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ELECTRONIC Clever IC provides rapid turn-on STARTER For fluorescent lamps Do your fluorescent lamps go blinkety-blink blink blink when you turn them on? Or do they flash on to blind you and then plunge you into darkness? Solve these problems with this new electronic starter which gives a rapid start every time. It fits in a standard starter case so the lamp wiring does not have to be altered. By JOHN CLARKE L ET’S FACE IT, fluorescent lights are bright and effi- cient but they can be very annoying when they don’t start as soon as you switch them on. This “blink blink blink - nothing - flash - Ah! it’s on!” sequence can be particularly frustrating if you need to leave your warm bed on a cold night for a “comfort break”. Fluorescent lamps are much harder to start when the temperature is low which adds to the problem. What can be really frustrating if you have a cantankerous fluorescent lamp is that changing to a new starter or even a new tube may not help much. Modern slimline 18W and 36W tubes are hard to start, even when new, and they are a real problem if they are used in a batten fitting intended for older style 20W or 40W tubes. Up till now, there has been no solution to this problem but Philips has just released a surface mount 8-pin chip which appears to be a real ripper. Designated the UBA2000T, it is specifically designed to start slimline “TL” tubes and incorpo­rates features which overcome all the disadvantages of conven­tional 14  Silicon Chip “glow switch” fluorescent starters. Before delving into the operation of the electronic starter we need to see how a fluorescent lamp circuit works and why the conventional starter it has its disadvantages. So let’s refer to Fig.1. A fluorescent lamp is connected to the 240VAC mains supply via a “ballast” which is an iron cored inductor. In more detail, the current from the 50Hz mains passes through one of the tube filaments, then through the starter, through the other filament and then via the ballast. The starter, as its name implies, gets it all going. If you pull a conventional starter apart, and you will if you build this project, you will find that it contains what looks like a conventional miniature neon lamp connected in parallel with a high voltage capacitor, typically .005µF 2kV ceramic disc. This very simple construction has quite a complex function. Similarly, the fluorescent tube itself looks very simple but there is more to it than meets the eye. A fluorescent tube is coated with a phosphor on the inside of the glass and it contains a minute quantity of mercury and a mixture of inert gases. As well, it has a filament heater at each end. This made with triple coiled tungsten wire and coated with an emissive material such as barium or strontium oxide. When power is first applied to the circuit of Fig.1, current is passed through the two filaments to raise them to red heat and this causes them to emit electrons, just like the filament in a radio valve. The electrons rapidly disperse along the tube so that when a high voltage is applied to the tube, an electric discharge can occur through the inert gases. Once this discharge starts, the mercury in the tube is vaporised and it begins to emit ultraviolet light. The ultravio­let causes the tube phosphors to fluoresce and so visible light is produced. The job of the starter is twofold. First, it has to let current pass through the filaments so they can heat up and emit electrons. Then after a short delay, the starter interrupts the current Fig.1: the circuit a conventional fluorescent lamp with a glow switch starter. The starter enables filament current to flow at switch-on and it opens after a short delay. The back-EMF then generated by the ballast inductor then fires the tube. That’s the theory anyway. to the filaments. Since the ballast inductor is also in series with the filaments, this sudden interruption of current causes it to produce a brief high voltage spike. This high vol­tage is applied to the tube to cause the electric discharge referred to above. If all goes well, the tube lights up and then the starter is effectively out of circuit. Clearly though, while the glass tube in the fluorescent starter might just look like a largish neon lamp, it is more than that. The starter has two contacts, one of which is bimetallic. When voltage is first applied to the circuit of Fig.1, the inert gas inside the starter ionises and a small amount of current flows. This heats up the interior of the starter and so the bimetallic contact bends over slightly to meet its mate and so current can flow through the two filaments and the ballast. Meanwhile the interior of the starter cools down, the bime­ tallic contact opens the circuit, the filament current stops and the ballast fires the tube. If all goes well, that is. Generally though, the starter has to make several tries before the fluores­cent tube fires properly and that leads to the blink, blink problem that we all know and hate. Features • • • Fig.2: functional diagram of the UBA2000T TL lamp starter. It counts the cycles of the 50Hz supply to give a precise filament heating time and it also monitors filament current to ensure that the lamp has the best chance of starting. • • • Starts 18 and 36W slimline fluorescent tubes Compatible with standard fluorescent starters Fast start without excessive flicker Precise preheat time Minimal radio interference Timeout if lamp fails to fire August 1996  15 Fig.3: the UBA2000T lamp starter IC (IC1) switches a 1000V Mosfet (Q1) to reliably start slimline and conventional fluorescent tubes. The IC repeats the start sequence up to six times, after which the Mosfet is turned off as a safety measure. 