1. A portable tube lamp and power supply therefor comprising a pair of transistors each having a base, a collector and an emitter; a saturating-type switching transformer which exhibits a square-loop hysteresis effect and having a center tapped primary winding, a center tapped feedback winding and a secondary; a battery; an inductor; means connecting the center tap of the primary winding in series through said battery and said inductor to the emitters of said transistors; means connecting the feedback winding across the bases of said transistors; forward bias means connected between the center tap of said feedback winding and the battery for providing base-emitter bias; means connecting the primary winding across the collectors of said transistors; a tube lamp and a capacitor connected in series across said secondary winding; said inductor and capacitor providing sufficient power to cause ionization of said lamp while, at the same time, serving to limit current through said lamp subsequent to ionization.
The present invention relates to a portable lamp. More particularly, the present invention involves a novel and efficient circuit for lighting a fluorescent lamp in such a manner that the lamp power supply and circuit are packaged as a small portable unit.
The most popular type of portable lamp today is the petroleum fuel lamp which burns kerosene, gasoline, or the like. The use of the well known flashlight is well recognized, but the flashlight is limited by its poor efficiency. There have been some attempts to design and market portable fluorescent lights, but to the present time, these attempts have been met with rather inadequate results because of poor efficiency in the direct current to alternating current inverters necessary for their operation.
The present invention takes advantage of recent advances in the design and construction of ferrite magnetic material as well as the production of low cost silicon power transistors. One of the important features of the present invention is the employment of a saturating-type switching transformer having a core which exhibits a square-loop hysteresis effect. A pair of transistors are connected across the primary winding of this transformer, the common emitters being connected to the center top of the primary, the collectors being connected to the ends of the primary, the bases being connected across a feedback winding, the center tap of which is connected across a current divider network to provide bias for the base-emitter junctions of the transistors. A battery and an inductance are connected in series between the common emitter connection and the center tap of the primary and the divider network referred to above is connected across the battery. The fluorescent lamp and a capacitor are connected in series across the secondary of the transformer. The combination of the inductance in the primary circuit and the capacitor in the secondary is such as to provide sufficient power for igniting the fluorescent tube without requiring any special starter circuit or starter filaments. The inductor and capacitor also serve to limit the current through the tube lamp after ignition.
Therefore, it is a principal object of the present invention to provide a portable fluorescent lamp.
It is another object of the present invention to provide a portable fluorescent lamp and circuit therefor which is efficient and also inexpensive.
It is a further object of the present invention to provide a circuit for a portable fluorescent lamp utilizing a saturating-type transformer in combination with a DC to AC inverter in the primary wherein there is provided an inductor in series with the primary and a capacitor in the secondary in series with the tube lamp, the inductor and capacitor providing sufficient power to permit ignition of the lamp, at the same time limiting the current through the lamp after ignition.
Other and further objects and advantageous features of the present invention will hereinafter more fully appear in connection with a detailed description of the drawing which constitutes an electrical circuit diagram of the invention.
Referring to the drawing in detail, the electrical circuit shown therein employs two transistors Q1 and Q2 and a saturating-type switching transformer T1. The transformer is provided with a core which produces a square-loop hysteresis effect or characteristic. The transformer T1 includes a primary winding 10, a secondary winding 12 and a feedback winding 14. A pair of resistors R1 and R2 are connected in series with each other and in series with a switch SW1 across the terminals of a battery B1. The emitters of Q1 and Q2 are connected in common to the negative terminal of the battery. One side of the switch SW1 is connected to the positive terminal of the battery and the other side of this switch connects with R2 as shown and with one side of an inductor L1. The other side of the inductor L1 connects with the center tap of the primary winding 10 of the transformer. The end terminals of the primary 10 of the transformer connect with the collectors, respectively of Q1 and Q2 as shown. The ends of the feedback winding 14 of the transformer connect with the bases of the transistors Q1 and Q2 respectively. The center tap of the feedback winding 14 connects to the junction between resistors R1 and R2 as shown. The secondary winding 12 of the transformer T1 connects in series with a capacitor C1 and a fluorescent tube lamp 16.
