A thermostatic control unit 13 has a back-up battery 23, for a clock 24, requiring a small current to maintain the circuit active during the open cycle of the thermostat switch 20. The control unit 13 is connected in series with an operating unit 11, which has a coil 30 and contacts 31 which operate a furnace (not shown). A positive temperature coefficient (PTC) resistor 32 ist placed in parallel with the coil 30 to provide a low input impedance for maintaining the small current without activating the coil 30. During the operation of the thermostat 15 with the switch 20 closed, a larger current to the PTC resistor 32 causes the resistor to increase in temperature and thus its impedance; so the wattage dissipated by the PTC resistor 32 is reduced to minimize the heat dissipation and thus the temperature of the operating unit 11.
展开▼
