Circuit arrangement to detect signals indicating a change in current through a needle lift sensor of an injection nozzle in combustion engines, which is connected to a constant direct voltage supply

A control pulse generating circuit responsive to the position of a valve needle for indicating the position of the needle in a combustion engine fuel injector comprising:

• a reference voltage source, a differential circuit having a differential output between a first output terminal and a second output terminal, a first regulating operational amplifier having a first minus (-) input, a first plus (+) input and a first signal output (3') supplying a first regulating voltage, a second regulating operational amplifier having a second minus (-) input, a second plus (+) input and a second signal output (3") supplying a second regulating voltage, said reference voltage source being coupled to said first plus input and to said second plus input so that said first regulating amplifier and said second regulating amplifier are operated symmetrically about a symmetric voltage, a first sensing resistor (R'.sub.M) connected between said first operational amplifier output and said first regulating output and supplying a first source voltage to said first output terminal (4'), and a second sensing resistor (R".sub.M) connected between said second operational amplifier output and said second regulating output and supplying a second source voltage to said second output terminal (4") whereby a regulating constant DC-voltage is provided between said first and second output terminals (4', 4");
• a needle sensor, having a sensor impedance (Z.sub.S), operatively connected to a fuel injector so that the sensor impedance of said needle sensor changes between the open position of said valve needle of said fuel injector and the closed position of said valve needle of said fuel injector, said needle sensor being connected between said first output terminal (4') and said second output terminal (4") to receive a differential source voltage and consuming a sensor current;
• a differential band pass amplifier having a differential input and a differential output, said differential inputs being connected individually to said first and second signal outputs (3', 3") and receiving the difference between said first regulating voltage and said second regulating voltage, the difference between which is responsive to said sensor current so that an amplified regulated differential signal voltage is provided to said differential output, and
• a Schmitt trigger, having a trigger input and a trigger output, said trigger input being coupled to said amplifier output to receive the amplified differential signal voltage so that the trigger output is in the form of pulses of varying width indicating the position of the valve needle of the fuel injector.