How to Calculate the Resistor Value for Open Wire Detection in BMXSDI1602 using DDP 2 or 3 wires?
Issue
Open wire detection in BMXSDI1602 requires to create artificially a leakage current (Ileakage) on the line with a resistor in parallel to the sensor. If this leakage current (0.4 mA < 1.3 mA) is out of range, the external line is considered as cut (or in a short-circuit to ground condition).
Resolution
For a dry contact sensor, set in parallel with the sensor a Rext = 33 K Ω.
However, using DDP 2 or 3 wires, the leakage current needs to fall within the limits defined above. You need to define the value of the resistor (Rext) to set in parallel with the sensor, considering
- the natural leakage current of the sensor
- the Rint (internal resistor) of the input, equal to 7.5 K Ω
Then, in order to obtain a current in the range of :
The calculation of Rext is performed with the following formula :
where :