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Three inverters for fluorescent lights Battery powered fluorescent lamps are commonly available as a feature of some torches but they are flea-power units. Here we present three DC-AC inverters, suitable for fluorescent lights rated up to 40 watts. Design by OTTO PRIBOJ Fluorescent tubes are one of the most efficient forms of electric lighting but because of their high voltage requirements, they are rarely used unless 240VAC mains power is available. Yet there are many applications where the high efficiency of fluorescent tubes could be highly useful. These range from portable emergency lighting to lighting for recreational vehicles, caravans, boats and farms. In fact, if a pinpoint light source or a 46 SILICON CHIP concentrated beam is not required, then fluorescent lighting should be the first choice, because of its efficiency, glare-free light and freedom from shadows. This series of three inverters provide the necessary high AC voltage to drive a fluorescent tube from a 12V DC supply and at a high efficiency. Not only that, the circuits are low profile and use readily available components. DC to AC inverters intended for driving fluorescent tubes have two quite different requirements. First, they must generate sufficient voltage to fire the tube. Once the discharge is established through the tube, it becomes a negative resistance which means that unless there is some means of current limiting, the tube will destroy itself. In conventional mains powered fluorescent lamp fittings, the ballast performs two duties. First, in conjunction with the starter bulb, it produces the voltage to fire the tube arid then, its inductive reactance limits the current through the tube to a safe level. This is a very effective and time proven circuit which is used in billions of fluorescent lamp fittings throughout the world. But using a ballast to limit the lamp current in a battery-powered fluores cent circuit is not practical because of the relatively high losses involved (about 10 watts for a 40 watt tube). This is too much to tolerate for a battery operated circuit. Fortunately, as the saying goes, "there is more than one way of skinning a cat". When designing a transistor driven inverter, it is desirable to make it run at a high frequency. This enables a very compact ferrite cored transformer to be used and also allows losses to be minimised. Using a high frequency to drive the fluores cent light fitting then gives a further advantage - we don 't have to use a conventional inductive ballast. Instead, we can use a capacitor. This will limit the current but the capacitor will have very low interna l losses which translates to higher efficiency. As well, the capacitor will be more compact and cheaper than an inductive ballast. It is also possible to design the inverter to have a higher internal impedance in order to limit the discharge current through the fluorescent tube but that tends to lead to higher losses and hence, higher current drain. Let's now have a look at the ci rcuit. The circuit Simplicity is the byword for this circuit although as with many ostensibly simple circuits , there is more to it than at first meets the eye. As shown in Fig.1 , the circuit is essentially a straightforward push-pull transistor inverter. The positive DC supply connects to the centre-tap connection on the primary winding of the transformer and the collectors of the two power transistors are each connected to the ends of the primary winding. When one transistor, say Ql, turns on, it saturates and applies the full 12V DC across half the transformer primary winding. By transformer action, thi s applies 12V (in opposite polarity) to the other half of the transformer winding , so that we have a total of 24 V across the full winding. The current builds up in the transformer winding associated with Ql until it reaches its maximum value whereupon Ql is suddenly forced to turn off and QZ turns on. Why? Because transformer action also applies to the other windings of the transformer and this means that the base drive voltages for Ql and QZ are generated by the associated 2-turn windings. When the current pulled by Ql reaches its maximum value, it effectively becomes DC and since transformers don't work on DC, the base drive voltage to Ql suddenly