US2476843A - Contact protection network - Google Patents

Description

1, 1949. A M -n5 v 2,476,3 2-3 CONTACT PROTECTION NETWORK Filed Feb. '7, 1946 INVENTOR A. M CURT DECEASED LAl ALETTE sm/mso/v cum/s H/S exg cum/x ATTORNEY Patented July 19, 1949 CONTACT PROTECTION NETWORK Austen M. Curtis, decease N. 1., by Lavalette Ste South Orange, N. J Laboratories, Incorp corporation of New York d, late of South Orange, venson Curtis, cxecutrix,

assignor to Bell Telephone orated, New York, N. Y., in

Application February 7, 1946, Serial No. 648,065 3 Claims. (Cl. 175294) This invention relates to circuit makers and breakers and particularly to electrical networks to protect such devices from the erosive effects of current flow during the operations thereof.

It has long been known that a condenser bridged about a pair of contacts forms an effective protector against sparking on the opening of the contacts and for this purpose the larger the capacity the better the protection. But this introduces a new defect, for the greater the capacity the condenser has, the greater amount of energy can be stored which must be discharged through the contacts when they are again brought together. Therefore a resistance must be introduced in the condenser circuit to prevent an undue rush of current at this time and hence the greater the resistance, the better the protection. However these considerations are mutually conflicting.

The ideal arrangement would be one in which the resistance in series with the condenser is extremely low when the contacts are opened to avoid hindrance to the functioning of the condenser and very high when the contacts are closed to limit the initial current flow from the discharge of the condenser to a value that could be handled by the contacts without undue heating.

A rectifier or asymmetric cell has such properties and its use in substitution for the usual resistance in the condenser resistance contact protection network would be possible provided an appropriately sized device of this nature having the required electrical constants were available. Applicant has discovered that a rectifier constructed of a crystal of germanium may be produced having physical dimensions so small that the whole unit may be mounted as part of a relay assembly or potted with a condenser. Since such a small unit may be constructed with an electrical resistance of about ohms in one direction and several thousand ohms in the other directions it forms an ideal unit for the purposes set forth.

A general characteristic of most known types of rectifiers or asymmetric cells is that there is a practical limit to the surge peak potential applied thereto. The said germanium crystal appears to operate indefinitely if the surge peak is limited to 150 volts but has been found to fail after several hundred operations with current in the neighborhood of 1 ampere at a surge peak between 500 and 1,000 volts. The rectifier combined with an appropriate condenser therefore provides a satisfactory contact protection unit for most uses in communication systems.

In situations where especially severe service conditions will be encountered, a third element in the form of a varistor, an element having a naturally high electrical resistance but which changes to a low resistance when a comparatively high potential is applied, will be added. Thus a varistor may be bridged about a relay winding or other inductive load element to block the surge peak which will be produced when the circuit therethrough is opened.

A feature of the invention is therefore the combination of a varistor, an asymmetric cell and a condenser for the protection of a pair of contact makers and breakers. In this combination, the condenser acts as an absorber to hold the potential across the opening contacts down and thus prevent the formation of a spark or an arc, the asymmetric cell acts as a gate to provide a low resistance path to the condenser on the opening of the contacts and a high resistance path on the closing thereof and the varistor, normally of little effect in the circuit acts as a drain when abnormally high voltages appear, thus protecting the asymmetric cell.

Other features will appear hereinafter.

The drawings consist of a single sheet having four figures, as follows:

Fig. 1 is a schematic circuit diagram showing the prior art arrangement;

Fig. 2 is a similar schematic circuit diagram showing the fundamental arrangement of the circuit elements according to the present invention;

Fig. 3 is a similar schematic circuit diagram showing a modification of the arrangement in which a series resistance and a shunt leak of substantially the same resistance values as exhibited'by the asymmetric cell are used for the protection thereof and Fig. 4 is a similar schematic circuit diagram showing the use of a, plurality of rectifiers used in parallel.

