## Circuit Theory

Figure 15-2 shows a high DC voltage being produced by a blocking oscillator circuit consisting of a trans­former (Tl) being switched on and off by a transistor (Ql).The current through the primary winding (I) rises as a function of Et/L (when Q1 is on), where E is the applied voltage, in this case 12 volts-direct cur­rent (VDC), and L is the primary inductance of Tl. This rise in current induces a voltage in the feedback winding further holding Ql on, due to supplying the base current through resistor R2 and the speed-up capacitor C2. When the core of Tl saturates because of a high primary DC current, the induced base volt­age goes to zero, turning off Ql. This results in a reverse voltage induced in the secondary forward – biasing diode (Dl) and charging capacitors (C3 and C4). When the charging capacitor reaches the trigger voltage of the SIDAC silicon switch (around 300 volts), it now turns on, dumping the energy stored in the capacitor into the primaries of pulse transformers (T2 and T3). This energy causes a rapid rise in the current “forward inducing the high-voltage output pulses required for the lightning display.

You will note that two pulse transformers are con­nected out of phase (reversed connected) relative to one another. This approach generates twice the out­put of what is possible from one transformer, now being in excess of 50,000 volts. You will note switch SI controls the primary power. Switch S2 selects the display texture. The base of Ql is the control port for the output of current sink Q2, which is controlled by switch S3.

Construction Steps

To begin the project, follow these steps:

1. Identify all parts and pieces and verify them with the bill of materials.

2. Insert the components, starting from one end of the perforated circuit board, and follow the locations shown in Figure 15-3, using the indi­vidual holes as a guide. Use the leads of the actual components as the connection runs.

which are indicated by the dashed lines. It is a good idea to trial-fit the larger parts before actually starting to solder.

Always avoid bare wire bridges, messy solder joints, and potential solder shorts. Also check for cold or loose solder joints.

Pay attention to the polarity of the capacitors with polarity signs and all the semiconductors. The transformer position is determined using an ohmmeter as instructed in Figure 15-3 . Note the SIDAC may have two or three pins. Only the outer ones are used and may con­nect in either way, as the part is not polarized.

3. Cut, strip, and tin the wire leads for connect­ing to SI, S2, and S3 and solder them. These leads should be 4 to 5 inches long.

4. Fabricate the plate section from a 43A X 2’Aj X.06 piece of plastic. This is the base plate for mounting and gluing the T2 and T3 pulse

coi Is/transformers.

5. Prewire T2 and T3, as shown in Figure 15-3, at the separation distance of 2 inches. Use short pieces of vinyl wire to extend these leads. Splice in 5-inch leads for interconnecting to the board.

6. Carefully position the wired pulse coil assem­bly to the plastic plate and secure it with sili­con rubber cement (a room temperature vulcanizing [RTV] adhesive). Clamp it in place to hold it in position as the cement sets. It is important to keep these coils straight for aesthetics.

7. Attach the discharge electrode wires using the wire nuts. You may solder these wires but must use care as excessive heat can internally damage T2 and T3.

8. Preconnect the pulse coil assembly to the board as shown. Connect the wall adapter using the wire nuts, observing proper polarity.

Electrical Pretest

Го test the project’s wiring, follow these steps:

1. Separate the ends of the discharge wires to approximately 2 inches. Preset the trimmer

 Figure 15-2 Testa lightning generator schematic

 pots to midrange and the slider switches SI and S3 to “off.” 2. Turn on SI and note a discharge occurring between the pulse coils. Change the position of the toggle switch S2 and note the discharge texture changing. Identify the switch position for heavy or light spark display. 3. Turn on S3 and note the display cycling on and off at an approximate rate of 100 seconds on and 100 seconds off. These times are inde­pendently variable over a wide range. Our suggested setting is 100 seconds on and 100

 Final Assembly For the final assembly, follow these steps: 1. Fabricate the enclosure from a 6’/4 – X 5’A – X.06-inch piece of plastic, as shown in Figure 15-4.

 seconds off. Тіїe device can be left on continu­ously with these low-duiy cycle sellings.
 Figure 15-3 Assembly hoard
 Figure 15-4 Final assembly

2. Glue the pulse coil assembly, followed by the assembly board. Mount the controls and dress the wire leads for a neat-appearing assembly.

