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An electronic ballast for fluorescent lamps Do you hate fluorescent lights with their inevitable flick, flick, flicker at switch-on, the flicker while they are running & the buzz or hum of the ballast? Now you can replace the internals of your fluorescent light fittings with this elec­ tronic ballast. It is highly efficient, gives instant starting & has no flicker, buzz or hum. By JOHN CLARKE Fluorescent lights are good. They are much more efficient than any incandescent light, they are free of glare and cast very little shadow. But fluorescent lights can also be a pain, espe­cially when they are first turned on. If the tube or the starter is a bit old or the temperature is low, there will be this inevi­table flick, flick, flicker and then maybe it will come on fully. These and the other irritations associated with fluorescent lights can be eliminated with this electronic ballast. It fits directly into a stand- WARNING! This circuit operates at voltages which are potentially lethal. No part of the circuit should be worked upon while it is connected to the 240VAC mains. If the project is to be used in a permanent domestic installation, it should be connected to the 240VAC mains by a licensed electrician. 42  Silicon Chip ard fluorescent light batten and can be built to suit 18W, 20W, 36W and 40W tubes. The electronic ballast gives a virtually instant start and since the tube is run at a very high frequency (around 100kHz), there is absolutely no sign of flicker. Nor is there any audible, buzz hum or whistle. As a bonus, electromagnetic interference to radio reception is low. Power factor control The electronic ballast design can also be said to be “green” in that it has less impact on the environment. This comes about because of its use of a power factor controller chip. Let’s discuss this point. Electricity supply authorities are constantly after ways to reduce power losses. This not only improves power station effi­ciency (meaning that less coal is burnt) but can keep costs down for the consumer. One of the major ways is to maintain the load current directly in phase with the supplied voltage. For loads such as incandescent lights and bar radiators, the current is in phase with the voltage but for inductive loads such as Shown almost actual size, the PC board is designed to fit into a standard 18W or 36W fluorescent batten fitting. It lights the tube almost instantaneously & produces no audible buzz or hum. motors and conventional fluorescent lights, the current lags the voltage considerably. Fig.1 shows roughly how the current lags the voltage for a conventional fluorescent tube. Here the current lags the voltage by 45° so that the power factor is 0.7 (cosine 45°). Since the supplied power is the RMS voltage x the RMS current x the power factor, the supplied current must therefore be some 41% greater than if it was exactly in phase (ie, power factor of 1 or unity) with the applied voltage. • • • • • • • • • • • Fig.1: this diagram shows the phase relationship between the voltage & current in a conventional fluorescent light fitting; the current lags the voltage. Note that in reality, the fluores­cent light current is not sinusoidal but it is shown in this way for simplicity. Features Suitable for 18W/20W and 36W/40W tubes Replaces existing ballast and starter High efficiency Fast start without flicker Noiseless operation High frequency drive Filament preheat Constant lamp brightness from 200V-280VAC input Fuse protection for faulty tubes 0.99 power factor Low electromagnetic radiation Fig.2: in a conventional electronic ballast, pulses of current are drawn at the crests of the 240VAC 50Hz waveform. This leads to poor line utilisation & a less than desirable power factor. October 1994  43 2.5V REF ZERO CURRENT 5 DETECT INPUT CURRENT 4 SENSE INPUT 8 UNDER VOLTAGE DETECTOR ZERO CURRENT COMPARATOR 7 DRIVE OUTPUT LATCH, PWM, TIMER, LOGIC VREF OVER VOLTAGE COMPARATOR 1.08xVREF 1 MULTIPLIER 3 INPUT Fig.3: block diagram of the MC34262 power factor controller IC. The heart of the chip is the two input multiplier. VCC MULTIPLIER ERROR AMP QUICKSTART 2 VOLTAGE FEEDBACK INPUT VREF 6 A +353V 0V 240VAC C1 HIGH FREQUENCY BYPASS N Dx Lx 3 1 Q1 7 IC1 MC34262 4 6 C2 STORAGE Iavg +400V LOAD R1 VIN ILI