US2928994A - Transistor inverters for feeding fluorescent tubes - Google Patents

Description

March 1960 M. WIDAKOWICH 2,928,994

. TRANSISTOR INVERTERS FOR FEEDING FLUORESCENT TUBES Filed 0ct. 22, 1957 2 Sheets-Sheet 1 Fig.1

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TRANSISTOR INVERTERS FOR FEEDING FLUORESCENT TUBES Filed Oct. 22, 1957 2 Sheets-Sheet 2 United States Patent i TRANSISTOR INVERTERS FOR FEEDING FLUORESCENT TUBES Marius Widakowich, Bromma, Sweden Application October 22, 1957, Serial No. 691,714

Claims priority, application Sweden October 22, 1956 6 Claims. (Cl. 315-205) For feeding fluorescent tubes from a direct current source (e.g. a battery) it is necessary first to invert and then to transform the voltage in order to supply the fluorescent tubes with a voltage suitable for their operation. For the inversion of the direct current devices are generally used which operate with moving contacts and/or windings. Either the inverter then feeds a transformer to which the various fluorescent tubes are connected by way of their series impedances, or each fluorescent tube or group of fluorescent tubes is fitted with a special transformer connected to the inverter, said transformer being of a current limiting design. The problem in this case is to maintain the current in the fluorescent tube constant and independent of the voltage fluctuations of the direct current source which are often great.

According to the invention a transistor inverter is used which shows such a characteristic that the current through the fluorescent tubes remains almost constant and independent of the voltage fluctuations of the direct current source. This may for instance be attained by the frequency of the inverter being changed approximately in proportion to the battery voltage, a reactive current limitation being then used. The advantage with an inverter operating according to the said principle is that it is very simple and chiefly consists only of the transistor and the transformer. If the latter is provided with a stray field and an iron core whose magnetization curve shows a pronounced knee, the fluorescent tube may be connected directly to the transformer without interposing further current limiting elements.

The invention will be described in greater detail with reference to the attached drawings showing various current circuit arrangements and diagrams in transistor inverters used for feeding fluorescent tubes.

'Fig. 1 is a schematic diagram of the invention;

Figs. 2 through 4 are diagrams illustrative of various electrical values in operation of circuitry arrangements according to the invention;

Fig. 5 is a schematic diagram of an embodiment of the inverter according to the invention illustrative of transistor circuitry with the transistor connected in a push-pull configuration;

Fig. 6 is a diagram of a modification of the transistor inverter according to the invention;

Fig. 7 is a diagram of another modification of the transistor inverter according to the invention.

Figure 1 illustrates the principle of the transistor inverter. When the switch 1 is closed, the battery 2 is connected via the transistor 3 to the primary winding 4 of the transformer, the transistor then operating with joint (grounded) emitter coupling. In the core of the trans former now a flux change proportional to time arises, said flux change coming to an end, however, as soon as the exciting current reaches the limit valve determined by the collector saturation current. At this point the transistor is cut off by the voltage of the feedback winding 5 falling to zero or changing direction, respectively, after which a new pulse starts. The duration of each S ce 2 pulse is thus limited by the point of time when the exciting current reaches the critical value, and the higher the battery voltage is the sooner this will take place.

Figure 2 is an idealized diagram showing the progress during the positive half-wave of magnet flux, collector current and voltage across the Winding 4 in an unloaded inverter and at a battery voltage E1 as well as at the double battery voltage E2. When switch 1 is closed a small current starts to flow from the +terminal of the battery 2 via the emitter base collector of the transistor to the primary winding 4 of the transformer and back to f the -terminal. Thereby a voltage .is induced in the the whole voltage E1. The voltage induced in the winding 5 is therefore constant and almost equal to the product of the battery voltage and the tranformation ratio between windings 4 and 5. As a consequence, the current in the emitter base circuit will be-constant for invariable battery voltage. It is primarily restricted by a resistor 8 which thus determines the collector saturation current Ik. In order to balance the voltage E1, the flux FEl, Figure 2, in the core of the transformer must grow proportionally to the time. the no-load exciting current IoEl which, to begin with, grows slowly, but more rapidly according as the transformer iron beomes saturated, As soon as this exciting current has reached the value of the collector saturation current IkEl, it will no longer grow. As the flux will then also stop growing the voltage of the winding 5 and therefore also the emitter base current disappears,-the-" Thus it is the peak value of the primary current of the transformer which determines the moment of commutation said peak value being substantially equal to the collector saturation current of the transistor. This current is primarily determined by the base emitter current flowing in that moment. If a resistor 8 of a constant value is chosen, the base emitter current becomes substantially proportional to the voltage of the battery 2. a

