Ohio State Flag
United States Flag


  Hal's 4 Inch Tesla Coil Project
Updated February 21, 2007

Coil Plans   Coil Assembly   Transformer Assembly   Control Panel
July 1, 2006
January 10, 2007
Click on photos above for an enlargement.

BE WARNED...      BE WARNED...      BE WARNED...
All of the devices shown on this page use HIGH VOLTAGE, which may inflict serious injury or death.
This page is intended solely as entertainment.  DO NOT ATTEMPT to reproduce any of the following devices.
Halice Internet Services will not be held liable for injury or death.

Click here to see video of my coil.

  Coil Plans

Coil Schematic
Click on image above for an enlargement.
  Here is the schematic for my 4 inch Tesla coil.  The basic design is from Gary Lau's site (with permission) but I have made substantial change.  Note that the schematic reflects 3 separate sections to the coil assembly's overall construction: the "Control Panel", the "Transformer Assembly" and the "Coil Assembly.  Three lines of thought go into this modular construction: inevitable future changes will be easier, the power source lies outside the strike zone and, breaking down and transporting the coil will be easier and faster.

I got most of my information, concerning the construction of my coil, from Gary Lau and his Web site.  For more about EMI Filters, and the correct way to connect them, see  For more about Terry's NST Protection Filter, see  For more about Power Factor Correction Caps, see  For more about MMC Tank Capacitors, see

The photos and info that follow are categorized to reflect the 3 sections of my coil design, starting with the "Coil Assembly".

  Coil Assembly

Secondary Coil Secondary coil   I bought my secondary coil, as shown in the photo to the left, for $20.00.  Fortunately, the previous owner did not seal the winding, so making modifications wasn't difficult.  The PVC pipe is thin walled and 4.22" in outer diameter.  The secondary is wound with 22 AWG enamel coated copper wire.  The height of the winding is 22.11", with an inch of pipe exposed at both ends.  I custom fitted 4" PVC end-caps over both ends to help support the secondary coil assembly, thereby reducing the stress on the end plugs (shown below).
topcap1 topcap2
Click on photo above for an enlargement.

In the photos at the top of the page, you can see the secondary assembly after being reconstructed.  I installed two 3/8 inch Lexan baffles, one 4 inches from each end of the winding.  Each was constructed from two layers of 3/16 inch Lexan and set in place with 5 minute epoxy.  To improve their insulating capability, I sealed both baffles, on the outer sides, with a thick coat of clear silicone caulking.

From 3/16 inch clear Lexan, I cut disks that were assembled into the end plugs.  In the top photo, the four disks that make up the top end plug were laid out.  I soldered braided stainless steel wire to a 1/4 X 20 brass nut by drilling a hole into the nut (but not far enough to get into the threads), inserting the wire and using a propane torch to melt the solder.  I drilled a 1/2 inch hole in the middle of one disk and routed a groove from the hole to the outer edge of the disk for the wire to pass through.  I press fitted the nut into the 1/2 inch hole.  I glued one disk, with a 1/4 inch whole in its center over the top of the nut and glued two disks under it, to improve insulation to the interior of the PVC tube.

Primary Coil & Table
Primary1 Primary2 Primary3
  I built the primary coil from 1/4 inch copper tubing, with 1/4 inch spacing.  Originally, there were 12.75 turns, with a total length just under 50 feet.  The inner diameter is 6 inches center-to-center and the outer diameter was approximately 18.75 inches.  The combs are made from 3/16 inch clear Lexan, set in routed grooves 3/8 inch deep.  The round board the coil is assembled upon is 3/4 inch 6 ply plywood.

I built the table from two 24 inch square sheets of 3/4 inch 6 ply plywood that I purchased from a carpenter neighbor for $5.00.  The legs are made of 1 1/4 inch schedule 40 PVC.  I finished the wood with the same oil based polyurethane that I will used to coat the secondary coil.

Instead of building a strike rail around the perimeter of the primary coil, I built 3 strike posts, using threaded brass rod, clear acrylic tubing and solid brass drawer knobs.
Primary4 Primary5
Click on photos above for enlargements.
  After finishing the rest of the coil assembly and running the coil's dimensions through Bart Anderson's JavaScript Tesla Coil Designer program, I found that I needed only 7.8 turns to match the secondary coil.  After firing the coil for the first time on July 1, 2006, I found that those calculations were very precise.  On July 5th, I trimmed the outer loops of the primary, leaving only 8.5 turns.

