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A high efficien for fluorescen This high efficiency inverter will power 36W or 40W tubes from a 12V battery and it is dimmable by about 20% for even more power saving. Overall inverter efficiency is about 70%. It can be used for camping, recreational vehicles, emergency lighting or as part of a solar power installation in remote areas. F luorescent tubes use far less energy than incandescent lamps and fluorescent tubes last a great deal longer as well. Other advantages are diffuse, glare-free lighting and low heat output. For these reasons, fluorescent lighting is the natural choice in commercial and retail buildings, workshops and factories. For battery-powered lighting, fluorescent lights are also the first choice because of their high efficiency. The main drawback with running fluorescent lights from battery power is that an inverter is required to drive the tubes. Inverter efficiency then becomes the major issue. There are many commercial 12V-operated fluorescent lamps available which use 15W and 20W tubes. However, it is rare to see one which drives them to full brilliance. For example, a typical commercial dual 20W fluorescent lamp operating from 12V draws 980mA or 11.8W. Ignoring losses in the fluorescent tube driver itself, it means that each tube is only supplied with 5.9W of power which is considerably less than their 20W rating. So while the lamps do use 20W tubes, the light output is well below par. Our new fluorescent inverter drives 36W or 40W tubes to full brilliance and has the option to dim the tube down to about 80% brightness. So not only do you get full brightness when you want it but you can dim the tube down when full brightness is not required and you want to conserve power drawn from the battery. Built on a long thin PC board, the inverter fits easily into a standard 36/40W batten. Drive for the fluorescent tube is controlled with a specialised IC which provides filament preheating before the tube is ignited. Once the tube is alight it monitors the tube current to maintain constant brightness. This current feedback control also provides for the dimming feature. It’s a long, narrow PC board, designed to fit inside a standard fluorescent batten (as shown on page 32). We haven’t shown a picture of the finished fluoro batten with lamp because it looks just like a . . . fluoro batten with lamp! 28  Silicon Chip www.siliconchip.com.au ncy inverter ent tubes By JOHN CLARKE +12V L1 GND Q1 T1 L2 IC3 BALLAST DRIVER IC1, IC2 PWM CONTROLLER & DRIVER 36W FLUORESCENT TUBE Q3 D1 – D4 BRIDGE RECTIFIER Q4 C1 FILAMENT 1 C2 Q2 470nF 630V R1 FILAMENT 2 (ERROR VOLTAGE) Fig.1: two switch-mode circuits are involved here: the DC-DC inverter involving IC1, Q1 & Q2 and the fluoro tube driver which converts high voltage DC to AC via IC3 and Q3 & Q4 in a totem-pole circuit. By the way, this project is quite similar in concept to the fluorescent inverter described in the November 1993 issue of SILICON CHIP. This earlier circuit is now superseded. Block diagram Fig.1 shows the general arrangement of the fluorescent inverter. The Warning: www.siliconchip.com.au 12V supply is stepped up to 280VDC using IC1 & IC2, Mosfets Q1 & Q2 and transformer T1. IC1 is the well-known Texas Instruments TL494 pulse width modulation controller. The internal functions of IC1 are shown in Fig.2. It contains a sawtooth oscillator, two error amplifiers and a pulse width modulation com- parator. It also includes a dead-time control comparator, a 5V reference and output control options for push-pull or single ended operation. Oscillator components at pins 5 and 6 set the operating frequency and for our circuit this is around 100kHz. This frequency was selected to enable use of a relatively small toroidal core for This circuit generates in excess of 275V DC which could be lethal. Construction should only be attempted by those experienced with mains-level voltages and safety procedures. September 2002  29 OUTPUT CONTROL +Vcc 13 Q1 6 5 RT SAWTOOTH OSCILLATOR D CK DEADTIME COMPARATOR DEADTIME CONTROL 4 Q FLIP FLOP CT 8 9 _ Q Q2 0.12V 11 10 0.7V 0.7mA Fig.2: this is the internal schematic for IC1, the TL494 switch-mode controller. 