June 28, 2016
June 28, 2016
Battery powered fluorescent lamp project was often found in hobby electronics magazines in '80s. I had thought that the fluorescent lamp is the device used by only AC power at that time and those articles light tiny fluorescent lamp with battery attracted me. I built some fluorescent light but I could not get good result for practical use from view point of efficiency and life time of lamp. That was because I was only following the articles without any knowledge about theory of dirscharge lamp and even electronics. I had no idea what to change to get good result and I lost interest in it after all.
However I found a battery powered fluorescent lamp project on the web and it was just same schematic as I did at that days. Therefore I 'resumed' the project after 30 years with modern drive circuit and right theory.
The fluorescent lamp is a kind of low-pressure mercury-vapor lamp. It produces visible light by fluorescent coat on inner surface of the tube. The fluorescent coat is excited by ultraviolet emmission of mercury-vapor excited by electrical discharge. The fluorescent lamp can be classified in operation mode of cathode, hot cathode fluorescent lamp (HCFL) and cold cathode fluorescent lamp (CCFL). The 'fluorescent lamp' generally means the HCFL and this project uses the HCFL. There is main application for each type of fluorescent lamps. The almost HCFL is used for generic lightings and the CCFL is used for backlights of LCD display.
Figure 1 shows the fluorescent light circuit often found in the hobby electronics magazines. Because they are for radio boys, it uses a generic power transformer for the inverter and no special component is needed for the project. The primary windings of the transformer works as a blocking oscillator in switching frequency of 1-2kHz. The secondary winding is tied to the fluorescent lamp directly and the high voltage pulse at the secondary winding starts to discharge instantly. However there are problems in stability and efficiency, as the result the lamp ends get darken quickly due to insufficient power output.
Figure 2 shos a simple electronic fluorescent lamp driver used for small fluorescent lights. The V1 is output of inverter, half-bridge or transformer, the LR and CR make a series resonant network with a resonant point near the driving frequency. When V1 is applied on start-up, the large current heats the filament (cathode) and the high voltage at the resonant capacitor depends on the current and capacitance is applied between the both cathodes. This produces the condition to start to discharge, pre-heating the cathode and applying high-voltage to the lamp. When the cathode is heated and start to thermionic emission, a small discharge occures on the filament and both ends of the tube light. If lamp voltage is sufficient to start discharge, it changes to main discharge between both cathodes. Because the lamp is in parallel with the CR, Q is lost and series resonanse quenches automatically. The LR works as a ballast at normal operation.
The circuit diagram in Figure 2 needs to be set to apply a sufficient lamp voltage to certainly start discharge at the various conditions. However, in most case, the discharge starts instantly at cold cathode mode operation below its thermionic emission temperature and then it changes to hot cathode mode operation when cathode temperature rises. The cold cathode mode operation damages the electrodes of HCFL lamps not designed for cold cathode mode operation, so that it needs to start with pre-heating of the cathodes for longer lamp life. There are some methods for hot-starting and a simple one is that controling the driving frequency that discribed below. The microcontroller is sutable for the start-up sequence and additional feature, such as constant wattage control and some safety controls, can be easily added.
Figure 3 shows a simulation of the HCFL driver. There are two load circuits in the schematic is because circuit changes by discharge at the lamp. The upper circuit is at pre-heat and the lower one is at normal operation. The resonant frequency is set to 55 kHz and f-I curve shows that resonant current at pre-heating significantly varies by changes of driving frequency. The lamp current at normal operation seems less sensitive to driving frequency but it should vary more than that because in fact the lamp impedance is negative resistance.
Figure 5 shows the built HCFL driver based on the simulation. The transformer T1 is an EI core got from junk bin and I measured the core characteristic. The winding, pri:0.5/42T, sec:0.13×6/180T, gap:0.1, makes 60V output voltage at 7.2V input voltage. The output voltage seems too low for fluorescent lamps but it is sufficient for small lamps upto 10W. The lamp voltage at normal operation is about 40V for those lamps and starting voltage rises upto 200V by series-resonance.
I designed it for 12V supply voltage first. However the primary winding (outer layer) could not be wound due to misestimation of winding thickness and I change the design voltage to 7.2V :-) It is not bad for battery powered operation rather than 12V. The resonant/ballast inductor L1 is wound with 0.13×6 UEW to reduce the power loss due to a skin effect. Litz wire is suitable for that use more than multifilar winding I did.
A microcontroller is used for switching controller to achieve arbitrary frequency contorl. The start up sequence is in three phases: pre-heating(65kHz/1.5sec), ignition(52kHz/0.5sec) and normal operation(44kHz). When driving the inverter at fixed frequency without any contorl, a half-bridge driver with oscillator, such as IR2153, will do.
