US3916264A

US3916264A - Time delay apparatus

Info

Publication number: US3916264A
Application number: US484380A
Authority: US
Inventors: Peter G Berg
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1974-07-01
Filing date: 1974-07-01
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28

Abstract

Apparatus for reducing the energization of an electrical load after a preselected time delay. A first self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level is interconnected to an electrical power source thereby rapidly to heat it above the given level. A second self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level is interconnected in series with an electrical load across an electrical power source. Both self-heating resistors are secured to heat conductive means so that after these resistors are initially energized they both self-heat with heat being transferred from the first to the second resistor thereby to accelerate the heating of the latter. The conductive means includes an adjustable heat-transfer path between the resistors whereby the delay period between initial energization of the resistors and the time the second resistor''s temperature rises above its given level to effect reduced energization of the load may be varied. Also disclosed is apparatus in which a third self-heating positive temperature coefficient resistor is positioned in heat-exchange relationship with the second self-heating resistor and which includes means for independently connecting the first and/or third resistors to a power so that any of three selected time delay periods may be obtained.

Description

United States Patent [1 1 Berg [ Oct. 28, 1975 TIME DELAY APPARATUS [75] Inventor: Peter G. Berg, Norton, Mass.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: July 1, 1974 [21] Appl. No.: 484,380

[52] U.S. Cl. 317/41; 3l7/132; 323/69; 338/22 R [51] Int. Cl. H0211 5/04; HOll-l 47/26; 1-1011-1 47/18 [58] Field of Search 317/41, 132, 133, 133.5,

317/l48.5 B, 148.5 R, 154; 307/117; 219/504, 505; 323/68, 69; 318/221 E;

[56] References Cited UNITED STATES PATENTS 3,307,167 2/1967 Race 317/41 X Primary Examiner-L. T. Hix

Assistant Examiner-Patrick R. Salce Attorney, Agent, or Firm-James P. McAndrews; John A. Haug; Russell E. Baumann [57] ABSTRACT Apparatus for reducing the energization of an electrical load after a preselected time delay. A first selfheating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level is interconnected to an electrical power source thereby rapidly to heat it above the given level. A second selfheating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level is interconnected in series with an electrical load across an electrical power source. Both self-heating resistors are secured to heat conductive means so that after these resistors are initially energized they both selfheat with heat being transferred from the first to the second resistor thereby to accelerate the heating of the latter. The conductive means includes an adjustable heat-transfer path between the resistors whereby the delay period between initial energization of the resistors and the time the second resistors temperature rises above its given level to effect reduced energization of the load may be varied. Also disclosed is apparatus in which a third self-heating positive temperature coefficient resistor is positioned in heat-exchange relationship with the second self-heating resistor and which includes means for independently connecting the first and/or third resistors to a power so that any of three selected time delay periods may be obtained.

10 Claims, 4 Drawing Figures I P/ F3 p3 US. Patent Oct. 28, 1975 3,916,264

FICJ.|

TIME DELAY APPARATUS BACKGROUND OF THE INVENTION This invention relates to time delay apparatus and more particularly to such apparatus which reduces the level of energization and therefore the power consumption of an electrical load after a preselected time delay.

Time delay apparatus for reducing the energization level of various electrical loads are widely used in many applications. Positive temperature coefficient (PTC) resistors having a resistivity-temperature curve which is steep-sloped above a threshold or anomaly temperature have been employed for this purpose by serially connecting the PTC resistor with a load, such as a solenoid-operated device, for example, to reduce its current to a value below its holding level after a delay dependent on the time period required for the PTC to selfheat to an elevated temperature at which its resistance is sufficiently high to decrease the load current to the desired reduced level. Such PTC apparatus provides only very course timing and is highly sensitive to and affected by variations in the voltage level of the electrical power supply, and in the resistance of the PTC resistors at ambient, and was limited as to the range of the respective resistances of the PTC resistor and the particular load that could be utilized. As the PTC resistor draws power away from the load as it selfheats and a very light load may draw little power, the trip time versus power characteristics in the longer (e.g., sec.) trip time ranges cannot be made reasonably constant. Also some electrical loads have an impedance which varies substantially between an initial value and a subsequent steady-state value after energization and this raises problems in obtaining predictable and reproducible time delays.

