GB2196805A

GB2196805A - Low voltage electroluminescent lamp driver circuits

Info

Publication number: GB2196805A
Application number: GB08716935A
Authority: GB
Inventors: Bruce H Kamens
Original assignee: Timex Corp
Current assignee: Timex Group USA Inc
Priority date: 1986-10-30
Filing date: 1987-07-17
Publication date: 1988-05-05
Anticipated expiration: 2007-07-17
Also published as: JPS63124390A; HK85090A; GB2196805B; GB8716935D0

Abstract

An electroluminescent lamp driver circuit for backlight illumination in watches comprises a CMOS monolithic integrated timing circuit 15 operating in an astable mode and driving a transistor 16 connected as an inverter-amplifier to provide repetitive pulses to the primary winding of a step-up transformer 12. The transformer has its secondary winding 14 connected across an electroluminescent lamp 11 to form a parallel resonator circuit. The components R1, R2, C1, C2 associated with the timer 15 are chosen so that transistor 16 is driven at the resonant frequency of circuit 11, 14, a capacitor C2 providing feedback to ensure that the driver circuit operates in synchronism with the resonant circuit 11, 14. One terminal of the lamp 11 and of the secondary 14 may be connected to the positive terminal of the battery supply (Fig. 2). The transformer provides a step up ratio of 30 to 1 and may be an autotransformer (Fig 3). <IMAGE>

Description

SPECIFICATION Low voltage electroluminescent lamp driver circuits This invention relates to low voltage electroluminescent lamp driver circuits for backlight illumination in watches.

More particularly, the invention relates to electroluminescent lamp driver circuits which are comprised of low cost, standard, commercially available components and employ a low voltage timing and drive circuit providing regenerative pulses to a voltage step-up transformer having its secondary winding connected in a parallel resonant circuit with the electroluminescent lamp, and which is arranged to illuminate the dial or display of a wristwatch.

US-A-4,253,097 describes an electroluminescent (EL) lamp panel wherein the phosphor bodies of the EL lamp are excited to luminescence by a low duty cycle, pulsed, high frequency potential developed in a tuned parallel resonant circuit comprised by an inductor connected in parallel circuit relationship with the EL panel. Resonant operation of the parallel tuned circuit thus comprised is achieved via a pulsing circuit and switching transistor wherein the switching transistor is directly connected in series circuit relationship with the parallel tuned circuit across a DC voltage source and requires the use of a relatively high voltage switching transistor. In this arrangement, driving pulses are supplied at a fraction of the resonant frequency of the parallel tuned circuit.

US-A-4,449,075 describes an EL lamp driver circuit for use with a large EL lamp panel rather than a small EL device for use as the background light for a wristwatch. In this circuit arrangement, an inductor is connected in series circuit relationship with a switching transistor which has to be a relatively high voltage switching device. The circuit further includes an astable multivibrator which interfaces with the switching transistor through two additional transistors for providing on-off switching potential to the high voltage switching transistor. The circuit arrangement provides pulses asynchronously and at a fraction of the resonant frequency of the inductor and capacitance of the EL lamp panel.This results in producing a periodic ringing of the resonant circuit and causes a high peak current to excite oscillation which makes the circuit unsuitable for use in watch applications.

US-A-4,527,096 describes an EL lamp drive circuit which includes a converter that provides current pulses to charge the capacitance of the EL lamp and produce a light. A switching circuit switches a predetermined number of successive current pulses at high frequency for progressively charging the EL lamp capacitance. The switching circuit also reverses the polarity of the EL lamp at low frequency when the EL lamp discharges at the end of a predetermined number of successive current pulses and before the EL lamp is charged again by current pulses produced by the converter. The switching circuit is connected to and is responsive to signals provided by a watch timepiece frequency divider circuit and the converter is connected to the timepiece battery for stepping-up the battery voltage to provide for the charging current pulses.Thus, the EL lamp is used as its own storage capacitor and receives substantially all of the energy provided by the converter. However, here again the pulsing transistor comprising a part of the converter charging circuit is directly connected to an inductor used to achieve the progressive charging. One disadvantage of this circuit is that a special customized integrated circuit is required to use the circuit in a wristwatch.

In order to overcome the problems and disadvantages encountered with the above briefly-described prior art driver circuits, the present invention was devised.

It is therefore an object of the present invention to provide a new and improved low cost, low voltage EL lamp driver circuit for watches which is manufactured from relatively low cost, standard, commercially available, proven, low voltage components interconnected in a manner to provide synchronized regenerative drive pulses to the high voltage EL lamp from the low voltage timing and drive.

circuit thereby improving reliability in service without excessive battery drain.