If the battery voltage is raised or doubled in value, to

E2, the growth of the flux must proceed twice as fast, for which reason the saturation of the transformer iron' will occur in half the time. The voltage induced in the winding 5 also obtains a doubled value on account of which the emitter base current is also doubled, The corresponding collector saturation current IkE2, however, will become somewhat less than the double IkEl due to the falling current amplification characteristic of the transistor. material, the magnetizing curve of which shows a pronounced knee, a very short interval will elapse, however, before Io has grown from IkEl to IkE2 in which moment the commutation at E2 takes place. As evident from Figure 2 the duration of the positive pulse will thus be practically inversely proportional to the battery. According to the invention the current limiting impedance necessary for the operation of fluorescent tubes I is chosen as an inductance, preferably as a stray field 1 in the transformer. The secondary current ofthe transformer, therefore, grows with the time, the curve for the This flux is generated by 5 If the core of the transformer consists of an iron the voltage of a corresponding primary load current In will then run in a similsr'maaaer'as'ro, as will be seen from Figured.

a very'she'r: moment earlier than at no-load running. In doubling the battery voltage the commutation will take place asbefore within half the time. It is evident that theistray field of the transformer between the primary winding 4 and the secondary winding 6, to which the flubrs'c'ent tube 7 is directly connected, will react more ata higher frequency caused by an increase of the batteryyoltag e, andvice versa, whereby an automatic con trol t or'maintaining a constant load current is attained A mode of ignition particularly efficient is obtained in an arrangement according" toFi'giire 6. According to the invention the rapid change in primary current occurring at the end respectively the beginning of each pulse (see Figures 2-4) is used for generating high peaks of striking voltage in anauxiliary transformer whose primary winding 10 is connected in series with one or both of"the' primary windings of the main transformer,

, and whose secondary winding-ll-is connected in series Itinstead the circuits are dimensioned in such-a way,

thatlthe excitingcurrent will play a smaller partin th egm oment of commutation, it will be the peakvalue of the load current that is limited by the collector saturation current. Hence, the commutation will take place as soon as the load current has reached the collector saturation current. hehce thev load current practically independent of battery voltage-fluctuations the arrangement is so devised that the base emitter current becomes independent of such variations, for instance, by adapting the impedance 8 as'a non-linear resistor operating to maintain a constant current. If the load is reactive, the commutation thus occurs earlier the higher the battery voltage is, and as a consequence thecurrent through the fluorescent tube-- In order to make this current and will --be-- almost entirely independent of the variations in the -ba ttery voltage. These facts are depicted in Figure 4 which shows that the commutation takes place at the same collector saturation current Ik at varying battery voltage. Conditions intermediating between the cases according to-Figures 3 and 4 may of course be applied in 1 practice.

In order to achieve larger power outputs and a symmetrical shape of curve it may be advisable to arrange the' 'inverter in push-pull connection, which does notchange; anything as regards the principle of the invention'.--- In Figure 5 such a push-pull connected inverter is shown which, in principle, operates in the same manner-as the inverter described in connection with Figure 1.

Asfsoon as'the transistor 3 is choked, due to the fact that the primary current of the transformer has reached the*c ollectorsaturation current value, the transistor 3' will-become conductive, in view of the fact that the voltagein the'winding 5' changes sign as soon as the ignited without employing a special ignition device, even if the tubes are so called hot cathode tubes, but there is nothing to prevent using the usual ignition devices. For

instance, as shown in Figure 5, the secondary winding 6 of-=the stray field transformer may be provided with two taps=-to'which the filaments of the fluorescent tube are connected.'- .Before the tube is ignited a voltage high enough is generated across the filaments in order to ex pedlte the ignition. The capacitor 9 assists in raising the voltage across 6, before the tube has ignited, whereupontheglow potentialof the tube determines the voltage across 6.

with the winding fi. The Winding 11-may also be connected in parallel with'the whole winding 6 or a part thereof; By this arrangement the transformation ratio between 4 and 6 may be considerably reduced with an accompanying improvement inefficiency. As the trans former 4, 6 is adapted as a stray field transformer, -it is possible in certain cases to omit a special pulse transformer 10, 11 by arranging the winding 10 on the winding 6 of the inverter transformer.- In this case the winding 6 will undertake the function of .thewinding 11.

