In the load description of Siemens PLC digital output, there are constant resistive load, lamp load and inductive load. These values in the data table relate to the digital output in each application.

Resistive load

The characteristics of a constant resistive load are generally represented in the technical data by the description of the maximum output current (when the signal is 1).

Lamp load

When using the lamp load, it should be considered that the on current of the lamp is n times of the rated working current. When the filament temperature rises, the impedance increases rapidly. The characteristics of the lamp load in the data sheet are expressed in watts.

This means lower power than the product at rated voltage multiplied by the allowable output current. The high turn-on current results in the maximum switching frequency of the lamp load being about 10 times lower than that of the constant resistive load.

Therefore, for digital output, only lamps with total rated wattage lower than the rated lamp load can be connected. The LED indicator is not affected and is equivalent to resistive load.

inductive load

If inductive load is used, the impedance of load (relay coil, contactor) depends on the switching frequency of digital output. Therefore, the allowable switching frequency is much lower than that of a constant resistive load,

This ensures the reliability of the relay switch. This is because the inductive load circuit releases the inductive energy generated when it is turned off through its protection circuit. If the switching frequency is too fast, the energy of turn off current can not be fully released, resulting in the output relay can not be turned off. For example, when there is no protection circuit, the digital output electronic transistor may generate surge voltage and damage the module.

Editor in charge: LQ

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