US2251763A - Electrical timer - Google Patents

Description

Aug. 5, 1941. R. L.* SCHOENE I ELECTRICAL TIMER Filed Jan. 50, x940 WMMCQMI m 3: :YE A

NVQN'T'OQ/ /Qudde// L. aSc/zoerza I cl 7 UITTOQZEYS Patented Aug. 5, 1941 UNITED STATES PATENT OFFICE ELECTRICAL TIMER Russell L. Schoene, Evansville, Ind.

Application January 30, 1940, Serial No. 316,454

2 Claims.

The invention relates to electric timers and more particularly to such timers of the type adapted to maintain a controlled circuit in a predetermined condition (1. e. closed or open) for a. selected variable interval of time.

Timers embodying the present invention have found particular utility when used to turn oif an exposure lamp after a selected interval of time in a photographic enlarger, contact printer, or similar photographic apparatus. These timers may, however, be utilized for a wide variety of other purposes, the use noted being simply exemplary. In the particular use suggested it is desirable that the apparatus be such that the operator can turn on an exposure lamp at will and that it be cut off automatically by the timer after a selected interval of time required by the particular photographic film or sensitized paper which is being exposed.

The general object of the invention is to provide an electric timer which is compact and thoroughly portable, can be operated from an ordinary lighting circuit as a source of power, has no moving parts except the switches and, hence, is not subject to wear and does not require any readjustment or rewinding as in the case of a clockwork timer, retains a high degree of accuracy permanently, and which is rugged in con struction though low in cost.

A more specific object of the invention is to provide a timer embodying a load device such as a main relay or electromagnetic switch which is to be deenergized after a desired time interval, together with a vacuum tube for supplying current to the relay, the vacuum tube having a control grid such that the conductivity of the tube may be cut off and the main relay thus deenergized by varying a small biasing potential on the tube control grid.

Another object of the invention is to provide a novel timer embodying a load device such as a main relay or electromagnetic switch which is to that one tube, adapted to carry heavy currents, serves to control directly the current supply of anassociated load device such as a main relay, while another of the tubes which has less .current capacity but greater sensitivity is in turnadapted to control the first tube under the guidance of a timing element which governs the more sensitive tube.

-A further object is'to provide an electric timer of such character that it will retain a high degree of accuracy despite changes in voltage of the electric supply circuit.

A further object is to provide a timer which affords means for varying in, say, units of time the interval to be measured, plus further means 'for effecting a selected and variable percentage change in the interval selected by the first means.

The invention also resides in various circuit improvements by means of which a wide variation'in time intervals to be measured can be accomplished with precision, flexibility of control is provided, and inadvertent recycling of the timer prevented.

be deenergized after a selected time interval, a.

bodying a plurality of vacuum tubes so arranged 'quirements of a particular'use dictate. 'quently, even though a particular embodiment of For purposes of exemplification, the'invention has been shown herein as embodied in a timer which is illustrated schematically in the drawing, and which is especially well adapted for use in making photographic exposures. It will be apparent to those skilled in the art, however, that timers embodying the present invention are susceptible of many other uses and that various changes in construction may be made as the re- C onse the invention has been shown and described in some detail, there is no intention to thereby limit the invention to such embodiment, but, on the other hand, the appended claims are intended to cover all modifications and alternative constructions falling within the spirit and scope of the invention.

In brief, the timer illustrated includes an electrically energizable load device in the form of a main relay or' electromagnetic switch in whose time of closure or actuation is governed by the associated parts of the device. Thus, this relay is utilized to complete a circuit through an output lead H for a selected interval or time. By

this means a photographic exposure lamp or other device connected to the output leads II and I2 by a plug receptacle I3 is energized for this selected interval. To control the timer five manually operable devices are provided in the illustrative device, viz; a main on-off switch H, a rotatable time interval selector |5 with a calibrated scale l6 indicating the time intervals corresponding to the settings of the selector knob IS, a rotatable control 59 with calibrated scale 60 indicating a desired percentage change (increase or decrease) in the time interval selected by manipulation of the selector l5, a push button ll for initiating the timing cycle, and a manual switch l8 for controlling the energization of the associated control lamp or other device manually rather than automatically. Current is supplied to the timer unit and finally to the output leads I and II from input leads L1 and L2 which may be connected to a suitable source of supply such as a 110 volt alternating current electric lighting system.