 You can use a clear or colored piece for this part. Fabricate holes forT2 and T3. observing the proper alignment with the glued plate assembly as in the prior step. Fabricate the remaining holes for switches, power leads, and trimpot access holes.

3. Verify the operation and preset controls for the desired spark display and cyclic timing.

Table 15-1 Tesla 2-inch spark coil parts

Ref. # Qty. Description DB#

Rl 4.7K, !/4~watt carbon film resistor (yel-pur-red)

R2,8 2 470-ohm, V«-watt carbon film resistor (yel-pur-br)

R3 27-ohm, ‘/4-watt carbon film resistor (red-pur-blk)

R4.6 2 1M trimmer resistor vertical mount

R5,7 2 10K. ‘/4-watt carbon film resistor (br-blk-or)

C5 100 mfd, 25-volt electrolytic capacitor vertical mount

C6 220 mfd, 25-volt electrolytic capacitor, vertical mount

C7 .01 mfd, 50-volt disk ceramic capacitor

Cl 10 mfd, 25-volt electrolytic capacitor, vertical mount

C2 .047 mfd, 50-volt polyester capacitor, marked 2A473 on green body

C3 3.94 mfd, 350-volt polyester capacitor

C4 .47 mfd, 250-volt polyester capacitor

Ql МГЕ3055 NPN transistor T0220

Q2 NPN PN2222 GP transistor

II 555-dual in-line package DIP timer

Dl 1N4007 1 Kv rectifier diode

SIDAC 300-volt SIDAC switch marked K3000; see text S1DAC

Tl Switching square-wave transformer 400V TYPE1 PC

T2.3 2 25 Kv pulse transformers CD25B

SI, 2,3 3 SPST 3-amp toggle switch or equivalent

PBl 5 X 1.5 .1 grid perforated circuit board

PCTLITE Optional PCB replaces PBl PCTLITE

WR20B 36 inches #20 vinyl stranded hookup wire, black

WR20R 36 inches #20 vinyl stranded hookup wire, red

WN1 2 Small wire nuts, #71В

EN1 Enclosure 6 ‘/4 X 5 ‘A X.062 plastic, see Figure 15-4

PLATE 21/4 X 4 Vа X.062 plastic partition

TAPE 6 x 1 x.125 double-sided sticky tape

12DC/.3 12-volt DC.3-amp wall adapter 12DC/ 3

This multifunctional electrical project is very suitable for science fairs, as the device is powered by a 12 volts-direct current (VDC) battery or a tow-voltage wall adapter lranstormer (see Figure 16-1). Many interesting action and display experiments are shown in detail with safety stressed throughout, thus suitable for the younger hobbyist. Construction involves basic wiring and soldering with some basic mechanical assembly.

Most parts are readily available. Those that are special, including a printed circuit board (PCB). may be obtained through www. amazingl .com. Expect to spend \$25 to \$50 to complete the project as shown. Table lfr-1 outlines the parts needed for the project.

You may perform the following experiments with your coil:

• Adjustment Suspend a grounded metallic object above the device. Start at about 3 inches of separation and make adjustments, increasing the separation until the optimum spark length is obtained. (Note grounding means connection to the metal base.)

• Effect on human body Use caution as this may cause a reflex secondary reaction even though it is painless. Hold a metal object tigbtly and advance to the coil terminal. Note

the painless, tingling sensation. Fake out your pals by letting them think it is really painful. This demonstrates the “skin" or surface effect of high-frequency electricity. Caution! Make sure you are standing on a dry, nonconductive surface.

• Effect on insulators Place various objects on top of the coil and note the effects of the high – frequency electricity. Glass or other usual insulators do not stop the sparks. Experiment using objects such as lightbulbs, bottles, glass, and so on.

• Effect on partial insulators Use some wood pieces about 12 X 1 X З-inches and notice red streaks and other weird phenomena occurring from within the piece. Try other materials as well.

Ionization of gases Obtain a fluorescent lamp or plasma globe and allow it to come within several feet of the device. It will glow and produce light without a direct connection, clearly demonstrating the effects of the elec­tric and magnetic fields on the gas. Note the distance from the coil that the lamp will glow. Experiment using a neon lamp in place.