Iavg Block diagram ON Q1 OFF Fig.4: simplified boost circuit employing the MC34262 power factor controller chip. Q1 is switched on & off for varying times during each AC half-cycle so that the current drain is evenly spread out. Fig.5: the fluorescent driver circuit. This takes the 400V DC from the boost circuit & uses an oscillator running at 100kHz to drive the fluorescent tube. This gives appreciably more light output than the same current at low frequency; eg, 50Hz. 44  Silicon Chip This extra current requirement when the power factor is less than unity contributes to substantial power losses in the mains distribution system all the way back to the alternators at the power stations. Since the power losses follow a square law (ie, I2R), increasing the current required by 41% will double the power losses! As a consequence, most commercial and industrial lighting installations are required to include power factor correction in the light fittings, by adding a capacitor across the supply. This is not required in domestic light fittings but perhaps it should be. Most electronic ballasts (and indeed all power supplies) have a similar drawback as far as the energy authorities are concerned. This is because they use a bridge rectifier and ca­pacitor filter. Typical electronic ballasts, as used in compact fluorescent lamps, use the circuit shown in Fig.2 to derive a 353VDC supply. These circuits draw a large pulse of current at the crest of each mains half-cycle and while the current is essentially in phase with the voltage, the fact that it has such a short duty cycle means that again, power losses are higher than they otherwise would be. The effective power factor for this type of circuit is between 0.5 and 0.7. By contrast, the SILICON CHIP electronic ballast incorporates a power factor controller chip which ensures that the current drawn from the 240VAC mains is spread more evenly over each half-cycle, and thus reduces losses in the distribution system. Fig.3 shows the internal details of the Motorola MC34262 power factor controller IC while Fig.4 shows how it is connected to boost the incoming mains voltage. It drives a boost converter using Mosfet Q1, inductor Lx, diode Dx and capacitor C2. The incoming 240VAC mains is fed to a bridge rectifier to provide positive-going half sinewaves. Capacitor C1 functions as a high frequency filter. Mosfet Q1 is rapidly switched on and off and each time Q1 is switched off, the energy stored in Lx is trans­ferred to capacitor C2 via diode Dx. IC1 monitors the DC output voltage, the current through Q1 (via resistor R1) and the raw DC input waveform. As a result, Q1 is switched on for longer times at the beginning and end of each A 47k 1W 750k 47k 1W 12-35VDC 470 35VW 750k 8 3 12k 22k 5 T1 S2 N2 .01 2 Q1 BUK547600B 10  G 7 S1 D6 BY229/ 600 D C2 6x1 400V S .018 1 150k 150k 150k 820k 680pF 3kV 12k ZD1 12V 1W 330  D8 1N5062 +2.5V 27k T2 N2 .018 27k 150k R1 43k 6 F2 2x 330  1W 820k D7 50822800 .0068 TP2 +400V 47  4 10 16VW F1 N1 F2 IC1 MC34262P T1 : EFD25/13/9 TRANSFORMER ASSY 3F3 CORE WITH 200um AIR GAP T2 : RCC 12.5/7.5/5 3F3 RING CORE L1, L2 : 26T 0.4mm DIA ENCU ON RCC/23/14/7 3F3 RING CORE L3 : 60T 0.4mm DIA ENCU ON EFD/20/10/7 TRANSFORMER ASSY 3F3 CORE WITH 150um AIR GAP D5 1N4936 DIAC1 ST2 0.1 63V 68k 2x 330  1W 0.12 A D1-D4 4x1N5062 F1 .01 250VAC L1 270k 240VAC ZD2 12V 1W 330  N3 0.1 250VAC 270k .01 250VAC L2 TP1 CASE +360V 0V C3 .001 3kV Q3 BUK457600B D G S D A C1 0.22 400V N FL1 FLUORESCENT TUBE C4 0.1 250VAC L3 900uH N1 22  100Hz NOTCH FILTER Q2 BUK457600B D G S GD S K A E CASE T1 FL1 F1 F2 36W 5A 500mA 7T 0.25mm DIA ENCU 18W 5A 250mA 10T 0.25mm DIA ENCU N2 T2 N1 N1 N2, N3 R1 84T 0.4mm DIA ENCU 14T 6T, 6T 0.4mm DIA ENCU 1.5  120T 0.4mm DIA ENCU 24T 3T, 3T 0.25mm DIA ENCU 3.3  ELECTRONIC BALLAST FOR FLUORESCENT TUBES Fig.6: the circuit is more complicated than typical electronic ballasts because it uses the MC34262 power factor controller (IC1). Note that the entire circuit is powered directly from the 240VAC 50Hz mains supply. half-cycle and for shorter times at the crest of each half cycle, as depicted in the waveforms associated with Fig.4. So in effect, the current drain of the circuit is spread more or less evenly over each half-cycle and the power factor is close to unity. The MC34262 has a