drops to zero. +12V Rx 1W 470 16\/V\' , + 0.47 TO FLUORESCENT TUBE ov ; BCE RX 40W Cx 120(2 + 22U .001 600VACl/.001 600VAC 20W 330\l 7-16W 680!1 .001 600VAC// .001 600VAC .001 600VAC 01 02 TIP3055 TIP3055 MJ E3055 VERSATILE FLUORESCENT INVERTER Fig.1: the circuit is a straightforward transistor inverter which steps up the 12V DC battery voltage to about 1000V AC to fire the fluorescent tube. Capacitor Cx limits the tube current to a safe value while flyback diodes D1 & D2 protect the transistors from voltage spikes when each one turns off. This causes Ql to turn off and since the current pulled by Ql is no longer DC but rapidly dropping to zero, transformer action now generates base voltage for QZ which then turns on fully. This reverses the 24V across the full primary winding and so the current pulled by QZ climbs to its maximum value whereupon the cycle begins again. The voltage across the transformer primary is 24 volts peak or approximately 12 volts RMS. (Actually, it will usually be higher than that, depending on the condition of the battery). This voltage is then stepped up by the 500-turn secondary winding to aro und 1000 volts RMS. This is enough to fire any fluores cent tube currently available. It will even fi re the 1.5-metre long 58 watt tubes. Once the discharge current through the tube is established, capacitor Cx in series with the tube limits the current to a safe value. Starting the inverter Balanced inverters such as the one depicted in the circuit of Fig.1 will usually not start by themselves. They need a fixed bias so that one of the transistors can turn on and start the switching cycle described above. This bias is provided by resistor Rx and the ZZQ 0.5W resistors in series with the bases of Ql and QZ. When power is first applied, both transistors try to turn on but the one with the highest current gain (beta) wins the race and starts the cycle. Diodes Dl and DZ are flyback protection diodes, included to protect the transistors from the peak voltages that are developed when each one turns off. Air gap in ferrite core All three versions use the same transform er but the gap between the ferrite cores is varied to set the current drain. This concept may be novel to some of our readers so we'll explain the principle. Normally the transformer used for DC-AC inverters is constructed without any gap which means that the core permeability is very high. This means that a relatively small magnetising force (ie, ampere-turns) is required to saturate the core. This results in a squ are hysteresis curve and a very square AC waveform in the primary and secondary windings. Such a waveform though is not really suitable if the current limiting element for the fluorescent tube is to be a capacitor - all it will let through will be very narrow current spikes FEBRUARY1991 47 which will not let the tube operate at normal brilliance. No, for the capacitor to correctly limit the current through the tube, the voltage waveform from the transformer should be more rounded, like a sinewave. This is achieved by having an air gap between the two halves of the ferrite transformer core. The oscilloscope photo accompanying this article shows the voltage waveforms which can be expected from the transformer. Note that they are nothing like square waves but much more like sinewaves. Having an air gap means that the inverter also draws a substantially higher current and that means the exact dimension of the air gap is quite important. We'll have more to say about this later. Construction All three inverters can be built using the same PC board which is coded PARTS LIST 16W Version 1 PC board, code SC11103901, 99 x 45mm 1 L-shaped mounting bracket (110mm in length) 1 TO-220 heatsink mounting kit 1 Siemens ETD29 transformer bobbin, N27 core & clips 1 25-metre length 0.2mm ECW 1 5-metre length 0.25mm ECW Semiconductors 2 MJE3055 NPN power transistors 2 1N4002 200V 1A power diodes Capacitors 1 470µF 16VW PC electrolytic 1 0.47µF 63VW metallised polyester (WIMA MKS, 5mm lead spacing) 1 1000pF 600VAC polypropylene (WIMA FKP 1) Resistors (5%) 1 6Bon ;w 2 22Q 0.5W Miscellaneous Hookup wire, screws, nuts, washers, solder, etc Note: ECW = enamelled copper wire 48 SILICON CHIP DV - TO FLUORESCENT • ~ ~ - TUBE Fig.2: all three versions of the inverter are built on the same PC board