Dual PTC resistors have also been used in a relay system as shown in US. Pat. No. 3,307,167 where a further use of'a pair of PTC resistors for providing a time delay effect on a controlled circuit is also proposed. However, difficulties are presented in economically mass-producing dual PTC resistor devices which have a preselected time delay within reasonably narrow tolerance limits.

SUMMARY OF THE INVENTION Among the several objects of this invention may be noted the provision of apparatus for reducing the level of energization and the power consumption of electrical loads after a preselected time delay in which the undesirable effects of variations in operating voltage and in the typical variations in the resistance of PTC resistors at ambient are minimized; the provision of such apparatus in which the time delay period is relatively insensitive to variations in load resistance and operating voltage and may be used in conjunction with a wide range of different loads; the provision of such apparatus which may be economically fabricated with a wide range of trip times and within good tolerance limits; the provision of time delay apparatus which permits the user to select any of several different trip times in effecting a delayed reduction in the energization of electrical loads; and the provision of such apparatus which is of simple construction, low in cost and reliable in operation. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, the time delay apparatus of this invention comprises a first self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level and means for interconnecting this resistor to an electrical power source thereby rapidly to heat it above the given level. Also provided is a second selfheating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level which is interconnected in series with an electrical load across an electrical power source. These self-heating resistors are secured to heat conductive means whereby after they are initially energized they both self-heat with heat being transferred from the first to the second resistor thereby to accelerate the heating of the latter. The conductive means includes an adjustable heat-transfer path between these resistors whereby the delay period between initial energization of the resistors and the time the second resistors temperature rises above its given level to effect reduced energization of the load may be varied. The invention may also include providing a third self-heating positive temperature coefficient resistor positioned in heat-exchange relationship with the second self-heating resistor and means for independently connecting the first and/or third resistors to a power source so that any of three selected time delay periods may be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of time delay apparatus of this invention;

FIG. 2 is an elevation of a dual PTC resistor device utilized in this apparatus;

FIG. 3 is an elevation of another embodiment of a dual PTC device employed as a component of apparatus of this invention; and

FIG. '4 is a circuit diagram of another embodiment of this invention.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, apparatus of this invention is shown in FIG. 1 to include a positive temperature coefficient (PTC) self-heating resistor Pl connected across an electrical power source V by means of a switch S and conductive leads. A second selfheating PTC resistor P2 is serially connected with an electrical load L across the electrical power source by switch S.

Self-heating resistors P1 and P2 are each conventional PTC resistors, usually in the form ofa pill, having a resistivity-temperature curve which is steep-sloped above a threshold or anomaly temperature. Such PTC resistors are formed from certain doped barium titanates, or carbon-black loaded cross-linked polyethylenes, etc. They have a relatively low resistance at usual ambient temperatures but after initial energization by a source of electrical power will self-heat and increase their temperature and resistance. Heat will be generated and the resistance will increase rapidly above the anomaly temperature until the heat generated balances the heat dissipated at which time the temperature and resistance stabilize with the resistance many times the initial value. Thus each PTC has a transition from a high heat generating state initially to a low heat generating state at an elevated temperature (e.g.,

80C.l35C.) at which it tends to self-regulate. These two PTC resistors are positioned in heat-exchange relationship with each other, such as by securing them to a common heat-conductive body.

Load L may be one with a purely resistive impedance or it may be one with reactive characteristics, such as a solenoid-actuated device, or a relay, or a motor, etc. Also, load L may have an impedance which remains substantially constant after energization or it may be one,- such as a relay or solenoid-actuated device, which has an impedance when electrically energized which is substantially lower than its impedance after an initial period of energization.

As illustrated in FIG. 2 the two-PTC resistor pills P1 andP2 are secured to a generally U-shaped piece of metal M by bonding them thereto. with conventional electrically conductive synthetic resin composition materials or by soldering. Soldered or otherwise electrically connected to the base portion B of this heat conductive yoke means M is a conductive lead 1 which serves as a common electrical terminal for P1 and P2.

