Overheating Protection Circuit (using PTC thermistor)

Published:2011/5/17 21:17:00 Author:Sharon | Keyword: Overheating Protection, PTC thermistor | From:SeekIC

For continuously operating mechanical and electrical equipment used in production like automatic lathes, electric oven, and ball mill and other and other unattended devices, due to the motor overheating or accident caused by thermostat failures have occurred not rare, it's necessary to take appropriate security measures. PTC thermistor overheating protection circuit can be easily and effectively to prevent such incidents from occurring. 1. Principle circuitFigure 1 is an example of motor overheat protection. It's a control circuit composed bythe PTC thermistor and the Schmitt circuits. In the figure, RT1, RT2 and RT3 are three step-type PTC thermistors of the same characteristics. They were buried in the stator windings. Under normal circumstances, PTC thermistor is in room temperature, and their total resistance is less than 1KΩ. At this point, V1 pauses, V2 turns on, and the relay K is through and open normally. The motor is powered by city electricity. Figure 1 Motor overheating protection control circuit 2. The main components election The choice of PTC thermistor depends on the motor insulation class. Components dimensions are shown in Figure 2. Usually, the choice of Curie temperature of PTC thermistor is 40℃ lower than the limit temperature which corresponds to the motor insulation level. Figure 2 PTC thermistor Dimensions 3 Installation and adjustmentThe recommended installation way is to bury PTC thermistor in the stator windings. Adjustment method: Put PTC thermistor in incubators, set the temperature TK, regulate RP to make that when PTC thermistor stays at TK-5 ℃, VD2 does not shine, K does not move; while when it stays at TK +5 ℃, VD2 lights, K acts. RP can be locked.



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