but some of the component values are different, as are the transformer winding details (see Fig.I). The completed board is mounted on an L-shaped bracket which is used for heatsinking but note that Qt & Q2 must be electrically isolated from this heatsink as shown in Fig.4. SCl 1103901 and measures 95 x 40mm. When you buy or make the board and before you commence construction, check it for breaks or shorts in the tracks. If there are any, correct them now. The table in Fig.1 shows the values for Rx and Cx for each model of the inverter. It also indicates that TIP3055s are used for the smallest inverter while the more rugged MJE3055s are used for the 20 watt and 40 watt models. After checking the board, you can install the wire link, the resistors, diodes and the capacitors. Make sure that you have the electrolytic capacitor inserted correctly. Winding the transformer when you rattle or crumple them them). You can now begin winding the second layer of another 100 turns, after which you repeat the above process. Continue doing this until you have wound fiv e layers of 100 turns, making up the 500 turns of the secondary winding. Note that if you are building the 16W version, only 400 turns are required. Once you've completed the last of the five windings, wind on five to six layers of polypropylene foil to ensure isolation. Now for the primary and base windings. The primary winding (that is the winding to the transistor collectors) contains two windings of six turns which are tapped in the middle. The gauge of the wire in this case depends on the inverter you are working on. For the 7-16W version, the wire is 0.35mm and 0.5mm for the other versions. The base windings are two wind- Before you continue any further with the board, you should begin winding the transformer. Fig.3 shows where the windings should be terminated on the former, as well as giving the correct number of turns for each inverter. Begin by winding on 100 turns of 0.2mm enamelled copper wire (or 0.25mm for the 40 watt inverter). This should get you from one side of the core to the other. Now, using a small brush, apply an even coating of petroleum jelly (Vaseline) to the winding. Once this has been applied, cover the winding with a layer or two of polypropylene foil. A This oscilloscope photograph shows the voltage good source of this is waveforms that can be expected from the those freezer bags you transformer. At top is the 200V p-p tube voltage can buy from your local while the bottom trace shows the 30V p-p supermarket. (These bags waveform which appears on the transistor collectors. The frequency is 17kHz (approx). are crinkly and noisy y X B rl-ol Lr-r-r , 2 31 A 0 0 0 O h ol 0 bifilar (ie, with two wires together) but in this case bifilar winding has not been used. When all the windings are complete, the starts and finishes must be cleaned of enamel, tinned and then terminated to the various pins of the bobbin. Before the wires are terminated, it is a good idea to fit some sleeving over them. Setting the air gap I C 1-2,2-3 A-B,B-C X-Y 40W 2T, 0.35mm ECW 20W 2T, 0.35mm ECW 6T, 0.5mm ECW 6T, 0.5mm ECW 7-16W ZT, 0.35mm ECW 6T, 0.35mm ECW 500T, 0.25mm ECW 500T, 0.2mm ECW 400T, 0.2mm ECW Fig.3: terminate the transformer windings exactly as shown here. The step-by-step winding details are given in the text. Be sure to use the correct gauge of wire & number of turns on the secondary for each version. ings of two turns each, again tapped in the middle. The wire gauge here is 0.35mm throughout. Note that some inverter transformers have the primary windings wound With the windings completed and terminated, the two ferrite half-cores can be placed into the bobbin and held in place by the spring clips. Before the second core half is placed though, the air gap needs to be set. Actually, it is not an air gap but a space between the two core halves, set by a given thickness of insulating material which may be paper or plastic. For the 7-16W inverter, the gap is 0.1mm; for the Z0W and 40W models, 0.44mm. How do you obtain a suitable thickness of material for that gap? If you have vernier caliper or micrometer, then it is simply a matter of measuring some plastic sheeting or paper and then folding it to give the right thickness. If you don't have one of these instruments, a single thickness of a page PARTS