Leads

2 and 3 are soldered to the usual metallized outer surfaces of self-heating resistors P1 and P2 so as to serve as electrical terminals therefor. A notch N is cut in base-portion B of M so as to modify the heat transfer characteristics of the heat conductive path between PI and P2.

PTC resistors P1 and P2 may have the same or different stabilization temperatures. Preferably Pl has a higher stabilization temperature, e.g., 120C. or 135C. and P2 has a somewhat lower one, e.g., 80C. Such PTC resistor components will have a lower resistance value, e.g., 1-2O at ambient, and a resistance at tem peratures above its anomaly temperature of many times that value.

When the circuit of FIG. 1 is energized by closing switch S, the substantially constant potential of power source V is applied directly across P1 and across P2 and load L in series.

As P1 is connected directly across source V it will draw a much higher initial current than P2 which has its current limited by the resistance of the load. P1 will very rapidly self-heat until in a brief period of time it approaches its stabilization temperature. Heat is transferred from P1 to P2 thereby accelerating the heating of the latter to a temperature at which its resistance increases steeply and reduces the energization level and power-consumption of load L. P1 and P2 will then stabilize at some temperature depending on the ambient and the heat transfer and dissipation characteristics of the assembly shown in FIG. 2. P2 may be thus maintained at an elevated temperature, e.g., in the order of 85C.ll0C. by this arrangement.

Resistor Pl performs an important function in the advantageous operation of the apparatus of this invention. Without its heat generation and transfer to P2, the time period for P2 to effect the desired reduction in load energization would be much longer and quite sensitive to variations or fluctuations in the level of V, and in the resistance of the particular PTC pill as well as the specific initial resistance of the load and any rising impedance characteristics it may have. By relying solely on the self-heating of P2, the time delay period would vary greatly as its input power and dissipation would fluctuate because of the above-mentioned factors.

The apparatus of this invention utilizing Pl therefore has markedly improved and closer limits of time delay tolerance. However, thesetolerance limits may be made even closer and more predictable and reproducible by the adjustability of the heat transfer path between P1 and P2. Typically, the variation in PTC resistor pill production samples is such that the initial or ambient low resistance value may vary from say 10 to 20. or as much as depending on numerous variables in the processing. This variation of only an ohm or so, while quite small on the absolute basis, may cause an appreciable difference in the time delay characteristics of mass-produced time delay apparatus. Another variable which tends to widen the production tolerances of such time delay apparatus is the heat transfer characteristics of one unit as compared to the next. If two pills happen to be secured to bracket M to effect slightly more or less heat transfer thereto, this too will undesirably widen the time delay period tolerances. By providing an adjustable heat transfer path, such as by notching M at N to a greater or lesser extent, successive units of this time delay apparatus may be calibrated by thus varying this heat transfer path so as to insure close tolerance units while employing inexpensive mass production techniques.

FIG. 3 illustrates another embodiment of the present apparatus wherein pill portions P1 and P2 are utilized rather than complete pills P1 and P2. P1 and P2 may be halves of the same pill or halves of pills with different stabilization temperatures, thus further reducing the costs of this assembly. In this embodiment heat conductive means M is merely a circular metal blank with half pills P1 and P2 conductively secured thereto in spaced apart positions leaving a gap between the opposed broken edge surfaces. The heat transfer path between PI and P2 is adjusted or varied by cutting a notch N of varying depth, if desired.

By selecting PTC resistors of the desired temperature-resistance parameters relative to the characteristics of the load L a wide variation in time delay may be obtained as well as the desired level of power to be supplied to the load after the delay period. For example, if the load is a solenoid-actuated device such as a valve that is to be deactuated-after a predetermined time, the resistance of P2 or P2 is such at the temperature to which it is raised by P1 or P1 that the current supplied to the load after the delay period is substantially less than the hold-in value of the solenoid.

The time-delay period or trip-time of the above described embodiments are somewhat sensitive to ambient temperatures and this is useful in many applications such as in automotive automatic choke controls and the like. This sensitivity to ambient may be reduced and the ability of the apparatus to operate on even greater range of loads may be increased by shunt-connecting an optional fixed resistor R of substantially constant resistance across P2 as indicated in broken lines in FIG.