Sucker Gap (Version 1)
Sucker Gap
Click on photo above for an enlargement.
  I built my sucker gap after a trip to Home Depot.  It is attached to the suck end of my shop vac during coil operation.  The gap is fully adjustable, since the PVC pieces are not cemented together.  The pieces of the gap fit well enough that none of the pieces shift during coil operation.  Another advantage to not cementing the pieces together is that the brass fittings, that I used as the gap electrodes, get tarnished rather quickly from use.  Being able to easily remove them for cleaning and polishing is a plus.

Spark gag systems emit ultraviolet radiation that WILL permanently damage your eyes.  Never look at a sucker gap or rotary gap system during operation.  UV sun glasses do not provide enough protection.  As proof of my claim, the WHITE PVC pipe, that my sucker gap is made of, turned YELLOW from being exposed to the light from gap operation.  Comparison photos to be posted shortly.

Sucker Gap (Version 2)
Series Sucker Gap
  My first sucker gap made long hot streamers but it had one major drawback: it only functioned at full voltage.  In January 2007, I attended my first Teslathon, the Western Winter Teslathon.  There I met Jeff Mullins and saw his series spark gap system.  I decided then that I would make my own series gap.  Since the sucker system had worked well on my first gap, I chose to incorporate suction in my series gap design.  It took a month to accumulate the parts, finalize my design and build the thing.  On February 19, 2007, I fired my series sucker gap for the first time.  After several hours of testing and adjusting, I achieved some very good results, including the ability to turn the power down.  Now, I can run the coil on input voltage ranging from 50 to 135 volts, which makes the coil much more versatile.  I will be able to perform experiments and demonstrations that would not have been possible at full power.
Series Sucker Gap   I made the base of my gap from 4 layers of 3/16" white Lexan.  Phenolic would have worked just as well.  The power tools I used to make it were a drill press, a router, a grinder and a jigsaw.
Series Sucker Gap   The electrode holders are made of 1/2" threaded couplers.  I drilled a 1/2" hole in each, perpendicular to its length, to accommodate the electrodes.  The electrodes are Miller Welder points, part #020603.  The body of these points are 1/2" diameter steel, tipped with 3/8" diameter tungsten.  To hold the electrodes in place, I used 1/2" hex set screws.  The five grooves in the PVC pipe were drilled on a drill press using a 1/2" drill bit.  Each set of electrodes fit snugly into the grooves in the PVC pipe.  Flat washers are used as spacers to adjust the electrode holders to the proper height.
Series Sucker Gap   Half inch bolts secure each electrode holder to the top layer of Lexan.  The electrical connection between each set of electrodes is made with 3/4" wide by 1/16" thick brass strips.  Two layers of Lexan are required to match the height the heads of the 1/2" bolts, after the bolt heads are ground down by 1/16".  The fourth layer of Lexan (not shown in this photo), is a solid piece that covers the entire bottom of the series sucker gap and acts as insulation.
Series Sucker Gap
Click on photos above for enlargements.
  After several hours of testing, burn marks are seen on the PVC pipe, adjacent to each gap.  Much of this was caused by running the gap with the suction turned off, so to make comparisons.  My testing proved that the gap runs much more efficiently with the suction turned on, resulting in longer and stronger streamers.

In March 2007, I will build my own version of a Terry Blake style asynchronous rotary gap, to take the place of this gap.  Toward the end of 2007, I will build a table-top Tesla coil and use my series sucker gap with it.

Tank Capacitor
tank1 tank2
Click on photos above for enlargements.
  I built the tank capacitor from 22 Multi-Mini Polypropylene Capacitors (MMC).  They are 0.15 uF 2000 VDC Cornell Dubilier caps, Part Number 942C20P15K-F.  The "F" in the part number means that the caps are RoHS compliant: lead-free terminations.  I purchased them from Eilene at Richardson Electronics, phone number 1-888-735-7358.  If you try to order them from their Web site, you have to purchase a minimum quantity; not so if you call directly.