LIST 20W Version 1 PC board, code SC11103901, 99 x 45mm 1 SOT-93 transistor heatsink mounting kit 1 L-shaped mounting bracket (110mm in length) 1 Siemens ETD29 transformer bobbin, N27 core & clips 1 30-metre length 0.2mm ECW 1 1-metre length 0.5mm ECW 1 1-metre length 0.35mm ECW Semiconductors 2 TIP3055 NPN transistors 2 1 N4002 200V 1A diodes Capacitors 1 470µF 16VW PC electrolytic 1 0.47µF 63VW metallised polyester (WIMA MKS, 5mm lead spacing) 2 1000pF 600VAC polypropylene (WIMA FKP 1) Resistors (5%) 1 330.Q 1W 2 22.Q 0.5W Miscellaneous Hookup wire, screws, nuts, washers, etc Note: ECW = enamelled copper wire Below: the switching transistors are bolted to the L-shaped bracket to ensure adequate cooling. This is the 16W version which uses MJE3055 transistors. FEBRUARY1991 49 INSULATING MICA WASHER -~jl SCREW r ~ ----CASE 1 T0220 DEVICE Fig.4: mounting details for the switching transistors. Smear all mating surfaces with heatsink compound before bolting each transistor to the heatsink, then use your multimeter to confirm that its metal tab is correctly isolated. of this magazine is close to .05mm. A single page of a newspaper is close to .08mm while standard 80gsm photocopier paper is 0.1mm. With a combination of these materials, you can obtain the right thickness of material for the transformer gap. By the way, you can obtain a very cheap plastic set of vernier calipers for as littl e as $3.50. See your local hardware store. Now that you've completed the transformer, you can ins ert it into place on the PC board. Use the overlay diagram to make sure you have connected it up correctly. Check again for any solder splashes on the board as it will not be possible to do this after the two transistors have been mounted on the heatsink. Heatsink bracket As you can see from the photos, the prototypes had the two transistors I. ' li.J 0 mounted on an L-shaped aluminium bracket. The two transistors are insulated from the aluminium bracket with mica washers, insulating bushes and heatsink compound, as shown in Fig.4. The L-shaped bracket also has the PC board mounted on it, as you can see. Before you mount the transistors on the bracket, solder them into position on the board; then, using four mounting screws, spacers and nuts, secure the board to the bracket. Note that the 40W version of the inverter needs an additional finned heatsink mounted to the back of the aluminium bracket. Use a larger heatsink than the one shown in our photos, as our prototype ran a little too hot for our liking. When the transistors are mounted, switch your multimeter to the ohms range and make sure that both transistors are correctly isolated from the aluminium bracket. You should get an open circuit if you _measure between the bracket and the transistor tabs. Ideally, the whole board and bracket assembly should be mounted inside a standard fluorescent light fitting but before you do that, it's time to check the circuit operation. You will need a 12V power supply or battery capable of delivering sufficient current to suit the inverter model. When operating correctly, the inverters will draw the following currents: 7-16W 20W 40W 0.9A 1.4A 3.2A When connecting the power supply to the inverter, make absolutely sure you have the correct supply polarity. If you don't, you are liable to 00---. • OP-010 FLUORO INVERTER You can use this full-size pattern to etch your own PC board and to check your finished board for possible defects. 50 SILICON CHIP PARTS LIST 40W Version 1 PC board, code SC11103901 , 99 x 45mm 1 SOT-93 transistor heatsink mounting kit 1 L-shaped mounting bracket (110mm in length) 1 single sided finned heatsink (see text) 1 Siemens ETD29 transformer bobbin, N27 core & clips 1 30-metre length 0.25mm ECW 1 1-metre length 0.5mm ECW 1 1-metre length 0.35mm ECW Semiconductors 2 TIP3055 NPN transistors 2 1N4002 200V 1A diodes Capacitors 1 470µF 16VW PC electrolytic 1 0.47µF 63VW metallised polyester (WIMA MKS, 5mm spacing) 2 1000pF 600VAC polypropylene (WIMA FKP 1) Resistors (5%) 1 120Q 1W 1 22Q 1W 2 22Q 0.5W Miscellaneous Hookup wire, washers, screws, nuts, solder, etc. Note: ECW = enamelled copper wire damage the transistors. When you are connecting the fluorescent tube, you will find that it has two pins at each end. These pins are for the heater filaments which are not used in this