12 PWM COMPARATOR 1 1 2 ERROR AMP 1 the transformer. The PWM controller generates variable width output pulses at pins 9 and 10, to ultimately drive the gates of Mosfets Q1 and Q2 via the CMOS buffers in IC2, a 4050 hex buffer package. Mosfets Q1 and Q2 drive the centre-tapped primary winding of transformer T1. The centre-tap of the transformer’s primary winding connects to the +12V supply while each side of the primary winding is connected to a separate Mosfet. Each Mosfet is driven with a squarewave so that when Q1 is on, 2 3 FEEDBACK 15 16 14 REF OUT 7 ERROR AMP2 Q2 is off and when Q2 is on Q1 is off. With Q1 on, 12V is applied to the top half of the transformer primary winding. Similarly, when Q2 turns on, 12V is also impressed across the lower primary winding. The resulting square waveform on the primary is then stepped up by the secondary winding. High speed diodes rectify the AC output from the transformer T1, while a 470nF 630V capacitor (C4) filters the output to provide a stable DC voltage. A portion of the DC voltage output Scope1: The gate drive to Q3 and Q4 when the fluorescent tube is at full brightness. Top trace is the gate drive to Q4, a nominal 12V peak-to-peak signal. Lower trace is the gate drive to Q3, which is from 0-334V plus the gate voltage when switched on. The small step in the top of the waveform is when the gate goes to 12V above the 334V supply. (Note: the final design reduces the output voltage to 280V). 30  Silicon Chip REFERENCE (called the error voltage) is returned to IC1 for feedback control and the pulse width modulation is varied to maintain the 280V output. The high voltage DC from the inverter is applied to the fluorescent tube via Mosfets Q3 & Q4 and an LC network consisting of L2 and C1. Mosfets Q3 & Q4 are switched alternately by the ballast driver IC3, an L6574 fluorescent ballast driver, made by SGS-Thomson. The resulting squarewave signal is applied through inductor L2 and capacitor C1 to the fluorescent lamp. Scope2: These waveforms are identical to those in Scope1 except that now the frequency is much higher, at 65kHz, to dim the fluorescent tube. Notice the “dead time” between Q4 being switched off to Q3 switched on. This prevents high current pulses which would destroy the Mosfets if both were on at the same time. www.siliconchip.com.au HV 12 Vs OP AMP OP OUT OP IN– OP IN+ VBOOT 16 5 UV DETECTION 6 BOOTSTRAP DRIVER Q3 HV GATE DRIVER HVG 15 OUT 7 Imin VREF DEAD TIME 4 RIGN DRIVING LOGIC LEVEL SHIFTER G D LOAD 14 Vs Q4 LVG 11 LV GATE DRIVE CBOOT S G D S GND 10 IFS Imax IPRE VREF VTHPRE 2 VTHE CONTROL LOGIC RPRE EN 1 8 VTHE 3 VCO EN 2 9 CF Fig.3: the internal schematic for IC3, the LM6574 fluorescent tube controller. It varies the output AC frequency from the external Mosfet totem-pole driver to control the tube brightness. The inductor is included to provide AC current limiting while capacitor C1 blocks DC current flow. During the starting phase, Q3 and Q4 are driven at a very high frequency and this provides a current flow through L2 and C1, the top tube filament, through C2 and the lower tube filament and then to ground via the current sense CPRE resistor R1. This current is limited to a low value by the impedance of L2 and it heats up the lamp filaments so the tube start easily. After about one second, the drive frequency is lowered to the series resonant frequency of L2 and C2 and the resulting high voltage across C2 fires the tube. Once the tube is fired, the drive frequency Scope3. These are the gate drive signals to Q1 and Q2 when the fluorescent tube is driven to full brightness. Frequency is around 100kHz. Note the “dead time” between one Mosfet turning off and the second