Solid state clock

A solid state clock for an automobile or the like, which includes display means in the form of a vacuum fluorescent display. Current is supplied from the automobile battery to the display through an integrated circuit or "chip". The voltage from the battery is doubled by an inductor coil coupled between the battery and the integrated circuit. An electrical switching means in the form of a transistor is coupled between the integrated circuit and the inductor coil for supplying converted frequency thereto. A second switching transistor and a Zener diode is provided to regulate the duty cycle of the first transistor to maintain the voltage to the display substantially constant.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to a solid state clock for an automobile or the 
like, and, more particularly, to circuit means between the automobile 
battery and the display means of the clock. 
More specifically, this invention relates to such a circuit means for 
increasing the voltage from the automobile battery to the clock display 
means and for maintaining that voltage substantially constant. 
Solid state clocks in automobiles or the like have display means in the 
form of a vacuum fluorescent display device which derives current from a 
supply such as a 12-volt battery of the automobile. The display normally 
is energized by a voltage which exceeds that of the automobile battery, 
normally in the range of 18-20 volts. Thus, multiplying means is provided 
between the supply and the display, through an integrated circuit or 
"chip", to increase, normally doubling, the voltage from the battery. The 
multiplying means comprises an inductor coil or "doubler" which is 
connected between the supply and the integrated circuit. The integrated 
circuit, in turn, directs the increased voltage to the vacuum fluorescent 
display and components thereof. An oscillating switching means in the form 
of a transistor is provided between the integrated circuit and the 
multiplying means. However, the multiplied voltage preferably should be 
maintained substantially constant so as to avoid any fluctuations in the 
power to the display and integrated circuit. 
Heretofore, it has been proposed to employ a second power transistor on the 
order of four or five watts as a voltage limiter or regulator between the 
12-volt supply and the doubler in order to regulate the voltage to the 
doubler by limiting the same on the order of 111/2 to 121/2 volts. In 
other words, this power transistor provides direct means between the 
supply and the doubler to regulate the voltage thereto. 
With the present invention, the direct voltage regulator or power 
transistor having a larger current carrying capacity between the current 
supply and the doubler is eliminated, and the oscillating transistor 
between the integrated circuit and the doubler itself is regulated by a 
second switching transistor and a Zener diode. The second transistor and 
Zener diode is effective to shut the first transistor off whenever voltage 
at the doubler increases to a value set by the Zener diode, such as 18 to 
20 volts. 
In the exemplary embodiment of the invention, a solid state clock for 
automobiles or the like includes a display means in the form of a vacuum 
fluorescent display which includes one or more filaments to illuminate the 
display. Operating power is supplied to the display from an electrical 
power supply, such as a 12 volt battery for the automobile, through an 
integrated circuit or chip. The display requires on the order of 18 to 20 
volts and, consequently, voltage multiplying means is coupled between the 
current supply and the integrated circuit. The multiplying means comprises 
an inductor coil or "doubler" which increases or doubles the voltage from 
the supply. Converter frequency is supplied from the integrated circuit to 
the inductor coil through a first transistor which, in effect, runs or 
operates the doubler. 
Means is provided to regulate the duty cycle of the transistor between the 
integrated circuit and the doubler. This regulating means comprises a 
second, switching transistor coupled to the base of the aforesaid first 
transistor, and the base of the second transistor is coupled through a 
Zener diode to the doubler. The Zener diode has a predetermined value, 
such as on the order of 18-20 volts, which is equivalent to the voltage 
needed for operating the display of the clock. As the voltage at the 
doubler increases to the value set by the Zener diode, the diode turns the 
second transistor on which is effective to turn the first transistor off, 
and thereby, regulates the doubler by means of regulating the transistor 
between the doubler and the integrated circuit. 
Other features and advantages of the invention will be apparent from the 
following detailed description taken in connection with the accompanying 
drawing.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1 in greater detail, the solid state clock of the present 
invention thereshown is designed for use in an automobile or the like. The 
clock, like other auxiliary components in the vehicle, derives power for 
operation from a direct power current supply such as a 12 volt battery 10. 
The clock includes a display means 12 and, in its illustrated form, an 
integrated circuit 14, although it will be understood by those skilled in 
the art that an equivalent discreet component circuit may be used in place 
of the integrated circuit 14. The display 12 is shown schematically as a 
block in FIG. 1 and preferably comprises a well known form of a vacuum 
fluorescent display, for example Futaba Part No. 4-BT-18. The display 14 
includes at least two filaments, indicated diagrammatically by the 
filament inputs F.sub.1 and F.sub.2 in the drawing. The integrated circuit 
14 is likewise shown in the drawing in block diagrammatic form and may 
comprise a commercially available circuit board or "chip", such as 
American Microsystems, Inc. Part No. 1485, as is known in the art. Current 
is supplied from supply 10 to the display 12 through the integrated 
circuit by means of a line 15 connecting the integrated circuit and 
display. 