My Neon Sign Transformer array (NSTs) consists of four 15,000 volt 30 mA NSTs, with the secondaries connected in parallel, for a total current of 120 mA: more than enough to kill you DEAD!!  This amount of power requires the tank capacitor to have a mains-resonant value of  0.0159 uF.  Using the "Larger Than Resonant" premise (LTR), the tank capacitance value is multiplied by a factor of 1.5 to 2.0.  When the mains-resonant value of 0.0159 is increased by a factor of  2.0, the tank capacitance rises to 0.03 uF.  For more information concerning LTR, see Gary Lau's "Steps to designing your own coil".  Configuring the MMC array with two strings of eleven caps each, wired in series, with the two strings connected in parallel (see schematic), the capacitance becomes 0.027 uF.  There is a 10M ohm 1/2 watt resistor connected in parallel to each capacitor in the array, as a safety precaution to drain the caps once the coil has been shut off.

Each string of caps is mounted on separate clear Lexan panels.  The two Lexan panels have been mounted vertically, in 1/2 inch deep slots, on a 3/4 inch thick, 6 ply plywood board.  The sucker gap is mounted on the same plywood board.  The board sits on the lower shelf of the table that supports the coil assembly.

Gary Lau checked out my MMC strings and pointed out that there should be some space between each cap, to avoid them from arcing between each other.  To avoid a complete rebuild of my MMC strings, I purchased .030 inch clear polyester plastic sheeting from my local hobby store and slid a piece between each cap.  During the first firing of the coil, I didn't seem to have any problem with arcing.   However, I did notice that several caps on the power supply end of both strings were perceptibly warmer than the others.  This may be caused by my sucker gap design; it may not be quenching the plasma arc sufficiently.  I really don't know.  If any experienced coiler has an idea what's causing this, please email me.  FYI, when running four 15KV NSTs for a total of 120mA, with a variac output of 135 volts, the current draw is steady at 11 amps.

Tank Cap & Sucker Gap Assembly
  Here is the MMC tank capacitor array married to my old sucker gap.  The two Lexan MMC panels are mounted vertically, in 1/2 inch deep slots, on a 3/4 inch thick, 6 ply plywood board.  The sucker gap is mounted forward of the caps on the same board.  During operation, the board is located on the lower shelf of the table that supports the coil assembly.
Click on photos above for enlargements.
  Here is my series sucker gap mounted on the same board that was used for the first gap assembly.  To accommodate the larger gap assembly, the two MMC panels were moved back on the plywood board.

Toroids Toroid
Click on photo above for an enlargement.
  I built my first two toroids from flexible vent tubing.  The larger is 6 5/16 inches x 23 1/4 inches.  The smaller is 4 3/8 inches x 16 inches.  I used five minute epoxy to attach the ends of the vent pipe to each other, as well as to attach the disks to the centerline of the vent pipes.  The center disks are made of 2 layers of 3/16 inch Lexan sheeting, covered on both sides with aluminum tape, which also bonds the vent tubing surface to both sides of the disk.  A 1/4 inch hole through the middle of each disk physically and electrically connects the toroids to the business end of the secondary coil.  The photo at the top of the page shows the toroids sitting on top of the secondary.  I used a piece of 1 1/2 inch PVC tubing as a spacer to raise the bottom of the lower toroid to 2 1/2 inches height above the secondary coil winding.

After much experimenting, I found that I got longer, meaner streamers using the larger toroid alone.  Its elevation above the top winding of the secondary coil is about 1". Raising the toroid farther away from the secondary weakens the streamer output.  During tests run on January 9, 2007, the coil's streamers varied in length from 3' to 4', with an occasional 5' and 6' strike.

  Transformer Assembly

The Transformers
  Originally, as shown in the photo to the left, I planned to use three 15KV @ 30mA neon sign transformers, with the secondaries connected in parallel, for a total of 90mA current.  After firing the coil for the first time on July 1, 2006, I realized that I needed MORE POWER !!!!!!!
nst1 nst2 nst3
Click on photo above for an enlargement.
   Before my second test firing on July 5, 2006, I added a fourth 15KV @ 30mA transformer which brought the total current up to 120mA.  The results were dramatic.  The coil spat out more vicious looking streamers that made regular strikes at four feet.  Video of my first and second test can be seen on my "4 Inch Tesla Coil Video Page".  My "Terry's NST Protection Filter" is mounted on top of the transformers.  Fortunately, Terry designed his filter to withstand 120mA, so no modification was needed to the filter when I added the fourth transformer.  The four large capacitors sitting to the right are the "Power Factor Correction Caps".  Each is rated at 60uF @ 370 VAC, for a total of 240uF @ 370 VAC.

