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Build a Jacob’s Ladder display & amaze your friends Ever since scientific showmen like Tesla and Edison were able to generate really high voltages, the Jacob’s Ladder display has been creating awe amongst laymen. In this article, we show how you can build your own Jacob’s Ladder using a low cost cir­cuit. By LEO SIMPSON & JOHN CLARKE Virtually any high voltage power supply which generates more than about 10kV (DC or AC) can be used to provide a Jacob’s Ladder display. The display consists of two vertical wires close spaced at the bottom and splayed apart to increase the gap as the spark rises. It is the paradoxical nature of the ladder discharge which intrigues most people. Who would believe that the spark would want to become longer and travel upward, seeming- ly defying gravi­ty? And surely the spark would take the shortest path rather than extend itself as it travels upward. In reality, the spark discharge is taking the easiest route from one electrode to the other. Initially, the discharge does take the shortest path which is at the bottom of the wires. But the Jacob’s Ladder display works because the continuous spark discharge gets hot and heats up the air around it. This heated ionised air rises, carrying Fig.1: the circuit uses 555 timer IC1 to pulse transistors Q1 & Q2 on and off. Q2 in turn drives a standard automotive ignition coil and this delivers high voltage pulses to the ladder. JACOB'S LADDER L1 IGNITION COIL F1 10A 12V the discharge with it until the gap is too wide to maintain the spark. The discharge then starts at the bottom again and works its way back up and the cycle contin­ues. Why is it called a Jacob’s Ladder? We don’t know who first came up with the name but it is an allusion to the Bible story of Jacob: “He dreamed that he saw a ladder standing on the earth, with its top reaching into heaven; a stairway for the angels of God to go up and come down” (Genesis, XXVIII;12). The high voltage supply is easy –either of the plasma bottle displays from the August or November 1988 issues of SILI­CON CHIP will do the trick but there is a better approach – adapt the low cost Electric Fence Controller from the July 1995 issue of SILICON CHIP. A kit for this design is avail­able from Dick Smith Electronics and from Jaycar Electronics and only requires a few simple modifications. D1 1N4004 10 470 16VW ZD4 16V 1W 1k 7 12k 4 IC1 555 6 2 B E C E B C 8 Q1 BC327 E 3 2.2k B Q2 MJ10012 100  C C 5W B 5 1 0.1 0.33 VIEWED FROM BELOW JACOB'S LADDER EHT DRIVER 68  Silicon Chip E ZD1 75V 5W ZD2 75V 5W ZD3 75V 5W HT GND PARTS LIST 1 PC board, code 11306951, 171 x 79mm 1 12V ignition coil (see text) 3 280 x 5mm cable ties 5 PC stakes 2 3AG PC mount fuse clips 1 10A 3AG fuse 2 5mm ID crimp eyelet terminals 1 TO-3 transistor insulating cap 2 3mm screws, nuts and star washers 1 red battery clip 1 black battery clip 1 ignition coil EHT connector 1 2-way terminal block 1 2m length of twin red/black automotive wire 1 60mm length of red heavy duty hook-up wire 1 60mm length of blue heavy duty hook-up wire 1 370mm length of 1.5mm copper wire 1 40mm length of 0.8mm tinned copper wire Semiconductors 1 555 timer (IC1) 1 BC327 PNP transistor (Q1) 1 MJ10012 500V NPN Darlington (Q2) 1 1N4004 1A diode (D1) 3 75V 5W zener diodes (ZD1ZD3) 1 16V 1W zener diode (ZD4) Capacitors 1 470µF 16VW PC electrolytic 1 0.33µF MKT polyester 1 0.1µF MKT polyester This photo is really a composite of two separate photographs which were combined using a computer. It shows how the spark climbs the ladder formed by the two vertical wires attached to the ignition coil. Note that the multiple discharge paths shown here are a result of the ¼-second exposure time used when taking the photo. In practice, fewer sparks are visible at any one time. Actually, using the Electric Fence Controller to generate the spark discharge provides a big advantage in that the result­ing Jacob’s Ladder is much more spectacular. Instead of having just one spark discharge which climbs up the wires, our version produces about 130 sparks second, so you have a whole series of sparks which appear to be climbing up the wires, as shown in the accompanying photo. The result is noisy and smelly, and all those sparks look quite nasty and dangerous – as indeed they are. How it works The Jacob’s Ladder is based on an automotive ignition coil. These can be purchased new from automotive retail­ ers but will be cheaper if purchased secondhand from motor wreckers. Select one which requires a ballast