circuit. You can make the connection at each end to either filament pin - it does not matter which. When power is applied, the tube should immediately light up and then abruptly increase its brilliance after about one or two seconds. There is no flickering as with conventional starter/ballast fluorescent lights. Note that fluorescent tubes do not reach their full brilliance when they are first turned on. Depending on the ambient temperature, they make take 10 minutes or longer to reach full brilliance. Even so, if you are a making a di- BARGAIN PRICED LASER A spacial purchase ol lllls tuba, which Is priced at a fraction ol Its real value allows us ID offer Ibis mains (240¥) powered KIT SPECIAL. Use 11 lor disco affects, holography, axparlmanling, ale, ale. Includes a la111a 2·3mW HE-NE tuba, translormar, PCB, and all components, amins plug and lead plus Illa mains switch: Everything except the case. IMPORTANT: This product carries lethal voltages and Is not for kids. Strlctty during February and Man:h, or until stocks run out at this unrepeatable price ol: ONLY $249.90 !! The price even includes Certified P&P. Don't tell your friends because you could miss out yourself! This view shows the 20W version of the inverter. Note the mica washers & insulating bushes used to isolate the switching transistors from the L-shaped aluminium bracket (see also Fig.4). The completed converter should be compact enough to mount in a standard fluorescent light fitting. LASER SCANNER DRIVER KIT This PCB and all romponents kit can be driven using an audio source or an electret microphone, to produce magnificent visual displays which are synchronised to music. Drives speakers with mirrors attached, or professional X-Y scanner. Makes provision possible driving from a romputer (with rorrect software). The kit includes the PCB, all onboard components, battery holde, external potentiometer, and even an electret microphone for ONLY $20.00 Professional X-Y scanner: $220 Small front surfaced aluminium mirrors: $9.00ea (These are to attach to the front of speakers) SEE IN THE DARK! A large finned heatsink (preferably slightly larger than the one shown here) should be fitted to the 40W version to stop the switching transistors from overheating. Although rated for driving 40W tubes, this version can also be used with the 36W slimline tubes, while the 20W version above can be used with 18W slimline tubes. rect comparison between a fluorescent tube powered by one of these inverters and an identical tube powered from a conventional mains fitting, you will find the brilliance of the inverter-powered tube slightly down. This is normal and is the compromise we came to for best efficiency. If you find that your inverter-powered lamp is not as bright as it should be and the inverter is drawing less current than it should be, it is likely that the transformer gap is not correct. Increasing the air gap by adding another thickness of paper should increase the brightness and the current drain. However, the current drain should not be appreciably higher than the values we show above, otherwise the inverter transistors will rapidly overheat. When normal operation is confirmed, you can mount the inverter inside a standard fluorescent light fitting and the job is finished. Finally, you can use the n ewer slimline tubes with these inverters. The 20W inverter will happily driv e an 18W slimline tube while the 40W job will drive a 36W slimline tube. sc with our INFRA RED NIGHT VIEWER at a SPECIAL price of ONLY $349.00 Only while stocks last, . . .• . and during March and April only we will include a high quality round IR filter which is 3mm thick and has a diameterof75mm. Can be easily cut down to suit your torch We have good stock of the 6032A tubes. This is your opportunity to build your own night viewer. A product which commercially usually costs 1000's of dollars at a fraction of the price. We supply the electronic kit, a 6032A Image Intensifier tube, and the case components. The 6032A iubes are BRAND NEW and pertorm well wi1h a good infra red torch. Torch and Lenses are not supplied I VISA I OATLEY ELECTRONICS PO BOX 89, OATLEY, NSW 2223 Telephone: Fax No: (02) 579 4985 (02) 570 7910 Certified p&p $4-$6 Aust. NZ add $2 Melbourne Distributor - Electronics World (03) 723 3860 or (03) 723 3094 FEBRUARY1991 51