US2383328A - Time delay device - Google Patents

Description

TIME DELAY DEVICE Filed June 27, 1941 Load WITNESSES: INVENTOR W Frede r/c/r W. Ly/e. EJJE-Jbf ,v BY

ATTORNEY Patented Aug. 21, 1945 assaszs 'rmm mzuir nsvrcs Frederick W. Lyle, Irwin, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 27, 1941, Serial No. 400,009

7 Claims.

This invention relates to control circuits and, more particularly, to a time-delay circuit'i'or retarding the operation of electrical devices.

In control systems of the type to which this invention pertains, the discharge time of a capacitor through a resistance is utilized to eifect a time-delay in the actuation oi relays. The timeconstant produced by resistance and capacity has been utilized in various control circuits for efl'ecting a prolongation of time in the actuation of certain devices. When a considerable length oi time is desired between certain operations, a resistance-capacity delay network in effecting this time-delay must necessarily be of large ohmic and capacitive values, respectively. In practice there are certain limitations imposed upon such combinations, and it is diflicult and costly to construct large capacitors and large values of resistances.

The current of a discharging condenser which is shunted by a resistor is given by the equation and theoretically the time required to discharge depends entirely upon the electrical constants represented by the product of RC. In every condenser a certain leakage is unavoidable which physical size aswell as to the cost of the equipment.

is also likely to vary from day to day. The

shunting resistor used with the condenser must be small in comparison to this leakage resistance, otherwise, the time scale will become erratic and the. tm constant will not be uniform. Since the leakage resistance is substantially inversely proportional to the size of the capacitance. it is evident that the maximum shunting resistor w ch should be only a definite fraction of the leakage resistance is also inversely proportional to the capacitance. Consequently, there is a practical limit to the value of RC in any physical embodiment which flxes the maximum obtainable time constant.

It is often desirable to obtain a larger time- .constant than the maximum which the physical s es 0' t e components nermit. In such cases the practice heretofore was to employ several time-delay. networks, each including resistance and capacity and connect these successively in b circuit when the discharge time of one was very nea l reached. Generally, the use or resis r co-canacitv combinations involves also the use of vacuum tubes. Consequently, a time-delay device orerainc beyond the discharge time of one resistan e-capacity unit necessitated several tubes and component parts which adds to the.

The salient feature of this invention is that the time-constant oi a resistance and capacity network may be extended in efiect for any length of time and a control of the apparatus effected when the desired time has elapsed.

Another feature of this invention is that a single delay network of a definite time-constant within-practical limits may be utilized and automatically applied at arw desired number of times, whereby the eii'ective delay time may be extended to a predetermined multiple of the time-constant value of the delay network.

A particular advantage of the delay apparatus in accordance with this invention is that the delay network components, such as the condenser and the discharge resistor, may be so chosen that the latter represents a very small fraction of the inherent leakage resistance of the condenser. In this manner a stable and uniform discharge cycle can be obtained which, although inherently of comparativelyshort duration, is cumulative in effect so that the ultimate result can be extended to any desired length'of time.

Other features and advantages will be apparent from the following description of the invention, pointed out in particularity by the appended claims and taken in connection with the accompanying drawing, in which the single figure illustrates diagrammatically a circuit of the electronic time-delay device in accordance with this invention.

The invention herein described comprises a circuit arrangement which includes a vacuum tube for the actuation oi a relay which controls the switching-oil or the switching-on of a load supplied from a power line. The delay in the operation. or the relay is eiiected by means of a resistance capacity network which when the condenser is charged, biases the grid oi the vacuum tube to such anode current value which is insufvice in the form of a dash-pot B. The main switch 6 in series with the line I controls the supply of power to the load as well as to the time-delay device about to be described. The latter consists of a vacuum tube 1 having anode 8, control electrode 8 and cathode Iii. A voltage dividing network comprising resistors II and I2 connected in series across the line i and 2 provides operating voltages for the vacuum tube I, the cathode ID of which is connected to the Junction point of resistors H and I2. The anode circuit between anode I and cathode it includes the energizing winding I3 of the relay 4 and the energizing winding ll of an electromagnetic device 15. When the power line is an alternating current supply, as indicated here, the winding I! as well as the winding II is shunted by condensers 29 and 28', respectively. The windings l3 and I4 are connected in series by means of contacts I! and H which may be opened when the reset switch 8!, to be described later, is depressed. The control circuit of the tube 1 includes between the grid 9 and cathods ID the parallel combination of resistor ll and condenser i9. A charging circuit for the condenser from the voltage appearing across the resistor II includes a rectifier i8 in series with a current conductive path formed by conductor 2., brush II, collector ring 22, contact segment 13, and sliding contact 24 of the switch II and a connection by means of conductor 26, contacts 11 and Il operated by the electromagnetic device II and conductor 30 to the grid 0.