The timing element in the unit embodies a condenser l9 and a shunt connected variable leakage resistor 20. When a charge is applied to the timing condenser I9, the time required for it to leak oil through the resistor 20 to such a point that the potential across the condenser I9 and resistor 20 reaches a predetermined final value, is a function of the value of the resistor 2|]. It should also be borne in mind that, for any particular values of resistance and capacity, the time interval is a function of the ratio of initial to final potential on the condenser. This time interval can be calculated by the formula where t is the time in seconds and R and C are, respectively, the values of the resistance and capacity; E1 and E2 being the initial and final potentials. Hence, the time selector knob I is used to vary the setting of the resistor 20 so as to adjust the time interval. On the other hand the control 59 is utilized, in general, to vary the ratio of E1 to E2 thereby also changing the time interval. In the present instance the scale 60, associated with the control 59, is calibrated in terms of percentage change so that it shows the percentage change in the time interval (determined by the setting of selector |5) which is effected by the setting of control 59.

As examples of suitable values for the condenser 9 and resistor 20, the resistor may be variable between resistance values of zero to 10 megohms, and the condenser I9 may be of 2 microfarads capacity. With such an arrangement the selected time interval can be varied through a wide range of the order of magnitude of from 0.2 second to 100 seconds. The practical minimum time interval is only dependent on the speed of operation of the relay H]. The maximum time interval depends, among other factors, on the insulation resistance of the condenser I9 and intervals up to 100 seconds are easily secured in commercial installations.

'Io utilize the timed variation in potential across the condenser l9, as its charge leaks off through the resistor 20, to control the main relay |0, one or more vacuum tubes are preferably utilized as intermediate elements in the system. In general, advantage is taken of the fact that a vacuum tube embodying an anode, cathode and grid is conductive (between its cathode and anode) so long as a biasing potential applied to its control grid does not exceed, or in other words i=RClog,

become more negative, than a predetermined value. As soon. however, as this grid bias becomes more negative than the predetermined value noted the tube reaches its so-called cut-01f point and immediately becomes nonconductive. In the present instance a type 6V6G tetrode tube 2|, embodying a cathode 22, an anode 23, a grid 24, and a screen 25 is connected in the supply circuit of the main relays actuating winding 26. Consequently, so long as a zero (or small value of) grid bias is applied to the control grid 24, current will be supplied to the relay actuating winding 26 through the tube 2|, thus retaining the relay energized. As soon, however, as the grid bias reaches its cut-01f point the tube 2| will become nonconductive and the relay l0 thereby deenergized so that it opens its contacts 21 and 28. Incidentally, the relay |0 usually opens at a somewhat lower current value than that at which it closes. For example, it may open at 6 milliamperes and close at 12 milliamperes.

The tube 2| which supplies the relay winding 26 must carry a fairly heavy current for this purpose and should, therefore, be a low Rp tube. In other words, it should have a low plate resistance, or a large plate current for a given anode to cathode potential. This ordinarily limits the value of a which may be used and results in somewhat of a sacrifice in its voltage sensitivity and it is, therefore, desirable to interpose a second tube 29 between the timing condenser IQ and the tube 2|. This second tube 29 may be a high a tube, that is, a tube having a high amplification factor or greater voltage sensitivity but lower current carrying capacity. This latter tube has been shown as a type 6F5GT triode, including a cathode 30, an anode 3|, and a central grid 32. Both of the tubes 2| and 29 are of the indirectly heated cathode type and embody the usual cathode heaters (not shown) supplied from a suitable transformer 33.

