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BUILD A PLASMA DIS Imagine a pulsating high voltage discharge which continually changes its shape and colours. It's a Plasma Display which you can control directly with your finger-tips, providing endless fascination as it sizzles and arcs. One of the most striking attractions of the new Sydney Powerhouse Museum is a large plasma discharge sphere which is over 550mm in diameter. It capitalises on the endless fascination that everyone has with lightning or with any high voltage display. It has the inexorable element of danger but paradoxically, it is completely safe as its great fluorescing arcs continually dance over the inside surface of the sphere. The Powerhouse's Plasma Display is truly spectacular but few people could afford to pay the $15,000 or would have the space needed to own one. But now you can have your very own Plasma Display. It is capable of the same PARTS LIST 1 plasma display bottle 1 circular disc of particle board 1 aluminium heatsink, 7 5 x 75mm (see text) 1 TV horizontal output transformer (not a tripler type) 2 2N3055 silicon power transistors 1 1N4002 silicon diode 1 1N5407 silicon diode 1 1 000 5W wirewound resistor 1 270 1 W resistor 2 16U or 1 50 5W resistors 1 in-line 3AG fuseholder and 3A fuse 8 SILICON CHIP 6 solder lugs 1 cordgrip grommet 1 one-way insulated terminal block Power supply 1 1 2V battery charger or 12-15VDC power supply rated at 2 amps or more (see also Fig.2). Miscellaneous Mica washers and insulating washers to suit T0-3 power transistors, rainbow hook-up wire, screws, nuts, lockwashers, solder. spectacular visual effects which you and your friends can enjoy to your heart's content. In essence the Plasma Display we're talking about is a large glass container containing an inert gas or a mixture of inert gases at a very low pressure - almost a vacuum in fact. High voltage AC feeds a specially shaped electrode inside the glass container and this produces a continually varying electrical discharge. You can put your hands or finger-tips directly over the glass and the high voltage discharge will pass right through the glass into your body- it's spooky, mysterious and hair-raising but completely safe. After you have seen and experienced this Plasma Display it is hard to comprehend that it is not dangerous. After all, in air a spark 10cm long not only looks and sounds dangerous but can be a painful experience if it jumps to you. It requires about 12,000 volts to generate a 10cm arc in dry air. But here in the Plasma Display you can have continuous electrical ·discharges more than 20cm long jumping all over the inside surface of the container and they can't possibly hurt you! There are several reasons why the Plasma Display is so spectacular yet completely safe, even though you can put your hands directly on the glass container. First, the gas inside the container is at an extremely low pressure and By LEO SIMPSON + __-C,...._.0-_ ___,_ _ _ _ _ _ ___, 3A 1000 5W PLAY this means that an electrical discharge can jump over much larger distances than are possible in air at normal atmospheric pressures. But since the container is glass this means it is an excellent insulator. So how can any current flow at all? How can the electrical discharge take place at all? And ·where is the discharge path since there is only one high voltage electrode, inside the glass, with no visible return path? High voltage DC Well, if the Plasma Display was fed with a very high voltage DC supply it would not work. No current could pass and no visible discharge could exist. Instead, the Plasma Display is powered with a high voltage high frequency AC (alternating current) supply. This means 'that minute' alternating currents can flow, firstly via the low pressure ionised gas inside the glass container, then via the very small capacitance of the glass container to earth. How can there be capacitance in the glass container when there are no apparent metal electrodes either on the inside or outside of the glass container? One electrode is the atmosphere on the outside of the evacuated glass container. Air is not a perfect insulator - its water content and very low ion content prevent it from being so. So it is possible for very minute alternating currents to flow from the outside of the glass container to earth. The low pressure ionised gas inside the container actually provides the internal electrode and therefore what is. an apparently invisible path is quite able to sustain the visible electrical discharge. Q1 2N3055 C 12-15VDC 01 1N5407 160 5W 270 1W Q2 2N3055.,___ __ oz 1N41J07 C ~ VIEWED FROM PLASMA DISPLAY EHT GENERATOR eaow SC0B-2-0888 Fig.1: the circuit for the EHT generator uses two 2N3055 power transistors connected as a transformer coupled multivibrator. This produces about 20,000 volts AC at a frequency somewhere between 20kHz and 30kHz. The EHT generator uses a horizontal output transformer intended for a blaclc and white TV set. Before mounting the transformer, you have to remove the existing low voltage winding and wind on two new windings (see text). Even so, the currents involved are extremely small and that is the other reason they are harmless when they pass through your person. OK, so what is plasma? The very name is a mystery to most people and something they may have heard of in the attempts to produce nuclear fusion - an extremely hot plasma is thought to be the requirement for fusion to take place. AUGUST 1988 9 enough. Take one large glass container, evacuate it, introduce a small amount of gas and seal it. You don't have to worry about that side of it though because the plasma container is supplied ready to go. What you have to do is assemble the EHT generator and the stand for the display bottle and provide a suitable 12V DC power supply. EHT Generator The EHT lead from the coil is connected to a one-way terminal block and then to the lead from the lid of the plasma container. Be sure to isolate the transistors from the heatsink correctly (see Fig.3). Actually, plasma is the term used to describe a highly ionised gas. Because it contains roughly equal numbers of positive ions and electrons it is electrically neutral but highly conductive. In fact, plasmas occur in all fluorescent lamps, mercury discharge tubes and so on. They are designed to be efficient sources of light. But with the right mixture of gases at the right pressure, plasmas can be quite different, as the display shown on our front cover illustrates. Actually, photos can hardly do justice to the Plasma Display we are talking about. The display is so dynamically variable it is just not possible to capture its essence in a still picture. In the case of the Plasma Display under discussion the particular mixture and pressure of inert gases inside the glass container is a trade secret. We don't know it. It has been arrived at after much trial and error. So has the shape of the fancy wire electrode inside the glass container. In principle though, the process for producing the display is simple EHT stands for "extra high tension" and is the term used to describe high voltages of several thousand volts or more, whether DC or AC. The EHT generator for the Plasma Display produces about 20,000 volts [20kV) AC at a frequency somewhere between 20kHz and 30kHz. In essence, the EHT generator is a DC to AC converter which takes low voltage DC and steps it up to very high voltage AC at a high frequency. It uses two 2N3055 power transistors connected as a transformer coupled multivibrator. The transformer is a horizontal output transformer intended for a black and white television .set. Normally a transfornmer of this type would be driven by the horizontal output transistor at 15,625Hz. For our circuit, the transformer is modified by adding two centretapped windings and these are driven by the two power transistors, as shown in the circuit of Fig.1. Each transistor's base is driven by a one-turn feedback winding 6A BRIDGE RECTIRER (P04) AO-O 240VAC N· EO---;;;,,--. Fig.2: if you don't have a 12V battery charger, this circuit can be used to make up a suitable DC power supply. No filter capacitors are required. Fig.3: the 2N3055 power transistors must be insulated from the heatsink using mica washers and insulating bushes. This diagram shows the mounting details. 10 SILICON CHIP 0 0 HEATSINK ~ -INSULATING BUSH <§:::i>- SOLDER LUG <at>-WASHER ~-SPRING WASHER ~---NUT Fig.4: wire up the EHT generator as shown here. The text gives full details on adding the two low voltage windings. ~ EHT TO PLASMA TUBE 16!l 5W E 02 8 ~ EHT COIL • .--1-6!l_5_W_~ ~ mounted upside down with the lid a push-fit into the plastic stand. The stand is made from two plastic mouldings which are standard sewer pipe fittings. These, in turn, fit onto a disc of timber which provides a stable base. The whole stand assembly is sprayed matte black so as to be as unobtrusive as possible. The EHT generator fits inside the plastic stand and is connected to the external power supply (battery charger or whatever) via a length of figure-8 power flex. Assembling the EHT supply HEATSINK 3A IN-LINE FUSE while each collector drives half the main low voltage winding. When the circuit is oscillating, each transistor alternately turns hard on, which applies the full DC voltage across the associated winding while the other transistor is biased off. So the transistors apply a square wave voltage to this low voltage winding and this is stepped up by more than a thousand times in the high voltage secondary winding. The 160 1W resistors in series with the bases of transistors Ql and Q2 are there to limit the amount of feedback current. They therefore have the effect of reducing the amount of damping on the transformer and thus improve efficiency. Initial bias to start the circuit oscillating is provided by the 1000 5W and 270 lW resistors. Protection against reverse biasing is provided by diodes Dl and D2, in conjunction with the 3A fuse. DC power supply Any DC power power supply capable of delivering 12 to 15 volts at more than two amps is adequate for the EHT generator. In fact, virtually any 12V battery charger will do the job, provided it can deliver 