 Figure 14-15 Low-voltage wiring

Obtain other lamps and note the colors, dis­tances, and other phenomena.

Indnction fields Obtain a small filament – type lamp, such as a flashlight bulb, and con­nect it between a large Viz – to 2-inch-dia meter metal or wire loop. The lamp will light due to energy coupled by induction. You will note that a current is required to light this type of lamp and is entirely different than the radia­

tion field that ionizes and causes the gas lamps to glow.

• Ion motors Create and carefully balance the rotor as shown. Use a piece of thin, spring beryllium copper. Pinprick the center with a center punch for the bearing point. The rotor will spin at high speeds if it is carefully bal­anced, demonstrating ion propulsion.

Note that your Tesla coil output is hungry for “capacitance.” It is suggested that you obtain an 8- inch toroidal terminal such as ourT08 at www. amazingl. com. You may substitute two metal bowls and attach them together to produce a spherical­shaped object. The torus, however, provides electro­static shielding of the coil, eliminating a discharge at these points.

 FRONT

Cutaway view of bracket showing primary windings

– These wires are randomly positioned in mese slots. Figure 14-17 Top-plate layout showing primary coils

Tapped lead lo primary coil must be positioned close to surface of top plate as possible for avoiding arcing to secondary coil.

Tap position shown here is where the shop model is roughly tuned to arid may serve as an approximate starting point

Position and direction of tap lead will affect tuning. Experiment until you get 10-12" open air discharges.

 Front view showing the control panel

Figure 14-18

 Bet # Qty. Description TPLATEl і Top plale. fabricated as shown in Figure 14-9 BPLATE1 l Bottom plate, fabricated as shown in Figure 14-10 CBKT1 l Capacitor bracket, fabricated as shown in Figure 14-5 SGBKT1 i Spark gap bracket, fabricated as shown in Figure 14-6 SGFIN1 2 Heat fins, fabricated as shown in Figure 14-6 PANEL1 1 Control panel, fabricated as shown in Figure 14-8 PCBKT1 4 Primary coit brackets, fabricated as shown in Figure 14-7 LSI 1 Secondary coil form, fabricated as shown in Figure 14-4 RFC! 1 RFC choke coil form, fabricated as shown in Figure 14-5 P1L1 4 5 lh – X Ч2 – inch PVC tubing for pillar spacers, as shown in Figure 14-3 TRAN1 4 6500-volt/.02-miHiampere current limited core and coil transformer Cl .005 mfd.10 Kv AC special polypropylene capacitor TUNG141 2 4* – X 1-inch pure tungsten rod, cut and faced, as shown in Figure 14-12 SI Single pole single throw fSPST) b-amp toggle switch (see Figure 14-15) ПИ Panel-mount fuse holder (see Figure 14-15) FSl 3-amp, slow-hlow, 3 ag size fuse (see Figure 14-15) NEON Neon indicator lamp (see Figure 14-15) COl 3-wire #18 power cord (see Figure 14-15) BUI Cord clamp bushing (see Figure 14-15) BU2 Neon lamp bushing (see Figure 14-t5) Rl 39K, ‘A-wau resistor (see Figure 14-15) WN1 Small wire nut (see Figure 14-15) CAP158 15/n-inch plastic cap (see Figure 14-5) CAP350 3 ‘/2-incb plastic cap (see Figure 14-16) CAP 4 -’/•«-inch PVC cap used for feet (see Figure 14-13) PLTUB1 10 to 12 inches of Vs – inch OD plastic tubing (see Figure 14-3) TYE1 2 8-inch tie wraps (see Figure 14-11) CLIP1 Small crocodile clip (see Figure 14-17) WR12 15 feel #12 AWG stranded, vinyl-covered wire SWl 8 #6 X – Vs brass wood screws SW2 14 #6-32 X 1 screws, Phillips SW3 4 #6-32 X ‘A screws, Phillips NU1 20 #6-32 hex nuts WASH1 28 #6×1/2 flat washers; use under #6 screws and nuts SW4 2 ‘/«-20 X Ч2 – inch brass screw
 Table 14-1 Tujelve-inch-spark Tesla coil parts

