US2927224A - Temperature compensated r. c. network - Google Patents

Description

March 1, 1960 H. E. RUEHLEMANN 2,927,224

TEMPERATURE COMPENSATED R C .NETWORK Filed Aug. 25, 1955 H. E. RUEHLEMAN BY d ATTOR 13's United States Patent() TEMPERATURE COMPENSATED R.C. NETWORK Herbert E. Ruehlemann, Allentown, Pa., assignor to the United States of America as represented by the Secretary of the Navy Application August 25, 1955, Serial No. 530,650

13 Claims. (Cl. 307-109) i (Granted under Title 35, U.S. Code (1952), sec. 266) Work which compensates forresistance variations therein caused by temperature fluctuations.

In electric timing devices such as electric f ,uzes for example using RC time constant circuits, the time is some function of the time constant of the resistance and capacitance of the RC circuit. Any variation in the value of resistance or capacitance due to temperature changes causes a corresponding uctuation of time. There are capacitors commercially available which have a negligible temperature coetlicient of capacitance in the range of 60 F. to +l75 F., but, at this time, no resistors having negligible temperature coefficient of resistance are available for utilization in electric time fuzes.

Heretofore, the problem of resistance changes due to temperature variations has been solved by employing carbon deposit resistors which have a negative temperature coeicient in series with wire wound resistors which have a positive temperature coefficient. These resistors are arranged in such combination that anr increase in resistance of one, caused by a fluctuation in temperature, will be exactly counterbalanced by the decrease of resistance of the other resistor due to this same iluctuation in temperature. However, wire wound resistors of the magnitude of several megohms are too large and cumbersome for utilization in ordnance devices like fuzes. Furthermore, the best carbon deposit resistors, when of small size, still have a temperature coefficient of resistanceof -300 parts per million, and most of them have a temperature coetlicient in the range of -900 to -1200 parts per million; and, when carbon resistors are used in combination with wire wound resistors, the temperature coefficient range of the carbon resistors is such as to cause excessive time deviations due to temperature changes to be effectively employed in precision time devices like time fuzes for bombs, mines, or missiles.

The general purpose of this invention is to provide a time constant RC network which overcomes the above disadvantages. In accordance with the invention, there is provided a pair of equally charged capacitors of the negligible temperature coeicient type and a pair of carbon deposit resistors arranged to present a discharge path for the capacitors, the resistors being of the negative temperature coefficient type and having different temperature coefficients of resistance. The pair of resistors are so arranged with respect to the pair of capacitors that the resistor with the lesser temperature coeflicient solely provides the discharge path for one of the capacitors, the discharge path for the other capacitor being provided by both resistors.

It is an object of this invention to provide a new and improved resistance-capacitance time constant network for electric timing circuits.

With the foregoing in mind, it is an important object of the present invention to provide a time constant network in which the time constant thereof is not appreciably affected by temperature variations.

Another object of the invention is to provide a time constant network which compensates for time fluctuations caused by temperature variations to which the time constant network may be subjected.

A further object is to provide a time constant network having a substantially constant time constant.

An essential object of the invention is to provide, in a time constant network, a resistive discharge circuit f of non-uniform temperature coeicient of resistance for energy storage means, whereby any tendency of the storage means to deviate from its normal rate of discharge due to temperature variations is compensated by the non- 1 uniform temperature coeliicient characteristic of the discharge path.

A significant object of the invention is to compensate for time deviations introduced in a resistance-capacitance time constant network by temperature variations by providing a pair of resistive discharge paths of different temperature coefiicients of resistance. Another important object of the invention is to provide, as the resistive discharge circuit for a resistancecapacitance time constant network, a pair of resistors of the negative temperature coefficient of resistance type and of different temperature coeliicients of resistance, whereby any deviatory tendency of the time constant of the network due to temperature changes is compensated by regulatory resistive coaction of the pair of resistors.

A still further important object is to provide a temperature compensated time constant network having a pair of capacitors of the negligible temperature coehcient of capacitance type and a pair of resistors of different negative temperature coefficients of resistance so arranged with respect to the capacitors that the rate of discharge of one of the capacitors through both the resistors regulates the discharge rate of the other capacitor, whereby time deviations in the network due to temperature changes are compensated.