The present invention provides a low cost, low voltage electroluminescent lamp driver circuit for a watch is made available which provides synchronized regenerative drive pulses to a high voltage EL lamp from a low voltage drive circuit components and comprises: an electroluminescent lamp having a pair of electrodes at least one of which is light transparent and with electroluminescent material sandwiched between the electrodes; a voltage step-up transformer having the secondary winding connected across the electrodes of the electroluminescent lamp to form a parallel resonant circuit with the capacitance of the electroluminescent lamp; a timing circuit comprising an astable multivibrator having its output connected through an inverter-amplifier across the primary winding of the voltage step-up transformer; and a feedback capactior coupled between the primary winding and the input of the astable multivibrator for synchronizing operation of the astable multivibrator with the resonant frequency of the parallel resonant circuit comprised by the parallel connected electroluminescent lamp and the secondary winding of the voltage step-up transformer.

The above and other features and many of the attendant advantages of this invention will be appreciated more readily as the same becomes better understood from a reading of the following detailed description when considered in connection with the accompanying drawings, in which: Figure 1 is a schematic, functional block diagram illustrating the essential components of the overall driver circuit and their interconnections; Figure 2 is a partial schematic of one modification of the invention; and Figure 3 is a partial schematic of another modification of the invention.

Fig. 1 is a schematic functional block diagram which illustrates the details of construction of this invention. In Fig. 1 an electroluminescent (EL) lamp is shown at 11 in block diagram form and preferably is of the type used for background lighting of the hands or of the electro-optical display of a battery operated watch. For a more detailed description of the construction and operating characteristics of the electroluminescent lamp 11 (hereinafter referred to as EL lamp) reference is made to the above-noted US-A-4,253,097. Briefly, however, it can be stated that EL lamp 11 is comprised of two outer spaced-apart, parallel, protective, foundation plastic layers at least one of which is light transmitting.Suitable electrodes are deposited upon opposed, inner planar sides of the outer foundation plastic layer and electroluminescent phosphor granules are sandwiched between the two electrodes. Electrical lead conductors are electrically connected to respective ones of the two electrodes so that a supply electric potential can be applied across the electrodes. Typically, this supply electric potential must be of the order of 60-80 volts, peak-to-peak, for example, in order to assure adequate excitation of the electroluminescent phosphor granules sandwiched between the two electrodes of the EL lamp. The EL lamp 11 is shown as a separate device in US-A-4,253,097, but it can also be integrated with the dial of an analog wristwatch to provide background illumination for the hands.Alternatively, the EL lamp can provide illumination behind an electro-optical display in a manner such as that shown in US-A-4,208,869.

Supply electric potential is applied to the EL lamp 11 through a voltage step-up transformer 12 having its secondary winding 14 connected across the two electrodes of EL lamp 11 as shown in Fig. 1. Secondary winding 14 and EL lamp 11, which is largely a capacitive device, form a parallel circuit which will provide damped oscillation at a natural frequency.

Voltage step-up transformer 12 has a secondary to primary turns ratio of about 30:1 in order to achieve the required voltage step-up from a low voltage timing and inverter-driver amplifier circuit 15, 16 that is connected to supply periodic regenerative drive pulses to the primary winding 13 of volatage step-up transformer 12.

The timing and inverter-driver amplifier circuit supplying primary winding 13 is comprised by a standard, commercially available, CMOS monolithic integrated timing circuit 15 which, for example, may comprise a Texas Instruments type TLC551C connected to operate in an astable multivibrator mode. The timing circuit 15 is specially designed to operate at very low power consumption levels (typically 1 milliwatt at VDD equal 5 volts) over a wide range of supply voltages ranging from 1 volt to 18 volts. In the circuit of Fig. 1, a 1.5 volt DC current power supply is employed to excite the timing circuit. For this purpose, input supply potentials are obtained across a set of resistors R1 and R2 connected in series circuit relationship between the 1.5 volt DC supply source and a capacitor C1 to ground.

The values of R1, R2 and C2 provide an RC time constant for the timing circuit and are chosen, together with other values of the timing circuit to be described, to cause the timing circuit to operate at a frequency equal to the parallel resonant frequency of the EL lamp circuit 11, 12.

Timing circuit 15 has a number of input terminals 1-7 as shown in Fig. 1. Only certain of these input terminals are provided with excitation potentials as described hereafter in order to condition the timing circuit for astable multivibrator operation as noted earlier. For this purpose, terminal 1 of the timing circuit is connected to ground, terminal 3 serves as an output terminal, terminals 2 and 6 are directly connected to the juncture between resistor R2 and capacitor C1, terminal 7 is directly connected to the juncture between the two resistors R1 and R2 and terminals 4 and 8 are directly connected to the positive terminal of the 1.5 voltage DC supply source.

