Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1996-05-09
2002-06-18
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S2090SC, C315S240000, C315S283000
Reexamination Certificate
active
06407507
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to personal electronic devices, such as wristwatches, pocket pagers, calculators, and organizers, having an EL lamp and a piezoelectric buzzer powered from a single inductor.
An EL lamp is essentially a capacitor having a dielectric layer including a phosphor powder which glows in the presence of a strong electric field and a very low current. The dielectric layer is held between two electrodes, one of which is transparent. Because the EL lamp is a capacitor, an alternating current (AC) must be applied to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and the current through the EL lamp ceases.
For personal electronic devices such as wristwatches, pocket pagers, and cellular telephones, an EL lamp is driven by an inverter which converts direct current from a small battery into alternating current. In order for an EL lamp to glow sufficiently, a peak to peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder.
While there are many ways to increase voltage, e.g. by using a transformer or a voltage doubler, most applications for an EL lamp use what is known as a “flyback” inverter in which the energy stored in an inductor is supplied to the EL lamp as a small pulse of current at high voltage. The inverter typically operates at high frequency (4 khz. or more) to minimize the size of the magnetics, i.e. the inductor or transformer, in the inverter.
FIG. 1
is based upon the disclosure of U.S. Pat. No. 4,527,096 (Kindlmann). When transistor
14
turns on, current flows through inductor
15
, storing energy in the magnetic field generated by the inductor. When transistor
14
shuts off, the magnetic field collapses at a rate determined by the turn-off characteristics of transistor
14
. The voltage across inductor
15
is proportional to the rate at which the field collapses (
&dgr;i
/
&dgr;t
). Thus, a low voltage and large current is converted into a high voltage at a small current.
The current pulses are coupled through diode
16
to the DC diagonal of a switching bridge having EL lamp
12
connected across the AC diagonal. Assuming that transistors
18
and
19
are conducting, the same amount of energy is supplied to lamp
12
each time transistor
14
turns off and, therefore, the voltage on the lamp is pumped up by a series of current pulses from inductor
15
as transistor
14
repeatedly turns on and off. Diode
16
prevents lamp
12
from discharging through transistor
14
. If transistor
14
were switched on and off continuously, the pulses would charge lamp
12
to the maximum voltage available from inductor
15
, e.g. about 140 volts. Since an EL lamp needs an alternating current or a variable direct current, the lamp would glow initially and then extinguish when the capacitance of the lamp became fully charged.
To avoid this problem, the transistors in opposite sides of the bridge alternately conduct to reverse the connections to lamp
12
. The bridge transistors switch at a lower frequency than the frequency at which transistor
14
switches. The four bridge transistors are high voltage components, adding considerably to the size and cost of the circuit. In addition, the circuit is not single ended, i.e. one cannot ground one side of lamp
12
, which is preferred.
One could use separate inverters for driving an EL lamp and a buzzer. In many applications, particularly watches, a second inverter is difficult to add, primarily because of the cost of a second inductor. It is known in the art to power a piezoelectric buzzer and an EL lamp from a single flyback inverter.
FIG. 2
is based upon the disclosure of U.S. Pat. No. 4,529,322 (Ueda). In inverter
20
, transistor
14
is switched on and off at about 8 khz. to charge lamp
12
. When transistor
21
is conducting, lamp
12
is discharged.
There is an average DC bias across lamp
12
, approximately equal to one half the maximum voltage, because the lamp is charged in only one direction and then discharged. DC bias on an EL lamp can cause corrosion and shorting of the electrodes of the lamp, particularly at elevated temperature and humidity, decreasing the life of the lamp.
Another problem with inverter
20
is that transistors
21
and
22
draw current from terminal
13
through inductor
15
. This current is wasted since it does not contribute to powering lamp
12
, thereby reducing the efficiency of the inverter and decreasing battery life.
A third problem with inverter
20
is that switch
25
is necessary for isolating piezoelectric buzzer
26
from the high voltage pulses applied to lamp
12
. High voltage pulses stress the piezoelectric element and can cause failure. In the Ueda patent, switch
25
is one of two ganged switches actuated by undisclosed means.
FIG. 3
is based upon the disclosure of U.S. Pat. No. 5,313,117 (Kimball). Inverter
30
includes transistor
31
, inductor
32
, and transistor
33
connected in series between voltage source
34
and ground. Inductor
32
is alternately connected through transistors
35
and
37
to lamp
27
. Diode
36
is connected in a series with transistor
35
for preventing the transistor from operating in the inverse active mode, i.e. preventing transistor
35
from conducting current from the ground terminal through the forward bias based-collector junction when the voltage on lamp
27
is negative. Similarly, diode
38
prevents transistor
37
from operating in the inverse active mode when the voltage on lamp
27
is positive and greater than the battery voltage. The transistors, resistors, and diodes are implemented on a single chip. The inductor and capacitors are external devices coupled to the chip on a printed circuit board.
External logic circuitry provides a series of pulse bursts alternately on output lines “X” and “Y”. These bursts are coupled to the bases of transistors
31
and
33
and cause the transistors to conduct alternately, thereby providing positive and negative half wave voltages to lamp
27
. Inverter
30
produces alternating current at a single ended output and one side of lamp
27
can be grounded.
It remains a problem in the art to drive an EL lamp and a buzzer from a single inverter with as few components as possible with no waste current and with no DC bias.
In view of the foregoing, it is therefore an object of the invention to provide a personal electronic device with an inverter having a single inductor for providing alternating current to an EL lamp and direct current pulses to a buzzer.
Another object of the invention is to provide a personal electronic device with an inverter having no waste current.
Another object of the invention is to provide a personal electronic device with an inverter having no DC bias.
SUMMARY OF THE INVENTION
The foregoing objects are achieved in this invention in which a personal electronic device includes an inverter having an output coupled to the junction of an EL lamp and a buzzer. The lamp and the buzzer are coupled in parallel with each other to ground or are coupled in series between a DC supply and ground.
REFERENCES:
patent: 4527096 (1985-07-01), Kindlmann
patent: 4529322 (1985-07-01), Ueda
patent: 5313141 (1994-05-01), Kimball
patent: 5418434 (1995-05-01), Kamens et al.
Durel Corporation
Vu David
Wille Paul F.
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