Mosfet turning on. www.siliconchip.com.au 1 is further reduced to provide full tube brightness. As you might expect, there is a fair amount of circuitry packed into the ballast driver IC; its internal workings are shown in Fig.3. An oscillator section comprises the VCO (voltage controlled oscillator) and the current sources set by resistors Rign and Rpre Scope4: This waveform shows the firing cycle of the fluorescent tube and is an attenuated signal of the actual tube voltage. The voltage is initially high and then drops once the tube has fired. September 2002  31 The PC board mounted in the fluoro batten. It doesn’t take up much space – in fact, there’s plenty of room inside the batten for some gell cell batteries and maybe a charger for an emergency light. Gee, we could be onto something here . . . at pins 4 and 2 respectively. Frequency during starting is controlled by resistor Rpre in conjunction with capacitor CF at pin 3. This sets the maximum frequency. Once the tube is started, the frequency is set by Rign and capacitor CF. An op amp at pins 5, 6 & 7 can be used for frequency control. The duration of the tube filament preheat is set by capacitor Cpre at pin 1. The enable inputs at pins 8 & 9 can be used to reinitiate starting if the tube does not fire or to shutdown the circuit if a tube is not installed. The gate drive for the Mosfets is interesting. Mosfet Q4 is driven directly via the low voltage gate (LVG) driver at pin 11. When pin 11 goes high, Q4 is switched on and when pin 11 is low, Q4 is off. High side switching Mosfet Q3 requires a special gate driver to allow it to drive the high voltage (HV) supply. The special gate driver comprises the bootstrap diode, level shifter, high voltage driver (HVG) and capacitor C boot between the source of Q3 and Vboot. When Q4 is switched on, Q3 is off and so capacitor Cboot can be charged from the supply at Vs via the bootstrap diode and Q4 (to ground). Thus Cboot will have the supply voltage across it. When Q4 is switched off and Q3 is switched on, the entire gate drive section for Q3 is pulled up to the HV supply and the gate drive is higher than this by the Vs supply stored on Cboot. The gate drive circuit (HVG) thus maintains its supply from Cboot. The bootstrap diode is now reverse biassed and plays no further part in the operation. When Q3 is switched off and Q4 is switched on, Cboot can be topped up via the bootstrap diode again. The capacitor value needs to be sufficiently large to prevent the HVG driver supply from drooping as it needs to charge the gate capacitance of Q3. Circuit details The full circuit of the fluorescent inverter is shown in Fig.4. IC1 is the TL494 PWM controller. Its frequency of operation set at around 100kHz by the 4.7kΩ resistor and 1nF capacitor at pins 6 and 5 respectively. The emitter outputs at pins 9 and 10 are pulled down via 1kΩ resistors and they each drive three paralleled buffers in IC2. Mosfets Q1 and Q2 drive the transformer as described previously to develop the high voltage supply across T1’s secondary winding. High Scope5: These waveforms show tube voltage and current when the tube is in starting mode. Top trace is the tube current while the lower trace is the voltage across the tube. Operating frequency is 62kHz. 32  Silicon Chip frequency rectifiers D1-D4 convert the AC waveform into a DC voltage and this is filtered with a 470nF 630V capacitor (C4). The 10nF 3kV capacitor (C3) is included so that it can be placed directly between the drain of Q3 and the source of Q4 to provide decoupling of this supply. This limits voltage overshoot as Q3 & Q4 switch on and off. Left uncontrolled, too much voltage overshoot can damage the Mosfets. Feedback from the high voltage DC output is derived from a resistive divider comprising series 270kΩ and 180kΩ resistors and an 8.2kΩ resistor. The resulting voltage across the 8.2kΩ resistor is applied to internal error amplifier 1 in IC1 at pin 1. The divider