 6KV/02A.005 M/10 Kv ITJNGt41

 DB#

 DB#
 T08

Continued

Table 14-1

 Ref. # Qty. Description SW5 l ‘/«-20 x 1-inch brass screw NU2 4 ‘/4-20 brass hex nut NU3 4 #8-32 hex nut for threads of Cl COTUBE 7 feet 3/i6 coiled copper tubing ROD1 4 ‘/4-20 X 7 ‘/4 – inch threaded rod (see Figure 14-13) NU4 12 ‘/4-20 steel hex nut LUG1 8 ty4-20 X #12 solder ring lug (see Figure 14-13 and 14-14) LUG2 2 #6 solder lug for green lead of COl andTl ground (see Figure 14-14) LUGBLOK 2 ‘/4-20 block lug #YLA6 (see Figure 14-12) LABI General label (see Figure 14-18) LAB2 Spark gap danger label (see Figure 14-3) T08 8- X 2-mch spun aluminum toroidal terminal (optional)

 Figure 15-1 Spark Tesla coil with timer

Тії is low-cost and interesting project generates a con­tinuous spark discharge with a variable rate of cur­rent. It operates from a 12-volt wall adapter or a battery for portable use or a science project where outlets may not be available. The device (see Figure

15- 1) has a built-in timer, allowing you to set the off and on times. This property provides excellent display for desk or bar conversation pieces where it will automatically turn on for several seconds and recycle,

surprising those in the area as the unit generates this noisy visual display.

Construction involves minimal electronic experi­ence. Expect to spend around \$25 for this rewarding and interesting conversation piecc with most parts readily available. Those that are special, including a printed circuit board (PCB), may be obtained through www. amayingl. com. Table 15-1 outlines the parts needed for this project

## Special Notes

The tap lead is shown being 3 feet long, enough to wrap itself around the primary coil assembly, provid­ing an additional turn. You may shorten this lead to a shorter length, bearing in mind that it will require readjustment, as it is pait of the primary coil.

If you cannot obtain open-air discharges of 10 to 12 inches, it will be necessary to experiment with the tap position along with the spark gap spacing. Open – air discharges are where the sparks emit from the output terminal into the open air. These will be longer than if they were dircct to a discharge probe. Point-to-point discharge lengths of 7 to 10 inches are possible and will he more intense but shorter than the open-air discharges.

The spark gap should be set to a maximum length where reliable operation occurs without extinguish­ing. The coil should not be operated for periods over 30 seconds due to radio frequency interface (RF1) potentials and overheating. You may have to place the system in a Faraday cage as shown on our plans as #FARAl. This will be necessary around sensitive electronic and communications equipment.

The secondary coil leads must never be allowed to route inside the coil form, as breakdown and irre­versible damage will occur (see Figure 14-3). Figure 14-4 shows a wooden block with leads secured.

Note the coil is referenced TOP and BOTTOM, where one end is slightly spaced shorter than the other. The shop unit seems to work best with the short end at the bottom. Coupling between the coils is critical and you may experiment by placing the sec­ondary coil on wooden blocks with various thick­nesses and check the results. A toroid terminal is recommended for optimum performance and you may use metal bowls.

Rpplication and Operation

Please note this Tesla coil device produces electro­magnetic radiation that can damage or interfere with certain types of equipment. Operation inside a screen room or Faraday cage may be required for FCC com­pliance.

 1. Mote that only holes shown dimensioned must be precise Others may be eyeballed. 2 Corner holes are 1/4” and are located 1/2" from edges. 3. Use small pilot holes for #6 wood screws for securing the four ВКРС1 brackets. 4. Dimensioned holes are referenced from exact center

Figure 14-9 TPLATE1 fabrication

As stated previously, remove the power cord before making any of the following adjustments. The tap lead when ungrounded from the primary is a shock hazard and can produce 23 milliamperes to ground.

Optimum output requires the proper selection of the correct tap position on the primary coil. Start at the end of the outermost turn and reconnect the tap to the exposed sections, changing a quarter of a turn at a time while noting the increasing or decreasing output. You may secure it via soldering when the proper position is determined. Caution: Keep this

lead flat, as breakdown to the secondary will perma­nently damage the windings. Note the tap lead clip position and mark it when using different terminals. Note Figure 14-17 for proper routing of the tap lead, which we found tuned reasonably close with this ter­minal.