In some instances it is desirable to change the time interval measured by the unit by a predetermined percentage of the interval previously determined by the setting of the selector knob I5. For example, in color photography three exposures (for the three color separation negatives) are usually made but the exposure interval for each is ordinarily different, the difference depending on the density of the respective negatives, etc. In such case the difference is most easily determined in terms of a percentage change from the exposure required for one of the series of negatives which is chosen as a base for calculation. Hence, if the base negative requires an exposure of, say, 20 seconds the selector knob |5 of the present instrument would be set for that value. But to accommodate the instrument for quick resetting for the other negatives it is desirable that some supplemental control be provided for effecting a selected percentage change in the time interval determined by the selector |5 rather than compelling the operator to convert mathematically the percentage to seconds and then reset the knob l5. It is for this purpose that the supplemental control 59 is provided.

In the present instance the control knob 59 varies the setting of a potentiometer 56 connected across the output of transformer 33, the

latter being used simply as a convenient source" of potential. In general, the potential derived potential acts conjolntly with that of the condenser |9 in determining the point at which the tube 29 is rendered conductive. In other words, referring back to the previous formula, the potential derived from the potentiometer is used i to vary E2 and, hence, the ratio of E1 to E2 so as to accomplish a change in the time interval 1:.

The potentiometer 56 is desirably supplemented by voltage divider resistors 58, 6| so that the scale 5|] (which reads in percent, plus or minus from a central zero setting) may be generally symmetrical throughout its length. The plus signs on the transformer 33 indicate the terminals which are positive during the half cycles of current that the tube 29 can be conductive. In such case the potentiometer 5B is connectedacross the transformer leads 35, 51 and one terminal of the resistor BI is connected to the positive lead 35. One terminal of the other resistor 58 is connected to the slider of the potentiometer (shifted by the'knob 59) while the common terminal of the divider resistors 58, 6| is connected through lead 55 with the cathode of the tube 29.

To condition the timer unit for operation the on-off switch I4 is closed to connect the unit to a suitable source of supply. The time selector knob I5 is rotated until the selected time interval is indicated on the scale I6, the latter being calibrated in time units such as seconds. For the present it will be assumed that the control 59 is in its zero position. Of course, the output leads H, |2 are connected to a photographic exposure lamp or other device which is to be controlled. Connection of the unit to a source of supply in this manner energizes the transformer 33 so that the tube cathodes 22 and 30 are heated and the potentiometer 56 is energized. Furthermore, a charge is continuously applied to'the condenser l9, one terminal of the condenser being connected to supply line L1 through the switch l4 and the other terminal to supply line L2 (through a circuit |934-32--39--55-6l-- -36-3125--2238L2). In this way the condenser I9 is charged to approximately the peak potential of the alternating current line voltage. This charged condition is maintained until the timing cycle is initiated.

To initiate the timing cycle, after the preliminary conditioning of the unit described above, the operator momentarily closes the push-button switch H. In brief, closure of this switch completes the plate and screen circuits of the tube 2| and connectsthe condenser |9 between the grid and cathode of the other tube 29. In particular, the upper terminal of the condenser I9 is connected to the cathode 30 (of the tube 29) through the switch |1, conductor 35, resistor 6| and conductor 55; while the lower terminal of the condenser is permanently connected to the grid 32 through the conductor 34. In this way the high a tube 29 is biased far beyond the bias value for plate current cut-off. In particular, the bias potential is'made up of two components, namely, an alternating component and a direct component. The alternating component is derived from the drop across the divider resistor 6| and this voltage is in turn determined by the setting of the potentiometer 56. The direct component is equal to the potential of the condenser l9. Though the alternating component, of course, alternately bucks and boosts the direct component the crest value of the alternating component is so much less than the direct component that the grid 32 remains biased far beyond cut-of! over the entire'alternating cycle. Consequently, no plate current will flow in the tube 29 and the voltage developed across a load resistor 39 and condenser 39 connected to the plate 3| is zero. This resistor and condenser 39, 39" are connected to the grid 24 of the other tube 2| so the bias on this grid is essentially zero and the tube 2| rendered conductive. Plate current from the tube 2| flows through a conductor 49 to the relay winding 26 (its other terminal being connected to the supply line L1 through the push-button switch |1) so that the relay is energized and both of its contacts 21, 28 closed.