ratio is such that pin 1 will be 5V when the DC voltage is 280V. The DC gain of the error amplifier is 213 times, as set by the 1MΩ and 4.7kΩ resistors at pin 2. The 47kΩ resistor and 100nF capacitor across the 1MΩ feedback resistor provide fast AC response from the circuit. This op amp is referenced to +5V (pin 14) via the 4.7kΩ resistor. Thus its output at pin 3 will be +5V if the high voltage DC level is 280V but will go lower than this if the DC voltage falls. As mentioned previously, the op amp Scope6: The tube current and voltage at maximum brightness. The frequency has now dropped to 33kHz and current is higher. Notice that the voltage waveforms are reasonably clean, producing much less radio interference than from a fluorescent tube operated with a conventional ballast. www.siliconchip.com.au www.siliconchip.com.au September 2002  33 2 3 1nF 12 5 IC1 TL494 6 C2 C1 11 8 1 9 10 4.7k E1 E2 100F q + 1k 1k 11 9 7 14 5 3 12 IC2d 10 IC2c 6 IC2f 15 IC2b 4 IC2a 2 8 IC2e 1 470F 35V LOW ESR IC2: 4050 ZD1 16V 1W 10 L1 40W FLUORESCENT INVERTER 4 7 16 15 14 13 4.7k 1M 100nF 100nF F1 5A 10 10 100nF 100nF S G S D Q2 STP60NE06 G D Q1 STP60NE06 470nF 100nF 5T 5T T1 Q1-Q4 8.2k 180k 270k 130T D G VRx 50k S D 5.6k VR1 5k 100k 82k D1-D4 1N4936 K A D1qD4 100nF 10k C3 10nF 3kV 470pF 100k 47k D5 1N914 100nF C4 470nF 630V +280V RPRE 1F IC3 L6574 LVG OUT HVG 16 VBOOT K D5, D6 56k 9 11 14 15 A 10k EN2 GND EN1 1 10 8 RIGN 3 CF CPRE 4 5 OP OUT 7 OP IN+ 6 OP INq 2 12 VS 100nF 100 Fig.4: the full circuit of the fluorescent inverter. IC3 is the clever component, varying the tube drive frequency between 100kHz and about 30kHz to preheat the filaments, ignite the tube and then maintain the tube current at the correct value. 2002 SC 100nF 47k 0V +12V POWER S1 10 S 750k 330nF K + 3.9k A ZD1 L2 3mH 750k D S D D6 1N914 2.2 G Q4 STP6NB50 10 G Q3 STP6NB50 100nF 100F 25V q 36W TUBE C1 100nF 250VAC C2 3.3nF 3kV T1 180k 100nF S1 F2 470nF 10 ZD1 GND RETREVNI TNECSEROULF W04 CABLE TIE LOOPED UNDER CORE & HOLD DOWN TIE Fig.5: at 340mm long, the PC board component overlay is a tad long to fit on one page. If you need to cut the board to fit it into, say, an odd-shaped fluoro lamp (eg, circular), the logical place would be across the screw holes, four diodes and 270kΩ resistor. output is compared with the sawtooth oscillator waveform to control the PWM drive to the Mosfets. Power to IC1 and IC2 is supplied via a 10Ω resistor from the 12V supply and filtered with a 100µF capacitor. A 16V zener diode protects the circuit from high voltage transients. The main current supply to transformer T1 is supplied via inductor L1 and filtered with the 470µF electrolytic capacitor. The 100nF and 470nF capacitors are included to supply the high frequency peak currents demanded by the switch-mode operation of T1. Reverse polarity protection is provided with fuse F1 in conjunction with the substrate diodes of Mosfets Q1 & Q2. Should the battery connection leads be transposed, the diode within Q1 or Q2 conducts and the fuse will blow. IC1 and IC2 are protected via zener diode ZD1 which will also limit the positive supply voltage to -0.7V below ground. Supply to IC3 comes from the 12V rail via a 100Ω current limiting resistor which prevents possible damage to the internal zener diode at pin 12. This F1 S2 CABLE TIE SEPARATES WINDING ENDS 470F 1k 1k Q2 100nF 100F 10 IC2 4050 IC1 TL494 16V 10 1M 1nF + 100nF +12V 4.7k CABLE TIE 100nF 4.7k 100nF 8.2k 100nF F1 0V 47k L1 zener also protects the IC from reverse polarity connection. The supply is decoupled with 100µF and 100nF capacitors. The high side driver supply capacitor Cboot is 100nF in value. Frequency of operation during preignition is set at around 100kHz by the 470pF capacitor at pin 3 and the Rpre value at pin 2. Preheat time is fixed at 1.5s using the 1µF capacitor at pin 1. Note that this capacitor must have very low leakage since its charging current is only 2µA. For this reason, we have specified