The capacitance of the output terminal will greatly affect the tuning, requiring more turns the larger the terminal size and vice versa. To operate the coil, you need to perform the following steps:

1. Place the coil on a table away from sensitive electronic equipment. Computers must be

FRONT

Use 3/8" to 1/2" plywood

1 Corner holes are 1/4" and mate to those on TPLATE1 in Figure 14-9.

2. Tnal-position referenced subassemblies as shown and mark hole locations for drilling 1/8" clearance holes.

Figure 14-10 Fabrication ofBFLATEl

removed from the area and disconnected from power if on the same line. Observe, read, and heed all warning labels.

Make sure the switch is “off” on the front panel before plugging the coil into a three – wire grounded outlet. Defeating the ground on the plug will result in system failure, poten­tial fire, and an extreme shock hazard.

Attach the secondary coil as shown in Figure

14- 3. Make sure that leads from the coil are as short as possible, allowing only what is neces­sary to make the connections to the bottom grounding screw and repeating for the top ter­minal. Failure to do this will result in irre­versible damage to the coil.

The spark gap can be increased just before reliable firing becomes a problem. Further spacing may produce intermittently longer discharges but may overstress the capacitor and transformer. Always set for a reliable, steady firing of continued operation. It is a good idea to space the gap at 4s of an inch when first firing up, and increase as described previously.

Figure 14-11 Isometric view of CBKTASSY capacitor bracket assembly

 Assemble as shown and verify proper alignment of tungsten electrodes. Preset gap to 1/4′ and secure with lock nuts. Figure 14-12 ISO view ofSGAPASSY spark gap assembly
 Figure 14-13 Bottom plate assembly

The secondary coil should be terminated into a conductive object such as a sphere or torus A metal 13- to 16-ounce coffee can or salad bowl may also be used with reasonable results.

5. The trick now is to adjust the tap lead for the maximum spark output from the particular terminal used. Note the lead position along

with the spark output of the secondary coil: the discharge length increases or decreases as adjustments are made. Note that a variance of several inches along the primary coil winding can make a noticeable difference in output. The open-air discharges will be longer than the point-to-point ones.

 To safety probe. Use 36" WR12
 LUGS may be replaced by forming circular loops at the lead ends and securing under the screw and nuts using flat washers. If using stranded wire, always pre-tin before forming to keep intact.
 Figure 14-14 Bottom plate high-voltage wiring

## Construction Steps

 Note that the layout must be closely followed only where actual dimensions are shown. Otherwise, trial-

 input drive voltage times the ratio of primary Q to secondary Q. Information describing some of Tesla’s works are available on our site at www. amazingl. com.

 Drill the small holes in the form a start and finish leads.

 і shown for securing the

 Thread wire through small holes leaving about 6" and wind a 0 3/4" layer of #24 magnet wire on form evenly and without any kinks or overlaps Finish by threading through holes at other end Coat coil several times with orange shellac, makrng sure there is no water or other impurities that will contaminate the

 A suggestion in winding this coil is ю work with a buddy using a broomstick as a shaft and carefully dispense the wire as needed. Keep wire taut and avoid kinks and overlaps as these will cause output to be greatly reduced!! You may wind a section and then shellac holding the wire with tape to avoid unwinding. Approximata frequency of cal is equal to total winding length of wire in meters times 4 divided into 300 x 10 e8. This is the familiar quarter-wave antenna formula. Finished coil should resonate at 500 to 600 kHz without any output terminal attached The frequency decreases substantially when a terminal is added changing more or less with terminal size.

 Dnll three small holes 1.5′ from ends of form and thread leads as shown.

 Precut coil form from 3" (3.5" OD) thin wall PVC tubing. Thoroughly clean both inner and outer surfaces and make sure form is dry.

 % t t г

Figure 14-4 Secondary coil

position the components as shown in the figures and use the "‘eyeball” approach to locate them. Please note that you may make substitutions that may increase or decrease performance.

