Constant current supply for domestic cooker temp. measurement - uses single controlled current transistor feeding switched parallel array of transistors driving sensors under microprocessor control

摘要

The constant current supply (1) is switched among different resistive temp. detectors installed at various locations in an electric cooker. The constant current supply (1) is derived from a pnp transistor (5) with its base bias set by a resistive divider (2,3) and a temp. compensating diode (4). The collector of the transistor supplies a rail (7) coupled to a parallel array of switching transistors (8), each with a resistive sensor (9) in its collector circuit. A microprocessor (13) controls the successive application of turn on signals to the base of each of the transistors, allowing successive monitoring of each temperature sensor. The voltage over each temp. sensor is gathered by a conversion circuit (14) and supplied to the microprocessor (13) which processes the data and delivers control signals to the cooker (17). The use of a constant supply current makes the voltage measured over each resistive sensor directly sensor directly related to temp.