number of other features which we will discuss later. The 400VDC output from the power factor controller circuit drives the fluorescent tube but it must be converted into a high frequency AC voltage using the scheme depicted in Fig.5. This uses an oscillator to drive the tube via a resonant circuit consisting of inductor L3 and capacitor C3. A starter circuit is also required to fire the tube after which the oscillator is essentially free running. Main circuit Fig.6 shows the complete circuit of the electronic ballast. The 240VAC mains is applied via fuse F1 and an interference filter comprising L1 and L2 and associated capacitors. L1 & L2 are wound onto a common toroid in antiphase so that the inductor works to eliminate common mode high frequency signals without saturation from the line current. The .01µF capacitors act to shunt high frequency signals to ground while the 0.1µF capacitor in conjunction with the inductance of L1 and L2 forms a low pass filter to block high frequency signals which would otherwise be radi­ated by the mains wiring. October 1994  45 PARTS LIST 1 PC board, code 11309941, 362 x 45mm 1 18W or 36W fluorescent batten with tube fitted 1 EFD25/13/9 3F3 core, former and retaining clips (2 x Philips 4312 020 4116 1, 1 x 4322 021 3524 1, 2 x 4322 021 3516 1) - T1 1 RCC23/14/7 3F3 ring core (Philips 4330 030 3499 1) -L1,L2 1 RCC12.5/7.5/5 3F3 ring core (Philips 4330 021 3515 1) - L3 4 M205 PC-mount fuse clips 1 5A M205 fuse (F1) 1 500mA M205 fuse (36W version) 1 250mA M205 fuse (18W version) 1 3-way mains terminal block 1 transistor insulating bush 6 9mm tapped standoffs 1 3mm Nylon screw & nut 2 small cable ties 12 3mm diameter screws 4mm long 2 3mm diameter screws 12mm long & two 3mm nuts 8 PC stakes 1 cord clamp 1 mains cord and plug 1 150mm length of 0.8mm tinned copper wire 1 11.25-metre length of 0.4mm enamelled copper wire 1 1-metre length of 0.25mm enamelled copper wire Semiconductors 1 MC34262P power factor controller (IC1) 3 BUK457-600B Mosfets (Q1-Q3) The AC mains waveform is full wave rectified using diodes D1D4 and partially filtered with the 0.22µF 400V capacitor. The resulting raw DC waveform is fed to Q1 via transformer T1 and to pins 3 & 8 of IC1 via series-connected pairs of 750kΩ & 47kΩ resis­tors. The 47kΩ resistors provide the initial power for the chip to pin 8 but once it is in running mode, it derives its power from the secondary winding of T1 via diode D5. 46  Silicon Chip 5 1N5062 800V 2A transient protected diodes (D1-D4,D8) 1 1N4936 400V 1.5A fast recovery diode (D5) 1 BY229-600 600V 7A fast recovery diode (D6) 1 5082-2800 Schottky diode (D7) 1 ST2 Diac (DIAC1) 2 12V 1W zener diodes (ZD1,ZD2) Capacitors 1 470µF 35VW PC electrolytic 1 10µF 16VW PC electrolytic 6 1µF 400VDC metallised polyester (Philips 2222 368 55105 or equivalent) 1 0.22µF 400VDC metallised polyester (Philips 2222 368 55224) 1 0.12µF MKT polyester 2 0.1µF 250VAC metallised polyester film & paper (Philips 2222 330 41104) 1 0.1µF MKT polyester 2 .018µF MKT polyester 2 .01µF 250VAC metallised polyester film & paper (Philips 2222 330 1103) 1 .01µF MKT polyester 1 .0068µF MKT polyester 1 .001µF 3kV ceramic 1 680pF 3kV ceramic Resistors (0.25W, 1%) 2 820kΩ 2 12kΩ 2 750kΩ 4 330Ω 1W 5% 2 270kΩ 2 330Ω 4 150kΩ 1 47Ω 1 68kΩ 1 22Ω 2 47kΩ 1W 5% 1 10Ω 1 43kΩ 1 3.3Ω 5% 2 27kΩ 1 1.5Ω 5% 1 22kΩ The seriesed 750kΩ resistors and a 12kΩ resistor divide the raw DC waveform down to a level suitable for the multiplier input at pin 3. The multiplier has two inputs (which it multiplies together): the input at pin 3 which provides phase and voltage information on the incoming rectified AC waveform, and the output of the error amplifier at pin 2. The error amplifier input at pin 1 monitors the +400V DC output from diode D6 via two 820kΩ resistors which reduce the voltage to +2.5V before it is fed via a 100Hz notch filter (to pin 1). Thus, the internal multiplier has two jobs to do as it controls the pulse width modulation drive to the gate of Mosfet Q1 via pin 7. First, it must regulate the DC output to +400V and second, it must ensure that Q1 is turned on and off so that the current drawn from the AC mains is evenly spread throughout each AC half-cycle. Note that while Q1 is draws current from the raw DC input in the form of very short pulses (typically about 20 microsec­ onds) long, the pulses are longer at the start and finish of each AC half-cycle than they are at the crest. This pulse current is filtered by the input filter consisting of L1, L2, C1 and the associated 250VAC capacitors so that the actual current drawn from the AC mains is 50Hz with relatively low harmonic content. Q1 draws current through winding N1 of transformer T1 (equivalent to inductor Lx in Fig.4) and each time Q1 turns off, diode D6 is forced to conduct and deliver charge to C2 which consists of six 1µF 400V metallised polyester capacitors. The secondary winding of T1 drives diode D5 and a 470µF capacitor to provide power to the chip itself. Current limiting for Q1 is provided by pin 4 which monitors the voltage drop across R1. The current waveform is filtered by the 47Ω resistor and a .0068µF capacitor while Schott­ ky diode D7 is included to clip turn- off voltage spikes due to the inductance between ground and the source of Q1. These spikes would otherwise cause circuit instability. OK, so we have a +400V DC supply and this needs to be turned into high frequency AC to drive the fluorescent tube and a circuit is required to initially fire the tube. These functions are performed by the fluorescent driver circuit which is depicted schematic­ ally in Fig.5. The circuit we have used is very similar to that featured in the fluorescent inverter circuit published in the November 1993 issue of SILICON CHIP. Fluorescent driver The fluorescent tube driver comprises Mosfets Q2 and Q3, transformer T2 and associated components. The fluorescent tube is driven via inductor L3 and the N1 winding of transformer T2. The gates of Q2 and Q3 are driven 5 6 F2 S1 4 7 3 8 9 S2 2 10 F1 1 4 5 3 6 2 7 1 8 L3 T1 WINDING DETAILS L1 N2 N1 N3 T2 WINDING DETAILS L2 Fig.7: winding details for the toroid filters and ferrite cored transformers. Note particularly that the two windings of L1 & L2 are wound in different directions. from the N2 and N3 windings which are connected in antiphase. When power is first applied, there is 400V DC between the drain of Q2 and the source of Q3. The 0.1µF capacitor adjacent to Diac1 begins to charge via two series 150kΩ resistors. When the voltage reaches about 30V the Diac breaks down and dumps the 0.1µF capacitor’s charge into the gate of Q3. Zener diode ZD2 protects the gate from overvoltage. Mosfet Q3 now switches on and current can flow from the +400V supply via the fluorescent tube top filament, the .001µF 3kV capacitor, the second tube filament, the 0.1µF 250VAC capaci­tor, inductor L3 and the N1 winding of T2. The current flow in N1 will apply gate drive to Q2 via N2 and switch off gate drive to Q3 via N3 (due to the polarity of the windings of T2). If oscillation does not occur, the Diac will again fire Q3. Ultimately, when oscillation occurs, Mosfets Q2 & Q3 will switch on and off in alternate fashion. The frequency of operation is set by the combined inductance of L3 and N1 which resonates with the .001µF capacitor, C3. The oscillator current heats the fluorescent tube’s fila­ ments and after a short period (less than a second) the tube ignites. Capacitor C3 is now effectively shunted by the discharge within the tube and the oscillation frequency is set by the core saturation properties of T2. Current through the tube is limited by the saturation of T2 and the impedance of L3. Once normal oscillation occurs, the start-up circuit com­prising Diac1 and the 0.1µF capacitor is disabled by diode D8. This diode discharges the 0.1µF capacitor every time Q3 switches on and hence prevents the Diac from firing. Gate drive to Q2 and Q3 is limited using two parallel 330Ω gate resistors and 12V zener diodes which clamp the gate voltage to a safe value. The 330Ω resistor from This shows the mains voltage waveform (the larger of the two set to 10V/div) & the current waveform (set to 90mA/ div) when the electronic ballast is driving a 36W tube. Note that the current is directly in phase with the voltage. The flattening of the 240VAC waveform is not a circuit function but was present at the time these photos were taken. These are the starting pulses as seen at the drain of Q3 with no tube in circuit. Pulses from Diac1 drive the gate of Q3 & switch it on. The voltage scale is 100V/div & the frequency is