US 2482820 A
Description (OCR text may contain errors)
Sept. 27, 1949. H. WOLFSON El AL PERIODIC ELECTROMAGNETIC RELAY 2 Shets-Shget 1 Filed Apri1,l7. 1943 CURRENT I MILUAM'S Inventor tlomey Sept. 27, 1949. H. WOLFSON ET AL PERIODIC ELECTRQMAGNETIC RELAY Filed April 17, 1943 FIG. 2
1 Patentoil m 21.1949
PERIODIC ELECTBOMAGNE'IIC RELAY Henry Wolhon and Stanley Carden Shepard,
London, England, assignors, by
IIICSIIC ments, to International Standard Electric Cor: poration, New York, N. Y., a corporation oi' Delaware Application April 17, 1943, Serial No. 483,521 In Great Britain May 28, 1942 1 g The present invention relates to periodically operating electric relay circuits, and makes use of the heating delay in thermally sensitive resistance devices known as thermistors for timing the operation of the circuits.
Thermistors have been in use for some years and are characterised by a temperature coeillcient of resistance which may be either positive or negative and which is moreover many times the corresponding coeilicient for a pure metal such as copper. This property renders thermistors particularly suitable for a variety of special applications in electric circuits.
Various different materials are available for the resistance element of a thermistor, these various materials having -diil'erent properties in other respects; as one example, a resistance material having a high negative temperature coeiilcient of resistance comprises a mixture of manganese oxide and nickel oxide, with or without the addition of certain other metallic oxides, the mixture being suitably heat treated.
'I'hermistors have been employed in two dinerent forms: (a) known as a directly heated thermistor and comprising a resistance element of the thermally sensitive resistance material provided with suitable lead-out conductors or terminals, and (b) known as an indirectly heated thermistor comprising the element (c) provided in addition with a. heating coil electrically insulated from the element. A directly heated thermistor is primarily intended to be controlled by the current which flows through it and which varies the temperature and also the resistance accordingly. Such a thermistor will also be aflected by the temperature of its surroundings and may therefore be used for thermostatic control and like purposes with or without direct heating by the current flowing through it. An indirectly heated .thermistor is chiefly designed tobe heated by a heated thermistor.
More detailed information on the properties 01' thermistors will be found in an article by G. L.
Pearson in the Bell Laboratories Record Dec. 1940, page 106.
Periodically operating electric circuits are used for a variety of purposes, and the present speciiication describes several simple circuits in which an ordinary relay is combined with one or more 3 Claims. (Cl. 175+373) thermistors which control its rate of operation by reason of the time delay which occurs between any change of heating current and the completion of the corresponding change in the resistance of the thermistor element. Two series of circuits are described, suitable for ,thermistors with positive and negative temperature coeflicients, respectively.
According to the invention there is provided a circuit arrangement for periodically operating an electromagnetic relay from a source of electromotive force comprising a thermistor connected to the relay winding and adapted to vary the current flowing through the relay in response to changes in the temperature of the thermistor, and a pair of contacts controlled by the relay and adapted when operated to change the rate of heating the thermistor.
The invention will be more clearly understood by reference to the following detailed description and to the accompanying drawing in which:
Figure 1 shows characteristic curves of a thermistor with a negative temperature coeflicient of resistance, and of circuits including a thermistor; and
Figures 2 to 7 (inclusive) show' schematic circuit diagrams of various embodiments of the invention.
The curve marked T in Fig. 1 shows a characteristic curve giving the relation between the current I through the resistance element of a typical thermistor having a negative temperature coemcient of resistance, and the corresponding difference of potential V across the element. It is found that the voltage at first rises rather steeply to a maximum value E as the current increases, and afterwards falls off along acurve which is concave upwards and has a negative slope. There are thus two currents corresponding to any voltage less than E, and if the applied voltage be E or greater the arrangement is unstable and the current will increase indefinitely until the thermistor is damaged, unless limited by resistance in the associated circuit. By arranging to operate the thermistor on the unstable part of the characteristic, rather large changes in current may be obtained for small changes in applied voltage.