The relay contacts 21 connect the output lead H to the supply line L1, and since the other output lead I2 is permanently connected to the supply line L2, closure of the relay Ill thus serves to connect the output leads to the supply lines. The second set. of relay contacts 28 are in shunt with the push-button switch l1 and serve to maintain the circuits throughout the timing cycle even though the switch ll be closed only momentarily and then reopened.

As the timing cycle proceeds, the charge on the condenser I9 gradually leaks off through the shunt-connected leakage resistor 20. After a predetermined time interval, dependent upon the setting of the variable resistor 20 as-previously noted, the charge on the condenser I9 falls to a value such that the tube 29-becomes conductive in view of the decreased bias on its grid 32. In

the present circuit it will be noted incidentally that the particular value of condenser charge at which the tube 29 becomes conductive is governed by the value of the jointly acting alternating component of bias applied from the divider resistor 6 The instrument is calibrated so that-when the potentiometer control 59 is in its mid or zero position the scale l6 (associated with the resistor control knob I5) will read directly in terms of time units. plate current from the tube 29 causes a correspondingly increasing voltage drop across the resistor 39 and condenser 39. This latter voltage increases the bias on the grid 24 of the tube 2|, with a consequent decrease in the plate current from the latter, until this plate current becomes too weak to retain the relay l0 closed. Accordingly, the relay I0 is dropped out at the end of the selected time cycle and opening of its main contacts 21 interrupts the supply of current to the controlled device connected to the output leads l2.

Dropping out of the relay |0 not only opens its main circuit contacts 21 but also its auxiliary contacts 28. Opening of these latter contacts restores the timer unit circuit to its initial condition so that the condenser I9 is recharged as before for use in a subsequent timing cycle. It will be noted that alternative connections of the main timing condenser IQ for charging and discharging are accomplished by, in effect, connecting the cathode 30 of the tube 29 to the line L2 for charging and to the line L1 to discontinue charging. The latter connection also serves at the 2 same time to connect the condenser across the grid and cathode in controlling relation with the tube 29. Incidentally the grid input impedance of the tube 29 is very high so that any recharging of the condenser |9 during the timing cycle, or any leakage from thecondenser through the tube, is negligible.

So far it has been assumed that the control 59 was set in its mid ,or zero position, In the event that the operator wishes to time an interval The gradually increasing which diiiers by a determined percentage from a preceding interval he sets the control knob 59 accordingly. Thus if the knob i was previously set for say, twenty seconds and he wishes to time an interval ten percent greater, he leaves the knob as it is but turns the knob 55 until its scale 5B, which reads in plus and minus percentage values on opposite sides of zero, shows the desired setting. This changes the setting of the potentiometer 56 and hence changes the voltage drop across the divider resistor 61. Consequently the alternating component of the bias on the grid of tube 29 is changed. Therefore, the final potential on the condenser l9, at which the tube 29 becomes conductive, is changed. The circuit elements are so proportioned that the changes in timing which result can be read directly as percentage values on the scale 60.

The timing unit herein disclosed is characterized by a high degree of constancy of accuracy. One factor contributing to this result is the fact that the output or plate current of the tube 2| remains at full value for approximately 95% of the time cycle interval and then rapidly and sharply decreases through the value at which the relay opens. Consequently, changes in spring tension in the relay or other mechanical variations in it which change the value of current at which it opens will not appreciably affect the length of the measured time interval. Changes in the characteristics of the tube 2| will also not affect the accuracy of the instrument.

Also of importance in maintaining constant accuracy of the timer is its insensitivity to changes in input voltage. Since it is contem plated that the unit will be energized from an ordinary commercial lighting system it is likely that it will encounter somewhat different supply voltages on difierent systems and also variations in voltage in even the same system. Within reasonable limits, however, such changes in input voltage will not affect the accuracy of the timer. This is for the reason that the higher or lower plate voltages which result are accompanied by correspondingly higher or lower grid voltages. For example, if the line voltage on L1L2 goes down the peak voltage applied to the timing condenser !9 will be lessened and so will the biasing potential which the condenser applies to the grid of tube 29. In compensation, however, the applied potential on the plate 3! is also decreased, and as the plate voltage decreases the cut-off point for the tube changes correspondingly. Consequently, there is an automatic compensation for variations in line voltage.

