US3588595A - Fluorescent lamp ballast and thermal protective means therefor - Google Patents

Abstract

BALLAST CONTROL FOR ELECTRON DISCHARGE TUBES, SUCH AS FLUORESCENT LAMPS, IN WHICH THE LAMPS ARE CONNECTED TO THE SECONDARY OF A TRANSFORMER HAVING A PRIMARY WINDING INCORPORATING BLOCKING OSCILLATOR MEANS IN WHICH A SINGLE TRANSISTOR HAS ITS OUTPUT CIRCUIT CONNECTED THROUGH ONE PORTION OF THE PRIMARY WINDING TO THE TERMINALS OF A DC SOURCE, AND ITS INPUT CIRCUIT CONNECTED THROUGH ANOTHER PORTION OF THE TRANSFORMER PRIMARY WINDING TO THE TERMINALS OF THE DC SOURCE, THE INPUT CIRCUIT BEING PROVIDED WITH RESISTOR MEANS AND CAPACITOR MEANS WHICH COACT WITH THE PRIMARY WINDING PORTION OF THE INPUT CIRCUIT IN A MANNER TO EFFECT A TUNED RC

NETWORK AND AN LC NETWORK HAVING A RELATIVELY HIGH FREQUENCY OF OSCILLATION ACTING TO ALTERNATELY BIAS THE TRANSISTOR INTO CONDUCTING AND NONCONDUCTING STATES FOR UNIFORMLY PULSING CURRENT FLOW FROM THE DC SOURCE TO THE PRIMARY WINDING, AND IN WHICH THERMAL SENSING PROTECTIVE MEANS OPERATES TO AUTOMATICALLY CONTROL THE OPERATION AND NONOPERATION OF THE OSCILLATOR MEANS AND PREVENT THE BALLAST FROM REACHING DANGEROUSLY HIGH TEMPERATURES WHICH COULD RESULT IN THE CREATION OF EQUIPMENT HAZARDS.

Description

United States Patent [72] Inventor Jerry J. Silvers Los Angeles. Calif. [Zl] Appl No. 880,213 [22] Filed Nov. 26. 1969 [45] Patented [73] Assignee June 28, I971 Walter Kidde 8: Company, Inc.

Clifton, NJ.

Continuation-impart of application Ser. No. 682,380, Nov. 13, 1967.

[54] FLUORESCENT LAMP BALLAST AND THERMAL PROTECTIVE MEANS THEREFOR 7 Claims, 1 Drawing Fig.

[56] References Cited UNITED STATES PATENTS 3,263,] 19 7/1966 Scholl 315/127X 3/ 1970 Moore ABSTRACT: Ballast control for electron discharge tubes, such as fluorescent lamps, in which the lamps are connected to the secondary ofa transformer having a primary winding incorporating blocking oscillator means in which a single transistor has its output circuit'connected through one portion of the primary winding to the terminals of a DC source, and its input circuit connected through another portion of the transformer primary winding to the terminals of the DC source, the input circuit being provided with resistor means and capacitor means which coact with the primary winding portion of the input circuit in a manner to effect a tuned RC network and an LC network having a relatively high frequency of oscillation acting to alternately bias the transistor into conducting and nonconducting states for uniformly pulsing current flow from the DC source to the primary winding,,and in which thermal sensing protective means operates to automatically control the operation and nonoperation of the oscillator means and prevent the ballast from reaching dangerously high temperatures which could result in the creation ofequipment hazards.

BACKGROUND OF THE INVENTION The invention relates to a control device for electronic I discharge tubes, such as fluorescent lamps.

l-leretofore fluorescent lighting systems or ballast controls for such lamps, when of portable character for operation on DC have in the main involved complex circuitry whichutilized a number of transistors and other components arranged as inverter units with relatively large transformer for stepping up the voltage. Inherently this type of unit or control was designed for operation at-relatively low frequencies and did not permit of a reduction in size of the transformer and other components, as would be possible under high frequency operation, to provide a less bulky device which could be installed in a small space, and yet operate efficiently for all fluorescent lamp power levels.

In thepresent invention, the components are arranged in a novel blocking oscillator circuit which is capable of being alternately operated to on and off" states and at relatively high frequency, for example, frequencies in the order of 4000 to 35000 cycles per second. At such frequencies, the size of the transistor and transformer may be materially reduced, thus enabling the ballast to be economically produced as a very efficiently operating small compact device capable of operating a fluorescent lamp within all its required parameters.

Since electronic ballast circuitry with blocking oscillators inherently provide a potential equipment hazard due to their heat storage capabilities under certain operating conditions, the present invention overcomes this problem by further providing novel temperature sensing control components for the blocking oscillator means.