about 1kHz. This is the waveform at the drain of Q3 when driving a 36W tube. The vertical scale is 100V/div & the frequency is about 100kHz. October 1994  51 A TP1 D5 .01 250VAC 750k 0.22 400V IC1 1 .0068 0.18 TP0V MC34262 470uF 43k 270k N L2 10uF .01 12k E 0.1 250VAC 270k 47k 1W 0.18 0.12 27k 27k 10  1 22k Q1 820k 47k 1W 820k 47  R1 A L1 K D6 D1-D4 12k 68k .01 250VAC 750k F1 D7 T1 TO EARTH TERMINAL OF BATTEN Fig.8 (above & facing page): the component overlay diagram of the PC board. Note that quite a few different diodes & zener diodes are employed & they must not be mixed up. This close-up view shows how transformer T2 is secured to the PC board with a Nylon screw & nut & a transistor insulating bush. gate to source provides a load for the T2 windings to accurately set the core saturation. Q2 and Q3 switch on and off at about 100kHz (150kHz for the 18/20W version) but do not require heatsinks. However, during the switch-over process, the Mosfet which is switched off, is forced to commutate whereby its internal reverse diode briefly conducts. This commutation can lead to high dissipation in the Mosfets and must be prevented otherwise they would ultimately be destroyed. To reduce this dissipation to a low value, a snubber capacitor network comprising the 680pF 3kV capacitor and the series 22Ω resistor is connected from the source of Q2 to the 0V line. The two 150kΩ resistors connecting the snubber network to the 400V supply provide a load for the circuit if the fluorescent tube is not present or is effectively open circuit. Circuit variations Depending on whether the circuit is to be used with an 18W or 36W fluorescent tube, there are a number of variations to the winding details of transformers T1 & T2, and the values of fuse F1 and resistor R1. These are shown on the table included on the diagram of Fig.6. These changes are also relevant to 20W and 40W tubes. Transformer T2 has different wind­ ings to set the frequency of operation for each tube type. For the 18W tube load, the fre­quency is set to around 150kHz, while for 36W loads the frequency is set to about 100kHz. This difference in frequency allows us to keep the same value of inductance for L3. The input filter, comprising L1 & L2 on a common toroid, is secured using two plastic cable ties. 52  Silicon Chip Construction The PC board for the circuit is K Q2 TP2 150k 150k 330  1W 150k D6 150k A T2 ZD1 1uF 400V 1uF 400V 1uF 400V 1uF 400V D8 1uF 400V ZD2 TP0V coded 11309941 and measures 362 x 45mm. It is designed to fit inside a standard 18W or 36W fluorescent batten fitting. Construction can begin by winding the toroids and the transformers. Let’s start with the larger of the two toroids which has two windings, L1 and L2. Fig.7 shows how they are wound, using 26 turns of 0.4mm enamelled copper wire (ENCU). Note that each winding must be wound in the direction shown on the diagram; ie, L1 is wound in a different direction to L2 so that they end up in antiphase. Transformer T2 is wound on the smaller of the two toroids and again, its windings must be wound as shown. The wire gauge and number of turns depend on whether you are building the 18W or 36W version. Use the table on the circuit of Fig.6 to find the number of turns for N1, N2 and N3. 22W DIAC1 T1 N2 330  680pF 3kV 1 .001 3kV TUBE END L3 N1 0.1 1uF 400V F2 330  1W 330  TUBE END N3 330  1W 330  1W 0.1 250VAC Q3 Both T1 and L3 are wound on ferrite transformer bobbins. The larger of the two is for T1. Both require the centre leg of one of the core halves to be filed down so that a precise air gap is formed when the cores are clipped together. You will need a small file and a set of feeler gauges. Initially, for each core set place the two core halves together and observe that there is no gap between the mating surfaces of the two. Now file the centre leg of one core half, making sure that you are filing squarely and evenly across the face. The required gap is 200µm (0.2mm) for the larger core (T1) and 150µm (0.15mm) for L3, the smaller core. The whole process should not take more that a few minutes since the ferrite material is quite soft. Take care when filing down the centre leg of each core to ensure that you do not exceed the gap required. The secondary winding of T1 (N2) is wound first using 0.25mm enamelled copper wire – see