In January 2007, my nephew Craig made me a stout platform for my transformer assembly.  The platform has casters, which makes moving the assembly a breeze.

I have used this transformer assembly to power my Jacob's Ladder.  To see a video of it in WMV format, click HERE.  In November 2006, I connected the same Jacob's ladder to my 15KVA pole pig.  To see a video of it in WMV format, click HERE.

Terry's NST Protection Filter
protect2 protect3 protect4
Click on photos above for enlargements.
  Terry's NST Protection Filter is designed to protect the secondaries of the Neon Sign Transformers (NSTs) from being damaged during coil operation.  The original schematic can be found HERE.

Gary Lau made a variation of this filter for his coil.  You can see it by clicking HERE and scrolling to the bottom of the page.

I copied an innovation that Gary Lau designed to give warning if the ZNRs (MOVs) start to heat up.  I have siliconed a 120 degree Fahrenheit thermostat switch (Stancor STC-120) to the end of one of the two ZNRs closest to ground.  If the ZNRs heat up to 120 degrees, the thermostat switch will activate a 120dB siren (Radio Shack 273-079) that is powered by a 9 volt battery.  I have mounted the siren and battery to the bottom of the filter board, as shown in the last photo.

  Control Panel

Control Panel
panel1 panel2 panel3 panel3
Click on images above for enlargements.
  Here is the control panel.  The box is a rectangular aluminum parking lot lamp housing: the type you see shining down upon you from a tall pole in your WalMart parking lot.  I took the glass out of its access door and replaced it with translucent white 3/16 inch Lexan.  The volt and amp meters I purchased online at Action Electronics.  The main power switch in the lower right corner is keyed.  The toggle in the lower left corner is a SPDT momentary that controls the variac.  The white knob on the left controls the RPM of the shop vacuum that will be connected to the sucker gap.  Note that the unit is operating in the top photo: the meters showing voltage and current flow.  At the moment the photo was taken, the control panel was being used to operate my Jacob's Ladder.

The second and third photos show the interior of the box.  The variac, contactor and EMI occupy the right third of the interior, leaving the rest of the box for storage.  The gray metal box mounted in the middle of the open space holds the tools that will be needed to maintain the coil.  The space around the box is storage for the wires used to connect the three separate components of the coil.  A clear Lexan panel separates the circuitry from the storage area.

Mounted in the upper left corner of the interior of the box, is a 15 amp micro-switch, from a microwave oven door assembly, that shuts off the power to the entire box when the access panel is opened.  In the schematic, this switch is labeled "Safety Switch".

The schematic shown below depicts the physical layout of the control panel box.  The simplified electrical schematic of the Control Panel, as well as the entire coil can be found here.

Click on photo above for an enlargement.
  The heart of my control panel is this 140 VAC, 15 Amp variac, that is controlled by an actuator motor.  I bought it for $20.00 from Southwest Liquidators, Inc. in Tucson.  Its function is to vary the amount of voltage supplied to the primary of the neon sign transformers (NST), which power the coil.  After a bit of clean up and re-wiring, it looks and works as good as new.

Click on photo above for an enlargement.
  I bought this 30 Amp Electro Magnetic Interference Filter (EMI) for $5.00 from Southwest Liquidators, Inc. in Tucson.  See Gary Lau's EMI page for how best to use this type of unit.

I fired this coil for the first time on the afternoon of July 1, 2006 and a second time on July 5, 2006.  Video of both days can be seen on my "4 Inch Tesla Coil Video Page" in Media Player format (wmv).

Many of the ideas for the construction of my coil came from Gary Lau.   His coil circuitry and Terry's NST Protection Filter, are the basis for my coil's wiring.  For tons of great info and insight, read Gary's entire site carefully, especially "Gary's steps to designing your own coil".

Another great source for Tesla coil information is the "Tesla Coil Mailing List".  There you can communicate with the worldwide community of Tesla coil enthusiasts.

Hal's  Physics page      4" Tesla coil video page      Hal's Lifter page      Hal's Van de Graaff page      Hal's NST Repair page

Ohio State Flag
(520) 429-8572

Halice COMPUTER REPAIR & SERVICE for Grandvew Heights, Columbus, Ohio

Copyright © Halice Internet & Coomputer Services.  All Rights Reserved.