SUMMARY OF THE INVENTION The present invention relates to a fluorescent lamp ballast; and is particularly concerned with a portable ballast of small compact efficient construction capable of operating from a DC source.

In the present application, the term ballast" is used in the technical sense as comprising a device used with an electricdischarge lamp to obtain the necessary circuit conditions for starting and operating."

With the foregoing in mind, it is one object of the herein described invention to provide ballast control means for a fluorescent lamp, which operates from a DC source, which is of small size, light of weight, efficient in operation and economical to produce.

A further object is to provide a small portable ballast control for fluorescent lamps, which utilizes unique blocking oscillator circuitry and components to pulse a DC source.

A further object is to provide a ballast control as above,

wherein a blocking oscillator is transformer coupled to the lamp, the transformer having a primary winding in which one portion is in the collector circuit of a transistor, and another portion of the primary winding is in a base feedback circuit of the transistor.

A further object resides in the provision of an extremely compact and light portable device of the character described, which embodies a simple blocking oscillator circuit having but a few components and only one transistor, these components being so arranged as to be efficiently operated to produce relatively high frequency pulses from a DC source.

A still further object is to provide a control device for electron discharge tubes, such as fluorescent lamps, in which a transistor is caused to function as a switch at high frequency to alternately open and close an energizing circuit of the lamp transformer in such a manner as to operate the lamp within all its required parameters.

Another object is to provide in ballast control for fluorescent lamps, which utilize blocking oscillator circuitry,

unique thermal sensing components in the control for the blocking oscillator so as to maintainsafe equipment operating conditions and eliminate possible equipment-hazards.

Still another object is to'provide in such ballast control,

unique thermal sensing control means in connection with the blocking oscillator, which utilizes a positive temperature coefficient thermistor.

Briefly,.the above objects are accomplished by providing a blocking oscillator circuit which embodies a single transistor, this transistor having its output collector circuit connected through one portion of the primary winding of a step up transformer for the lamp, and an input base circuit connected through another portion of the primary winding and containing tuned RC and LC networks tuned to synchronously bias the transistor under normal operating conditions to its on and off states of operation at relatively high frequencies. Resistor means in the base circuit of the transistor are utilized as a source for sensed temperature changes, under abnormal operating conditions, by means of a positive temperature coefficient thermistor having thermal coupled relation with the resistor means. The thermistor is embodied as a component of a voltage divider circuit which operates through appropriate means connected to the base of the blocking oscillator transistor so that it will be automatically subject to an overriding control functioning in response to ballast temperature conditions.

Further objects and advantages of the invention will be brought out in the following part of the specification, wherein detailed description is for the purpose of fully disclosing a preferred embodiment of the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the accompanying drawings, which are for illustrative purposes only, the single view schematically illustrates the various components and connecting circuitry of an embodiment according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT vided with a primary winding 11 comprised of two sections as indicated at 11a and 11b. A secondary winding 12 is connected to an output circuit load which in this case comprises the fluorescent lamps L-L connected in series.

The blocking oscillator circuit includes a transistor Q1 having an emitter 13, a base 14 and collector 15. This transistor is disclosed as being of the NPN-type, but of course a PNP-type or other suitable type of transistor may be utilized in accordance with well-known practice.

In the disclosed embodiment, the junction point 16 of the sections 11a and llbof the transformer primary winding is connected through a relatively wide range protective thermal cutout switch 17 to the positive terminal of a DC electric supply source, which may be a battery, this terminal being indicated as B. This switch, for example, may be set to open at l l0 C. and close at 73 C.'The outer end of the winding section Ila is connected through a conductor 18 with the collector 15, while the emitter 13 is connected through a conductor 19 to ground as indicated at 20 which in this case comprises the negative terminal of the battery source. An output circuit for the transistor 01 is thus provided which includes the transformer section 11a of the primary winding and which in this case constitutes the main winding for power transformation through the transformer to the secondary winding 12 to provide stepped up voltage for the fluorescent lamps.

The outer end of the primary section 1 lb of the transformer is connected by a conductor 21 to the base 14 of the transistor Q1. This conductor provides a current flow channel to the base of the transistor and includes a resistor R1 which is connected across the terminals of a capacitor C to form an RC network as indicated at 22. A series resistor R2 shown as comprising parallel connected resistors R2a and R2b, is provided in this circuit and is of a value to provide the desired base drive current for the transistor.

It will also appear that the section 1112 of the transformer primary winding provides an inductance which with the capacitor C forms an LC network as generally indicated at 23. The capacitor C is selected to resonate the inductance of this section of the primary winding.