To smooth out the pulsating direct current from the plate of the tube 2|, and thereby prevent relay chatter, a condenser 4| is connected in shunt with the relay winding 26. Also, to stabilize the circuit, and particularly to prevent inadvertent reclosure of the relay ill after it has been automatically opened, a condenser 42 is preferably connected across the output leads H, l2. An 0.25 microfarad condenser is suitable for this purpose. It prevents any voltage surges in the output circuit incident to opening of the relay H] which might otherwise tend to cause a reclosure of the relay.

In some instances the operator may wish to retain the load circuit energized for an indefinite period. For example, if a photographic exposure lamp is connected to the load receptacle l3 he may wish to have the lamp on for some indefinite period during focusing of the camera, etc. To meet this requirement, the normally open switch i8 is provided for such manual rather than automatic control. This switch is connected in the output lead H in shunt with the main relay contacts 21. Accordingly, when the switch I8 is closed current is supplied to the output leads ll, l2 just as in the case of closure of the relay except that it is supplied for an indefinite period determined only by the time of closure of the switch.

In order to accommodate the timer for automatic control, a remote control unit 43 is provided. This unit may, for example, be incorporated in a photographic contact printer. In such case it embodies an exposure lamp 48, which is to be controlled, a pilot lamp 49 and a platen switch 50. Connections are established through a suitable four contact plug and receptacle. The receptacle on the timer itself has two load contacts 44--45 receiving corresponding plug prongs 46-41 connected to the unit 43. Similarly, two control circuit contacts 5l-52 in the receptacle receive mating plug prongs 53-54 connected to the unit 43. The load contacts 4445 are connected respectively to the output leads ll-IZ so that the prongs 46-4l connect the exposure lamp 48 to the timer output circuit. In the same way, the contacts 5|52 and prongs 53--54 connect the platen switch 50 in shunt with the timer push button I! so that closure of the switch 50 serves in initiate a timing cycle just as in the case of the push button I! described above. The pilot lamp 49, being connected between contacts and 5|, indicates when it is on that the onoff switch I4 is closed and voltage applied to the timer for operation.

To operate the timer from the unit 43 the time selector knob I5 is set for a desired time interval. Then the operator closes the platen switch to initiate the timing cycle. Thereafter the operation is the same as when the push button I! is used to initiate the cycle as previously described. At the end of the cycle the relay l0 open-circuits the supply line to the load as before, which in this case is the exposure lamp 48.

From the foregoing it will be seen that a timer of constant accuracy has been provided which does not entail the use of any of the usual clockwork mechanisms which are likely to become worn or misadjusted. The whole device is simple and rugged in construction, and very easy to operate.

I claim as my invention:

1. In a timing device the combination of an electrically energizable load device including a pair of contacts arranged to be closed in response to energization of such device, a vacuum tube embodying a cathode, an anode and a grid, means including a manually operable switch for connecting said tube to a source of current with its output in series with said actuating winding, means for completing a circuit through said contacts in shunt with said switch upon energization of said load device means for applying a biasing potential to said grid, and means for changing said biasing potential to a value below the cutoff point for the tube after a predetermined time interval, to thereby render the tube nonconductive and to deenergize said load device.

2. An electrical timer comprising, in combination, a condenser, a variable leakage resistor connected in shunt with said condenser, first and second vacuum tubes each having a cathode, an anode and a control grid, said first vacuum tube having a high amplification factor and said second tube having a low plate resistance, a load multaneously, not only connecting said anode and cathode of said second tube across a source of potential, but for also connecting said condenser across the grid and cathode of said first tube, to thereby apply a negative biasing potential to said last mentioned grid which diminishes gradually as the condenser charge leaks off through said resistor until said first tube is rendered conductive after a time interval deterl0 mined by the setting of said variable resistor.

RUSSELL L. SCHOENE.

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