Choice of direct current power supply for sensors and pre-actuators associated with Discrete I/O modules

At a Glance

The following is a presentation of precautions for choosing sensors and pre-actuators associated with I/O modules.

External direct current power supplies

When using an external 24 VDC direct current power supply, it is advised to use either:

regulated power supplies,
non-regulated power supplies but with the following filtering:
- 1000 μF/A with full-wave single phase rectification and 500 μF/A with tri-phase rectification,
- 5% maximum peak to peak ripple,
- maximum voltage variation: -20% to +25% of the nominal voltage (including ripple).

NOTE: rectified power supplies with no filtering are prohibited.

Ni-Cad battery power supplies

This type of power supply can be used to power sensors and pre-actuators and all associated I/Os that have a normal operating voltage of 30 VDC maximum.

While being charged, this type of battery can reach, for a duration of one hour, a voltage of 34 VDC. For this reason, all I/O modules with an operating voltage of 24 VDC can withstand this voltage (34 VDC) for up to one hour every 24 hours. This type of operation entails the following restrictions:

at 34 VDC, the maximum current withstood by the outputs must under no circumstances exceed the maximum current defined for a voltage of 30 VDC,
temperature downgrading imposing the following restrictions:
- 80% of I/Os at 1 up to 30°C,
- 50% of I/Os at 1 up to 60°C.