Fullscreen HTML5Med Res (??MB)HTML4

It sizzles!   It sparks!   It crackles!    It’s Build a Jaco High voltage displays have always been awe-inspiring. They not only look and sound spectacular – they even have a pungent smell, caused by the ozone which is generated by any high voltage discharge. One of the most fascinating high voltage displays is the Jacob’s Ladder, in which a series of sparks continually climb between two vertical wires. Warning! This Jacob’s Ladder display uses very high voltage which can give a nasty shock. Do not put your fingers anywhere near the display, the coil, nor any part of the circuit while ever power is applied. By Leo Simpson 32  Silicon iliconCChip hip At night it’s really spectacular: this photo of our new Jacob’s Ladder is a two-second time exposure. siliconchip.com.au siliconchip.com.au   Fascinating!   It’s electrifying! ob’s Ladder SO HOW DO YOU make an electric discharge spectacular they are also quite dangerous. climb a pair of wires? In practice, it is quite We got to thinking: how can we produce something just easy. The two vertical wires are spaced close as spectacular but not mains-powered? Our original Jacob’s together at the bottom and slightly splayed apart to increase Ladder circuit was based on a conventional 12V ignition the gap as the sparks rise. coil and we realised that today’s cars have very powerful So why do they rise at all? Surely the spark would always ignition systems. take the shortest route rather then extend itself as it travels So why not revise the circuit with a higher-powered coil upwards? out of a late model car? But the spark discharge is actually taking the shortest In practice, it turned out to be not quite so simple. While path, or rather, the easiest path from one electrode to the all current model cars use engine management and highother. Initially, the discharge does take energy ignition systems, they use the shortest path which is at the bota wide variety of ignition coil artom of the wires. But the continuous rangements. spark discharge is hot and heats up the Some use direct fire ignition air around it. This heated ionised air systems, with a coil right on top of rises, carrying the discharge up with it each spark plug. Others use a conuntil the gap between the two electrode ventional coil and distributor while wires is too large to maintain the spark. some others such as the Holden The discharge then starts at the bottom Commodore use three doubleagain and the cycle continues. ended coils to run a V6 motor. Back in September 1995 we proWe decided that the doubleduced a Jacob’s Ladder circuit which ended coil arrangement was probFig.1: this shows the spark has been popular ever since. But just ably the best for our purpose since plug firing arrangement for the Commodore V6 double-ended ignition coil. The two recently our attention was drawn to a it should have much higher voltage spark plugs are fired together (in series), so number of mains-powered discharge than a coil which only has to fire quite a high output voltage is needed. circuits on the internet. While quite one spark plug at a time. siliconchip.com.au April 2007  33 + F1 10A 0.47Ω FAST 5W D1 A T1 IGNITION COIL K JACOB'S LADDER 10Ω 1N4004 18k 12V BATTERY INPUT 7 470 µF 16V ZD5 16V 1W K 18k 2 6 A 8 4 VCC RES OUT DIS TRIG 3 2.2k Q1 BC327 B E CV 1W 5 B 1 ZD3 75V 5W 100nF ZD4 75V 5W – SC  2007 JACOB'S LADDER 555 DIODES (D1, ZD1-ZD5) A B K BAND BC327 8 4 1 E K ZD2 75V 5W E GND 330nF ZD1 75V 5W C C 150Ω IC1 555 THR Q2 MJH10012 BU941P A MJH10012 BU941P C B C C E Fig.2: the circuit uses a 555 timer (IC1) to pulse transistors Q1 & Q2 on and off at 75Hz. Q2 drives a Commodore V6 ignition coil and this delivers high voltage pulses to the Jacob’s Ladder wires. By way of explanation, the Commodore ignition coil has two high voltage terminals, each of which is connected to a spark plug. So when the coil fires, it drives two spark plugs in series; one will be on the power stroke while the other will be on the exhaust stroke and thus will be “wasted”. The arrangement is shown in Fig.1. The only drawback is that Commodore ignition coils come in an assembly of three, all attached to a common mounting plate. This assembly is quite expensive to buy, whether new or from a wrecker – you can expect to pay around $150 or more. Too much! However, you can purchase single ignition coils for a VN Commodore (the first with the 3.8-litre V6) and that is what we did. Even so, they typically cost around $50 although you might get one at lower cost from a wrecker. By the way, it may be possible to adapt other double-ended coils, such as from a Toyota V6 Camry or Avalon, but we have not tried them. main power transistor does not get too hot – it operates without a heatsink. 