A more specific object of the present invention is to provide a temperature compensated time constant network having a pair of capacitors of the negligible temperature coeiiicient of capacitance type initially charged in parallel and a circuit selectively adapted to present simultaneously a pair of discharge paths to the capacitors, one of the discharge paths being formed by a single resistor of the negative temperature coeicient of resistance tvpe and the other discharge path being formed by the aforesaid single resistor and a second resistor having a greater negative temperature coetiicient of resistance.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood bv reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:

Fig. l illustrates a conventional and simplified time delay circuit; and

Fig. 2 is the circuit arrangement, partly in schematic and partly in block diagram, of a timing circuit incorporating the time constant resistance-capacitance network of the invention.

Referring now to the drawings, there is shown in Fig. l, which illustrates a well known time constant network a capacitor C which is initially charged to a value E0 from a source indicated as a battery through switch arm S and its stationary contact a. When capacitor C `Patented Mar. 1,` 1960 apanage where E is the initial voltage on the capacitor, Ec is the voltage across the capacitor C at any time t, and RC is the time constant T of the circuit.

If the rated value of resistor R is varied an incremental amount Ar due. to a change in temperature, the new time constant, assuming that the changeY in capacitance is negligible, is:

T=(R:Ar)c (2) and the time is accordingly changed by an incremental amountk At, resulting in the expression From Equation 3, it isy readily apparent'that, in thearrangement of Fig. l, the time will deviate directly proportional to the change in resistance caused by temperature fluctuations.

Referring now to Fig. 2, there is shown a timing circuit incorporating the time constant resistance-capacitance network of the present invention. The time constant networkof the invention, shown in solid lines, consists of a pair of capacitors C1 and C2 initially connected in parallel. and in thev same. polarity through lead 8, switch arm S', contact a', conductor 9 and through conductors and 11. Initially, the capacitors C1 and C2, which are of. the negligible temperature coeicient of capacitance type, are equally charged to a value E0 from any suitable source such as a battery having its negative terminal connected through contact a' and stationarywiper arm to the negative plates of capacitors C1 and C2 and its positive terminal connected through contact a and switch arm S to the positive plates of capacitors C1 and C2.

It; is tobe understood that, if the timing circuit of Fig. 2; is employed in an electric fuze of an explosive missile, the contacts a and d are xedly disposed in the breech of the missile firing mechanism so as to engage wiper arm S and stationary wiper arm 15, respectively, upon insertion of the missile into the breech of the firing mechanism and the voltage source E0 is connected to contactsy a and d through conductors 13 and 14, respectively, and is mounted on a suitable supporting structureassociated with the tiring mechanism whereby the capacitors C1 and C2 are charged upon insertion of the missile into the tiringrnechanism breech. Switch arms S and S', whichY are shown as. ganged for movementl in unison, may be of the inertia type switches which are thrown to their b and b positions, respectively, by set-back force as the missile is'propelled by the tiring mechanism.

Although the invention is described herein as being used in electric fuzes for explosive missiles and is ofspecic utility therein, it is to be understood that the inventionl is not limited to such use but also is of utility in any type of electric timing device wherein precision timing is desirable, and in such cases the switches S and S may be of any type suitable for the purpose and' need not necessarily be ganged.

A resistor R1, of the negative temperature coetiicient of resistance type, has one end thereof connected to the negative. plates of. capacitors. C1 and C2A andthe other' end thereof connected to xed contact b'. A second resistor R2, having a greater negative temperature coeicient of resistance than resistor R1, is serially connected between the capacitors C1 and C2 and is initially shorted by switch S and conductor 9 during the time capacitors C1 and C2 are being charged. Resistors R1 and R2 may be of the boron carbon type or of the carbon deposit type. Also, in accordance with the teachings of the invention, the negative temperature coefcient of resistance of resistor R2 ymust be greater than that of resistor R1. For example, the temperature coefficient of resistor R2 may be -900 parts per million and that of resistor R1 may be -300 parts per million. These values are only by way of illustration, and the invention is not limited thereto.

