequency of the supply enables the skin effect and so all the electricity travels over the surface of objects and wires.
          So this is one reason why “Neon” is so efficient – once you use hi frequency supplies.

    1. I’ve enjoyed several other videos from him on Google+ over the last few months. They’re very low production quality, but excellent little builds from stuff you’re likely to have around the shop. Given how limited his budget clearly is, I’d be very surprised if it were fake.

      https://plus.google.com/104836136441664902744/posts

      Speaking from experience, I can tell you it’s incredibly irritating to put some hard work into something and post it online only to have the peanut gallery [incorrectly] accusing you of fraud. Please be careful with your criticisms.

      1. I second that, he is clearly using very budget equipment and I would be surprised if he bothered to fake anything. He is also responsible for the wind-up chicken powered pendulum that was featured here a while ago. I like his videos, interesting accent too. It would be good to see him get something out of it and maybe get some better film gear.

  3. Let me remind you that this is not a “Tesla” coil. It is simply an RF power oscillator (a small radio transmitter.) and if it generates and radiates more than a significant amount of unshielded RF energy it may exceed the radiation limits of the FCC Part 15 rules and regulations as well as FCC Part 97 which applies to Amateur Radio (Ham) regulations. If it can light up a CFL it will also generate radio and television interference (RFI and TVI). Tesla did not invent Tesla coils. They are simple auto-transformers, known from the days of Faraday. In the early days, when Edison only distributed DC current, a chopping circuit was needed to chop the DC at a high frequency in order to permit the auto-transformer to step-up the voltage. Very high voltages could be obtained by using very high ratios of coill turns and being sure that the insulation of the coil would not break down. Although some people experiment with full size high voltage coils, they are illegal because of the interference that they create, unless they are used in shielded rooms for horror movies. –W2ILP–

  4. i`m a tech of 40 some years and andf i`m a tesla follower in a matter of speaking… i`ve bulit 30 or more tesla coils and they have varied size and power , but i`m first 2 years i built a well known two transistor , two ceramic resistor to small fly back ,i took this to a neon shop and with a 9 volt battery i set on the bench and by complete chance 17 ft of neon lights lit up with one lead , wow boy how the hell are doing that i `m in aw my self… . his name was bill mcneon in el cajon ca his son pick my little tesla driver and started making 12volt neon systems for autos !!! true story

  5. This is a really cool circuit!

    Here is a circuit simulation model:
    https://www.systemvision.com/design/slayer-circuit-tesla-coil
    Note that the capacitance in the circuit (c_load) models the effect of placing objects near the Tesla coil (such as fluorescent tubes) — It’s not part of the circuit that you build.

    Also notice the way that the transformer is modeled in SystemVision. It’s constructed using magnetic building blocks — windings and a core. This maps more directly to the structure of the magnetic circuit and allows you to better understand how the circuit works.

  6. i am doing this experiment as shown as in figure, but i use KN 2222 transistor instead of 2N 2222 transistor, and 9 Volt battery. still it’s output is very less amount than input,, but it work,,,,so what is defect in my tesla coil……????

  7. Hey i made tesla coil refring with above circuit diagram when i touch the secondry coil then LED on bread borad is turn off and when i dont touch secondry coil then LED on bread bord will glow but Bulb is not glowing when Bulb is closes to secondry coil and LED on bread bord will sill on……. plase helpe me

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