resistor. The circuit comprises a 555 timer IC, two transistors, the ignition coil Resistors (0.25W 1%) 1 12kΩ 1 100Ω 5W 1 2.2kΩ 1 10Ω 1 1kΩ and several resistors, capacitors and diodes – see Fig.1. The revised circuit pulls a lot more current than the Electric Fence Controller and generates lots of fat, juicy sparks instead of the deliberately restricted high voltage transients of the original circuit. IC1 is a 555 timer used to produce the short pulses. Note that we used a standard 555 timer here since it is more rugged than the CMOS (7555) version and less likely to be damaged by any high voltage transients which September 1995  69 The ignition coil is secured to the PC board using plastic cable ties, while a plastic cap is fitted to Darlington transistor Q2 to prevent unexpected shocks during testing. Note that you don’t have to buy a new coil – a secondhand coil obtained from a wrecker’s yard will do the job quite nicely. may be present on the PC board. IC1 is connected to oscillate at about 133Hz, as determined by the 0.33µF capacitor at pin 6 and the associated 12kΩ and 1kΩ resistors. The two resistors set the duty cycle of the pulse train delivered by pin 3 at essentially 14:13; ie, close to a square wave. When pin 3 is high, transistor Q1 is held off and no base current flows in Q2. When pin 3 goes low, Q1 is switched on due to the base current flow through the 2.2kΩ resistor and Q1 switches on Q2 via its 100Ω base resistor. The coil now begins to charge via fuse F1. The instant pin 3 goes high again, Q2 switch­es off and the coil develops a high voltage and generates a spark across the gap. Q2 is an MJ10012 Darlington power transistor, specifically designed as a coil driver in automotive ignition systems. It has a 500V collector-emitter rating so it can withstand the high voltages developed across the coil’s primary winding. Depending on the spark gap, the coil’s peak primary voltage will only be about 200V or so, but if the gap is very large or the coil is operated without any EHT output lead, the secondary voltage can be excessive and there can be a flashover inside the coil. Not only can this damage the coil but it can also produce a very high primary voltage points. This done, solder in all the low profile components such as the IC, diodes and resistors. Table 1 lists the resistor colour codes but it is also a good idea to check the resistor values using a digital multimeter before soldering them in position. Now solder in the capacitors, taking care to ensure that the 470µF electrolytic is oriented as shown. Take care to ensure that the semiconductors are correctly oriented as well. In par­ ticular, note that D1 (1N4004) faces in the opposite direction to the three zener diodes (ZD1-ZD3). Note that ZD4 is mounted under the PC board across the 470µF capacitor. Pin 1 of the IC is adjacent to a notch in one end of the plastic body. Transistor Q1 should be pushed down onto the board as far as it will easily go before soldering its leads. Q2 is secured directly to the board (ie, no insulating washer) using 3mm ma­chine screws and nuts. As well as securing Q2 in place, these mounting screws and nuts also connect Q2’s collector (ie, the case) to a track on the PC board. To ensure reliable connections, use star washers under the screw heads and solder the nuts to their surrounding copper pads. This done, fit an insulating cap to Q2 – this will prevent any nasty shocks during the testing procedure. The 100Ω 5W wirewound resistor is mounted about 6mm above the PC board, to avoid any possibility of charring – it does get hot. which may damage Q2. Accordingly, three 75V 5W zener diodes, ZD1 to ZD3, are connected in series across Q2 to limit the primary voltage devel­oped by the coil to about 225V, well within the transistor’s rating of 500V. Power supply Power for IC1 is provided by the battery via fuse F1, the 10Ω resistor and diode D1. A 470µF capacitor filters the supply to provide reliable triggering for the timer. Transient protec­tion is provided with ZD4, a 16V zener diode. A 0.1µF capacitor at pin 5 filters the trigger point voltage to ensure that the timer does not false trigger. Diode D1 offers reverse polarity protection for IC1, while the fuse protects the battery from supplying excessive current should a fault occur. Construction The circuit is constructed on a PC board coded 11306951 and measuring 171 x 79mm. This board, together with the ignition coil mounted on it, fits neatly inside a 230mm length of 90mm plastic stormwater pipe (available from plumbing supply