a polyester type in this position; do not substitute an electrolytic. After the filament preheat, the frequency falls to about 33kHz, set by the 100kΩ resistor at pin 4. Before this low frequency is reached, the tube is ignited at the series resonant frequency of L2 and the 3.3nF capacitor across the tube. This occurs at around 60kHz. The resulting tube current flows through the 2.2Ω resistor at Q4’s source and the voltage developed across it is monitored via a 10kΩ resistor at pin 6, the inverting input of an internal op amp. The non-inverting input to the op PRIMARY1 amp is connected to the wiper of VR1 via a 10kΩ resistor. A 100nF capacitor between the inverting input to the op amp and the output filters the resulting output and this controls the value of Rign at pin 4 via diode D5. When pin 5 of the op amp is high, diode D5 is reverse biased and the frequency of operation is simply set by the 100kΩ resistor at pin 4, to 33kHz. When pin 5 is low, Rign is the 100kΩ resistor to ground in parallel with the 47kΩ resistor connecting to diode D5. The frequency of oscillation thus rises. The internal op amp can therefore control the frequency of operation in a feedback loop where it monitors the tube current against the reference set by potentiometer VR1. Varying the frequency also changes the tube current (and brightness) because the impedance of inductor L2 increases as the frequency rises. The enable 2 (EN2) input at pin 9 is used to cause the circuit to begin preheating again if the tube does not fire. Two series 750kΩ resistors and a 3.9kΩ resistor divide the voltage at the top of the tube down to a low value PRIMARY2 HINGE S1 F1 S2 CABLE TIE TO GIVE 1mm GAP WHEN CLOSED F2 SEC FINISH SECONDARY START L1: 6 TURNS OF 1mm DIA ENAMELLED COPPER WIRE ON POWDERED IRON CORE 28 x 14 x 11mm (JAYCAR LO-1244 OR SIM.) T1: SECONDARY 130 TURNS OF 0.4mm ENAMELLED COPPER WIRE ON FERRITE CORE 35 x 21 x 13mm (JAYCAR LO-1238 OR SIMILAR). PRIMARIES 2 x 5T OF 7.5A FIGURE-8 WIRE L2: 42 TURNS EACH HALF (84 TOTAL) 0.4mm ENAMELLED COPPER WIRE ON FERRITE CORE 32 x 30 x 30mm (JAYCAR LO-1290 OR SIMILAR) Fig.6: winding details for the inductors and inverter transformer. L2 is held in place with three small cable ties, daisychained to lock it in place. 34  Silicon Chip www.siliconchip.com.au which is then rectified by diode D6 and fed to pin 9. If the tube does not fire after the first preheat and ignition sequence, the voltage across the tube will remain much higher than if the tube had fired and started. If the voltage at pin 9 exceeds the 0.6V threshold, the ignition process will repeat until the tube fires and lights. In practice, the tube may need to undergo several preheat sequences when the temperature is low or if it is an old tube, but will fire on the first attempt when the tube is warm. Construction The Fluorescent Inverter is built on a long narrow PC board coded 11109021 and measuring 340 x 45mm. It fits easily into in a standard fluorescent 36/40W batten. Its wiring diagram is shown in Fig.5. You can begin assembly by checking the PC board for shorts between tracks and possible breaks in the copper pattern. Also check that the hole sizes are suitable for the components. The six mounting holes, the heatsink 10k 3.9k 56k FILAMENT2 TO FLUORO TUBE FILAMENT1 Q4 2.2 3kV L2 750k 10 3.3nF 12090111 C2 100nF C1 100nF 250V AC Q3 750k 330nF VR1 5k 10 C3 10nF 3kV 47k 100k 100F 100nF 100k 5.6k 100nF D1qD4 914 D5 100nF 10k 470pF 1F 270k C4 470nF 630V Q1 IC3 L6574 100 D6 914 mounting tab holes and cable tie holes should be 3mm in diameter, while holes for the screw terminals and fuse clips need to be 1.5mm in diameter. Insert the wire links and resistors first, using the resistor colour codes as a guide to selecting the correct values. You can also use a digital multimeter to check the values directly. Then install the ICs and diodes, taking care with their orientation. Install the capacitors next, using the Table as a