1. Assemble subassembly secondary coil (LSI), as shown in Figure 14-4.

2. Assemble the RFC1 subassembly and fabri­cate the CBKT1 capacitor bracket, as shown in Figure 14-5. Drill two small holes for threading the 4-inch connection leads. Evenly and tightly wind 40 to 50 turns of #2f> magnet wire. Note the mounting scheme of sleeving into a plastic cap (CAP15) screwed

 BPLATE1 Assemble (RFC1) subassembly as shown. Drill two email holes for threading 4" connecting leads through Evenly and tightly wind 40 to 50 turns of #24 magnet wire. Note mounting scheme by sleeving into (CAP1S) plastic cap screwed to the (BPLATE1) bottom piate esshown
 1ІЗ 46
 "16
 Figure 14-5 CBKT1 cap bracket and RFC1 assembly

to a base plate (BPLATE1) shown later in Figure 14-16.

3. Fabricate the SGBKT spark gap bracket and four primary coil-holding brackets (PCBKT1) from.065 polycarbonate (Lexan) plastic, as shown in Figures 14-6 and 14-7.

4. Fabricate the two spark gap cooling fins (SGFIN1) from.065-inch aluminum, as shown in Figure 14-6.

5. Fabricate front panel (PANEL1) from.065- inch aluminum, as shown in Figure 14-8.

6. Fabricate TPLATEl from % – to ‘/2-inch thick­ness plywood, as shown in Figure 14-9.

7. Fabricate BPLATE1 from Vs – to Vi-inch ply­wood, as shown in Figure 14-10. Drill holes when the layout is verified in step 12. More serious craftsman may want to replace the plywood with clear Lexan or Plexiglas. This approach creates a professional-looking project.

8. Imbricate four pillars (PILI) at 574-inch lengths of ‘/2-inch PVC tubing, as shown in Figure 14-3.

У. Attach capacitor Cl to the bracket, as shown in Figure 14-11.

10. Assemble the spark gap assembly (SGA – PASSY) as shown in Figure 14-12.

11. Add several 74-20 hex nuts to a length of threaded rod (TROD). Cut four pieces to 7’As inches and dress the cut ends with a small file. Reform the threaded ends using the nuts as a threading die.

12. Lay out bottom plate (BPLATE1) and verify the components’ locations. Drill the mounting holes for securing the components, as shown in Figure 14-13.

13. Assemble the four PVC caps with pillars at the corners using the threaded rods and hard­ware. Assemble the components using the 6- 32 X 1-inch screws with washers and nuts. Note the front panel is secured by the sand­wiching action of Tl and the capacitor bracket. Attach the solder lugs as shown in Figure 14-13.

14 Wire up the high-voltage section, as shown in Figure 14-14. Note all leads to the capacitor

Figure 14-6 SGPK1 spark gap brackets

and the spark gap must be direct. Avoid loops and use short pieces of stripped WR12 sol­dered to the respective lugaTheTl trans­former lead to the radio frequency (RFC) choke is soldered as shown. The secondary ground lead is soldered to LUG2. You should end up with the grounding lead to the second­ary, the primary coil center connection with the lug, a lead for the primary tap, and a lead to the safety discharge probe.

15. Assemble PANELI and wire the primary 115 VAC input section, as shown in Figure 1Ф15. Note that it is important to verify a proper ground of the green wire of the cord (COl) being electrically connected to the frame of Tl, the PANEL section, and the secondary return lead.

16. Attach a 3 ‘/2-inch plastic cap (CAP35) to the center via a ‘/4-20 X 1-inch brass screw and nut. This point is the earth-grounded common return of both the primary and secondary coils. Attach the four coil brackets using #6 78-inch brass, flathead wood screws. Verify that the bracket holes line up with the pilot holes so that everything fits as shown in Figure 14-16. If not, redo the pilot holes and assemble as shown.

17. Measure an 8-foot length of wire (WR12) and thread one end through the clearance hole as

 065 ALUMINUM

SGFIN1 Spark gap cooling fins

shown and wind six full turns, placing them into the larger clearance in the four brackets as you go. Shape for a neat circular shape, as shown in Figure 14-17. Strip off the end as it must be inserted and soldered into the 3/i6- inch copper tubing (COTUB316) for the remaining part of the primary coil.

18. Continue by winding the attached 7-foot sec­tion of the – Vie – inch copper lubing as shown, snapping the cutouts into the brackets. Attempt a neat circular look, as shown in Figure 14-17.

