<P>The emitter outputs at pins 9 and 10 are pulled down via 1kW 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 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 
      &amp; Q4 switch on and off. Left uncontrolled, too much voltage overshoot 
      can damage the Mosfets.</P>
      <P>Feedback from the high voltage DC output is derived from a resistive 
      divider comprising two series 270kW resistors and an 8.2kW resistor. The 
      resulting voltage across the 8.2kW 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 334V. The DC gain of the error amplifier is 213 
      times, as set by the 1M&#937; and 4.7kW resistors at pin 2. The 47kW resistor 
      and 100nF capacitor across the 1M&#937; feedback resistor provide fast AC 
      response from the circuit.</P>
      <DIV class=wpimg style="FLOAT: right; WIDTH: 302px"><A 
      href="http://us1.webpublications.com.au/static/images/articles/i305/30504_22mg.jpg"><IMG 
      height=227 alt="Click for larger image" 
      src="30504_22lo.jpg" width=302 
      border=0> </A>
      <DIV class=wpcaption>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.</DIV></DIV>
      <P>This op amp is referenced to +5V (pin 14) via the 4.7kW resistor. Thus 
      its output at pin 3 will be +5V if the high voltage DC level is 334V but 
      will go lower than this if the DC voltage falls. As mentioned previously, 
      the op amp output is compared with the sawtooth oscillator waveform to 
      control the PWM drive to the Mosfets.</P>
      <P>Power to IC1 and IC2 is supplied via a 10W resistor from the 12V supply 
      and filtered with a 100mF 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 470mF 
      electrolytic capacitor. The 100nF and 470nF capacitors are included to 
      supply the high frequency peak currents demanded by the switch-mode 
      operation of T1. </P>
      <P>Reverse polarity protection is provided with fuse F1 in conjunction 
      with the substrate diodes of Mosfets Q1 &amp; 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.</P>
      <P>Supply to IC3 comes from the 12V rail via a 100W current limiting 
      resistor which prevents possible damage to the internal zener diode at pin 
      12. This zener also protects the IC from reverse polarity connection. The 
      supply is decoupled with 100mF and 100nF capacitors. The high side driver 
      supply capacitor Cboot is 100nF in value.</P>
      <DIV class=wpimg style="WIDTH: 302px"><A 
      href="http://us1.webpublications.com.au/static/images/articles/i305/30504_21mg.jpg"><IMG 
      height=227 alt="Click for larger image" 
      src="30504_21lo.jpg" width=302 
      border=0> </A>
      <DIV class=wpcaption>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. </DIV></DIV>
      <P>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 1&#956;F capacitor at pin 1. Note that this capacitor 
      must have very low leakage since its charging current is only 2mA. For 
      this reason, we have specified a polyester type in this position; do not 
      substitute an electrolytic.</P>
      <P>After the filament preheat, the frequency falls to about 33kHz, set by 
      the 100kW 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.</P>
      <P>The resulting tube current flows through the 1.2W resistor at Q4's 
      source and the voltage developed across it is monitored via a 10kW 
      resistor at pin 6, the inverting input of an internal op amp. The 
      non-inverting input to the op amp is connected to the wiper of VR1 via a 
      10kW 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 
      R<FONT size=1>IGN</FONT> at pin 4 via diode D5. </P>
      <P>When pin 5 of the op amp is high, diode D5 is reverse biased and the 
      frequency of operation is simply set by the 100kW resistor at pin 4, to 
      33kHz. When pin 5 is low, R<FONT size=1>IGN</FONT> is the 100kW resistor 
      to ground in parallel with the 47kW resistor connecting to diode D5. The 
      frequency of oscillation thus rises.</P>
      <DIV class=wpimg style="FLOAT: right; WIDTH: 302px"><A 
      href="http://us1.webpublications.com.au/static/images/articles/i305/30504_7mg.jpg"><IMG 
      height=29 alt="Click for larger image" 
      src="30504_7lo.jpg" width=302 
      border=0> </A>
      <DIV class=wpcaption>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 . . .</DIV></DIV>
      <P>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.</P>
      <P>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 750kW resistors and 
      a 3.9kW resistor divide the voltage at the top of the tube down to a low 
