Patent GB-687136-A (GB)
Title: Improvements in temperature change responsive means for controlling electric circuits
IPC: G05B11/01
Filed: 1949-03-11
Abstract: 687,136. Fire extinguishing. SPEED DEVELOPMENT CO., Ltd., and RUFFLE, W. H. Oct. 14, 1949 [March 11, 1949], No. 6783/49. Class 47(ii) [Also in Groups XXXVI and XXXVIII) A device operated by a sensitive switch actuated by the low voltage output from a detector responsive to a rapid change in temperature has a holding circuit for the device which also removes the load due to the device from the switch and has means for injecting a voltage into the switch which tends to maintain the switch actuated when actuated and released when released..According to the Provisional Specification the invention is suitable for operating heaters, coolers or fire extinguishers as well as alarms. A fire alarm comprises a number of pairs of thermo junctions CS located at vulnerable points and arranged such that one junction of a pair is more exposed than the other so that a substantial response is only obtained for rapid temperature changes. The junctions are arranged in series with the coil 7 of a switch constructed like a moving coil meter and having two contacts associated with the pointer 2. A sufficient output from the thermo junctions causes contacts 2 and 6 to engage and thus to operate a relay comprising windings 8 and 14. Contacts 12, 13 provide a holding circuit for the relay and remove the load from the switch contacts and contacts 21, 22 operate the alarm lamp L. A resistance R<SP>1</SP> in series with the relay coil 8 causes part of the relay current to flow through the moving coil until the holding circuit operates and this ensures that a more positive contact is made. Relay coil 8 is connected in parallel with a resistance R 3  and in series with R 6  so that only a small part of the available voltage from supply S is used. Thus, when the output from the thermo-couples decreases sufficiently the moving coil switch releases and the remaining part of the voltage from the source S is connected through contacts 2, 3, and 16, to the second coil 14 of the relay and causes the relay to release. The current through coil 14 flows through the moving coil 7 increasing the pressure between contacts 2 and 3 until the relay first opens the holding contacts 12 and 13 and then opens contacts 15, 16, restoring the circuit to normal. In order to diminish the effect of vibration the supply for the moving coil and relay is impulsed by contacts 9a. When the circuit is switched on a relay L<SP>1</SP> operates and its contact l<SP>1</SP> operates a relay L<SP>3</SP>, the contact l<SP>3</SP> of which removes the short circuit from relay L<SP>2</SP> causing relay L<SP>2</SP> to operate. Contact l<SP>2</SP> causes relay L<SP>1</SP> to release which in turn causes relays L<SP>3</SP> and L<SP>2</SP> to release, and the operating sequence then recommences. Thus the three relays continuously operate and release in succession causing contact 9a to make and break. A further contact of relay L<SP>3</SP> may be used to cause the alarm lamp to flash intermittently when operated. The circuit may be tested by operating a button 18. This operates a relay L<SP>5</SP>, contacts 19, 20 of which open causing the current through R<SP>5</SP> to develop a voltage across R<SP>2</SP> equal to the output from the thermocouples in the case of fire. The voltage causes a current to flow through the thermo-couples and the moving coil and thus causes the alarm lamp to light. The upper contacts of the button disconnect winding 14 of relay L 4  preventing a false clear being given as a result of the pulsating current supply. Several detector circuits of the type shown in Fig. 1 may be arranged together, Fig. 2 (not shown), a common test panel (having a single button but individual lamps) and a common impulsing circuit being provided. Additional contacts on relays L<SP>1</SP> and L<SP>2</SP> are used to pulse the different circuits. A suitable thermo junction arrangement comprises a first junction 45 exposed at the top of a metal tube 47 and a muffle 55 supported on the wires and screening the second junction 44 from the flames. The tube is slotted at 53 at its base and the muffle is spaced from the tube so that air or gas can circulate through the tube allowing the second junction to heat up after a delay. Two insulating base discs 43 and 49-are provided and are secured to the base by a flange 50. Specification 678,502, [Group XXXVIII], is referred to.