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 may be present on the PC board. IC1 is connected to oscillate at about 75Hz, as determined by the 330nF capacitor at pin 6 and the two associated 18kW resistors. The two resistors set the duty cycle of the pulse train delivered by pin 3 at about 66%. When pin 3 is high, transistor Q1 is held off and no base How it works Our Jacob’s Ladder circuit does not in fact produce a continuous discharge. Since it is based on an automotive ignition coil, it produces continuous individual sparks, at a rate of around 75 sparks/second. So you have a whole series of sparks which appear to be climbing up the wires. The result is noisy and smelly (from the ozone) and looks quite dangerous, as it indeed it could be, if you are unlucky enough to inadvertently touch the high voltage terminals of the coil. You’d get much the same belt as you would if you touched a spark plug top while the motor is running. The circuit itself comprises a 555 timer IC, two transistors, the ignition coil and several resistors, capacitors and diodes – see Fig.2. This revised circuit (compared with September 1995) has been modified to ensure that the high-energy coil is driven to a reasonably high current of around 5A peak while still maintaining a duty cycle which means that the 34  Silicon Chip This scope screen grab shows the circuit operation. The upper trace (yellow) is taken at the collector of Q1, showing the pulse waveform fed to the base of Q2. The lower trace (purple) shows the high voltage waveform produced at the collector of Q2 and therefore the voltage across the primary winding of the ignition coil. Note that it is limited to 328V peak-peak by the four 75V zener diodes. siliconchip.com.au Fig.3: the component overlay for the PC board. The photo below is an early prototype (in fact, using the same PC board as our original Jacob’s Ladder) – hence the TO3 transistor and some other circuit changes. However, it does give a good idea of how the Commodore coil is mounted in the new version. At the bottom of the page is a section of the reverse side of the board showing how connection is made to the primary of the ignition coil via spade lugs passing through the board. current flows in Q2. When pin 3 goes low, Q1 is switched on due to the base current flow through the 2.2kW resistor and Q1 switches on Q2 via its 150W base resistor. The coil now begins to charge via fuse F1 and the 0.47W 5W resistor. 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 MJH10012 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 may only be about 300V 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 across the coil’s high voltage terminals. In practice, our scope measurements showed that this could siliconchip.com.au April 2007  35 produce a coil primary voltage well in excess of 400V, which leaves less safety margin than we would prefer for Q2. Accordingly, four 75V 5W zener diodes, ZD1-ZD4, are connected in series across Q2 to limit the primary voltage developed by the coil to about 300V, well within the transistor’s rating of 500V. Note that Q1 inverts the output signal from IC1 and therefore drives Q2 with a duty cycle of about 34%. As noted above, the duty cycle is set to provide sufficient “on” time for Q2, so that the coil current can build to a value of about 5A peak, ensuring hot, juicy sparks. By the way, the specified Commodore VN ignition coil has a very low primary resistance of about 350 milliohms so we have added the 0.47W 5W resistor into the collector circuit of Q2, to limit the primary current and reduce heat dissipation in the power transistor. Power for IC1 is provided by the 12V battery via a 10A fuse (F1), the 10W resistor and diode D1. A 470mF capacitor filters the supply to provide reliable triggering for the timer. Transient protection is provided with ZD5, a 16V zener diode. A 100nF 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. Note that you will need a heavy-duty power supply to run this circuit; ie, one capable of providing about 5A or more, with low output impedance. Alternatively, use a sealed lead acid battery rated at 7Ah or more. You will need to keep it charged up between short periods of use. Construction The circuit is constructed on a PC board coded 11104071 and measuring 170 x 76mm. This board, together with the ignition coil mounted on it, can be mounted on a suitable piece of timber or MDF. Fig.3 shows the assembly details for the PC board. Begin the assembly by installing and soldering in all the low profile components such as the IC, diodes and resistors. It is a good idea to double-check the resistor values using a digital multimeter before soldering them in position. Now solder in the capacitors, taking care to ensure that the 470mF electrolytic is oriented as shown. Take care to ensure that the semiconductors are correctly oriented as well. Pin 1 of the IC is adjacent to a notch in one end of the plastic body. Transistor Q2 should be push­ ed down onto the board as far as it will easily go before soldering its leads. Q2 is secured directly to the board (ie, with 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 36  Silicon Chip on the PC board. To ensure reliable connections, use star washers under the screw heads and solder the nuts to their surrounding copper pads. Note that our circuit and the PC board overlay diagram show a BU941P or MJH10012 plastic TO-218 power transistor fitted instead of the MJ10112 TO-3 version shown in the photos of our prototype. This is because we built our prototype on the PC board for the September 1995 original version of our Jacob’s Ladder. If you have the original PC board (coded 11306951), you could adapt it to the circuit shown here but you will need to have four 75V zener diodes connected in series rather than the three zeners used in the 1995 design. The 150W 1W and 0.47W 5W resistors are mounted about 6mm above the PC board to improve heat dissipation – they do get warm. 