outlets). Fig.2 shows the assembly details for the PC board. Begin the assembly by installing PC stakes at the five external wiring TABLE 1: RESISTOR COLOUR CODES ❏ No. Value 4-Band Code (1%) 5-Band Code (1%) ❏ 1 12kΩ brown red orange brown brown red black red brown ❏ 1 2.2kΩ red red red brown red red black brown brown ❏ 1 1kΩ brown black red brown brown black black brown brown ❏ 1 10Ω brown black black brown brown black black gold brown 70  Silicon Chip ▲ JACOB'S LADDER Fig.2 (left): install the parts on the PC board as shown in this wiring diagram, making sure that all polarised parts are correctly oriented. The EHT connection to the coil is made using a brass EHT ignition coil connector. Warning! TERMINAL BLOCK This Jacob’s Ladder display uses very high voltage which can give a nasty shock. Do not put your fingers near the display or coil while ever the power is applied. Fig.3 (below): check your PC board for defects by comparing it against this full-size etching pattern before installing any of the parts. CABLE TIE IGNITION COIL CABLE TIE CABLE TIE 10  100  5W Q1 IC1 555 F1 D1 1k 12k 12V BATTERY POSITIVE Q2 2.2k 0.1 1 0.33 ZD1-ZD3 470uF ZD4 12V BATTERY NEGATIVE September 1995  71 The fuse clips can now be installed. Note that these each have a little lug at one end to retain the fuse after it has been installed. These lugs must go to the outside ends, otherwise you will not be able to fit the fuse. The ignition coil is secured to the PC board using three cable ties (see photo), after which the leads can be run to its primary terminals. These leads should be terminated using 5mm eyelet connectors to allow for easy connection to the coil. Don’t just crimp the connectors to these leads – solder them as well to ensure long-term reliability. Finally, complete the construction by fitting the twinlead battery cable (red to positive, black to negative). The free ends of this cable are fitted with large (30A) battery clips the battery leads and carefully slide the assembly into its 90mm tubular plastic hous­ing. This done, feed the battery cable through the hole in its end cap, secure it using a cordgrip grommet and reconnect the leads to the PC board. The board assembly will be held in position when the end caps are fitted and, generally, this should be sufficient. Howev­er, if you wish the board to be held even more securely, wrap a small amount of foam rubber around the top of the coil so that the assembly is a tight fit within the tube. We made our Jacob’s Ladder spark gap with a 220mm length and a 150mm length of 1.5mm copper wire. The shorter length was soldered to an ignition connector which plugs into the coil EHT, while the longer wire was soldered to the GND terminal. We used a 2-way terminal block to separate the wires at the base of the ladder. If enamelled copper wire is used, scrape the insulation away along the inside edges to allow the spark to travel free­ly. Testing Before you apply power, you must provide a temporary spark gap for the ignition coil, otherwise it could be damaged, as noted above. The gap can be made quite simply with a paper clip. Extend the paper clip so that you have a hooked section at each end. Fit one hook into the EHT socket on the coil and make sure that it cannot fall out easily. This done, bend the other end of the clip so that it is close to (less than 5mm) but not touching the negative primary connection of the coil. Now for the smoke test. Immediately, there should be a continuous spark across the temporary spark gap. Do not attempt to touch any part of the coil while power is applied because it can give you a very nasty shock! If everything works OK, disconnect M FRO NEW N CHIP O SILIC No kinks A large coffee jar placed over the ladder will prevent high-voltage shocks, although it does tend to diminish the spectacle of the display. Note that the two wires should be as straight as possible without any kinks. Any slight kinks will mean that the sparks will not progress smoothly up the ladder but will tend to “stick” at the kinks. So keep the wires as straight as possible and splay them apart very slightly so that the gap at the top is no more than about 20mm. You can also place a large coffee jar over the complete assembly for safety’s sake (see photo) although this does tend to diminish the spectacle of SC the display. 20 Electronic Projects For Cars On sale now at selected newsagents Or order your copy from Silicon Chip. Price: $8.95 (plus $3 for postage). Order by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 72  Silicon Chip