The time actuated system 5, which may consist of one of many known conventional electrically actuated circuits controlled by a time constant network, is connected to contact b. through lead 6 and to the negative plates of capacitors C1 and C2 through conductor 7. Upon switchesS and S being thrown to their respective b and b' positions, the time actuated system 5 is connected across capacitors C1 and C2 which, at the instant switches S and S' are thrown to the b and b positions, start to discharge through resistors R1 and R2 and continue to Y discharge therethrough. until a predetermined interval of time has elapsed, as determined by the selected values of resistors R1 and R2 and. capacitors vC1 and 2, whereupon the magnitude of potentials on capacitors C1 and C2 have been reduced to such values as to enable actuation of. time. actuated system 5 in its prescribed manner such, for example, as igniting a detonator or energizing a relay or solenoid. It is torbe noted that the ohmic values of resistors R1 and R2 and the capacitive values of capacitors C1 and C2 may be of any desired values selected to produce a predetermined time delay, as is well known to those skilled in the art, and have no. bearing on the concept of the invention.

In the operation of the invention as to compensating for time constant variations due to temperature fluctuations andassuming that capacitors C1 and C2 have been equally charged to the value E0, movement of switches S and S', either by set-back force in the case of a missile fuze orr by suitable meansv either manual or automatic, in the case of conventional electric timing devices, initiates the timing cycle of the time constant network, and capacitor C1 starts to discharge through resistor R1, producing a. current flow i1 which results in an IR value across resistor R1 determined by the. values of i1 and resistor R1. However, capacitor C2, which initially was charged to the same potential as C1, can discharge only when the potential across C1 becomes lower than the potential on C2. As soon as capacitor C1 has reduced to a potential below that of capacitor C2, capacitor C2 discharges through resistors R2 and R1 in series with a current i2. This current i2 through resistor R1 causes an additional IR drop across resistor R1 which increases the voltage drop across resistor R1 and thereby delays the discharge of capacitor C1. Assuming that the time constant network is at such temperature that resistors R1 andl R2 are at their rated values, the capacitors C1 and C2 continue to discharge in this manner through resistors R1 and R2 at the rate determined by the rated time constant of the network.

On the other hand, if the temperature increases, the resistance values of resistors R1 and R2 will decrease incrementally in proportion to their temperature coeicients of' resistance. Accordingly, since the resistive value of R1 has decreased, capacitor C1 will start to discharge through. resistor R1 at a faster rate than its normal rate of discharge. But, since the temperature coethcient of resistor R2 is greater than that of resistor R1, the resistance of resistor R2 has decreased incrementally more than has resistor R1, and, as a consequence, capacitor C2 will also discharge more rapidly than. its` normal discharge rate; resultingiiran increase iri magnituieof current i3. This increased current i2 appearing across resistor R1 in'turn increases the IR drop across resistor R1, thusA proportionallyV delaying the discharge of capacitorCl and therebycounterbalancing the tendency of capacitor C1 to discharge faster due to temperature effects ovnlrlesistorlRl. In this manner, the time constant network of the invention compensates for resistive variations therein introduced by temperature fluctuations. In the case of a decreasing temperature, the capacitors C1 and C2 function in a converse manner from that described above to attain the same result.

It has been found that excellent temperature compensation in a medium time range is obtained when the resistor R2 has a negative temperature coeflicient of resistance twice, or thrice, that of resistor R1. For medium and long time range, complete temperature compensation is obtained by employing a resistor R2 having a negative temperature coefficient of resistance which is 3.6 times greater than that of resistor R1.

The teachings of the invention are not limited solely to the utilization of negative temperature coeiiicient resistors, but, if desired, the invention can be practiced with positive temperature coefiicient resistors. If positive temperature coefficient resistors are employed as resistors R1 and R2 in the time constant network of Fig. 2, resistor R2 must have a greater positive temperature coefiicient of resistance than resistor R1 in order to obtain temperature compensation as taught herein.

Briefiy stated in summary, the invention contemplates to compensate for time variations introduced in time constant networks by temperature fluctuations by providing a pair of resistors of different temperature coefficients to form the discharge p-aths for a pair of capacitors, the resistors being of either both negative or both positive temperature coeicient types with the resistor having the lesser temperature coefficient solely presenting a discharge path to one of the capacitors and both resistors in series presenting a discharge path to the other capacitor.