The display 12, in actual practice, requires a higher operating voltage 
than the 12 volt D.C. supply 10 supplies, generally in the range of 18-20 
volts. For purposes of providing an increased voltage and for providing, 
as well, an appropriate operating voltage at the filament inputs F.sub.1 
and F.sub.2 of the display 14, a step down transformer, generally 
designated 16, is provided having a primary winding 16a and a secondary 
winding 16b, the primary to secondary windings having a turns ratio of 
approximately 10 to 1 to thereby provide a 1.75 RMS voltage for the 
display 12, as presently being described. The opposed ends of the primary 
winding 16a are coupled between the high voltage terminal 10 of the power 
supply and an input of the integrated circuit 14 by conductors 18 and 20, 
respectively. A diode 22 is provided in line 18 between the supply 10 and 
the primary winding 16a as a safeguard in the event that the polarity at 
the supply 10 inadvertently is reversed. The opposed end terminals of the 
transformer secondary winding 16b are coupled by respective conductors 24a 
and 24b to the filament inputs F.sub.1 and F.sub.2. Line 17 supplies 
voltage to the grid of the display. The center tap 16c of the secondary 
winding 16b is coupled to ground 36 of the battery by conductor 25. The 
primary winding 16a of the transformer 16, in addition to providing an 
appropriate voltage input for the secondary winding 16b, serves also as a 
voltage doubler coil to develop a constant, approximately 18 volt signal 
for the input 26 of the integrated circuit 14 in a manner to be explained. 
The aforesaid 18 volt input signal for the integrated circuit 14 is applied 
to the terminal 26 from the supply 10, through a series electrical path 
comprising diode 22, line 18, the primary or doubler coil portion 16a of 
the transformer 16, line 20 and a second diode 19 disposed along line 20 
in like polarity to diode 22. The integrated circuit 14 includes a 
conventional crystal controlled oscillator circuit and a frequency divider 
therefor as well as conventional clock counter logic for generating a 
necessary signal input to actuate the display unit 12 to provide a correct 
visual indicia of time. An output 30 of the integrated circuit 14, leads 
through line 32 to a switching transistor, generally designated 34. 
The clock crystal and certain associated circuitry are disposed externally 
of the integrated circuit 14, as indicated in the dashed block 45. The 
circuit within the dashed block 45 comprises a quartz crystal 45a having a 
natural resonance frequency of 4.194304 megahertz coupled in parallel with 
a 10 megaohm biasing resistor 45b. The opposed, parallel, connected ends 
of the crystal 45 and the biasing resistor 45b are coupled to respective 
input terminals 45c and 45d of the integrated circuit 14 and, as well, to 
ground through a 20 picofarad capacitor 45e and an adjustable 5-35 
picofarad trimming capacitor 45f. The variable trimming capacitor is 
denoted in standard symbolism by the arrow through the capacitor 45f. A 
65.536 kilohertz pick-off on the frequency divider portion of the 
integrated circuit 14, labelled as terminal 30 in the drawing, is coupled 
by a conductor 32 to the base 34a of the transistor 34 through a 10,000 
ohm current limiting resistor 42. The emitter 34b of the transistor 34 is 
connected to ground 36 through conductor 38. The collector 34c of the 
transistor 34 is coupled to the common junction of the doubler or primary 
transformer coil 16a. In essence, transistor 34 is provided between the 
integrated circuit 14 and the inductor coil 16 to provide, in conjunction 
with associated control circuitry, to be described, a substantially 
constant 18 volt input at the terminal 26 of the integrated circuit 14. 
The ignition of the automobile 43 of the automobile is effective through 
the line 43a to shut down the display 12 when the ignition is turned off, 
but the integrated circuit remains on regardless of whether or not the 
ignition is on. The integrated circuit continues to function except that 
it does supply voltage to the display. When the ignition is off there is 
no output at terminal 30 to the doubler or transformer 16. 
As mentioned above, it is desirable to maintain the voltage at terminal 26 
on the integrated circuit substantially constant as the voltage from 
supply 10 is increased beyond its nominal 12 volts. One example of such an 
increase in voltage might occur when the automobile battery or supply 10 
is charged from the alternator. Thus, the multiplying action of the 
doubler coil 16a must be modified or regulated to sustain a constant 
output independently of voltage fluctuations above the 12 volt value. It 
has been proposed that this be done by regulating the voltage directly 
from the supply 10 to the doubler or inductor coil 16 by means of a power 
transistor between the supply 10 and the doubler 16. Such a power 
transistor would have to be capable of handling on the order of four or 
five watts to provide a necessary power dissipation effective to achieve 
the desired voltage regulation function. 