guide. Make sure that the high voltage 470nF and 10nF capacitors are installed in the correct positions. If you inadvertently put the low voltage capacitors in the wrong positions, they will blow at switch-on. When inserting the two fuse clips, note that they have little end stops which must be placed to the outside edge to allow the fuse to be clipped in place. The screw terminals can be inserted and soldered in place. When inserting the two heatsinks, bend the mounting lugs over on the underside of the PC board to secure them in place. Insert the Mosfets, taking care to put the correct type in each position. Q1 and Q2 are screwed to their heatsinks with an M3 screw and nut before they are soldered to the PC board. Potentiometer VR1 can now be installed. Winding the toroids Three cores need to be wound, for L1, L2 and transformer T1. The winding details are shown in Fig.6. Beginning with L1, use a 28 x 14 x 11mm iron powdered toroidal core and wind on six evenly spaced turns of 1mm diameter enamelled copper wire. Strip the wire ends of insulation and tin them (with solder) before soldering to the PC board. Secure the toroid with two 100mm cable ties daisy-chained to extend the length and through the holes allocated on the PC board. Transformer T1 is wound on a 35 x 21 x 13mm ferrite toroid. First wind on the secondary 130 turns of 0.4mm diameter enamelled copper wire. Wind these tightly together around the core, leaving a few millimetres spacing between the start and finish ends of the windings. Fit a cable tie between the start and finish of this winding to maintain the Close-up photos of L1, T1 and L2 (as drawn at left) to help you with their construction. The winding on L1 occupies only about 3/4 of the toroid while the secondary of T1 (which goes on first) occupies all of its toroid. www.siliconchip.com.au September 2002  35 Parts List – 12V Fluorescent Light Inverter 1 36/40W fluoro batten with tube 1 PC board, coded 11109021 (340 x 45mm) 1 Powdered iron toroidal core, 28 x 14 x 11 (L1; Jaycar LO-1244 or equivalent) 1 Ferrite core, 32 x 30 x 30mm (L2; Jaycar LF-1290 or equivalent) 1 Ferrite toroidal core, 35 x 21 x 13mm (T1; Jaycar LO-1238 or equivalent) 1 16mm 5kΩ linear potentiometer with knob (VR1) 1 50kΩ trimpot (for calibration) 2 M205 fuse clips 1 M205 quick blow 5A fuse (F1) 1 2-way PC-mount screw terminal blocks (Altronics P-2101 or equivalent) 2 2-way PC-mount screw terminal blocks (Altronics P-0234A or equivalent) 2 Mini-U TO-220 heatsinks 25 x 30 x 12.5mm 1 150mm length of 0.8mm tinned copper wire 1 250mm length of 1mm diameter enamelled copper wire 1 15m length of 0.4mm enamelled copper wire 1 500mm length of 7.5A-rated figure-8 cable 1 500mm length of green (or green/yellow) hookup wire 1 2m length of red and black automotive figure-8 wire, 1mm square section 2 automotive battery clips (1 red and 1 black) 6 M3 tapped metal spacers x 6mm long 2 M3 x 6mm screws Ideally, the maximum cur6 M3 x 15mm screws rent for the fluorescent tube 8 M3 nuts should be adjusted using a 1 cord-grip grommet trimpot. To do this, replace 13 100mm cable ties the 100kΩ resistor between 1 PC stake pin 2 of IC3 and the top of Semiconductors 1 TL494 switch-mode controller (IC1) 1 4050 hex CMOS buffer (IC2) 1 L6574 fluorescent ballast driver (IC3) 2 STP60NE06 60V Mosfets (Q1,Q2) 2 STP6NB50 500V Mosfets (Q3,Q4) 1 16V 1W zener diode (ZD1) 4 1N4936, UR104 fast diodes (D1-D4) 2 1N914, 1N4148 switching diodes (D5,D6) Capacitors VR1 with a 50kΩ trimpot and series 82kΩ resistor, as shown in Fig.4. Adjust this pot for 3A, measuring the current as shown in Fig.8 and described in the text. Wait a while for the inverter to fully warm up then re-adjust it. You can then switch off, measure the voltage between pin 2 of IC3 and VR1 and replace the trimpot/resistor with a similar value fixed resistor. 