The resistors R1 and R2 form a divider network which provides an initial forward bias to the base-emitter junction of both transistors. This bias will tend to cause both transistors to commence to conduct when the battery is connected into the circuit by closing the switch SW1. One transistor, for example Q1, will conduct more than the other due to inherent imbalance in the transistors and in the circuit. Regeneration will then drive Q2 into cut-off and Q1 will be driven into saturation. The collector current of Q1 will increase through the inductor L1 and the upper half of the transformer primary 10 until the transformer core becomes saturated. When the core becomes completely saturated, there is no longer a time rate of change in the magnetic flux and the voltage in the feedback winding 14 will drop towards zero. Q1 will now cease to conduct as the voltage in the feedback winding, which is holding the transistor in conduction, drops rapidly towards zero. At the same time, Q2 will commence to conduct and will be quickly driven into saturation so as to produce the second half of the alternating current cycle through the inductor L1 and the lower half of the primary winding 10.
The presence of the inductor in the circuit as shown alters the square wave which is normally produced across the primary to that of an increasing voltage exponential. The time constant of the circuit is determined by the value of the inductance and the series resistance of the circuit which will also include the resistance of the fluorescent lamp as reflected back into the transformer primary from the secondary. Thus, the initial current flow in response to any sharp increase in load will be limited by the presence of the inductor. On the other hand, a sharp increase in load during each half-cycle of the voltage waveform is characteristic of the ionization of the fluorescent lamp; conversely, when the proper ionization level of the fluorescent lamp is reached, the resistance of the device drops sharply. The capacitor, which is in series with the lamp and the transformer secondary, acts as a ballast by assuming the role of limiting the lamp current after the initial limiting action of the inductor has diminished. Thereafter, the action of the capacitor in conjunction with the effect of the inductor in the transformer primary circuit constitute an effective means of providing the high voltage necessary for automatic starting without using auxiliary starting filaments in the lamp and, at the same time, constitute a means for limiting the peak current through the lamp after ionization has taken place.
For illustrative purposes only, the circuit diagram could include a pair of 2N3402 transistors; the transformer could have a transformer core ferroxcube No. 846T250-3E2A, a primary winding including 28 turns of NO. 22 AWG wire with center tap, a feedback winding having eight turns of No. 26 AWG wire with center tap and a secondary winding with 420 turns of NO. 32 AWG wire. The battery could be a NO. 732 "Eveready" 12-volt dry cell battery. Resistor R1 is preferably one-half watt resistor of 150 ohms and R2 is a 1,000 ohm, one-half watt resistor. The lamp is a "Sylvania" F8T5/CW. The capacitor C1 is 0.001 μf Mylar capacitor and the inductance L1 is a 3.5 MH inductor. The switching frequency of the circuit disclosed herein using the components referred to above would be approximately 20 KiloHertz.
Obviously, different transistors, different sizes of resistors, capacitors, inductors, transformers and lamps could be employed within the scope of this invention providing, however, the principles enunciated herein are utilized in the circuit. Also, although the "common emitter" circuit is shown, it is possible to use other recognized inverter circuits such as a common collector circuit.
The circuit disclosed herein allows efficient operation of a fluorescent lamp from a portable, low voltage, direct current power source. Because the circuit includes an automatic starting feature, it is unnecessary for the operator to hold down a starting switch until initial ionization of the lamp has taken place. In installations such as on boats and in campers where the on-off switch is often placed some distance from lamp, the self-starting feature eliminates the need of running high voltage secondary wire external to the lmap itself in order to accomplish starting. The self-starting feature of this circuit eliminates the need for starting filaments provided in the ends of certain types of fluorescent tubes, these starting filaments often times being the cause of failure of the lamp itself. Thus, the self-starting feature of the circuit increases the life of the fluorescent tube itself.
When used in a boat or vehicle, the lamp can be powered from the vehicle battery itself, in which case it might be desirable to use equivalent but huskier components than those specifically described above. At this point it should be mentioned that the circuit is designed so that the lamp can operate satisfactorily even when the battery voltage drops considerably below that of the fresh battery level. The lamp in the above mentioned circuit will start and continue to run when the terminal voltage of the battery has dropped even thirty five per cent below fresh battery voltage; the latter effect is accomplished by selecting a proper turns ratio on the transformer.
Whereas the present invention has been described in particular relation to the circuit illustrated herein, it should be understood that other and further modifications, apart from those described or suggested herein, may be made within the spirit and scope of this invention.