US2641701A - Timing device - Google Patents

Description

June 9, 1953 1'. MOORE TIMING DEVICE Filed Aug. 30. 1947 3 Sheets-Sheet 1 In venior i w d fiv y I I I ttb rjey' June 9, 1953 'r. MOORE 2,641,701

TIMING DEVICE File d Aug. 30, 1947 3 Sheets-Sheet 2 ab 25:33 k5 6* I xi 2 8 Inventor A ttprney June 9, 1953 1'. MOORE 2,641,701

TIMING DEVICE Filed Aug. 30, 194'? 3 Sheets-Sheet 3 m I cg v z 4 vIzvenlor A Homey Patented June 9, 1953 TIMING DEVICE Thomas Moore, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application August 30, 1947, Serial No. 771,457 In Great Britain November 29, 1946 2 Claims.

This invention relates to electronic timing devices. In such devices it is known to use a valve normally conducting and to initiate the timing by the operation switch which applies a potential through a resistance across a condenser connection between grid and cathode of the valve so that, the condenser charges up to a potential which eventually raises the grid to a negative potential of a value sufficient to render the valve nonconducting and to regulate the time taken to cut off the valve by adjustment of the said resistance or condenser or both of them.

It is a disadvantage of such devices as heretofore designed that the time interval between switching on and the cut off of the valve is varied by fluctuations in the potential of the source providing the anode supply potential to the valve and the potential applied via the resistance condenser combination to the grid of the valve.

This invention consists of a device of the type above described in which fluctuation in the supply potential are compensated in a novel man ner. It is furthermore sometimes required that such device shall be capable or" automatically compensating for irregularities in the functioning of the apparatus controlled. An alternative form of the invention is described in which this object is attained in a novel manner.

The two embodiments to be described have been developed for controlling the exposure time of X-ray apparatus but the invention is capable of measuring time intervals which vary within wide limits and of controlling a variety of different things.

These embodiments will now be described in relation to the accompanying drawings in which Fig. l is a circuit diagram of an embodiment which compensates for fluctuation in supply potential;

Fig. 2 is a circuit diagram of an embodiment similar to that illustrated in Fig. l but including in addition a means of automatically compensating for irregularities in the operation of the X-ray generator;

Fig. 3 is a diagram of a typical X-ray generator circuit showing the manner in which the circuit of Fig. 2 is connected thereto so as to compensate for irregularities in the operation of the X-ray generator.

In the drawings, the relays and their contacts are shown in accordance with the detached contact method, the contacts being shown where they are used in the circuit and not necessarily next to the corresponding relays. The relays are marked with capital letters and the corresponding 2 contacts with the same small letters and in addition, a numeral distinguishing the various contacts of the same relay. The numeral appearing under a capital letter associated with a relay indicates the number of contacts controlled by the relay.

In Fig. 1 a valve VI has a relay A connected in series in its anode circuit and is supplied with anode current from an alternating current supply source, which may be the electric supply mains, at X and Y. This alternating current is rectified by rectifier SR! and smoothed by resistance R2 and condenser C2, VI is shown in the figure as a directly heated cathode pentode though any type of valve may be used having a second grid the potential of which affects the control grid potential necessary to cut off the anode current of the valve. A directly heated cathode has the advantages that it heats up quickly so that the device can be used immediately it is required but in some applications of the invention this is not important. Resistances R5 and R5 are inserted to provide an electrical mid point of the cathode for connection of the negative high tension supply and the cathode is returned to a point on the potentiometer constituted by the resistances R9, R1, so that the grid of the valve is negative in respect of the cathode but not to a degree sufiicient to render the valve nonconducting, when the press button S is first operated into the on position. This standing grid bias may be varied by adjusting R7.

The screen of the valve is connected to the junction of resistance Eli) and a neon tube V2, the combination Rio V2 and potentiometer RS constituting a potential divider in which V2 operates to hold the screen-to-cathode potential of the valve constant under difiering conditions of supply voltage potential to a degree depending on the adjustment of R8. Large values of R8 increase the permissible fluctuation of the screen potential of Vi, when the supply potential varies.