1 470µF 35V or 50V low ESR PC electrolytic 2 100µF 16V PC electrolytic 1 1µF MKT polyester 1 470nF (0.47µF) MKT polyester 1 470nF (0.47µF) 630V polyester (C4) 1 330nF (0.33µF) MKT polyester 10 100nF (0.1µF) MKT polyester 1 100nF (0.1µF) 250VAC class X2 MKT polyester (C1) 1 10nF (0.01µF) 3kV ceramic (C3) 1 3.3nF (0.0033µF) 3kV ceramic (C2) 1 1nF (0.001µF) MKT polyester 1 470pF ceramic Resistors (0.25W, 1%) 1 1MΩ 2 750kΩ 1 270kΩ 1 180kΩ 2 100kΩ 1 82kΩ 1 56kΩ 2 47kΩ 2 10kΩ 1 8.2kΩ 1 5.6kΩ 2 4.7kΩ 1 3.9kΩ 2 1kΩ 1 100Ω 5 10Ω 1 2.2Ω 5% 36  Silicon Chip 4-band code 5-band code separation, then insert the wire ends into the relevant PC board holes and temporarily tie them together, under the PC board. The primary windings are wound over the secondary. Use figure-8 wire rated at 7.5A with a polarity stripe. Insert one end through the S1 & F1 holes nearest Q2 and wind five turns onto the core, starting up through the centre and anti-clockwise toward S2 & F2. Insert the wire ends into S2 & F2 with the same wire between S1 and S2 and the second wire between F1 and F2; i.e, if the polarity stripe on the wire goes to S1 then it terminates into S2. The toroid is secured using a cable tie wrapped around the core as shown and spaced above the PC board using another looped cable tie placed side on. This lifts the core so that it is at the same height as the primary winding side. Inductor L2 is wound on a split ferrite core with a gap of 1mm. This gap is necessary to prevent core saturation and also to reduce its Q. This gap is set by inserting a cable tie in the hinge portion of the split core. This is shown in the detail diagram for L2 in Fig.5. Wind 42 turns of 0.4mm enamelled copper wire onto each core half, so that in effect, you have an 84-turn coil split between them. Insert the cable tie and snap close the core. The core is secured to the PC board with a daisy-chained length of cable ties around the top and through the holes in the PC board. Then strip, tin and solder the two winding ends to the PC board. Installing the board The PC board is installed into a standard 36/40W batten and mounted on 6mm high metal spacers. Before you can do that, you must remove the original ballast and the starter components. Find a suitable position within the batten for the PC board. We positioned our PC board so that three of the wires Capacitor Codes Value OR 1µF 470nF 330nF 100nF 10nF 3.3nF 1nF 470pF Old Value 1µF 0.47µF 0.33µF 0.1µF .01µF .0033µF .001uF 470pF IEC EIA Code Code 1u 105 470n 474 330n 334 100n 104 10n 103 3n3 332 1n0 102 471 470 www.siliconchip.com.au TUBE ‘TOMBSTONE’ SOCKET CORD GRIP GROMMET TUBE ‘TOMBSTONE’ SOCKET FLUORESCENT INVERTER POTENTIOMETER HEAVY DUTY AUTOMOTIVE WIRE TO 12V BATTERY ORIGINAL TERMINAL BLOCK CHASSIS CONNECTION Fig.7: here’s how the PC board is wired into a standard 36/40W fluorescent light batten. The starter and its holder are discarded but the original tombstones and terminal block are retained. Any power factor capacitor is also removed. from the tube mounting tombstones reached the PC board terminals. The remaining wire was extended using the existing terminal block. Drill holes to mount the PC board at the six mounting positions. You will also need to drill a hole in the side of the batten for the dimming potentiometer. The shaft on this pot-entiometer may need cutting down to size. Also drill and file a hole for the cordgrip grommet which can be positioned on the end of the batten or in the base. Cover up any slots and holes on the underside of the batten base where the PC board will be located. We used Gaffer tape for this. Attach the PC board using M3 screws and nuts. Make sure that the heatsinks on the PC board do not make contact with the batten top cover when it is fitted otherwise the fuse will blow. Follow the diagram of Fig.7 which shows how to connect the batten wiring to the PC board. Do not forget the earth wire which connects between the batten case earth and the negative terminal on the PC board. Secure the 12V power leads with a cordgrip grommet. Testing The fluorescent