Fig.6. Start the N2 winding on pin 2 and wind on the required number of turns before terminating at pin 6. Now apply a layer of insulating tape over the winding. The primary (N1) of T1 can now be wound using 0.4mm enamelled copper wire. This must be wound in the same direction as the secondary winding. Start at pin 4 and wind on the requisite number of turns neatly, side by side, placing a layer of insulating tape over each layer. The end of the winding terminates on pin 1. The transformer can now be assembled by fitting the core halves into the bobbin and securing with the clips. L3 is wound using 60 turns of 0.4mm ENCU wire, starting on pin 2 and finishing at pin 3. Again, insulate RESISTOR COLOUR CODES ❏ No. ❏  2 ❏  2 ❏  2   ❏  4 ❏  1 ❏  2 ❏  1 ❏  2 ❏  1 ❏  2 ❏  4 ❏  2 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 Value 820kΩ 750kΩ 270kΩ 150kΩ 68kΩ 47kΩ 43kΩ 27kΩ 22kΩ 12kΩ 330Ω 330Ω 47Ω 22Ω 10Ω 3.3Ω 1.5Ω 4-Band Code (1%) grey red yellow brown violet green yellow brown red violet yellow brown brown green yellow brown blue grey orange brown yellow violet orange brown yellow orange orange brown red violet orange brown red red orange brown brown red orange brown orange orange brown brown orange orange brown brown yellow violet black brown red red black brown brown black black brown orange orange gold brown brown green gold brown 5-Band Code (1%) grey red black orange brown violet green black orange brown red violet black orange brown brown green black orange brown blue grey black red brown yellow violet black red brown yellow orange black red brown red violet black red brown red red black red brown brown red black red brown orange orange black black brown orange orange black black brown yellow violet black gold brown red red black gold brown brown black black gold brown orange orange black silver brown brown green black silver brown October 1994  53 between each layer with insulating tape and apply a layer of tape over the final windings. Assemble the ferrite cores into the bobbin and secure with the clips. PC board assembly Fig.8 shows the component layout for the PC board. Before installing the components, check the board for shorts or breaks in the copper tracks and make any repairs that may be necessary. Also check the holes for correct sizing for each component. You will need 3mm diameter holes for the six PC board mounting holes, inductor L3 and the cable tie holes for the large toroid input filter (ie, L1 & L2). Start the board assembly by inserting all the PC stakes plus the four M205 fuse clips. This done, insert the resistors, links and diodes, followed by IC1. The diodes and IC must be oriented as shown, while the ST2 (Diac1) can be inserted either way around. Take care with the diodes since there are several types used on the board. The 1N5062 diodes (D1-D4 and D8) are axial lead types with spherical bodies. The 1N4936 diode (D5) is an Why is it called a ballast? The circuit presented here is called an “electronic ballast” because it replaces the ballast choke found in all conventional fluorescent lamp fittings. Electronic ballasts are more efficient than conventional ballast chokes and the fact that they operate the tube at very high frequencies also improves the efficiency. Which leads to the question “Why is the choke in a fluorescent fitting referred to as a ballast?” A ballast or more correctly, a ballast resistor, is used in a circuit to limit the operating current to a safe value. A fluo­rescent tube requires a ballast because its mercury vapour dis­charge has a negative resistance characteristic, ie, if the current increases, the voltage across the tube decreas54  Silicon Chip es. If the ballast choke was not in the circuit, the current through the tube would not be limited and it would be burnt out. Hence, the ballast choke maintains the current through the tube at a more or less constant value. And why are fluorescent light fittings called battens? Standard fluorescent light fittings for use in domestic and commercial installations are usually referred to as “battens”. This is because they are screwed to the timber battens which secure the Gyprock or fibrous plaster ceiling material to the rafters. In the same way, incandescent lamp holders which screw to a wall or ceiling are usually sold as “batten