The conventional contact protecting means is shown in Fig. 1. Here a, relay is shown in control of an armature and back contact. This contact controls a load here illustrated as an inductance 2- and a resistance 3 in circuit with a source of current 4. The contact protection is provided by a circuit bridged about the contacts and consists of a condenser 5 and a resistance 6. The values of these two elements must be always a compromise for the condenser 5 should be of great capacity to protect the contacts when they aaraecs are opened and the resistor should be or high resistance to protect the contacts when they are closed. These two requirements are mutually conflicting for the higher the resistance, the less.

Y eflective is the condenser.

In accordance withthe present invention, an asymmetric cell 1 is substituted for the resistonce 6. with this arrangement, when the armature of relay I is moved away from its back contact, the asymmetric cell presents little or no hindrance to the flow 0! charging current in the condenser 5. r However, when the contacts are closed, this asymmetric cell 1 constitutes a high resistance to the discharge current of the condenser and by thus limiting the rush of current through the in radio receivers may be constructed to present a resistance of about 10 ohms in one direction and about 10,000 ohms in the other direction, these figures, of course, being subject to wide variation and being given only by way of example.

A varistor l2 may be bridged about the load 2 and 3 in situations where especially severe service conditions are expected. This will be normally of little effect but will act as a drain when abnormally high voltages appear.

Fig. 3 shows a variation of the arrangement" in which a series resistance 8 which may be in the order of 10 ohms and a shunt leak resistance 9 which may be in the order of 10,000 ohms are used in combination with the asymmetriccell I.

These resistances of substantially the same values as exhibited by the asymmetric cell 1 are employed for the protection thereof. The series resistance 8 adds 10 ohms to the inherent resistance of 10 ohms of the asymmetric cell so that the charging current oi. the condenser 5 will encounter a low resistance of substantially ohms. The shunt resistance of 10,000ohms in parallel with the substantially 10,000 ohms of 45 the asymmetric cell then presents a resistance of 5,000 ohms to the discharge path of the condenser. Again it must be noted that these values are given by way of example and are also subject to variation in accordance with difierent 5 circuit conditions encountered.

Fig. 4 shows two asymmetric cells in parallel each with its series resistance and a singleshunt resistance designed to afford greater protection where the value of the load and other factors in the circuit'require it. Other arrangements which will be clear irom this teaching may be made within the, scope or this invention.

What is claimed is:

. 1.. A contact protection means consisting of a condenser in series with an asymmetric cell bridged about a pair of contacts to be protected and protecting means ior said cell comprising a series resistance of a value to limit the current through said cell substantially to the value produced by said cell in normal operation, and a shunt resistance of a value to limit the potential across said cell substantially to the maximum value to which said cell is subjected in normal operation.

2. A contact protection means consisting of a condenser in series with an asymmetric cell bridged about a pair of contacts to be protected and protecting means for said cell comprising a series resistance of a value to limit the currentthrough said cell substantially to the value produced by said cell in normal operation, a shunt resistance of a value to limit the potential across said cell substantially to the maximum value to which said cell is subjected in normal operation and a varistor responsive to abnormal potentials caused by the operation of said contacts in series with a load having a substantial inductive component to minimize surges produced thereby.

3. A contact protection means consisting of a condenser in series with a germanium crystal rectifierbridged about a pair of contacts to be protected and protecting means for said rectifier comprising a series resistance of the order of ten ohms and a shunt resistance of the order of ten thousand ohms.

. LAVALETTE STEVENSON CURTIS, Executrirc of the Estate of Austen M. Curtis,

Deceased.

REFERENCES CITED The following referemces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,357,257 Slepian Nov. 2, 1920 1,664,225 Robinson et al Mar. 27, 1928 1,857,160 McEachnon May 10, 1932 2,011,395 Cain Aug. 13, 1935 I 2,049,177 Rumpel July 28, 1936 FOREIGN PATENTS Number Country Date 759,031 France Jan. 27, 1934

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