inverter circuit generates high voltages which can give you an electric shock. Take care when taking measurements and disconnect the 12V battery before touching any part of the circuit. With 12V applied and without the MEASURING THE CURRENT DRAIN 0.358V TO +12V 100nF 22k + – 0.1 5W + WIRE FROM INVERTER DMM Fig.8: connect this circuit in series with the inverter if you want to check the operating current. www.siliconchip.com.au fluorescent tube installed, check that there is about 280V DC between the metal tab of Q3 and ground. This voltage should be within 5% of 280V, between 266V and 294V. Now disconnect 12V, insert the tube and reapply 12V. Check that the tube starts within a few seconds. The circuit may make several attempts before the tube lights, particularly in cold weather. As with all fluorescent lights, the tube will not reach full brightness until after five minutes or so and during this time the tube may exhibit a series of darker bands (striations) along its length. These will disappear once the tube has warmed up fully. The bands will be more noticeable if the dimming How to run an 18W tube As night follows day, we know that people will soon be asking us how to run this circuit with different sizes of fluorescent tube. Well at least we can forestall one of the queries – how to run an 18W tube. The changes required are simple: Increase the turns on each half of the split inductor for L2 up to 50 (total of 100). These changes will also have the effect of making the dimming control more effective. control is set to minimum brightness. With the fluorescent tube driven to full brightness the current drain is around 3.7A at 12V. This means that some 45W is drawn from the battery and so the fluorescent tube drive will be a little less due to losses in the inverter. This is similar to the standard mains fluorescent drive circuitry which uses an iron-cored ballast (inductor) to limit tube current. If you wish to check the tube current, use the circuit of Fig.8. This is connected in series with the positive supply to the inverter PC board and uses a 0.1Ω 5W resistor as a current shunt. The 22kΩ resistor and 100nF capacitor filter the current drawn from the battery so that the multimeter will be able to read the average current. Connect a clip lead across this resistor and only disconnect it when taking measurements as otherwise the resistor will overheat. It is recommended that the inverter not be used while charging the battery from a high current charger e.g, an automotive alternator or mains-powered unit. If the inverter Mosfets still run excessively hot it is recommended to reduce the current drain to 2.5A (250mV across the 0.1Ω resistor) slightly reducing lamp brightness. The current drawn from the battery is the voltage across the capacitor divided by 0.1. For 3.7A, the reading will be 370mV across the 100nF capacitor. Note that this current will only be reached after the tube has been lit for a few minutes. When fully dimmed, the current will be around 3A or 300mV across the 100nF capacitor. If the current is substantially different to these two values, check the battery voltage. It should be around 12.3V or more when driving the fluorescent inverter circuit. If it is below 12V, the battery will require charging. Also check that the 1mm gap is present between the core halves of L2. Then check the number of turns. If these are correct add more turns to the inductor if the current is too high and remove turns if the current is too low. Remember that it is the impedance of L2 in conjunction with the drive frequency from IC3 which set the overall circuit operating conditions. During operation, the heatsinks for Q1 and Q2 will run warm – and the transformer core for T1 will also run warm. Q2’s heatsink will also be slightly warmer than that for Q1 since it is close to the heat from T1. Inductors L1 and L2 will not be noticeably hotter than the ambient temperature. SC September 2002  37