19. Assemble and check the integrity of all the wiring and mechanical assembly. Label the unit as shown in Figures 14-3 and 14-18.

20. Open up the spark gap or place a piece of insulating material between the electrodes to prevent firing. Verity the proper assembly and plug the unit into a 115 VAC, three-wire, grounded outlet. Check the action of SI and note the lamp NE1 igniting. Quickly shon out the spark gap electrode attached to Cl to the chassis ground with the safety probe. Note a loud, bright discharge occurring. Only per­form this momentarily as it subjects compo­nents to unusual stress and only serves to verify proper operation at this point. Also remove the power cord and material between the spark gap electrodes.

 Dimensions shown must be followed on this drawing to obtain proper coupling of the primary and secondary coils.
 Follow this drawing to get the correct slotting. Use as a pattern or template. Slots must retain wire and tubing but allow snapping into position.
 Figure 14-7 PCBK1 primary coil holding brackets (four required)
 Material is.065 clear polycarbonate (Lexan) plastic. Three pieces required.

21. Feed the tap wire through the appropriate hole and attach a crocodile clip (CLIPl). Insert the secondary coil assembly into cap CAP350, attaching the ground return wire to the screw. This lead should be as short as pos­sible, as breakdown will occur inside of the coil. Attach the output terminal to the top of the coil using the same procedure. See Figure

14-3.

Before proceeding to the next step, you must always remove the power cord before making adjustments. The tap lead is a shock hazard, producing 23 milliamperes to earth ground. The 115 VAC power lines must be avoided, as they can be lethal.

22. When firing up your coil, start with the spark gap set at ■/» inches and the adjustment tap at the outermost winding tap to provide maxi­mum inductance. Note that this lead also con­tributes to total inductance when routed as shown (the same direction as turns of the pri­mary coil). It should be possible with this coil to easily obtain 10- to 12-inch streamers when properly adjusted. Careful adjustment of the

tap location and gap adjustment will greatly enhance performance. When operating for a prolonged period, always note the potential breakdown points, indicated by heavy corona or premature sparking. These points must be corrected or a burning tracking condition will result.

 Figure 14-В Panel made of A)65~inch aluminum

## Tuielve-lnch-Spark Tesla Coil

Use Caution Rround Sensitive Electronic Equipment

This fascinating visual and audible display project never fails to attract attention as fiery bolts of light­ning jump into the open air. The project, when prop­erly built and tuned, will generate some 12-inch spark discharges (see Figure 14-1). Operation is done from a standard 3-wire 115 volts-cilternciting current (VAC) outlet and requires caution in assembling and opera­tion. The transformer secondary output is 23 mil­liamps and is not considered lethal but can provide a nasty shock and burn lhal the victim will remember. The system output, while quite spectacular, can be contacted by firmly holding a metal object and bring­ing it near the output terminal. You will experience a very mild shock.

Expect to spend around \$100 for this very reward­ing project. Assembly is more mechanical than elec­trical and will require basic skills in the use of hand tools. The winding of the secondary coil may require building a simple jig and fixture to allow turning for applying the fine wire windings.

Tesla Coil

A Tesla coil is one of the most fascinating electrical display devices to see in operation; a large unit can produce a continuous spark exceeding the height of the coil. Electric discharges that simulate lightning bolts will produce cracks of noise louder than a rifle shot. These sparks, as well as being very Impressive and attention getting, can also can produce bizarre effects in most common materials. For example, wood may explode into splinters or be made to glow with an eerie, reddish light from within. Insulating materi­als seem to be useless against this energy. Lights ener­gize without wires, and sparks and corona in the form of St. Elmo’s fire occur within proximity of the device. High-energy electric and magnetic fields ren­der electronic equipment useless. Phenomena not normally associated with standard high-voltage elec­tricity become apparent in the form of many weird and bizarre effects.

Brief Theory

A Tesla coil is a high-frequency resonant transformer It differs from a conventional transformer in that the voltage and current relationships between the pri­mary winding and secondary winding are independ­ent of the winding turn’s ratios. A working apparatus basically consists of a secondary (LSI) and a primary (LP1) coil. It is obvious that the primary circuit is capacitance dominant, and tuning the primary circuit via taps along the primary coil alters the frequency accordingly. However, this relative fine-tuning of the primary circuit to the secondary is mandatory for proper operation. Force driving the secondary coil will produce hot spots and an interwinding break­down along with other negative results.