An advantage of thedevices hitherto described resides in the-fact that the transistors.themselves, do not actas current limiters beyond the'very moment of commuta tion, for whichreason the losses in-thertransistors are small and :the load capacity of ithe tra-nsistors is large..

In those cases described above, more or less saw-tooth shaped currents are obtained in the'fiuorescent tube. However, it is possible to causethe. current to assume a square shape of-curve which results-in a higher yieldof light in thefiuorescent tubes as compared with that obtained with a sinusoidal orsaw-toothshaped current. Inthiscase the limiting-of the current through the tubes 4 is not attained by a series inductance but by dimensioning the controlcircuit-of the transistor in such a way that-the-current obtains a square course. This'is most simply-carried out by feeding said control circuit from a preconnected oscillatorcontrolled-inverter"which at the same time t operates to maintain -a Constant current-- amplitude onthetoutput-side. I Figure"? shows byway of example such a device. 14 represents a push-pull connected oscillator operating in the samemanner as describedin connectionwith Figure l or S. Throughthe secondary windingof the transformer 15 the oscill lator delivers a square alternating voltage to the control circuit oftheinverter 16.- The control voltage '-is i maintained constant, for instance byconnecting in paral 1 lel'a voltage deperldentresistor 17. Thetinverter trans-' former 18 has a leakage as small as possible'so as not to'aifect' the shape: ofcurve of the current-throughthe fluorescent tube, which current will depict the control current.

An advantage of the deviceaccording to the invention is the-fact that thespace requirement of the transformer and the "other elements is so small that the comautomatically varying the frequency of alternating current applied to =the--transforiner substantially proportionallyto the voltagevariations of thesource of direct current comprising, a transistor element having a base electrode, a collector circuit in-the inverter-including said source of direct 'curr'ent and a primary winding on said transformer, an'- emitter-base" control circuit in the in verterincluding a potential "generating Windingon said transformer, a stabilizing resistance connected in said, emitter-base control circuit, said transformer having a core and a suitable stray field operatively associated with the secondary winding acting to compensate flux varia-t tions in the transformer core by corresponding frequency variations to maintain the lamp current substantially constant.

2. The combination according to claim 1, in which said transistor circuits are constructed to maintain the collector saturation current greater than the flux saturation current of the transformer core for actual variations of the direct current source voltage.

3. The combination according to claim 2, in which said transformer core has a magnetization curve with a pronounced knee.

4. The combination according to claim 3, in which the stabilizing resistance in the emitter-base control circuit has a fixed value sufliciently high to allow the control current to vary substantially proportionally to voltage generated by the potential generating inding of the control circuit. g

5. The combination according to claim 1, in which the stabilizing resistance in the emitter-base control circuit is variable to maintain the collector saturation current substantially constant for actual variations of the direct current source voltage, said transistor circuits being constructed to maintain the collector saturation current below the flux saturation current of the transformer core.

6. A circuit arrangement for energizing a fluorescent lamp with alternating current comprising, in combination with the lamp, a source of direct current, a transistor inverter, a transformer having primary winding connections to the transistor inverter and secondary connections to said lamp, said transistor inverter comprising a transistor element having a base electrode, a collector circuit including said source of direct current and a primary winding on said transformer, an emitter-base control circuit including a potential generating winding on said transformer, and a stabilizing resistance connected in said of direct current and the lamp circuit having inductance values so chosen as to compensate the frequency variations thereby to maintain the lamp current substantially constant.

References Cited in the file of this patent UNITED STATES PATENTS 2,016,790 Perlman Oct. 8, 1935 2,086,668 Fedor July 13, 1937 2,121,829 Seaman et al. June 28, 1938 2,300,916 Furedy Nov. 3, 1942 2,447,304 Atkins Aug. 17, 1948 2,488,169 Browner Nov. 15, 1949 2,497,534 Campbell Feb. 14, 1950 2,681,996 Wallace June 22, 1954 2,727,146 Fromm Dec. 13, 1955 2,745,012 Felker May 8, 1956 2,780,767 Janssen Feb. 5, 1957 2,816,230 Lindsay Dec. 10, 1957 2,837,651 Schultz June 3, 1958 FOREIGN PATENTS 1,123,405 France June11,'1956 1,129,507 France Sept. 10, 1956 OTHER REFERENCES Transistor Power Supply," by L. H. Light, Wireless -World, December 1955, pages 582 to 586.

Transistor Power Supply for Geiger Counter, A. R.

:5 Pearlman, Electronics, August 1954, pages 144 and 145.

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