Other voltage stabilising devices could be substituted for V2.-

When the press button S is operated, the supply source is connected to the valve which conducts and relay A operates by virtue of the valve anode current flowing in its windings.

The X-ray generator is switched on directly by at or indirectly via another relay operated by the closing of a! and the time period commences.

Contacts (12 connect the grid of VI to a source of negative potential provided by the potentiometer RI bridged across the alternating current supply source, via the rectifier SR2, connected s0 as to conduct the negative half cycles of the alternating current supply, smoothed by resistance R3 and condenser C2 and via the tapped resistance R4.

At the same time at contacts open and remove the short circuit across condenser CI.

The negative potential across grid and cathode is the same as that across condenser CI which commences to charge at a rate depending upon the potential supplied from SR2, the value of resistance R4 and the capacity of condenser CI.

When the potential across CI reaches the point at which the valve becomes nonconducting, the anode current ceases, A releases, aI switches off the X-ray generator, a2 disconnects the grid of VI from the source of negative potential at SR2, and when the press button S is released a3 short circuits the condenser CI, the circuit being thus restored to normal.

The potentiometer RI is adjusted in the initial setting up of the equipment and is capable of varying the timing range of the device.

Resistance R7 is an adjustment primarily intended to recalibrate the device when a valve is changed, though it, again may be used to vary the timing range of the device.

The time setting control, accessible to the operator, is the resistance R4, alterations of which affect th time taken for CI to charge up to the critical value.

The manner in which fluctuations in the supply potential are compensated will now be described.

Suppose that the supply potential rises. The following factors tend to lengthen the time measured by the device:

(a) The filament current rises tending to increase the valve emission and raise the grid cutoff voltage of VI.

(b) The anode voltage rises tending to raise the grid cut-01f voltage of VI.

Screen voltage rises to a greater or less degree depending on the setting of RB, a large value of R8 causing a larger rise and a zero setting of R8 causing practically no variation of screen voltage due to the voltage regulation action of V2. A rising screen voltage raises the grid cut off voltage of VI.

The following factors tend to reduce the time measured by the device:

(d) The standing grid bias, due to the potentiometer Re, R: rises, accelerating the rise of negative grid potential to the cut-off value.

(c) The charging voltage applied to CI rises owing to th increased potential across RI.

When the supply voltage falls all these factors operate in the reverse direction.

The values of the components are chosen, so that these conflicting factors balance one another and three adjustments are provided to adjust the device so that accurate balance is obtained. These are RI, R7, and R8. The first two are primarily intended as calibratory adjustments for the initial setting up of the equipment to compensate for tolerances in components and variations in supply potential. The resistance R8 however can be used to adjust the degree of compensation for supply potential fluctuations and it can be set to give any desired characteristic to the device, varying from completely stable operation, to a decreasing time measurement for a rising supply voltage, on the one hand, or the converse, on the other hand. This can be understood when it is seen that variation of R8 varies the effect of factor (0) above described. This may be a useful adjustment where .the apparatus controlled has an effectiveness not directly proportional to the supply potential supplied to it and where the device and the controlled apparatus can be energised from the same supply source.

This is particularly applicable to X-ray equipment where the rise of X-ray emission is more than proportional to any rise in the supply potential.

In the second embodiment, illustrated in Fig. 2 the circuit is the same as that illustrated in Fig. 1 except that the source of charging potential applied to CI is derived from a potentiometer bridge not across the power supply source but across the secondary windings of a transformer TI the primary of which is connected in series with the current flowing in the apparatus controlled, in this case an X-ray tube.

Any apparatus in which an alternating current or pulsating direct current flows which is approximately proportional to the output of the apparatus, may be controlled in this manner.

The current passing between cathode and anode of an X-ray tube is an approximate index of the emission of X-rays by the tube and is a pulsating direct current. A part of this current is passed through Ti primary windings and thus the voltage across RI. is approximately proportional to the X-ray emission from the tube. As this rises, the charging potential applied across CI rises also and the grid cut-off potential of VI is more quickly reached, reducing the timing period for a given setting of R4. As the tube emission falls, the timing period is corresponding 1y increased.

