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.