Thus, the LC network and RC network provide means whereby with properly adjusted components will have a periodic frequency and time constant compatible with each other and aid in the starting and stopping of current flow in the transistor of the blocking oscillator circuit. Moreover, the

capacitor is so chosen that the rate at which its charge builds up will be matched to the rate at which the transformer section 11b discharges its stored energy upon collapse of the magnetic field of the transformer during the operation of the oscillator as will hereafter be explained more fully. The collapsing and expanding magnetic fields of the transformer and the charge and discharge of the capacitor are synchronized and locked in step so to speak by the proper choice of values in the components. It is thus possible to control the on and off operations of the transistor and provide a switching operation of relatively high frequency which combine with a faster rise time so that the physical size of the transistor and transformer may be reduced to provide the small compact arrangement as obtained in the present invention.

In operation of the basic arrangement as thus far described, it will be seen that the section 11a of the transformer primary winding functions in the output loop of the transistor for the main power transformation, and that the section 11b is in a feedback loop providing the base bias for controlling the switching of the transistor of the blocking oscillator. To start the operation, the switch 17 being closed, it is only necessary to connect the power source by switch or other means, not shown. A positive potential is thereby simultaneously applied to the collector l5 and base 14 of the transistor with the result that the transistor is activated to a conducting or switch closed state. The current flow in the collector circuit through the transformer winding section 110 produces an expanding magnetic field in the transformer until the core reaches a point of saturation. The section 11a induces a voltage in the section 11b which supplements the beginning of the initial charge on capacitor C, and as the capacitor C is charged, a negative going bias is applied to the base of the transistor which drives the transistor to cutoff.

Upon cessation of current flow through the winding section 11a of the transformer, the magnetic field begins to collapse, a reverse current and voltage being thus induced in the winding section 11b. The charged capacitor C will discharge through resistor R1, and since the rate of charge buildup in the capacitor C is matched to the rate at which the winding section 11b now discharges its stored energy as the field collapse, a large positive bias will be applied to the base of the transistor under normal operating conditions to turn it on. The cycle is thus repeated on a synchronized frequency basis in a manner to energize the fluorescent lamps at a relatively high frequency.

Further refinement features in the basic arrangement as described above include the provision of a capacitor C2 in the energizing circuit of the fluorescent lamps connected to the secondary winding 12. This capacitor permits of fine tuning of the lamp load so that the transformer sees both a resonant load and a more appropriately matched impedance. Operation is also improved by a diode 2 connected between the base 14 of the transistor Q1 and grounded negative side of the power source. This diode provides a current flow path between the base and emitter of the transistor, and operates to dump the space storage charge which would otherwise limit the speed of operation of. the transistor. By the utilization of this diode, the operating frequency of the ballast may be increased.

While the basic arrangement thus far described has excellent operating characteristics, it is desirable in ballast devices of the type embodying the present invention to provide an overriding protective control which will automatically provide against overheating under abnormal operating conditions. For example, it is known that electronic ballast circuitry utilizing blocking oscillators, embodies inherent undesirable operating characteristics which potentially create an equipment hazard. Blocking oscillators are inherently energy storing devices, and if their output path for some reason becomes obstructed or in the event of a lamp or load failure, the stored energy normally supplied to the external load is dissipated in the circuitry and can cause overheating to the point of damage. it is therefore a further important feature of the herein described invention to provide improved heat sensing means and an overriding control for the transistor Q1 of the blocking oscillator.

For such purpose, there is provided a temperature responsive overriding control for the transistor Q1, as generally indicated at 30. Temperature changes of the ballast device are sensed by physically mounting a thermistor RTl, of the type having a positive temperature coefficient, in thermally coupled relation with the resistors R221 and R2b to produce an isothermal mass wherein the thermistor will sense changes of temperature due to variations in the heating of the associated resistors under normal and abnormal conditions of operation. This thermistor is connected in series with a resistor R3 across the DC electric supply source to provide a voltage divider which is connected at 31 with a base 32 of a transistor Q2. This transistor has a collector 33 which is connected through a resistor R4 to the positive side of the DC source, and an emitter 34 which connects through a resistor R5 with the negative side of the power supply. A transistor Q3 is utilized to control the operation of the main power transistor 01. Transistor Q3 has a base 35 connected at point 36 of the emitter circuit of transistor Q2, a collector 37 which connects with the base 14 of transistor 01, and emitter 38 which is connected directly to the negative side of the supply circuit.

The overriding control will be inoperative under normal operation of the ballast device, and the oscillator will function as previously described. The resistors R2a and R2b will be the first components to get hot upon malfunction of the circuit. Thus, under ordinary operating conditions, the transistor Q2 will be nonconducting because the value of the thermistor is very low with respect to total voltage divider, and the base 32 of this transistor will be electrically close to ground or negative potential.