2A or more. (Note: some chargers with internal monitoring circuits are not suitable for this job). If you don't have an adequate DC supply or battery charger, the circuit of Fig.2 will do the job. It uses a 15V 2A transformer driving a bridge rectifier. No filter capacitors are necessary. Construction Kits for the Plasma Display will be available from Jaycar stores at $299. This will include the plasma bottle which is evacuated, charged with inert gas and sealed. At the time of writing, two different plasma displays were being considered. One, which we suspect contains mainly helium, gives a striking lightning-like display, leaping in three jagged tonges from the specially shaped discharge electrode. The other, which the staff at SILICON CHIP preferred, is probably a mixture of mainly helium and neon and gives a wavy aurora-like display which dances over the entire inside of the container in flares or pink, reds, violets and blues. Really, though words (or still photos) can't do justice to the displays. You have to see them in the glass! As supplied, the plasma container is a large glass jar with a sealed lid having a short length of wire attached to it. The jar is This is wired and assembled onto the circular piece of timber which serves as the base for the plasma display stand. In our prototype we used a circular piece of particle board 210mm in diameter and 15mm thick, although the exact dimensions are not important. The two transistors are mounted on a small heatsink fashioned from a piece of aluminium scrap about 75mm wide by 75mm high, with a foot section 10mm wide which mounts on the timber with a couple of small self-tapping screws. The details can be seen in the photos. The TV horizontal output transformer comes complete with a mounting bracket which makes it easy to mount on the timber with a couple of screws. Before you do that, remove the existing low voltage winding (on the opposite leg The EHT generator is mounted on a circular disc of timber and fits neatly inside the plastic stand. It is connected to the power supply via a length of figure-8 power flex. AUGUST 1988 11 When the EHT lead has been connected, the lid of the plasma bottle can be pushed into the top of the stand. Note the fancy shape of the wire electrode. to the large circular EHT encapsulated winding). Now wind on the two new low voltage windings. We used ribbon cable for this purpose as it makes bifilar winding easy. Wind on the two 4-turn windings together and do the same with the two one-turn windings. Secure the windings in place by running some hot wax from a candle over them. Four solder lugs should be secured to the timber base, adjacent to the transistor heatsink. 12 SILICON CHIP These take the 1000 and 270 resistors and the two diodes. The diagram of Fig.3 shows how the transistors are mounted on the heatsink using mica washers and insulating bushes. The transistor mounting surfaces and the heatsink should be lightly smeared with heatsink compound before they are assembled together. Fig.4 shows how the wiring of the EHT generator goes together. The two 160 resistors stand off the bases of the transistors while the other ends go to the relevant transformer connections in ''midair" . Nothing fancy, but it works. Make sure you connect the diodes the correct way around. An in-line fuseholder is used for the 3-amp fuse and the power flex is secured to a hole in the plastic base using a cordgrip grommet. When you have finished the wiring check your work carefully to see that it agrees with the circuit and wiring diagrams. Now use an insulated terminal connector to connect a short length of stiff wire from the EHT output wire over to the earth end of the EHT coil. Arrange the two ends so that there is a gap of about 7mm or so. Apply power from your 12V battery charger or power supply to the EHT generator and note that a continuous arc occurs across the spark gap. OK, so it works. Let it run for 15 seconds or so and then turn the power off. Feel the transistor cases to make sure that they are not running hot. After running for quite some time they will become pretty warm but not so hot that you can't touch them. Now it is merely a matter of connecting the short wire from the lid of the plasma bottle to the EHT output from the transformer. Carefully place the plasma bottle into the stand and apply power. A colourful discharge should immediately appear in the container. It is best viewed in darkness or dim light. You can put your hands all over the container to "control" the plasma display but keep your hands and fingers away from the lid of the container. If your hands stray too close you may get a nasty shock directly from the lid. We advise that the Plasma Display be placed in the centre of a suitable table, not near the edge. You should avoid any chance of the glass container being knocked to the floor. Replacement would be expensive if it was broken. [!II;! Where to get the parts: a .complete kit of parts for the Plasma Display is available from Jaycar Electronics Pty Ltd for $299.00. This will not include the 1 2V DC power supply.