Introduction to Your Easy – to-Build Tesla Coil

The circuit schematic in Figure 14-2 shows a step-up transformer producing 6,500 volts at 23 milliamps from the 115 AC line. This voltage-current combina­tion can produce a painful shock. The builder must use adequate caution, just as being around any live 115 VAC circuit. When in doubt, consult someone experienced with this equipment. Safety rules should be followed at all times.

The device also produces ozone; therefore, use it in a well-ventilated area. Do not use it for prolonged periods of time. Thirty seconds at any one time is ample for any demonstration. Avoid eye exposure to the spark gap and use ample protection, such as

 1 Disconnect ground end of secondary coil and inserts 1k resistor In series with a variable frequency generator Connect e scope across resistor and determine resonanl frequency by noting a sharp dip In signal amplitude Record Ihls reading Note tbal output terminal must be solidly connected to the coll lead and must be away from conductive objects to obtain an accurate reeding Approximate frequency of this coll Is around 500 kHz.

 Note that the direction of routing the tap lead around the secondary coil will cause considerable difference in performance It is suggested to experiment with these settings You will note that a static tuning setting will vary in actual operation dua to capacitance gainad by the output spark. Prlmaiy resonant frequency should be preset to a slightly lower frequency than that oT stepl.

 2 Short out spartt gap with a short clip leaa ana disconnect lead from the RF choke Connact scope and generator combination lo tap of primary coll Startat maximum turns and note a sharp rise in the voltage wave form at some frequency Record and repeat at various tap points.

 safety glasses or shielding, as dangerous ultraviolet light is emitted. The unit, when constructed, can develop voltages up to and in excess of 250,000 volts. (This is the DC value ot voltage that would be necessary to produce the arc lengths obtainable with this Tesla coil model.) It will cause a gas discharge lamp, such as a regular household fluorescent lamp, to glow up to a distance of several feet from the unit. The high-voltage output coil terminal can actually be touched with a piece of metal held securely in the demonstrator’s hand, cre­ating quite a conversation piece.

 Circuit Theory The device, as shown in Figure 14-3, consists of a sec­ondary coil, LSI, containing approximately 500 turns wound on a polyvinyl chloride (PVC) form 13 inches long. This coil possesses an inherent resonant fre­

 Figure 14-2 Circuit schematic

 quency determined by its inductance and capacity, usually around 500 kHz. A primary circuit consisting of a drive coil (LP1) and a capacitor (Cl) are impulse driven by a spark gap (SGAPl).This primary circuil should also have a resonant frequency equal to that of the secondary coil for maximum performance. It is possible to “force drive” the secondary coil with fewer results. The output voltage of the device is dependent on the ratio of Q between these two coils. The primary coil has an adjustable tap that allows for fine-tuning. It should be noted that it does not take much in the way of added capacitance to the secondary to alter its resonance point. Even a change in the output terminal will require a readjustment of the tap. Transformer Tl supplies the necessary high volt­age. It is rated at 6,500 VAC at 20 milliamperes. (A larger-capacity transformer will produce more output but may stress the other circuit components.) This voltage charges the primary resonating “Lank” capac­itor (Cl) to a voltage where it fires the spark gap
 WR12

 Method showing proper attaching of secondary leads to grounding screw and toroid terminal.

 SGAP1

 CAP1

 Figure 14-3 Completed Tesla coil rear view

 Discharge probe is made from a 12" section of plastic tubing. Thread the wire WR12 and ball up the end to prevent from slipping back. Balled-up end is now contect point

 (SGAP1), producing an impulse of current through the primary inductance (LP1) where oscillations take place. The frequency is determined by the inductance and capacity values of the primary circuit. The volt­age output of the secondary coil, LSI, is usually approximately related to the primary voltage Vp multiplied by C1/C2 where Cl equals the primary capacity, VP equals the spark gap discharge voltage, and C2 equals the secondary coil capacitance (usually relatively small). Another way of expressing this rela­tion is that the output volts are dependent on the