Current Loop Sensor Interface

  
This is a very simple circuit to interface a current loop sensor to an input which is designed for a voltage, such as that from a standard potentiometer. This page is technical so that interested persons can build the circuit themselves, but 4QD can also supply the interface on a 'postage stamp' sized (22mm x 27mm) board as shown in the first diagram. It is a relatively trivial circuit! This circuit was developed to interface one of the new type contactless (inductive) throttle sensors to 4QD's motor controllers for use in a conversion to an Eze-Go golf car.
Current Loop Sensor Interface - schematic

The throttle itself consists of a coil and some electronics with a steel jacketed brass slug which is moved in and out of the coil by the throttle movement. The assembly has only two contacts and draws a current of between 600 microamps and 2 milliamps: the current varies with slug position as the throttle is moved. As such it is a system which borrows much from the industrial 4-20mA current loop sensor standard. The sensor will work down to about 5 to 6 volts minimum. 4QD's NCC and Pro-120 use an internal 9v rail, so there is not a huge 'overhead' to allow any voltage swing across the sensor, hence this circuit. The sensor is shown by the standard symbol for a current source (two intersecting circles). Tr1 and Tr2 are a current mirror arranged to source about 1mA into the sensor, via Tr2. This current can be adjusted by the preset to 'back-off' the zero position current. This allows an electrical zero adjust (in practise the customers usually prefer to use the throttle's mechanical zero adjust). Tr3, 4 and 5 are a second mirror which reflect the sensor's output current (less the backed off zero portion) into the 4K7 resistor - a resistor is a current to voltage converter! We have therefore developed an output voltage proportional to throttle position and it has the same 3 wires as a standard potentiometer.



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