General protective measures of Discrete I/O modules

At a Glance

The following is a description of the general protective measures integrated into the channels of I/O direct current modules.

DC outputs

Every transistor output (except where specifically labeled "Non-Protected"), features a protective device which allows the following to be detected when an output is active:

an overload or short circuit; failures such as these cause the output to be deactivated (tripped) and the failure to be indicated on the display on the front panel of the module (the LED corresponding to the channel flashes, the I/O error LED comes on),

a polarity reversal; a failure such as this causes the power supply to short circuit without damaging the module. In order to obtain optimal protection, a quick-blow fuse must be installed on the power supply and upstream from the pre-actuators,

an inductive overload; each output is individually protected against inductive overloads and has a fast electro-magnet demagnetization circuit using a zener diode which allows the mechanical cycle of certain fast machines to be reduced.

DC inputs

24 and 48 VDC dc inputs are of constant current type. For any input voltage in excess of 11 V (for 24 VDC inputs) or 20 V (for 48 VDC inputs), the input current remains constant.

This characteristic has the following advantages:

guaranteed minimum current in active state in accordance with IEC standards,
limited consumed current when input voltage increases, to avoid the module overheating unnecessarily,
reduced consumed current to the power supply sensor supplied by the PLC power supply or a process power supply.