The switch 25 is so constructed that the segment 23 mounted on an insulating disk II is driven by a shaft 32, The sliding contact 24 is afllxed to a holder 33 which may be moved in the frame 35 along the segment is from the starting position in which it is shown to any desired position and secured there by the locking screw 34. The shaft 32 is displaced by means of a ratchet and pawl arrangement against the biasing force of a spring ID. The ratchet and pawl assembly comprises the ratchet wheel ll amxed to the shaft 12, the ratchet l2 depressed by the spring "and carried by the lever 44, the latter being connected to the arm 45 of the electromagnetic device ii. A pawl is pivoted around the pin 48 engages the ratchet wheel II. It may be lifted by means of the rod connected to the plunger 5!. The latter is actuated by means of the winding 52 energized from the supply line when the switch 55 is closed. The-lifting of the pawl 48 permits the reverse rotation of the ratchet wheel ll provided the lever I4 is in the position shown. The ratchet I2 is then out of engagement being lifted by the pin 45.

The circuit above described illustrates the example of an operative arrangement for an alternating current power line supply. When a direct current line feeds the load, the rectifier I. may be omitted. It is understood, of course, that the polarity of the line must be taken into consideration and the positive side connected to the anode circuit. The heater for the cathode III is not shown inasmuch as the heater connection depends largely on the type of tube used. In certain tubes such as the RCA Type 117N6GT the heater may be connected directly across the 110-volt alternating current supply.

Considering the operation of the circuit, it is assumed that the system is at rest, as shown in the drawing, and a maximum time delay is desired. Consequently the sliding contact 24 is placed near the end of the holder 33 making contact with'the starting end of the se ment ll. Conductors 20 and I! are in this manner interconnected completing the current path from the source, that is, the resistor II, to the grid 0 provided that the contacts 21 and 28 are also closed. The current ilow in the charging circuit just mentioned is always in one direction because of the rectifier ll. The latter is so polarized that only the negative half cycles of the alternating current voltage will be impressed on the condenser II.

In closing the main switch I the vacuum tube becomes energized, and since the condenser I. is not charged, the grid I is effectively at cathode potential. This permits anode current to now which will energize the windings II and H. The energization of the winding II will cause actuation of the arm 4! prior to the actuation of the relay 4. The reason for this is that the relay 4 is of the slow acting type and must overcome the retardation of the dashpot I. When the arm ll is pulled down, the segment 23 is moved in the direction of the arrow one fraction of its length which is equivalent to the movement eifected by one step of the ratchet wheel 4|. At the same time the contacts II and 21 are closed, and the circuit is completed for the charging of the condenser ll which is now effectively connected across the resistor II. The condenser will charge practically instantaneously to the peak voltage of the negative halt cycles of the alternating current flowing in the resistor 12. The latter is so in less time than required for the relay 4 to overcome entirely the retarding force of the dashpot 5 so that the circuit of the load is not completed as yet. The anode current again is of such low value that the force acting on the relay 4 is released so that it returns to its original starting position.

As stated before, the condenser II is free to discharge. The time of discharge will take place in accordance with the time-constant produced by the parallel combination of the condenser II and the resistor ll, represented by the product RC. When this time elapses the slowly rising plate current again reaches a value which actuates the arm 4!. The segment is moved another step in the direction oi the arrow, eflecting again contact with the insulating support II, the circuit between conductors II and 2| is broken. The next discharge of the condenser ll results again in the operation of the arm '45, closing the contacts II and 21. However, no recharge oi the condenser II will be effected due to the open circuit position oi the switch 25. The sustained plate current ultimately operates the relay l closing the contacts I and I and thereby completing the circuit to the load from the supply lines I and 2. The load will remain energized until the mechanism is reset. This is effected by the winding I4 is deenergized, the arm 45 returns to the position which lifts the ratchet 42. The ratchet wheel ll is now free to return propelled by the force exerted by the spring 40 to the starting position determined by the stop pin iii. The switch 25 will now complete the contact between conductors 20 and 28 so that when the switch 55 is released, the movement of the arm 45, in closing contacts 21 and 28, will again recharge the condenser IS. The entire operating cycle will then be repeated. The delay-time may be ad- Justed at will by the replacement of the sliding contact 24 to the desired number of movements of the ratchet wheel I for the opening of the contact between conductors 20 and 26.