      value which is then rectified by diode D6 and fed to pin 9. </P>
      <P>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.</P>
      <H3>Construction</H3>
      <P>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.</P>
      <DIV class=wpimg style="WIDTH: 302px"><A 
      href="http://us1.webpublications.com.au/static/images/articles/i305/30504_16mg.jpg"><IMG 
      height=85 alt="Click for larger image" 
      src="30504_16lo.jpg" width=302 
      border=0> </A>
      <DIV class=wpcaption>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 270kW 
      resistor.</DIV></DIV>
      <P>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. </P>
      <P>The six mounting holes, the heatsink 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. </P>
      <P>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.</P>
      <P>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.</P>
      <DIV class=wpimg style="FLOAT: right; WIDTH: 302px"><A 
      href="http://us1.webpublications.com.au/static/images/articles/i305/30504_15mg.jpg"><IMG 
      height=97 alt="Click for larger image" 
      src="30504_15lo.jpg" width=302 
      border=0> </A>
      <DIV class=wpcaption>Fig.5: ...continued.</DIV></DIV>
      <P>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.</P>
      <P>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. Potent-iometer VR1 can now 
      be installed.</P>
      <DIV style="CLEAR: both"></DIV>
      <TABLE class=breakout>
        <TBODY>
        <TR>
          <TD class=breakoutCell>
            <DIV class=breakoutTitle>How to run an 18W tube</DIV>
            <P>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.</P>
            <P>The changes required are simple:<BR>Increase the turns on each 
            half of the split inductor for L2 up to 50 (total of 100) and 
            increase the 1.2<FONT face=Symbol>W</FONT> current sensing resistor 
            to 2.2<FONT face=Symbol>W</FONT>.</P>
            <P>These changes will also have the effect of making the dimming 
            control more effective.</P></TD></TR></TBODY></TABLE>
      <H3>Winding the toroids</H3>
      <P>Three cores need to be wound, for L1, L2 and transformer T1. The 
      winding details are shown in Fig.6.</P>
      <DIV class=wpimg style="WIDTH: 302px"><A 
      href="http://us1.webpublications.com.au/static/images/articles/i305/30504_11mg.jpg"><IMG 
      height=106 alt="Click for larger image" 
      src="30504_11lo.jpg" width=302 
      border=0> </A>
      <DIV class=wpcaption>Fig.6: winding details for the inductors and inverter 
      transformer. L2 is held in place with three small cable ties, 
      daisy-chained to lock it in place.</DIV></DIV>
      <P>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.</P>
      <P>Transformer T1 is wound on a 35 x 21 x 13mm ferrite toroid. First wind 
      on the secondary 134 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. </P>
      <DIV class=wpimg style="FLOAT: right; WIDTH: 302px"><A 
      href="http://us1.webpublications.com.au/static/images/articles/i305/30504_4mg.jpg"><IMG 
      height=147 alt="Click for larger image" 
      src="30504_4lo.jpg" width=302 
      border=0> </A>
      <DIV class=wpcaption>Close-up photos of L1, T1 and L2 (as shown in Fig.6) 
      to help you with their construction. The winding on L1 occupied about 3/4 
      of the toroid while the secondary of T1 (which goes on first) occupies all 
      of its toroid.</DIV></DIV>
      <P>Fit a cable tie between the start and finish of this winding to 
      maintain the separation, then insert the wire ends into the relevant PC 
      board holes and temporarily tie them together, under the PC board.</P>
      <P>The primary windings are wound over the secondary. Strip a 3mm length 
      of insulation from one end of some 7.5A-rated (0.75mm2) hookup wire and 
      solder it into the S1 hole of the PC board, as shown on Fig.5. </P>
      <P>Now wind on four turns in the direction shown and then cut and strip 
      the wire insulation at a length suitable for inserting into the F1 hole. 
      Solder this wire in position. Note that the windings must be reasonably 
      tight around the toroid.</P>
      <P>Then wind the second primary winding in the same manner, starting in 
      the S2 hole and finishing at the F2 hole in the direction shown. It sounds 
      tricky bit you will find that you can easily thread the hookup wire under 
      and through the toroid.</P>
      <DIV class=wpimg style="WIDTH: 302px"><A 
      href="http://us1.webpublications.com.au/static/images/articles/i305/30504_5mg.jpg"><IMG