holders”. axial lead type with a black and light grey cylindrical body. D6 is a two-lead TO220 encapsulation, while the two zeners (ZD1, ZD2) are axial lead types with an orange body. D7 is a small axial lead type with a clear transparent cylindrical body. When installing the capacitors, take care with the orienta­tion of the electrolytic types which are polarised. Note that the capacitors must be as specified. In particular, don’t substitute 630V DC capacitors for those specified at 250VAC. Transformer T1 and inductor L3 must be installed with pin 1 oriented correctly. The input filter toroid is mounted using two cable ties as shown in the photos, while T2 is secured using a transistor mounting bush together with a Nylon screw and nut. Install the Mosfets (Q1-Q3) and fit the fuses into the fuse clips. The terminal block is mounted using two 3mm screws and nuts. Connections from the PC board to the terminal block are made with short lengths of tinned copper wire. Installation of the PC board We recommend that the PC board be installed into the fluo­rescent batten before testing because the voltages on the board are potentially lethal. Before installation, the existing ballast, starter and starter socket should be removed from the batten. The existing three-way insulated terminal block should be left in place as it will still be required to terminate the incoming mains supply wiring. Testing Now it is ready for testing. Insert a fluorescent tube into the fitting and apply power. The tube should initially start with a blue glow at the tube ends and then light up. After about a second the power factor controller will start up and the tube will reach full brilliance. If the circuit does not power the tube, switch off immediately and dis- connect it from the mains. Check that the fuses are intact and if so check your board for incorrectly located components. You should also check that the inductors and transformers have been wound correctly. Voltage checks Note that this circuit is potentially lethal to work on and that all points of the circuit float at mains voltage. If you do use a multimeter to make voltage checks, make sure it has shrouded probes and do not handle the meter while you are actual­ ly measuring voltages. Under no circumstance should an oscilloscope be connected to the circuit unless it has differential inputs or the circuit is powered via a line isolating transformer. You can check that the DC supply section of the circuit is operating by connecting a multimeter (set for 1000VDC) between TP0V and TP2. At switch-on, the voltage will initially be some­what lower than 400V and after a second or so it will settle at 400V DC. The power supply for IC1 can be measured between TP0V and on the cathode of D5. This voltage should gradually rise to about 12V, whereupon the circuit will start and the voltage should then sit at about SC 20-25V. Fig.9: this is the PC artwork reduced to 70.7%. To reproduce it full size, use a photocopier with an expansion ratio of 1.41. Check the board carefully before mounting any parts. Drill holes in the base of the batten to accommodate the six PC board standoffs. If the unit is to be used as a free standing lamp, then any holes in the metalwork of the batten should be covered to prevent accidental contact with the live PC board or its components. After installing the PC board into the batten, the tube leads and mains wiring should be connected to the PC board. Use a 2-way insulated terminal block to make the extension in the wires to the far-end tombstone (tombstones are the sockets used at each end of the fluorescent tube). It is important to earth the metal case of the batten to the green/yellow Earth wire in the mains lead. This should be done using the earth contact provided on the batten via the insulated terminal block mentioned earlier. The centre terminal of this contact is screwed onto an integral lug in the batten. The Active (brown) lead and the Neutral (blue) lead should connect to the A and N inputs on the PC board. Clamp the cord so that it cannot be pulled out of the terminal block. The assembled PC board fits neatly at one end of the batten fitting and is secured with six screws. Remember that the whole circuit is potential lethal since it is powered directly from the 240VAC mains supply.