Control circuit for heater with variable resistance associated with sensor for detecting oxygen in exhaust gases

Abstract

A circuit comprises a measurement resistor (Rs) in series with a heater (16, 18); a detector circuit (20) for providing a signal (VOP) indicative of the current flowing in the resistor (Rs); switching devices (PW1, PW2, PW3, PW4) for controlling the connection of the heater (16, 28) to a voltage source (Vbat) and the connection of the measurement resistor in the supply circuit comprising the source (Vbat) and the heater (16, 28); and a control unit (18, 32) arranged to drive the switching devices in a manner such that each time the heater (16, 28) is activated, the measurement resistor (Rs) is kept disconnected from the supply circuit of the heater (16, 28) for a predetermined period of time and the measurement resistor (Rs) is then connected in the supply circuit of the heater (16, 28).

Description

technical field [0001] The invention relates to a control circuit for a heater with a variable resistor connected to a sensor for detecting the oxygen content of the exhaust gas, in particular for an internal combustion engine of a motor vehicle. Background technique [0002] Current O.B.D. II standards for emissions (exhaust) require that the operating temperature of the lambda probes be kept within well defined limits. This requirement cannot be met simply by selecting the resistance value of the heater and / or by its automatic compensation. [0003] The functionality of the heater can be monitored by accurately detecting the current flowing through a measuring resistor connected to the heater and, if desired, the functionality of the probe for subsequent adjustment operations can be deduced therefrom. [0004] Given the tight tolerances imposed by the O.BD.II standard, this current measurement must be very precise, which contradicts two opposing requirements for the resis...

AI Technical Summary

Problems solved by technology

However, in steady state, when the current is low, the voltage drop across the measurement resistor with low resistance is only a few millivolts (mv), resulting in large measurement errors and due to the non-negligible offset compared to the useful signal and drift also increases the error

[0006] On the other hand, the use of a measuring resistor with a high resistance value will produce a loss of efficiency of the heater in the phase immediately following start-up (it will take longer to reach a steady state), especially during the initial phase. Produces large power dissipation with significant size issues

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