US 3321666 A
Description (OCR text may contain errors)
May 23, 196 J. R. GARNETT ETAL 3,321,666
DIMMER CIRCUIT FOR GAS DISCHARGE TUBES Filed Dec. 21, 1964 CONTROL CIRCUITS I INVENTORS JAMES R. GARNETT HARRY ROWN 3 1 3 BY A T TORNEYS United States Patent 3,321,666 DIMMER CIRCUIT FOR GAS DISCHARGE TUBES James R. Garnett and Harry Brown, both of Philadelphia,
Pa., assignors to the United States of America as represeuted by the Secretary of the Navy Filed Dec. 21, 1964, Ser. No. 420,221 3 Claims. (Cl. 315-269) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present invention relates to a brightness control circuit and more particularly to a dimmer circuit for a gas discharge tube and the like.
In the field of instrumentation it has been the general practice to employ light tubes of various kinds as means for signalling the condition of various instruments. A significant example is the well-known nixie tube which is a gas discharge device containing numbers from 0 to 9 or letters or other indications. The tube is capable of lighting any one of these numbers or letters as desired according to the condition of the instrument. The individual figures in the tube are gas discharge devices which require a voltage above a specified minimum in order to light. A difiiculty has arisen in that in certain circumstances, such as, for example, in the low light level of an airplane cockpit the light emanating from the nixie tube or other device is too bright and is annoying and distracting to the operator. Standard means for dimming such devices has simply consisted of lowering the voltage across the tube. However, since the tubes must have a specified minimum voltage in order to light, it has been found that lowering the voltage across the tube quickly results in impairment of the function of the tube and failure of the tube when the voltage goes below the specified minimum.
The general purpose of this invention is to provide a circuit by which the amount of power fed into the tube may be lowered without lowering the peak voltage put across the tube, so that the net light eifect of the tube is lessened without failure. To attain this, the present invention provides a circuit having an oscillator connected to a variable resistance element whereby the circuit alternately has a very high and very low resistance in series with the tube. The result of this is to provide to the tube a series of oscillations comprising peaks of sufficient voltage to keep the tube 11 lit, in which the amount of power fed to the tube is a function of the percentage of the cycle during which the peak voltage is on, which percentage is controlled by the oscillator. The resistance element may be a transistor used as an amplifier, or a relay. The control of the oscillator is provided by variable resistances contained in the oscillator which control the frequency of the oscillator or the percentage of the oscillator cycle in which the condition of the oscillator is sufticient to turn off the transistor or relay.
Accordingly, it is an object ofthe present invention to provide a dimmer circuit for a gas discharge tube such as a nixie tube, whereby the peak voltage of the circuit is above the minimum voltage of the tube but the amount of power fed in is a function of a variable device controlled by the operator.
Another object of the invention is to provide an oscillator for a variable resistance element connected to a gas discharge tube to control the power fed into the gas discharge tube.
A further object of the invention is the provision of control means within the oscillator to control the On-Off ratio of the oscillator cycle.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detiled description when considered in connection with the accompanying drawings in which like reference nurnerals designate like parts throughout the figures thereof and wherein:
FIG. 1 shows a circuit diagram of an oscillator, resistance element, and gas discharge tubes according to the present invention.
FIG. 2 shows a voltage-current diagram of an oscillator element in the oscillator of FIG. 1.
FIG. 3 shows the variation with time of the voltage signal on the oscillator of FIG. 1.
FIG. 4 shows an alternative embodiment of the resistance element in FIG. 1.
In FIG. 1 there is shown a set 11 of gas discharge tubes numbered in the drawing from 1 through 0. These numbers represent the components of a gas discharge tube used for readouts such as a nixie tube and all ten numbers will generally be contained within a single tube. A set of control circuits 12 is connected to the ten numbers in the tube and controls the application of a voltage to the tubes as shown. All of the tubes are connected at their opposite sides in parallel to a resistance element 13 which is shown as a transistor in FIG. 1. The-base of transistor 13 is connected by a pair of resistances 14 and 15 to a voltage source. Connected between the junction of resistances 14 and 15 and ground is a second transistor 16 controlling the signal on the base of transistor 13. Connected to the base of transistor 16 is a resistance 17 which as shown may be variable. Variable resistance 17 is connected to the voltage source through another resistance 18 which as shown may also be variable.
The operation of the oscillator is conducted through a unijunction transistor 21 the poles or bases of which are connected between the voltage source and ground 'by a pair of resistances 22, 23. A capacitance 24 is connected between the emitter of transistor 21 and ground.