Figure 2 shows one circuit according to the invention. The winding of a relay A is connected in series with a directly heated thermistor T and two adjustable constant resistances R1 and R2 to a source S of electromotive force. R: is shortcircuited by the contacts a of the relay A when it is unoperated. The relay may have any numaseasso 3 beret othersetsoicontacts (notshown) rot-periorming any desired switching operations.
It will be asumed that the relay resistance is included in R1. If the relation between the current I and the voltage V be plotted for the constant resistance R1, the straight line 08.1 in Fig.
1 will be obtained. By adding the ordinates oi the curves T and OR; the curve 'I'Ri is obtained. giving the relation between the current I and the voltage V applied to the circuit of Pig. 3 when the resistance R: is short-circuited as shown.
The straight lineORz in Fig. 1 gives the characteristic for the resistances R1 and Rs together, the contacts a of Fig. 2 being open. ,By adding the ordinates of curves T and OR: the complete circuit characteristic TR: is obtained (or the condition when contacts a are open.
Suppose that the source S supplies a constant voltage F (Fig. 1) which is greater than the initial maximum E. If the relay A (Fig. 2) is at first prevented from operating, the current obtained will be FM from the curve TRi. when the thermistor has reached a constant temperature. Assuming that the relay is adiustcd to operate on the current FM, and is now ireed, the contactsa will open. introducing the resistance R2. The circuit resistance is now given by the point P on the curve TR: where it is cut by the ordinate of M. Thus at the instant when the contacts a open, and before the thermistor temperature has had time to change, the dotted straight line OP gives the instantaneous characteristic of the total circuit resistance. As the voltage applied to the circuit is only F, the current must drop instantaneously to FN, where N is the point of intersection of FM and OP. Assuming now that the current FN is sumcient to hold the relay A operated, the thermistor will cool until the current is reduced to FQ, the point Q being on the curve TRz. It the relay be adjusted so that it just releases on the current FQ, the contacts a close and short circuit the resistance R2. In the same way as just explained in connection with the points M and P, the current will suddenly increase to FK, the point K being the intersection of the dotted straight line L with FM, where L is the point on the curve TR1 having the same ordinate as Q. If it be assumed that the current FK is insufllcient to operate the relay A, the thermistor T 'will heat up until the current reaches the value 4 thanthereleaseciu-rentandmalittiemcre thantheoperatecurrentalongperiodwillbe obtained. Variousperiods and various ratios or operate to release times are clearly possible.
I: the thermistor has a positive temperature coeilicient of resistance, a corresponding parallel arrangement will give a similar result. This is shown in Fig. 3. When the relay A is wowatedtheoontactsaareinthiscaseopen. A suitable resistance R: is connected in series with the source S, and R1 is adiusted so that when the thermistor T has heated up and increased its resistance the voltage across the relay A rises sumciently to operate it. This throws the shunt resistance R: across R1, reducing the voltage, but not enough to release A. The thermistor T cools and its resistance ultimately decreases suiilciently to release A, and the cycle is repeated. The period may be set as beiore by suitably adjusting R2.
Fig. 4 shows another embodiment of the invention using two directly heated thermistors T1 and T2 having negative temperature we!!- cients of resistance.
The thermistor Tl is connected in series with the relay A across an adjustable portion oi the resistance R4 which is connected in series with a resistance Re to the source S.
The contacts a are adapted to be closed when the relay A is operated and to connect the thermistor T: across the resistance R4 in series with an adjustable resistance Rs.
FM again. If it now be assumed that the relay just operates on the current FM, the contacts a open, and the current falls to FN' again and the process is repeated indefinitely.
It will be seen that the conditions for adjustment of the relay A are that it should operate on a current between FK and FM, and that it should release on a current between FQ and FN. It will also be evident that the range QM may be varied by suitably choosing the resistances R1 and R22 by increasing R1 the point M may be moved to the left, and by decreasing R1+R2 the point Q may be moved to the right. It will be evident, therefore, that the operate and release times may be separately controlled.
