GB558861A - Improvements in or relating to generators of periodically varying electric currents - Google Patents

Abstract

558,861. Converters. STANDARD TELEPHONES & CABLES, Ltd., CHATTERJEA, P. K., and HOUGHTON, L. W. July 21, 1942, No. 10151. [Class 38 (ii)] Periodically varying electric currents of determined wave form are generated from a direct current source by means of an electromagnetic switch and two or more thermistors, the switch having an operated position for each of the thermistors in which position the thermistor is heated and then serves to vary the current to the electromagnet so that after a time interval the switch is moved to its next operated position. In the form shown in Fig. 2, the electromagnetic switch comprises a polarized two position relay B with two windings W1, W2 in series with thermistors T3, T4 of negative temperature coefficient. In the position shown current from the battery E2 flows through the thermistor T4 and winding W2 but its resistance is too high for the relay to operate till the thermistor has been heated. During this heating process the voltage across a resistance R7, which voltage supplies the output circuit 1, 2 steadily rises. Eventually the relay operates and its contacts b connect T3 and W1 in circuit instead of T4 and W2. Since T3 is cold, the current drops immediately but rises gradually as the thermistor heats up till it is sufficient to operate the relay back to its original position. The voltage 1, 2 will thus have a saw toothed form. In a modification, the thermistor has a positive temperature coefficient and is indirectly heated. Fig. 3 shows a form in which the electromagnetic switch has a single winding W which when energized moves round, one step at a time, three contact arms on three banks of contacts S1, S2, S3. Each bank has four fixed contacts connected as shown. In the position shown current flows from the battery E3 through a movable contact segment of bank S1 to contacts I and 4 and thence respectively through thermistors T11 and T14. Some current will also flow in a parallel circuit through the winding W and thermistor T11. As the thermistor T11 heats up its resistance decreases till the current through W is big enough to move the three contact arms on one step. Thermistors T12, T11 will now be in circuit and when T12 is sufficiently heated the switch will move on the next step and so on. The output circuit 1, 2 only receives voltage when the contact arm of bank S2 engages contact 2 or 4. The output terminal 2 is then connected to a point between the thermistor T11 and a resistance R11 (or T13 and R13). During this engagement, the thermistor T11, which is already heated becomes heated still more, and the voltage of the above-mentioned point varies in a corresponding manner. The resultant wave form at the output terminals 1, 2 thus approximates to a rectangular form, Fig. 4 (not shown).