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 teachings herein and the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A temperature compensated time constant network comprising energy storage means adapted to exponentially discharge a predetermined potential, a resistive circuit of non-uniform temperature coefiicient of resistance adapted to present a discharge circuit to said storage means and to form therewith a network of predetermined time constant, and selective circuit means coupling said storage means and said resistive circuit to form sa-id network of predetermined time constant whereby said storage means discharge said predetermined potential through said resistive circuit, the non-uniform temperature coefficient characteristic of said resistive circuit compensating for any time fluctuations introduced in said predetermined time constant by temperature variations.

2. A time constant network which inherently compensates for time fluctuations introduced by temperature variations comprising energy storage means adapted to exponentially discharge a predetermined potential,

a pair of resistive paths of different temperature coefficients of resistance, and selective circuit means for discharging said storage means through said resistive aths.

p 3. A time constant network which inherently compensates for time fluctuations introduced by temperature variations comprising an energy storage network having a pair of discharging branches, a pair of re- 6, sistors of differen'ttemperaturecoefficients of resistance, and switching means for serially interconnecting said resistors across one of said branches and for connecting the lresistor with thelesser temperature` coefiicient across the other of said branches.

4. The network of claim 3 wherein each of said pair of resistors has a negative temperature coefiicient vof resistance.

5. The network of claim 4 wherein said discharging branches include capacitors which inherently have negligible temperature coefficients.

6. The network of claim 4 wherein said pair of resistors are of the positive temperature coefficient of resistance type.

7. A time constant network which inherently compensates for time fiuctuations` introduced by temperature variations, comprising a pair of capacitors adapted to be equally charged from a source of energy, a pair of resistors of different temperature coeficient of resistance adapted to provide a discharge circuit for said capacitors, and circuit means including a switch for connecting said resistors to said capacitors to provide a discharge circuit therefor, said resistors being so connected with respect to said capacitors that the rate of discharge of one of said capacitorsl through both said resistors regulates the discharge rate of the other capacitor whereby time deviations in the network due to temperature changes are compensated.

8. The network of claim 7 wherein said resistors have negative temperature coefficients of resistance and said capacitors have a negligible temperature coefficient of capacitance.

9. A time constant network which inherently compensates for time fluctuations introduced by temperature variations, comprising a pair of capacitors of negligible temperature coefficient of capacitance, a discharge path for each of said capacitors, a first resistor of predetermined temperature coefficient of resistance defining the discharge path for one of said capacitors, and a second resistor in series with said first resistor forming the discharge path for the other capacitor whereby the discharge of said other capacitor through said first and second resistors regulates the discharge of said one capacitor through said first resistor to thereby compensate for time fluctuations caused by temperature variations, the temperature coefiicient of said second resistor being greater than said predetermined temperature coefficient of resistance.

10. The network of claim 9, wherein the temperature coefficient of resistance of said second resistor is 3.6 times greater than said predetermined temperature coeficient of resistance.

ll. The network of claim 10, wherein said first and second resistors have negative temperature coefficients of resistance.

l2. In a resistance-capacitance time constant network, means for compensating for time fluctuations caused by resistance changes due to temperature variations and comprising a first resistive discharge path adapted to have a first potential exponentially discharged thereacross, and a second resistive discharge path adapted to have a second potential equal to said first potential exponentially discharged thereacross, said second path having a greater temperature coefiicient of resistance than said first path and said second path including said first path as a portion thereof whereby time uctuations caused by resistance changes in said resistive paths due to temperature variations are compensated by commutual resistive coaction of said paths.

13. A time constant network which inherently compensates for time fluctuations introduced by temperature variations, comprising a pair of capacitors having a negligble temperature coefiicient of resistance, circuit means for equally charging said capacitors in parallel, and selective circuit means for discharging said capacitors through a pair of discharge paths, one of said paths being formed by a single resistor of predetermined negative temperature coeicient of resistance connected across one of said capacitors, the other of said paths being formed by a second resistor serially connected with said single resistor across the other capacitor, said second resistor having a negative temperature coecient of resistance greater than Said predetermined temperature coeicient.

References Cited in the file of this patent NITED STATES PATENTS Ruhlemann 1211119, 19,32 Stansbury Dec. 17, 1935 Haynes Sept. 5, 1944 Southeimer Oct. 26, 1948

Images