Tesla coils
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STOP PRESS  This is my old site last updated June 2005.  Enjoy the pics here but it is best to shift direct to the new site. Looks the same but lots more stuff and regularly updated.  The full size pictures  are only available there.

CLICK HERE to go to tesladownunder.com index page

Miscellaneous topics on this page include:


Australian safety requirements













Australian safety requirements: The Dept of Energy Safety inspected my 'shed' on Nov 21, 2003.  My equipment runs from 50 V AC to make it pass the Western Australian requirements.  Higher voltages require a qualified electrician unless they are 'downstream' of the 50 V AC. This is provided by an arc welder stepping down the 240 V AC mains to 50 V AC (120 A peak).  The 50 V AC is then stepped up (by another arc welder wired in reverse) to 240 V AC again.  Actually 225 V with losses.   Various projects then raise this voltage a lot further.   Assume that all home made mains equipment described runs from this arrangement.  Note that this is no safer, just legal or rather not covered well by existing laws.

(click to enlarge)

My two arc welders showing 225 V output. The internal view of the smaller (110 A) welder showing the adjustable shunt giving variable current limiting.

Measurements: I have been a bit limited with the technical measurements but hope to add these as I obtain them. Of particular interest is the peak current to the coil gun and confirmation of the calculated tuning details for the TC.  Any one care to donate a digital storage CRO? I have kept most measurements in inches and feet etc rather than our metric units for the benefit of US coilers. And yes we do spell aluminium like that in Australia. 

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  Taken with a HP 812  4 mega pixel digital camera with tripod for estimated 10 sec exposures from what my wife regards as a safe distance. (There is not inconsiderable risk to electronic devices and some of the close up photos were taken with the camera wrapped in aluminium foil). MPEG video on website also taken with same camera. It can even take a picture of itself! (in a mirror on auto timer in a green room).  I have a custom made tinplate box which can cover this camera or my Nikon 501 or Sony digital video camera which allows them to be used more safely with a tripod closed to the action.

I feel that many Tesla coil shots on the web are blurred, out of focus or are just don't convey the size of the sparks/streamers.

I have a list of criteria that I use for digital cameras (I did win an award for my cover shot on my site - well a small one anyway from ACD Systems Nov 2003 photo contest)


Always use a tripod (or stack of bricks or the previous model TC). For medium exposures of seconds a hand held camera will blur a lot. The sparks will be sharp but the rest will be blurred.


Make sure you get the end points of all the sparks to either side. Chopped off sparks don't really give the impression of size. That is unless you do a Richard Hull and have sparks ONTO the camera.


Try to get the sparks at a right angle to the camera by using a breakout point or grounded object.  Only by doing this will you see the full spark length in comparison with a known fixed measure (which is usually the secondary winding height).


Have a person in the background for scale (at a safe distance).  Even Tesla himself got this right in his photos (admittedly with double exposures).


Adjust the background lighting to be able to see the surroundings for scale. It is best to at least partially cover the spark gap as this tends to wash everything out.  It should have a shrapnel and UV guard on anyway shouldn't it?


Fix the focus on the TC toroid with lights on, hold the focus, then lights off, sparks on and shoot. This is needed as you can't autofocus in the dark. You may need to grow extra hands for this.


Let the camera do an auto exposure and it will finish when it has enough light (may be 10 seconds on my camera).


Don't put a large number of shots on your site. You choose the best one(s) to save others the trouble of downloading many.


I use thumbnails linking to a largish size of compressed photo of 200 - 400k but I am very choosy with the photos and lots get discarded.  I keep the resolution full (4 megapixel before cropping) but reduce the colour spectrum from 24 bit to 16 bit to save space. I can't tell the difference.


I use some RF protection if my camera close up to the coil with a metal can made for the purpose as above.


Avoid windy times (unless you want to show the banjo effect) as it makes the streamers look blurred.


(I might add if in Australia, don't hold your baby up in front of the TC for publicity shots to emulate Steve Irwin).


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Glossary:  (If you can't find it below, try here)

ARSG Asynchronous rotary spark gap.  This switches the capacitors charge into the coil rapidly and frequently but not in synchrony with the mains 50Hz.  In my case the rotary disc is driven by an angle grinder motor.  See SRSG.

Ballast  is a method of limiting otherwise excessive current draw from the mains particularly when using large transformers 'pole pigs'. Typically inductors (large coils) are used but in other situations, resistors or capacitors can be used.

Capacitor stores electrical charge and allows alternating current to pass.  In its simplest form it has 2 conductive plates separated by an insulator. The main capacitor in a Tesla coil is the tank capacitor.  This can be homemade (in my case with aluminium foil and polyethylene sheet) or by multiple mini capacitors (MMC). Other capacitors are used to filter voltage spikes or as power factor correction.  The value is typically measured in microfarads (uF), nanofarads (nF) or picofarads (pF).  The other main capacitor is the toroid although it might not look like one.