Power transistors having such capability are relatively expensive. With the 
present invention, such a regulator (in the form of a power transistor in 
line 18 between supply 10 and the inductor coil 15) is eliminated by 
providing means to regulate the voltage by regulating the duty cycle of 
transistor 34 and thus the effective duty cycle of doubler 16a. More 
particularly, referring to FIG. 1, a second switching transistor, 
generally designated 46, is provided for actuating the transistor switch 
34 in accordance with a sensed voltage condition. Specifically, the 
collector 46a of transistor 46 is connected directly to the base 34a of 
transistor 34. The emitter 46b of transistor 46 is coupled to ground 36 
while the base 46c of transistor 46 is coupled through a pair of series 
resistors 50 and 56 to ground. An anode of a Zener diode 52 or similar 
break-down device is coupled to the common junction of resistors 50 and 56 
while the cathode of the Zener diode 52 is coupled to the common junction 
of the coil 16a and the emitter 34c of the transistor 34. Resistor 50 
limits the base current to protect transistor 36 from an overload. 
Resistor 56 provides a "bleeder" which maintains transistor 46 turned off 
when Zener diode 52 is not conducting. 
The switching transistor 46, through the circuit described above, regulates 
the duty cycle of transistor 34 in accordance with the selected break-down 
voltage of the Zener diode 52 to the end that the capacitor 64 is 
sustained at a substantially constant charged value corresponding to the 
selected break-down voltage of the Zener diode 52. Specifically, the 
approximately 65 kilohertz square wave signal varying between zero volts 
and the positive voltage input at terminal 26 is applied to the base 34a 
of the transistor 34 to turn the transistor on during positive voltage 
half cycles and to turn the transistor off on alternate half cycles of 
zero voltage. This provides a uniform on-off switching function for the 
transistor 34 at the 65 kilohertz rate to provide a nominal 50% on duty 
cycle for the transistor 34. However, switching transistor 46 is effective 
to prematurely turn transistor 34 off on each "on half cycle" at a point 
in time when the break-down voltage of Zener diode 52 is exceeded. 
Specifically, as voltage at the output terminal 60 of inductor coil 16 
increases to a value of approximately 18 volts, by known voltage doubler 
action of the collapsing magnetic field in the coil, Zener diode 52 breaks 
down to turn on the switching transistor 46. Transistor 34 is abruptly 
turned off when transistor 45 is turned on. In other words, the break down 
of Zener diode 52 at its selected break-down voltage of, for example, 18 
volts, causes transistor 34 to be turned off prematurely in its duty cycle 
resulting in a discontinuation of charging current through coil 16a. 
Without transistor 46 and Zener diode 52, the duty cycle of transistor 34 
would have a uniform square wave curve with its on time equaling its off 
time. This is exemplified by the following formula: 
EQU V.sub.O =V.sub.I (1+t.sub.on /t.sub.off) 
Where: 
V.sub.O =Voltage output; V.sub.I =voltage input; 
t.sub.on =time on; and t.sub.off =time off. 
It can be seen that the output voltage would be doubled when the on and off 
times of transistor 34 are equal. However, this ratio is charged by the 
Zener diode 52 dependent upon the voltage at point 60. In other words, the 
longer the voltage at point 60 remains equal to or higher than the value 
of Zener diode 52, the longer switching transistor 46 remains on and 
switching transistor 34 remains off. 
One terminal of a charging capacitor 64 is coupled to the common junction 
of isolation diode 19 and the input terminal 26 of the integrated circuit 
14 with its opposite terminal coupled to ground 36. The capacitor 64 
serves to store charging current and maintain the voltage uniform during 
off periods in the charging cycle. 
With the above described circuit, it can be seen that the present invention 
not only provides means for doubling the voltage from supply 10 to the 
vacuum fluorescent display 12 through integrated circuit 14, but the 
voltage is maintained substantially constant without employing a power 
regulator such as a power transistor directly between the supply 10 and 
the inductor coil 16. This is done by providing regulating means in the 
form of transistor 46 and Zener diode 62 which regulate the duty cycle of 
the transistor 34 which runs the doubler coil 16 from the integrated 
circuit 14. By eliminating the aforesaid power transistor between supply 
10 and doubler coil 16, there is a considerable cost saving because the 
switching transistor 46 is considerably less expensive than a power 
regulator between the supply and the doubler coil. It has been found that 
a transistor on the order of a one-half watt power dissipation capacity 
may be employed for transistor 46 rather than a four or five watt 
transistor between the supply 10 and the doubler coil 16. Not only is the 
power size decreased, along with the relative cost thereof, but such 
factors as packaging, mounting boards and other components are directly 
affected which further reduces the overall cost of the solid state clock 
with which the invention is utilized. 
While a particular embodiment of the present invention has been shown and 
described, it is apparent that various changes and modifications may be 
made, and it is therefore intended in the following claims to cover all 
such modifications and changes as may fall within the true spirit and 
scope of this invention.