4x1N4007 So why is the capacitor inside the starter? One reason is that it helps prevent arcing across the contacts as they open. The other is that it helps reduce the radio interference both from the starting operation and from the electric discharge inside the fluorescent tube. These conventional starters are very simple to manufacture but they have a number of drawbacks. First, the pre­ heat time is set by the thermal lag of the bimetallic contact. This is the time it takes the contact to cool and reopen and it can vary depending on ambient temperature and manufacturing tolerances. In some cases the preheat time will not be enough to allow the filaments to warm up sufficiently to fire the tube. Naturally, this problem gets worse as the starter and fluorescent tube get older. A more serious problem is that when the starter contact opens, the induced voltage from the ballast may not be sufficient to fire the tube. This is because the bimetallic contact can open at any time within the mains cycle and the ballast current may be very low when this happens. So that is why even a new starter may need several tries to fire the fluoro tube. PARTS LIST 1 PC board coded 10308961, 17 x 28mm 1 fluorescent starter container and lid with terminals (see text) 1 12mm diameter x 12mm long piece of heatshrink tubing Semiconductors 1 UBA2000T TL-lamp starter (IC1) (Philips) 1 TO-220 1000V Mosfet, BUK456-1000B, STP3N-100 (Q1) 4 1N4007 1000V rectifier diodes (D1-D4) Capacitors 1 3.3µF 63VW PC electrolytic 1 .0056µF 2kV ceramic Resistors (0.25W, 1%) 1 1MΩ 1 100kΩ 500V MAX (Multicorp) 1 62kΩ Thirdly, there is no provision to stop the starter sequence if the lamp fails to start. This repetitive starting can eventu­ally burn out the ballast Fig.4: this diagram illustrates the starting sequence of the UBA2000T. 16  Silicon Chip due to overheating. Alternatively, if the starter’s contacts weld up, the ballast will be burnt out and this means an expensive repair. Generally, it is cheaper to replace the whole lamp fitting. Clever chip Our new electronic starter circuit is shown in Fig.2. It plugs in directly to the starter socket on a fluorescent lamp fitting. As well as using the Philips UBA2000T lamp starter chip, it has a 1000V Mosfet, a bridge rectifier and a few resistors and capacitors. While the UBA2000T is a teensy little chip, it has quite a lot of circuitry inside it, as indicated by the function­al diagram of Fig.2. Looking at Fig.2, the UBA2000T has Vin and Vsense pins which monitor the mains voltage and filament current, respective­ ly. By monitoring Vin the UBA2000T knows whether the tube is ignited or not; the voltage level is lower once the tube is ignited. By monitoring the filament current, the UBA­ 2000T can fire the tube at the optimum time. Pin 3 drives the gate of a 1000V Mosfet which is used to switch the filament current on and off. The Mosfet is not switched on if the Vcc supply is too low (below 40-49V) or the current through the filaments is excessive (above 2.2A peak). Fig.4 shows the typical start sequence waveform. When power is first applied to the circuit, the capacitor at the Vcc pin is charged through the internal switch S1. When Vcc reaches the start voltage, (Vcc(sl)) and when the mains voltage is at its peak value, the Mosfet will be turned on. The UBA2000T now counts the mains cycles until 1.52 seconds (ie, 76 cycles at 50Hz) has elapsed. Also during this time the capacitor at the Vcc pin discharges. The Mosfet is switched off provided the current through the internal sense resistor is greater than 285mA. This allows the ballast inductor to produce sufficient voltage to fire the tube. Typically, this firing voltage will be somewhere between 700 and 800V! If the fluorescent tube does not fire, the UBA2000T tries again, as shown in Fig.4. It must first recharge its own supply capacitor at pin 6 (Vcc) and then filament current is applied again. This second preheat period is set to 0.64s since the filaments are already assumed to be warm. After the tube fires, the peak voltage across it will typical­ly be about 100V which is considerably lower than the mains voltage peak (around 340V) and so the Vcc(sl) threshold for the UBA2000T can no longer be reached. The Mosfet is therefore held off and the starter circuit is effectively out of action until the mains voltage is turned off and reapplied. The UBA2000T will repeat the start sequence six times after which the Mosfet will be turned off. This is a very good safety feature since it prevents the ballast inductor from being burned out. As a further safety feature, the Mosfet will be turned off if the sensed preheat current exceeds 2.2 amps peak. Circuit description The circuit of Fig.3 shows how the UBA2000T is used in practice. Diodes D1-D4 are connected in a bridge to rectify the mains voltage. This applies the correct voltage polarity to both IC1 and Q1. The 100kΩ and 62kΩ resistors across this rectified mains supply divide the voltage down for the pin 4 input and limit the charge current to the 3.3µF Vcc capacitor at pin 6. The 1MΩ resistor between pin 6 and the gate of Q1 provides a small pullup This photo shows the copper side of the assembled PC board. The surface mount IC means that you will need a fine-tipped soldering iron to mount it in place. Fig.5: the parts layout diagrams for both sides of the PC board. Note that the four diodes and the 100kΩ resistor are mounted underneath the .0056µF ceramic capacitor. Only three parts are mounted on this side, the main one being the 1000V Mosfet. This should be sleeved with heatshrink tubing before the board is installed inside the starter case. current to keep the Mosfet gate high once it is triggered by a pulse