ISOLATED MEASUREMENT CIRCUIT FOR SENSOR RESISTANCE

摘要

A circuit and method is disclosed for measuring the resistance of a resistive sensor, such as a PTC or NTC temperature sensor used for monitoring the temperature of the windings of an electric motor. The measurement circuit is based on an electronic circuit in which conductors from a sensor located in the object to be monitored are connected to an amplifier circuit in a feedback configuration that reduces the DC level supplied to the voltage divider when the sensor resistance increases. The measured signal is amplified and, using a comparator to compare it with the output voltage of a sawtooth generator, a continuous PWM (Pulse Width Modulation) signal is generated and transmitted in digital format to a SELV electronic circuit, for example through an opto-isolator. The essentially logarithmic signal amplification makes it possible to reliably distinguish between a short circuit in the sensor circuit and low values of sensor resistance.