Fig. 3 shows a typical X-ray circuit with the connections of transformer TI.

T4 is an auto transformer connected to an alternating current source.

Secondary tappings at C and D are connected to an X-ray transformer T2, on one side through contacts bl and D2 in parallel of relay B which is operated on closure of (II contacts, bl closing first and completing the circuit through a resistance RIZ to be shortly afterwards short circuited by 122 contacts. The secondary of T2 is in two halves joined in the middle by a resistance RII one end of which is earthed and the other connected to the primary windings of TI being the same as TI of Fig. 2. The other end of T- primary is earthed making this Winding effectively in parallel With RI I. A meter may also be connectedacross RI I.

An X-ray tube X is connected across the outer ends of the secondary winding of T2 and when the tube is operating, a voltage is developed across RI I proportional to the current passed by the tube and a current passes through TI primary winding also proportional to the tube current. The voltage appearing across A, B is therefore proportional to the tube current for any given setting of RI. A and B are connected to SR2 and the negative supply lead shown in Fig. 2.

Secondary tappings E and D of T4 are connected to the primary winding of an X-ray filament transformer T3, the circuit being normally broken at switch hook H from which hangs press button S. Leads from s are connected to C3. R3 and a3 as shown in Fig. 2.

When it is desired to make an X-ray exposure the press button S is lifted from its hook and the filament of the X-ray tube is automatically switched on, R4 having been first set to the correct exposure time.

A brief wait ensues, to allow the X-ray filament and the cathode of valve VI to heat up. The button is then pressed and the exposure is regulated by the device.

The button is released after a short pause or after the re-lighting of a lamp operated on release of relay A (a feature not shown in the figures), and the press button should then be promptly replaced on its hook to disconnect the X-ray tube filament as soon as possible.

The purpose of including contacts a2 in the circuit between R4 and the grid of VI is to start the timing process in accurate synchronism with the switching of the controlled apparatus by al and to isolate CI from any stray voltages which might partially charge it prior to the operation of A.

The purpose of including contacts a3 in the connection from the upper end of CI and press button S, in Figs. 1 and 2, is to disconnect Cl from the leads to the press button S, which may be of some length, and thus to avoid interference with the timing by stray effects from such leads which would otherwise be efiectively in parallel with Cl. 7

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. An electronic timing device comprising a current supply source, a source of biasing voltage, an electron valve having control grid, cathode and anode electrodes, a relay connected in series between said anode electrode and said current supply source, said relay operating responsive to flow of current in said valve, a charging circuit comprising a condenser and resistor in series, said condenser being connected between said control grid electrode and said cathode electrode, and means including said relay for connecting both said condenser and said control grid electrode to sa'd source of biasing potential upon operation 0 said relay.

-2. An electronic timing device for controlling the operation of associated apparatus, said device comprising a current supply source, an electron valve having a control grid, a cathode, an anode and an auxiliary grid, a relay having a pair of contacts, said contacts being closed when said relay is energized, means connecting said relay in series between said anode and said source, means connecting said cathode to a positive voltage point in said source whose voltage varies with the voltage applied to said anode, means connecting said auxiliary grid to a. positive voltage point in said source whose voltage varies with the voltage applied to said anode, a condenser connected between said control grid and said cathode, a resistor and means connecting said resistor, said condenser and said pair of contacts in series to a negative voltage point in said source whose voltage varies with the voltage applied to said anode.

THOMAS MOORE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,946,615 Demarest Feb. 13, 1934 2,147,781 Ward Feb. 21, 1939 2,165,048 Gulliksen July 4, 1939 2,167,529 Scaife July 25, 1939 2,250,202 Matusita July 22, 1941 2,259,331 Vedder Oct. 14, 1941 2,295,297 Schneider Sept. 8, 1942 2,361,845 Hutchins Oct. 31, 1944 2,384,647 Schwarzschild Sept. 11, 1945 2,431,284 Stadum Nov. 18, 1947 2,471,834 McDowell May 31, 1949 2,532,508 Menkhaus Dec. 5, 1950

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