MMC  multiple mini capacitors.  In my case using 30 (or 60) polypropylene capacitors rated at 0.33 uF at 1600 VDC to obtain a total value of 0.044 uF (or 0.088 uF) at 24 kVDC.  With this particular series of Cornel  Dubillier capacitors it has been found that this DC rating can reasonably be used as the AC rating with Tesla coil use.

MOT Microwave oven transformer typically rated at 250 VAC in and 2000 VAC out at perhaps up to 1000W. Four of these are used in my supply.

MOV (metal oxide varistors) used to protect circuits from transients, such as across the mains to protect spikes of voltage above mains voltage.

NST  Neon sign transformer.  Typical rating 250 VAC in and 12,000 VAC out at 30mA.

Power factor correction capacitors are capacitors across the mains which improve the phase angle of currrent versus voltage and will reduce the apparent current draw of a transformer.  Typically 30 uF for an NST.

RCD's (residual current devices) are a safety device on the mains that switch off power if more than typically 30mA is leaked to earth to prevent electric shock.

RQ (Richard Quick) static gap is a stationary spark gap for smaller Tesla coils using a number of short lengths of copper pipe nearly touching so that there are multiple small sparks in series.

SCR (silicon controlled rectifier) Is an electronic switching device in some respects like a large transistor. They can switch large amounts of power .

SRSG Synchronous rotary spark gap.  This switches the capacitors charge into the coil rapidly and frequently in synchrony with the mains 50Hz.  In my 18 inch coil this will be from a modified motor running at 3000 RPM.  See ARSG.

Sparks What Tesla coils are all about.  Lightning like high voltage discharge through air to another object.

Streamers Sparks that leave the toroid and branch off in to the air without reaching another object.

TC  Really, you should work this one out for yourself.

Variac A variable 'autotransformer' used in the mains circuit used to vary the mains voltage (110 in USA, 250 in Aust) from 0 to full mains voltage up to about 110% i.e. 120 V / 270 V.

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A selection of formulae that I have had to use:

Ohms Law                     v (Volts)   = i (current in Amps) * r (resistance in Ohms)

Power                            P (Watts) = v (Volts) * i (current in Amps)

Power                            P (Watts) = 1/2 C (capacitance in Farads) * v (volts)

Impedance                    Z (Ohms) = 2 * pi * F (freq in Hertz) * L (inductance in Henries)

Resonant frequency    
               F (freq in Hertz) = 1 / {2 * pi * sqrt [ L (Henries) * C (capacitance in Farads)]}

Secondary Voltage       Vsec = Vprimary * sqrt [ L sec / L primary]


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Three people are known to have died using Tesla coils.
My safety tips include:
          Be afraid, be very afraid. 
          Have the main switch distant from your setup.  I plug and unplug the extension cord for preference. A distant momentary push-for-on switch is preferred so you can't get to the HV while it is on. Alternatively a 'lock out, tag out" system if there are other operators.  It should be child resistant such as having 2 momentary switches.
          Have a bright "on" light close to any HV wires to visually indicate that they are alive.  This is especially important in the breadboarding stage when you don't have the noise and sparks of the TC to tell you it is on.
          Work with one hand in your pocket if possible. 
          Use a shorting stick to discharge your capacitors and use draining resistors.
          Have an earth wire dangling in front of you if you have to work close to a running TC (as I do).
          Have it 100% safe for observers especially children with a physical barrier preferred.
          Use an RCD (residual current device) but this will only protect you from the mains voltage not the far more dangerous primary voltage of the TC.  I also use a second RCD after the variac but this only trips if the variac voltage is set over 160 V.
          Have a fire extinguisher suitable for electrical fires available.
          Don't drink and drive (the TC).  Drink your Corona beer to get the bottles for the salt water capacitors before you make the TC.
          Make sure your rotary gap is enclosed in a near bullet proof casing. The kinetic energy of a 10,000 rpm disc shattering with electrodes is enough to penetrate mild steel sheet of the same gauge as my shed is made of.
          Use ear protection.
          Consider eye protection for the UV particularly from an unshielded spark gap.
          Ozone protection with adequate ventilation.
          Public displays require another level of safety and (in Australia) should include a Faraday cage and various safety interlocks.

More comprehensive and interesting safety information is here 

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Humour (?)
My accountant on seeing the first photo of the TC at full power " ...is that some sort of burglar alarm?".

2 A short cartoon  (1.3 Mb)- must see  : )

3 A sign on the outskirts of a nearby town.  Many of my projects result in IONS - sparks, lasers etc.  LIONS are a local service organisation but have lost part of their "L".  I am probably the only person in the world that finds this even faintly amusing.

This page was last updated August 28, 2005