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 two 25mm long M4 screws, with nuts and lockwashers. The connections to its primary winding are made underneath the PC board, through holes, using crimped spade connectors. The specified type is red with a 5mm wide spade section. To do this, cut two 50mm lengths of heavy-duty hookup wire with a wire size up to 1mm in diameter. Strip both ends of each wire and crimp a spade connector to one end of each – these go into the underside of the ignition coil via 8mm clearance holes on the underside of the PC board. The other ends of the wires are soldered to their respective points on the top of the PC board – see Fig.3. Then fit the twin-lead battery cable (red to positive, black to negative). The other end of this cable is fitted with large (30A) battery clips. Now you are ready to test the circuit. Testing Before you apply power, you must provide a temporary spark gap for the igniThe two tion coil, otherwise it may lengths of PVC tube be damaged by an internal shown here discharge. The gap can be do a great made quite simply with job of moving a paper clip. Push it over the spark one of the high voltage up the wires, terminals and then posiaway from tion it so that any spark the soldered can jump about 20mm terminals. There across to the other high was just one tiny voltage terminal. problem: after prolonged use Now for the smoke they started to test. As soon as you catch on fire . . . connect the power, so we are not there should be a recommending they continuous stream be used! of sparks across the siliconchip.com.au temporary spark gap. Do not attempt to touch the coil (nor anything else!) while power is applied because it can give you a nasty shock! If everything works OK, disconnect the battery leads and mount the whole PC board assembly on a suitable piece of timber or MDF (medium density fibreboard). We mounted our prototype using four woodscrews and some plastic spacers. We made our Jacob’s Ladder spark gap with two 30cm lengths of springy steel wire. These were attached to the high voltage terminals of the ignition coil by soldering each to bare spark plug connectors. These connectors are not particularly easy to find these days but we tracked them down at a specialist auto parts supplier. Your local auto electrician could be another possibility. If you cannot find any, perhaps you could “rat” some old spark plug leads and extract the connectors. Being designed for crimping, they may also not be easy to solder to – we managed by filing the surface of the connectors to a bright surface and then immediately soldering the wires on with a large (100W) hot soldering iron (normal 30W-ish hobby electronics irons don’t stand a chance!). Note that the two wires should be as straight as possible without any kinks but are slightly splayed apart to make the spark discharge run smoothly up the wires. 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. Coat-hanger wire would probably work just as well, bearing in mind that it can be difficult to get coat-hanger wire absolutely straight. Don’t use electrical conduit We originally placed a 50mm length of 20mm electrical conduit over both high voltage terminals of the coil (as seen in the photographs). This stopped any tendency for the spark to jump between any slight bumps or protuberances on the spark plug connectors and made the sparks climb up the wires much more smoothly. Unfortunately, though, after a prolonged period of use, these got carbonised and started to catch fire. Well, it seemed like a good idea at the time. If you find the sparks jump between the terminals and do not rise up the wires, try using proper spark plug insulating sc boots. DO NOT use electrical conduit. PARTS LIST – JACOB’S LADDER 1 PC board, code 11104071, 170 x 76mm 1 VN Commodore V6 12V ignition coil (see text) 2 3AG PC mount fuse clips 1 10A 3AG fuse 2 red 5mm crimp spade terminals 2 25mm M4 screws, nuts and star washers 1 red battery clip 1 black battery clip 2 bare spark plug connectors (see text) 2 spark plug insulating boots (if required – see text) 2m length of twin red/black automotive wire 2 300mm lengths of 1mm steel or copper wire 1 timber or MDF baseboard 1 12V DC 5A power supply or SLA battery (see text) Semiconductors 1 555 timer (IC1) 1 BC327 PNP transistor (Q1) 1 MJH10012, BU941P 500V NPN TO-218 Darlington transistor (Q2) 1 1N4004 1A diode (D1) 4 75V 3W or 5W zener diodes (ZD1-ZD4) 1 16V 1W zener diode (ZD5) Capacitors 1 470mF 16V PC electrolytic 1 330nF MKT polyester 1 100nF MKT polyester Resistors (0.25W 1%) 2 18kW 1 2.2kW 1 150W 1W 1 0.47W 5W wirewound 1 10W Fig.4: actual size artwork for the PC board. The corner mounting holes and the two ignition coil mounting holes should be drilled at 5mm while the three clearance holes for the coil primary wires should be drilled at 8mm. siliconchip.com.au April 2007  37