It is apparent that the effective time delay, will be a certain multiple or the time-constant of the network comprising resistor I8 and con denser l8, and may be expressed by the formula RC+RC in which n is the number of rechargings of the condenser l8 and RC the product of the capacitance value of the condenser and the ohmic value of the resistance.

What is claimed is:

1. In a time delay circuit for retarding the actuation of a relay, a vacuum tube having anode, cathode and control electrodes, a circuit between said anode and cathode including in series a source of anode potential, the energizing winding of said relay and the energizing winding of an electromagnetic device; said device comprising an actuating arm, a pair of contacts and a lever operable by said am, a second pair of contacts in series with said first pair comprising a conductive segment and a slidable member adapted to be positioned at various distances over said segment, means for moving said segment from a certain starting position upon each actuation of said lever for a fractional portion of its peripheral length whereby said last-mentioned contacts may be held in physical engagement for repeated actuation of said lever depending upon the displacement of said sliding contact with respect to the length of said segment and said starting position, a resistance and a capacity connected in parallel between control electrode and cathode of said tube, a voltage source for applying a potential to said control electrode which is negative with respect to said cathode, thereby charging s'aid condenser, a charging circuit including said two pairs of contacts in series whereby, upon actuation of a said electromagnetic device due to anode current, said first pair of contacts complete said charging circuit to said condenser, said tube being so biased thereby as to release said device until repeated actuation thereof following each discharge-time of said condenser displaces said second pair of contacts to an open position, said relay being operatively energized upon anode current fiow maintained in said tube when said sliding contacts are disengaged.

2. A time delay circuit in accordance with claim 1 characterized in that means are provided for 3. A time delay circuit in accordance with claim 1, wherein said means for moving said segment comprises a ratchet afilxed to said lever, a shaft carried at one end, said segment mounted on an insulating support, and at the other end a ratchet wheel cooperating with said ratchet, a stop mechanism therefor comprising a pawl, spring means biasing said shaft upon rotation by said lever for resetting said shaft when said pawl retarding the acting time of said relay in comis disengaged, mean for disengaging said pawl, and stop means engaging said support upon resetting to said starting position,

4. A time delay circuit in accordance with claim 1 wherein said source of anode potential is derived from a portion of a voltage dividing resistor connected across an alternating current supply line, and said charging potential source includes another portion of said resistor and a rectifier, said cathode being connected to an intermediate point of saidvoltage divider and the energizing wind ing of said relay and the winding of said electromagnetic device being each shunted by a capacity.

5. In an electrical time delay circuit, a current source, a load device energized therefrom, switching means for controlling the energization of said device from said source, means for actuating said switching means comprising a capacitor, a first and a second circuit-interrupter, a source of voltage connected to charge said capacitor only when said circuit interrupters are closed, means for discharging said capacitor at a predetermined rate, means maintaining said first circuit interrupter normally open but actuating it to closed position when the charge on said capacitor falls below a predetermined value, means for maintaining said second circuit interrupter normally closed but for moving it to open position after a predetermined number of actuations of said first circuit interrupter to open position, and means for actuating said switching means only after the opening of said second circuit interrupter.

6. In an electrical circuit, a capacitor, a first and'a second circuit interrupter, a source 01' voltage connected to charge said capacitor only when said circuit interrupters are closed, means for discharging said capacitor at a predetermined rate, means maintaining said first circuit interrupter normally open but actuating it to closed.

position when the charge on said capacitor falls below a predetermined value, and means for maintaining said second circuit interrupter normaily closed but for moving it to open position after a predetermined number of actuations of said first circuit interrupter to open position.

'7. In an electrical circuit, a capacitor, a first and a second circuit interrupter, a source 01' voltage connected to charge said capacitor only when said circuit interrupters are closed, means for discharging said capacitor at a predetermined rate. an electrical discharge device having a pair of principal electrodes and a control electrode, means for maintaining said first circuit interrupter closed when current flows between said principal electrodes and open at other times. means for impressing the voltage of said capacitor on said control electrode, and means for maintaining said second circuit interrupter normally closed,.but for moving it to open position after a predetermined number of actuations oi said first circuit breaker to open position.

. FREDERICK W. LYLI.

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