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

Circuit Theory

Circuit Theory

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

Circuit Theory

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.

Circuit Theory

Figure 15-3 Assembly hoard

Circuit Theory

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.

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