The period 01' the cycle depends on the rate of The resistance R4 is first adjusted so that a suitable current for operating the relay is obtained. When the relay operates, the load comprising R5 and T2 is shunted across R reducing the operating current, but not enough to release the relay. T2 thereupon heats up and reduces its resistance, and T1 cools and increases its resistance, both reducing the current through the relay until its releases. T: then cools, and T1 heats up until the original operating current is again obtained when the cycle is repeated. Thus the operate time of the relay is controlled by T1 and the release time by both T1 and Ta, and these times may be controlled by adjusting R4 and R5.
Fig. 5 shows an arrangement corresponding to Fig. 4 for use when the thermistors have a positive temperature coefllcient or resistance. The thermistor T1 is in parallel with the relay A and T: and R5 are now in series instead of in shunt and are arranged to be shortcircuited by the contacts a when the relay is unoperated. The resistance R4 is also in series with the main circuit. The thermistor T1 will heat up and will increase its resistance so that the relay A ultimately operates. When this happens the contacts a are opened, and the thermistor T: and resistance Rs are introduced, reducing the current without at once releasing the relay. As T1 cools and T: heats up, the current through the relay becomes reduced until it releases. Tl heats up again and the cycle is repeated. The adiustment of R4 controls the rate of heating of Ti which determines the'operating time of the relay. The release time is controlled by both thermistor-s and is adjusted by means of Re.
Another embodiment employing one indirectly heated thermistor with a negative temperature coeflicient oi resistance is shown in Fig. 8. The
relay A is connected to the source S in series with 76 is connected to the source 8 through the closed The resistance R7 is first adjusted so that with the contacts 4: held open, the current through the relay does not become sunlcient to operate it when the thermistor is hot. On allowing the contacts a to close the thermistor is further heated and reduces its resistance sufflciently to allow the relay to operate. This switches of! the heating coil, the thermistor cools and the relay presently releases again, and the cycle is repeated as before. The release time is-then controlled by adjusting R1 and the operate time by adjusting Ra.
In Fig. 6, if the thermistor T3 has a positive temperature coeilicient, the circuit will operate it the contacts a are closed when'therelay is operated and open when it is released. In this case it can be seen that if the source S is switched on when the thermistor is cold, the relay.will operate at once, so that the thermistor will be heated by the current in the heating coil as well as by the current flowing in the resistance element. Its resistance will thus increase until the relay releases, switching off the heating coil,-
thereupon the thermistor cools, allowing the relay to operate again. Adjustment of R1 now. controls the operate time and Rs the release time.
Another embodiment is shown in Fig. 7. The
' relay A is connected in series with resistances If T4 has a negative temperature so that Rm controls the releasing time and Re the operating time.
What is claimed is:
1. A circuit arrangement for periodically operating an electromagnetic relay from a source of electromotive force comprising, an indirectly heated thermistor, a winding for the electromagnetic relay, said winding and the resistance element of said thermistor being connected in series to the source of electromotive force, a first adjustable constant resistance and a second adjustable constant resistance, a. heating coil for said thermistor, the said heating coil being connected to if T4 has a positive temperature coefiicient the contacts a will be open when the relay is operated. In the first case, T4 having a high resistance when cold, the relay operates directly the source S is switched on and T4 is heated by both currents. When its resistance has fallen to release the relay, the heating coil is switched off and it cools so that the relay presently operates again. Thus the cooling controls the operating time of the relay, which may be adjusted by means of R10; and the heating which is controlled by Re determines the releasing time of the relay. If T4 has a positive temperature coeflicient the operation is inverted the said source in series with the said second adjustable resistance, and a pair of contacts for said electromagnetic relay, said contacts being in series with said heating coil and said second adjustable resistance.
2, A circuit arrangement according to claim 1 in which the thermistor has a positive temperature co-efiicient of resistance, the said pair of contacts being closed when the relay is operated.
3. A circuit arrangement according to claim 1 in which the thermistor has a negative temperature co-efficient and the said pair of contacts are closed except when the relay is operated.
HENRY WOLFSON. STANLEY GARDEN SHEPARD.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS 7 Date Germany Dec. 23, 1936