1. This resistance provides a shunt path around P2 so that it does not have to carry the entire load current. The resistance of P2 (at ambient) is not substantially greater, and is preferably less than, the resistance of R. But at the increased temperatures to which P2 is raised P2 has a resistance much greater than that of R. This modified circuitry makes it very convenient to control the reduced level of energization and the power consumption of the load over wide ranges. For example, if the load is a solenoid-actuated device, the energization thereof after the. time delay may be reduced to a level which is only somewhat above the hold-in current. The

increased load Current at elevated temperaturesof' P2 is carried in a substantial portion by the resistor R.

In certain time delay apparatus applications it is desirable to be able to select any of several different delay periods. The apparatus of FIG. 4 provides this multiple function. This embodiment utilizes a third self-heating PTC resistor P3, positioned in heat-exchange relationship with P2, and switches S1 and S3 which constitute means for independently connecting Pl and/or P3 across power source V concurrently with the closing of S. P1 and P3 may be PTC pills with the same or different temperature-resistivity characteristics. The conductivities of heat transfer paths between P1 and P2 and between P3 and P2 may be the same or different. If S and S] are closed simultaneously, one time delay period will be obtained for reduction of the energization and power consumption of load L, while a different trip time will result if switches S and S3 are concurrently closed. A third and even shorter time delay will be obtained if all three switches, S, S1 and S3 are closed at the same time so that P2 has heat transferred to it from both P1 and P3.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for reducing the energization of an electrical load after a preselected time delay comprising:

a first self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level; means for interconnecting said resistor to an electrical power source thereby rapidly to heat it above said given level; second self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level; means for interconnecting said second self-heating resistor in series with an electri cal load across an electrical power source; and

heat conductive metal plate means having said selfheating resistors secured in spaced relation to each other adjacent respective opposite ends of said heat conductive means whereby after said resistors are initially energized they both self-heat with heat being transferred from the first to the second resistor thereby to accelerate the heating of the latter, said conductive means having notches of selected proportions intermediate said resistors providing a selected heat-transfer path between said resistors whereby the delay period between initial energization of the resistors and the time the second resistors temperature rises above its given level to effect reduced energization of the load is selectively predetermined.

2. Apparatus as set forth in claim 1 in which the electrical load is the actuating coil of an electromagnetic device having a movable armature. I

3. Apparatus as set forth in claim 2 wherein the electrical load is the coil ofa solenoid-actuated valve or the like.

- v5. Apparatus as set forth in,claim 2. wherein the maximum rated'steady-state current of. said device is substantially less than that which is carried by said coil during initial periods of energization and corresponds to said reduced energization effected by said heating of said second resistor and wherein said notch in said conductive means is proportioned to assure heating of said second resistor to its given level within a selected time to avoid overheating of said device.

6. Apparatus as set forth in claim 2 which further includes a resistor of substantially fixed resistance shuntconnected with said second resistor, the resistance of said second self-heating resistor being not substantially more than that of the fixed resistor at temperatures below its given level but substantially greater than that of the fixed resistor at temperatures above its given level whereby after initial energization of said load the power supplied to said load decreased as a function of time to a predetermined lower level as the temperature and resistance of said second self-heating resistor increase.

7. Apparatus as set forth in claim 6 wherein the combined resistance of shunt-connected fixed and second self-heating resistor at the increased temperature to which the latter resistor is heated is such that the current supplied to the load is substantially less than that initially supplied to the load and is ofa value not greatly in excess of the minimum current required to hold in the armature.

8. Apparatus for reducing the energization of an electrical load after a preselected time delay comprising:

a first self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level;

a second self-heating positive temperature coefficient resistor positioned in heat-exchange relationship with said first resistor and having a relatively low initial resistance which increases abruptly as its temperature rises above a given level;

means for interconnecting said second self-heating resistor in series with an electrical load across an electrical power source;

a third self-heating positive temperature coefficient resistor positioned in heat-exchange relationship with said second resistor and having a relatively low initial resistance which increases abruptly as its temperature rises above a given level; and

means for independently interconnecting said first and third self-heating resistors across an electrical power source substantially concurrently with the interconnection of the second resistor and load across its electrical power source whereby any of three time delay periods for reducing the energization of the load may be obtained by so energizing the first or third or the first and third self-heating resistors simultaneously with the energization of the second self-heating resistor and the load.

