GB946637A - Means for compensating temperature and voltage dependence in thermistor-delayed relays - Google Patents

Abstract

946,637. Time delay switching circuits. ALLMANNA SVENSKA ELEKTRISKA A.B. March 21, 1962 [March 29, 1961], No. 10785/62. Heading H2H. To reduce the effect of variations in ambient temperature and operating voltage on a thermistor 2 used to produce a time delay in the action of a relay, the thermistor is provided with a heating coil 3 energized by a second thermistor 4 connected in series with a resistor 5 across the supply U for the relay coil 1. The compensating effect may be increased by connecting a movable contact a short distance from the right-hand end of resistor 5, so that when, for example, the ambient temperature increases (at normal voltage), the voltage applied to the coil 1 and thermistor 2 decreases to augment the action of coil 3. To counteract the effect of the voltage drop across the compensating circuit caused when the relay is energized for long periods, the circuit is modified as shown in Fig. 4, in which the thermistor 2 is shunted by a contact 8 closed by energization of the relay coil 1, and the operating voltage is supplied through a contact 6a. This contact may be movable, Fig. 5 (not shown), for adjustment if two-fold compensation is not desired or if the relay is to be connected to a resistor which does not influence the voltage drop in the compensating circuit when the relay is operated. To improve the voltage compensation, the circuit shown in Fig. 1 may be modified by connecting a varistor, of the type whose resistance decreases with increase of voltage, in several ways depending on its voltage dependence and that of the thermistor 4. Thus the varistor may be in parallel with the thermistor 4, or with this thermistor and part of the resistor 5, Fig. 3 (not shown).