Cross-Physics Sensor Replacement in Industrial Control Circuits
Cross-physics sensor retrofits require matching phase delay, thermal drift coefficients, and input filter bounds before changing the transduction element.
Off-State Leakage Current
Semiconductor Residual Flow
Small amounts of electricity that continue to pass through a solid state switch even when it is turned off can affect the behavior of a circuit. While a mechanical contact provides a complete break, off-state leakage current is a characteristic of the transistors used in electronic sensors. This current provides the power for the internal sensing logic of two wire devices.
High Impedance
Resistance of the output transistor is not infinite when the device is in the inactive state. A few milliamperes of current may still flow through the load. If the load has a very high input impedance, this small current can generate enough voltage to keep a controller input in the on state.
Input Triggering
Digital inputs on some programmable logic controllers are sensitive enough to be activated by this residual flow. This leads to false signals where the controller thinks the sensor has triggered when it is actually idle. Resistors are sometimes added in parallel with the load to drain this extra current to ground.
Load Compatibility
Selection of a sensor must match the electrical requirements of the connected device. Three wire sensors typically have much lower leakage because they have a dedicated power wire. Monitoring the leakage levels ensures that the system remains reliable across the entire temperature range where the leakage current often increases.