The output terminal 3 of the timing circuit 15 is supplied through a current limiting resistor R3 to the base electrode of a PNP transistor 16 that operates as an inverter-driver amplifier or switching means for supplying periodic excitation current pulses to the primary winding 13 of voltage step-up transformer 12.

For this purpose PNP transistor 16 has its emitter directly connected to the positive terminal of the 1.5 volt direct current supply source and has its collector connected to one terminal of the primary winding 13, the remaining terminal of which is connected to ground.

In accordance with one aspect of the invention and to assure synchronization of operation of the timing circuit with the natural resonant frequency of the parallel tuned circuit comprised by secondary winding 14 and EL lamp 11, a feedback path is provided from the juncture of the collector of PNP transistor 1 6 with the postive end of primary winding 13 through a feedback capacitor C2 to the number 6 input terminal of timing circuit 15.

As noted above, the timing circuit 15 is connected to operate in an astable multivibrator mode and the parameters of the excitation circuit are such that it will operate at a frequency equal to the parallel resonant frequency of the parallel circuit comprised by the inductance of the secondary winding 14 and the capacitance of the EL lamp 11 which are connected as a parallel resonant circuit.

With the above-described arrangement, the output from the timing circuit 15 drives inverter-driver amplifier 16 into saturation at a duty cycle of approximately 10%, which causes it to act as a switching means for connecting the voltage source to primary winding 13.

This results in the production of a current pulse through primary winding 13 during each cycle which by proper selection of circuit components R1, R2, C1 is in phase with the resonant frequency voltage developed across EL lamp 11. Feedback through feedback capacitor C2 which also affects the frequency of the timing circuit 15, is used to synchronize oscillation of timing circuit 15 with the resonant frequency of the parallel resonant circuit formed by the inductance of the parallel connected secondary winding 14 and capacitance of EL lamp 11.

In one particular embodiment of the abovedescribed circuit, which has been reduced to practice, the various components of the circuit shown in Fig. 1 have the following parameters: R1 =64 kilohms R2 =20 kilohms R3=510 ohms C1=.005 microfarads C2=.002 microfarads Transformer 12, using a TDR miniature potentiometer core, type H 5C2 P5.8/3.3, has a primary winding 13 constructed of 90 turns of 51 wire with an inductance of 4.23 millihenries and a resistance of 35.6 ohms. Secondary winding 14 is constructed of 2600 turns of 51 wire with an inductance of 3.2 henries and a resistance of 1.2 kilohms. PNP transistor 16 comprises a Motorola PNP silicon annular transistor type 2N4402 and EL map 11 comprises a .4-L lamp R20-3 manufactured by Timex Corporation.

Fig. 2 shows a modification of the invention, wherein the parallel resonant circuit comprising the secondary winding 14 of the transformer 12 and the EL lamp 11 is connected by a jumper 17 to the low voltage battery positive terminal. In this case the high voltage resonant circuit is not isolated from the low voltage timing and drive circuit as it is in the Fig. 1 embodiment. This arrangement may be desired in some wristwatch applications where one electrode of the EL lamp is electrically connected to the dial of the watch which is held at a common potential with other watch elements connected to the positive battery terminal.

Fig. 3 shows another modification using an autotransformer 18 having a first winding 19 and a second winding 20 connected in series and wound in mutually inductive relationship on a common core. The junction between one side of capacitor C2 and the collector of tran ,sistor 16 is connected to a junction between the series connected winding 19, 20 by a lead 21. Winding 19 of the autotransformer serves as the primary winding and series con nected first and second winding 18, 19 in series serve as the secondary winding of a transformer to step-up voltage supplied to EL lamp 11. The inductance of winding 18, 19 and the capacitance of EL lamp 11 primarily determine the resonant high voltage circuit frequency as in the other embodiments of the invention.

There has been described a low cost, high efficiency electroluminescent lamp driver circuit which is constructed from off-the-shelf, stan dard, commercially available components. The circuit is intended principally for use in operat ing a lighting source for watches to provide illumination for the hands from a lighted dial, or to provide backlight for electro-optical displays.

Having described three embodiments of a low cost, low voltage electroluminescent lamp driver circuit for watches constructed in accordance with the invention, it is believed obvious that other modifications and variations of the invention will be suggested to those skilled in the art in the light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention described which are within the full intended scope of the invention as defined by the appended claims.

Claims

1. A low cost, voltage electroluminescent lamp driver circuit for a watch which provides electrical isolation between the high voltage electroluminescent lamp and the low voltage drive circuit, comprising a low voltage battery power supply; an electroluminescent lamp hav ing a pair of electrodes at least one of which is light transparent and having electroluminescent material sandwiched between the electrodes; a voltage step-up transformer having its secondary winding connected across the electrodes of the electroluminescent lamp to form a parallel resonant circuit with the capa citance of the electroluminescent lamp; timing circuit means comprises an astable multivibrator having its output connected to switching means, said switching means being connected to supply current pulses from said low voltage battery power supply to the voltage step-up transformer; and feedback circuit means coup led between the primary winding and the input of the astable multivibrator for synchronizing operation of the astable multivibrator with the resonant frequency of the parallel resonant circuit comprised by the parallel connected electroluminescent lamp and the secondary winding of the voltage step-up transformer.