Upon the occurrence of malfunction and increased temperatures of the resistors R20 and R2b which ensues, the ohmic value of the thermistor increases at a predetermined rate to a specific level and the base 32 of transistor Q2 will now acquire a positive voltage sufficient to exceed the emitter-base barrier potential of transistor Q2. This transistor turns on, and the current flowing through resistor R5 acts to turn on and activate transistor Q3 which hitherto has also been in nonconducting state. With the transistor Q3 in a conducting state, it effectively connects the base of the main power transistor O1 to negative, thereby turning off and deactivating transistor 01, whereupon the entire circuit ceases to oscillate. Transistor Q1 remains in nonconducting state until such time as the thermistor RT] cools sufficiently and thereby returns transistor O2 to its nonconducting state for starting normal operating cycle of operation again. This cycling can go on indefinitely, since at no point will the temperature be permitted to exceed a dangerous limit.

As exemplary of a ballast control, as described above, the disclosed embodiment has been built and tested in which the various components were as follows:

Fluorescent lamp: General Electric, Sylvania or equivalent- T5, 21', 13 watts.

Transistor Ql-Type 2N305 5 Transistor Q2, Q3Type-2N5 l 83 Diode-Type lNS39l Transformerprimary section 11a, 32 turns; primary section 11b, 32 turns; secondary winding 12, 580 turns.

Battery 1 2 volts Resistors-Rl-68 ohms; RZa and R2b-47 'R36.8 K; R4 and RS-l K each.

Capacitors-C1.0 mf; C2.0068 mf.

While the above values of the components have been given, it will be appreciated, of course, that these components may be varied to meet varied requirements and specific installations.

The present device eliminates the possibility of equipment hazards due to abnormal operations, and in practice has been found to operate at a sufficiently high output frequency and with a sufficiently high peak voltage to ionize the gas within the conventional fluorescent lamp, without the necessity of having the normally provided heated cathode. The normal life of a fluorescent lamp based upon three burning hours per start is approximately 7500 hours. With the device of the present invention the normal life of the lamp, since not being dependent on starts and being independent of cathode heating, results in greatly prolonged lamp life. This becomes a very important factor where the ballast of the present invention is required to operate under emergency and standby conditions.

From the foregoing description and drawings, it will be clearly evident that the foregoing objects and features of the invention will be accomplished.

Various modifications may suggest themselves to those skilled in the art without departing from the spirit of my invention, and, hence, I do not wish to be restricted to the specific form shown or uses mentioned, except to the extent indicated in the appended claims.

I claim:

1. In control means for electronic discharge tubes:

a. ballast means energizable from a DC source and normally operable to energize a connected electronic discharge tube, said means including synchronously operable pulse generating tubes; and

b. overriding control for maintaining the normal operation of said ballast means within safe operating temperature limits, said control including temperature sensing means responsive to operating temperature changes generated externally thereof in said ballast means for automatically energizing and deenergizing said generating means as the ohms each;-

operating temperature varies from one side to the other of a predetermined safe operating value.

2. Control means according to claim 1, wherein the generating means includes a main power transistor having a base element, and the temperature responsive means includes switching means operable to connect and disconnect said base with respect to a negative potential source.

3. Control means according to claim 2, wherein the switching means are'actuatable in response to a potential variable in accordance with a sensed operating temperature characteristic of said ballast means.

4. Control means according to claim 3, wherein said variable potential is generated by voltage divider means including a thermistor for sensing operating temperatures of said ballast means.

5. Control means according to claim 4, wherein the generating means are normally pulsed by a control circuit connected to the base element of said transistor, said control circuit having resistor means therein, and said thermistor being positioned in heat coupled relation to said resistor means.

6. Ballast means for fluorescent lighting tubes, comprising:

a. a saturable core transformer having a primary winding with a tap connection between its ends, and a secondary winding adapted for connection with a fluorescent lamp circuit;

b. a transistor having an output circuit including a collector element electrically connected with one end of said pri- 'mary winding, an input circuit including a base element electrically connected with the other en of said primary winding, and an emitter element;

c. a DC power source common to said output and input circuits, connected between said tap connection and said emitter;

d. timing control means in said input circuit for normally synchronously biasing said transistor into conducting and nonconducting states to energize a fluorescent lamp connected in circuit to said transformer secondary at a relatively high sustained frequency, said means including current carrying means having a temperature variation dependent upon changes in current flow therein; and

. overriding control means for biasing said transistor into nonconducting and conducting states in response to temperature changes in said current carrying means.

7. Ballast means according to claim 6, wherein the overriding control means includes a thermistor having thermal coupled relation with said current carrying means.

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