GB567225A - Improvements in or relating to circuit arrangements for periodically operating an electromagnetic relay - Google Patents

Abstract

567,225. Electric supply ; automatic control systems. STANDARD TELEPHONES & CABLES, Ltd., and SHEPARD, S. C. July 30, 1943, No. 18578/44. Divided out of 567,222, [Group XXXVII]. Addition to 557,707. [Class 38 (iv)] [Also in Group XXXVII] A circuit arrangement for periodically operating a relay B and providing adjustable on and off periods comprises a thermistor T2 adapted to vary the current flowing through the relay in response to changes in the temperature of the thermistor and two sets of contacts cl, c2 adapted to be simultaneously operated directly or indirectly by the relay, the contacts c1 controlling the heating circuit r2 of the thermistor and so tending to vary the operating current of the relay while the contacts c oppose this effect. In the form shown in Fig. 1, the heater r2 is supplied with current from an adjustable tapping R9 and after a time lag, the thermistor having a negative coefficient, the current through R2 becomes sufficiently great to operate the relay B. This closes at b the circuit to a relay C which cuts out at c1 the heater r2 and cuts in at c2 the circuit to a heater r3 of a second thermistor R3. Thus, as resistance R2 increases, resistance R3 diminishes, but not quite so much and after a longish time interval relay B releases, releasing relay C in turn so that the cycle begins again. In a modification, Fig. 2, a thermistor T4 has both direct and indirect heating and when the relay D operates, the indirect heater r4 is cut off at e2, while the opening of contacts el serves to increase the operating voltage to the thermistor and relay. Thus, the cooling of the thermistor is delayed and only after a longish interval does the relay D release. To give a greater time lag, one or more additional thermistors T33 may be included by moving a switch S1, S2 to the position 1. The heater r33 of this thermistor T33 must now be heated before the current flowing through it is sufficient to heat the heater r4 of the first thermistor. Compensation for variations of ambient temperature is also provided by an opposing winding g in series with another thermistor T5. Alternatively, when the arrangement is used to control an oven, the thermistor T5 is subject to the oven temperature and the winding g supports the main winding of the relay. The arrangement diminishes the proportion of on " to " off periods as the oven gets hot, till a limiting temperature is reached.