The operation of the relaxation oscillator is as follows. As shown in FIG. 2, the characteristic of unijunction transistor 21 is that its current increases with voltage up to a specified point, identified as V on the base thereof. When the voltage on the base of transistor 21 as .measured at the junction V in FIG. 1 attains the value of V the current will suddenly increase into the negative resistance section of transistor 21, as shown in FIG. 2, which will cause the charge on capacitor 24 to discharge into transistor 21 through the emitter thereof. This discharge also causes the resistance across the poles of transistor 21 to decrease greatly, and voltage thereacross is greatly reduced. The elfect of this is to cause the condition of transistor 21 to return to the positive resistance side of the curve in FIG. 2 and the cycle is repeated. The frequency of the cycle is determined by the time that it takes charge to build up on capacitor 24 to a voltage V This time is determined mainly by the resistance of variable resistance 18. If variable resistance 18 is high, the frequency will be low and the period to charge up will be high. If resistance 18 is very low, capacitor 24 will charge up quickly and the frequency of the oscillator will be high. FIGURE 3 shows the relationship of the voltage at junction V with respect to the time on and time olf. During time on, transistor or resistance element 13 has an essentially zero resistance and the tube 11 is on. During the time 011, the resistance of element 13 is essentially infinite and tube 11 is oil. If resistance element 13 is a transistor, the transition from zero to infinite resistance will not be instantaneous but the transition is quite quick and for practical purposes considering the range of voltages involved in FIG. 3 it may be considered is substantially instantaneous at voltage V If the resistance 18 is decreased, it will increase the rate of charging up of capacitor 24 and the base of transistor 21 will arrive at V quicker. The result of this will be to cause transistor 16 to be conducting a major part of the time. When transistor 16 is conducting, the connection to the base of transistor 13 is essentially shorted and it is off. Thus decreasing resistance 18 increases the time during which transistor 13 is blocking tube 11 and thereby decreases the brightness. Increasing resistance 18 causes the curve in FIG. 3 to be longer, lowers the frequency and causes a greater percentage of the time of each cycle to be in the On stage. Therefore increasing resistance 18 causes the tube 11 to be brighter. If resistance 17 is varied, this varies the proportion of voltage V which is placed across the base of transistor 16. Resistance 17 and the input resistance of transistor 16 form a voltage divider. The effect of raising resistance 17 is to place a lower percentage of the voltage at junction V on the base of the transistor 16 and thereby to cause junction V to charge up to a higher voltage before conduction of transistor 16 takes place. By this means a greater percentage of the cycle time will be in the On stage, and increasing the resistance 17 increases the brightness of the tube 11. The effect of increasing resistance 17 may be seen in FIG. 3 if it is considered that the effective voltage V, is raised toward the voltage V which, as will be seen, will cause a greater percentage of the time to be shifted into the T section. In FIG. 3, when the voltage is between V and V the tube 11 is on and when the voltage between V and V the tube 11 is off.
In FIG. 4 is shown an alternative embodiment for the resistance element 13. Resistance element 13a is provided, which comprises a reed relay 25 operated by a coil 26. The reed relay 25 and coil 26 operate in the same manner as the transistor 13, in that when there is voltage on the coil 26 corresponding to a voltage on the base of the transistor 13, the reed relay 25 is engaged and resistance is 0. When there is no signal on the coil 26 from transistor 16, i.e., when transistor 16 is conducting and is shorted out, then reed relay 25 is disengaged and resistance is infinite. Reed relay 25 and coil 26 provide a substantially instantaneous transition from zero to infinite resistance which will cause the shape of the On and Off portions of the voltage on tube 11 to be almost square.
It will be understood, of course, that control circuit 12 may be connected to a whole series of tubes 11, each of which will contain a number of distinct gas discharge elements which may be 1 through or may be letters, or may be specific readouts. One side of all of these elements will be connected in common to one side of resistance elements 13 or 1301.
Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
-What is claimed is:
1. An apparatus for controlling the brightness of a light source comprising:
a light source requiring a predetermined minimum voltage across it for conduction;
a first voltage source connected in circuit to said light source; and means for periodically interrupting the connection between said first voltage source and said light source, the ratio of connection time to nonconnection time being variable, said means comprising:
a second voltage source;
a unijunction transistor having two base electrodes and an emitter electrode, one base electrode coupled to said second voltage source;
a resistor-capacitor network connected in series between the other base electrode and the emitter electrode of said unijunction transistor;
a first variable resistance connected between said second voltage source and said emitter electrode for providing a variable charging current for said capacitor, said capacitor being discharged by said unijunction transistor upon reaching a prescribed voltage;
a first transistor;
a second variable resistance connected between the emitter electrode of said unijunction transistor and the base of said first transistor thereby forming a voltage divider with said first transistor for varying the point of conduction thereof; and
means operatively connected to the collector of said first transistor and said light source for periodically interposing a substantially infinite resistance between said light source and said first voltage source as said resistor-capacitor network is charged and discharged.
2. An apparatus as recited in claim 1 wherein said means operatively connected to said first transistor comprises:
a second transistor having its conduction controlled by said first transistor and providing a substantially infinite resistance between said light source and said first voltage source.
3. An apparatus as recited in claim 1 wherein said means operatively connected to said first transistor comprises:
a reed relay means for varying the resistance between said light source and said first voltage source in accordance with the conduction of said first transistor.
References Cited by the Examiner UNITED STATES PATENTS 9/1965 Nielsen 30788.5 12/1965 Phelps 30788.5
References Cited by the Applicant UNITED STATES PATENTS 3,063,004 11/1962 Vic.
J. D. CRAIG, Assistant Examiner.