9. Apparatus as set forth in claim 8 which further includes a resistor of substantially fixed resistance shuntconnected with said second self-heating resistor, the resistance of said second self-heating resistor being not substantially more than that of the fixed resistor at temperatures below its given level but substantially greater than that of the fixed resistor at temperatures above its given level whereby after initial energization of said 10. Apparatus as set forth in claim 9 in which all the load the power supplied vto said load decreases as a function of time to a predetermined lower level as the temperature andresistance of said self-heating resistor increase.

electrical power sources are constituted by a single common power source.

Claims

1. Apparatus for reducing the energization of an electrical load after a preselected time delay comprising: a first self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level; means for interconnecting said resistor to an electrical power source thereby rapidly to heat it above said given level; a second self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level; means for interconnecting said second self-heating resistor in series with an electrical load across an electrical power source; and heat conductive metal plate means having said self-heating resistors secured in spaced relation to each other adjacent respective opposite ends of said heat conductive means whereby after said resistors are initially energized they both selfheat with heat being transferred from the first to the second resistor thereby to accelerate the heating of the latter, said conductive means having notches of selected proportions intermediate said resistors providing a selected heat-transfer path between said resistors whereby the delay period between initial energization of the resistors and the time the second resistor''s temperature rises above its given level to effect reduced energization of the load is selectively predetermined.

2. Apparatus as set forth in claim 1 in which the electrical load is the actuating coil of an electromagnetic device having a movable armature.

3. Apparatus as set forth in claim 2 wherein the electrical load is the coil of a solenoid-actuated valve or the like.

4. Apparatus as set forth in claim 2 wherein the electrical load is the coil of an electrical relay.

5. Apparatus as set forth in claim 2 wherein the maximum rated steady-state current of said devIce is substantially less than that which is carried by said coil during initial periods of energization and corresponds to said reduced energization effected by said heating of said second resistor and wherein said notch in said conductive means is proportioned to assure heating of said second resistor to its given level within a selected time to avoid overheating of said device.

6. Apparatus as set forth in claim 2 which further includes a resistor of substantially fixed resistance shunt-connected with said second resistor, the resistance of said second self-heating resistor being not substantially more than that of the fixed resistor at temperatures below its given level but substantially greater than that of the fixed resistor at temperatures above its given level whereby after initial energization of said load the power supplied to said load decreases as a function of time to a predetermined lower level as the temperature and resistance of said second self-heating resistor increase.

7. Apparatus as set forth in claim 6 wherein the combined resistance of shunt-connected fixed and second self-heating resistor at the increased temperature to which the latter resistor is heated is such that the current supplied to the load is substantially less than that initially supplied to the load and is of a value not greatly in excess of the minimum current required to hold in the armature.

8. Apparatus for reducing the energization of an electrical load after a preselected time delay comprising: a first self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level; a second self-heating positive temperature coefficient resistor positioned in heat-exchange relationship with said first resistor and having a relatively low initial resistance which increases abruptly as its temperature rises above a given level; means for interconnecting said second self-heating resistor in series with an electrical load across an electrical power source; a third self-heating positive temperature coefficient resistor positioned in heat-exchange relationship with said second resistor and having a relatively low initial resistance which increases abruptly as its temperature rises above a given level; and means for independently interconnecting said first and third self-heating resistors across an electrical power source substantially concurrently with the interconnection of the second resistor and load across its electrical power source whereby any of three time delay periods for reducing the energization of the load may be obtained by so energizing the first or third or the first and third self-heating resistors simultaneously with the energization of the second self-heating resistor and the load.

9. Apparatus as set forth in claim 8 which further includes a resistor of substantially fixed resistance shunt-connected with said second self-heating resistor, the resistance of said second self-heating resistor being not substantially more than that of the fixed resistor at temperatures below its given level but substantially greater than that of the fixed resistor at temperatures above its given level whereby after initial energization of said load the power supplied to said load decreases as a function of time to a predetermined lower level as the temperature and resistance of said self-heating resistor increase.

10. Apparatus as set forth in claim 9 in which all the electrical power sources are constituted by a single common power source.