Siemens PLC digital output load description - Database & Sql Blog Articles

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There are constant values ​​of resistive load, lamp load and inductive load in the load description of the Siemens plc digital output. These values ​​in the data table relate to the digital output in each application.
The characteristic of a resistive load with a constant resistive load is generally indicated by the specification of the maximum output current (when the signal is 1) in the technical data.
Lamp load When using the lamp load, consider that the lamp's turn-on current is n times the rated operating current. Heating the filament causes a rapid increase in impedance. The characteristics of the lamp load in the data table are expressed in watts.
This means that the power obtained by multiplying the rated voltage by the product to allow the output current is lower. The high turn-on current causes the maximum switching frequency of the lamp load to be approximately 10 times lower than the constant resistive load.
Therefore, for digital outputs, only lamps with a total rated wattage below the calibrated lamp load can be connected. The LED indicator is unaffected and is equivalent to a resistive load.
Inductive Load If an inductive load is used, the impedance of the load (relay coil, contactor) depends on the switching frequency of the digital output. Therefore, it allows a much lower switching frequency than a constant resistive load.
This ensures the reliability of the relay switch. This is because the inductive load circuit is to release the induced energy generated when it is turned off by its protection circuit. If the switching frequency is too fast, the shutdown current energy cannot be fully released, and as a result, the output relay cannot be turned off. For example, when there is no protection circuit, the digital output transistor may generate a surge voltage and damage the module.