from pin 3. The gate is switched off when pin 3 goes low. Note that the source electrode of the Mosfet is connected to pin 1 so that its current (ie, the filament current) is sensed by the internal 26mΩ resistor between pins 1 and 2 of IC1. Capacitor C1 is included to suppress radio frequency inter­ference caused by discharge in the tube. Note that all components are rated for the high voltages involved. The 3.3µF 63VW capacitor can have up to 49V across it, while the voltage across the tube at the instant Q1 is switched off can be 800V or more. Consequently, C1 has a 2kV rating while Q1 and D1D4 have a voltage rating of 1kV. The 100kΩ resistor must have a minimum rating of 500V. Construction Fig.6: this is the full-size etching pattern for the PC board. Check your board for etching defects by comparing it with this pattern before installing any parts. The electronic starter is constructed on a PC board coded 10308961 and measuring 17 x 28mm. This is designed to be a snug fit inside a standard fluorescent starter container. Even so we had to mount components on both sides of the board and in some cases RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ No. 1 1 1 Value 1MΩ 100kΩ 62kΩ 4-Band Code (1%) brown black green brown brown black yellow brown blue red orange brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown blue red black red brown August 1996  17 The electronic fluorescent starter is mounted inside a dud fluorescent lamp starter case. It will rapidly start slimline (25mm) and conventional (38mm) fluorescent tubes without flashing. they lie on top of each other, as you will see from the diagram of Fig.5 and the photos. To put this PC board together you will need either a very keen pair of eyes or better still, a pair of close-up specs or a mag-lamp. IC1 is a surface mount IC so you will need a finetipped soldering iron since the IC’s legs are spaced only 1.27mm (.050") apart. Note that since IC1 is a surface-mount type, it is mounted on the copper side of the board. Other components on the copper side are the four diodes, C1 and the 100kΩ and 62kΩ resis­tors. Check that the PC board is correct by comparing it with the published pattern. Correct any shorted or broken tracks at this stage. Before soldering anything to the board we suggest that you pre-tin the copper tracks for the IC pins. Then place the IC in position making sure it is oriented correctly. How do you do that? We did say you will need very good vision. Notice that one end of the IC is chamfered along one side; pin 1 is at the top, if you hold the IC with the chamfered edge at left. This can be seen in Fig.5. Once you have the tracks for IC1 tinned and it is in posi­tion, solder each pin quickly with just enough heat to melt the solder on the PC board. Then check each solder connection is good by measuring the resistance between each pin and the PC track. On the component side of the PC board side insert and solder in the 1MΩ resistor and the 3.3µF capacitor, taking care with polarity. The capacitor should lie over the 1MΩ resistor. Keep a few mm of lead length between the PC board and capacitor so that it lies more or less parallel with the board. Cut the leads short on the copper side after soldering. Now install the remaining parts on the copper side. Make sure that the diodes are oriented as shown and cut the leads flush with the PC board side. Now place the .0056µF capaci­tor over the diodes and bend its leads so that the capacitor body can lie parallel with the board. Solder this in place. Next, mount the Mosfet on the component side, with its leads bent at right angles so that it lies parallel to and close to the board. It is oriented so that the metal tab faces away from the top of the board. Solder and trim its leads. Lastly, fit a 12mm length of 12mm diameter heatshrink tubing over the Mosfet to com­pletely insulate it. Starter container You will need to disassemble a starter for its case and lid with terminals. Use a small screwdriver to carefully prise the Bakelite lid from the cylindrical container. You will need to gradually work around the whole circumference of the container with the screwdriver until the baseplate can be removed. Withdraw the lid and components and cut the wires close to the capacitor body. These leads are then attached to the PC board of your new electronic starter. Cut the starter tube wires close to the baseplate lug. The capacitor leads can now be inserted into the PC board from the PC board side and soldered in place. Next, carefully inspect the PC board assembly for any solder dags or splashes or pigtails which are too long. This aspect is most important when you consider the peak voltages which can occur between the leads to the Mosfet and diodes. None of your soldering should diminish the gaps between conductors of the bare board. When you are satisfied that all aspects of the soldering and assembly are correct, insert the PC board and starter lid assembly into the container and clip in place. Note that the components may need to be pressed closer to the PC board if the fit is too tight. Finally, test your new electronic starter in a fluorescent light fitting. The tube should initially glow orange at the filaments, then glow white at the tube ends and then light up fully, usually SC after the first attempt. Especially For Model Railway Enthusiasts Includes 14 projects for model railway layouts, including throttle controllers, sound simulators (diesel & steam) & a level crossing detector. Price: $7.95 plus $3 for postage. Order today by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or send cheque, money order or credit card details to: Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 18  Silicon Chip