2. An electroluminescent lamp driver circuit according to Claim 1 wherein said low voltage battery source is on the order of 1.5 volts and wherein the switching means and battery power supply comprise an inverter amplifier and wherein the astable multivibrator and said inverter/amplifier are standard, commercially available low voltage circuit components capable of operation from said low voltage battery power supply.

3. An electroluminescent lamp driver circuit according to Claim 2 wherein the astable multivibrator is a monolithic integrated timing circuit fabricated in accordance with CMOS technology such as the Texas Instruments TLC 551 timing circuit operated in an astable mode.

4. An electroluminescent lamp driver circuit according to Claim 3 wherein the inverter-amplifier is a commercially available PNP silicon transistor such as the Motorola 2N4402 silicon annular transistor having its base connected to the output from the astable multivibrator, its collector connected to the primary winding of the voltate step-up transformer and its emitter connected to said low voltage battery power supply.

5. An electroluminescent lamp driver circuit according to Claim 1 wherein the voltage step-up transformer has a secondary to primary winding turns ratio of about 30:1.

6. An electroluminescent lamp driver circuit according to Claim 4 wherein the voltage step-up transformer has a secondary to primary winding turns ratio of about 30:1.

7. An electroluminescent lamp driver circuit according to Claim 1 wherein the voltage step-up transformer has a primary winding comprised of 90 turns of 5 1 wire with an inductance of 4.23 millihenries and a resistance of 35.6 ohms and the secondary winding is comprised of 2600 turns of 51 wire with an inductance of 3.2 henries with resistance of 1.28 kilohms.

8. An electroluminescent lamp driver circuit according to Claim 6 wherein the voltage step-up transformer has a primary winding comprised of 90 turns of 51 wire with an inductance of 4.23 millihenries and a resistance of 35.6 ohms and the secondary winding is comprised of 2600 turns of 51 wire with an inductance of 3.2 henries with resistance of 1.28 kilohms.

9. An electroluminescent lamp driver circuit according to Claim 1 wherein the feedback circuit comprises a capacitor having a capacitance value of about .002 microfarads with one of its terminals connected to the juncture between the output of the inverter-amplifier and the primary winding of the voltage stepup transformer and the other terminal of the capacitor being connected to an input terminal of the astable multivibrator.

10. An electroluminescent lamp driver circuit according to Claim 8 wherein the feedback circuit means comprises a capacitor having a capacitance value of about .002 mixcrofarads with one of its terminals connected to the juncture between the output of the inverter-amplifier and the primary winding of the voltage step-up transformer and the other terminal of the capacitor being connected to an input terminal of the astable multivibrator.

11. An electroluminescent lamp driver circuit according to Claim 1, wherein said secondary winding is connected to one terminal of said low voltage battery power supply.

12. An electroluminescent lamp driver according to Claim 1, wherein said voltage stepup transformer comprises an autotransformer having first and second windings connected in series, said first winding comprising said primary winding, and both said first and second windings comprising said secondary winding.

13. An electroluminescent lamp driver according to Claim 12, wherein one terminal of said low voltage battery power supply is connected via said switching means to a junction between said first and second autotransformer windings.

14. A low voltage electroluminescent lamp driver circuit substantially as hereinbefore described with reference to the accompanying drawings.

CLAIMS Amendments to the claims have been filed, and have the following effect: Claim 1 above have been deleted or textually amended.

New or textually amended claims have been filed as follows:

1. A low cost, low voltage electroluminescent lamp driver circuit for a watch comprising a low voltage battery power supply; an electroluminescent lamp having a pair of electrodes at least one of which is light transparent and having electroluminescent material sandwiched between the electrodes; a voltage step-up transformer having a primary winding and having its secondary winding connected across the electrodes of the electroluminescent lamp to form a parallel resonant circuit with the capacitance of the electroluminescent lamp; low power switching means connected in circuit relationship with the primary winding of the voltage step-up transformer; timing circuit means comprising an astable multivibrator having its output connected to control said switching means, said switching means being connected to supply current pulses from said low voltage battery power supply to the primary winding of the voltage step-up transformer in response to the output from said tim ing circuit means; and feedback circuit means coupled between the primary winding and the input to the astable multivibrator for synchronizing operation of the astable multivibrator with the resonant frequency of the parallel resonant circuit comprised by the parallel connected electroluminescent lamp and the secondary winding of the voltage step-up transformer.