When designing the power supply system for your NX series PLC and NX Units, it is important to accurately design both the NX Unit Power Supply System, and the I/O Power Supply System.

These two systems are separated from each other for reasons explained in the below article:

Why do NX IO units have separated Unit Power and IO Power?

For the NX Unit Power Supply System, determining the correct NX-PD cards and their placement on the PLC rack is relatively simple, with the calculations being handled through Sysmac Studio, as explained in the below article:

Do I need Power Supply units for my NX CPU?

The above article briefly mentions that for the I/O Power Supply, the system will require an NX-PF card if it contains specific NX-IO cards. This is normally adequate for smaller systems, however for larger and more complicated systems, it is preferable to calculate the requirements and use one or more NX-PF cards according to those requirements.

Additionally, it should be noted that Sysmac Studio does not perform calculations for the I/O Power Supply System, but NX-PF cards do require NX-Unit power, and must be factored into the calculations for determining the number and position of NX-PD cards.

How to Design the I/O Power Supply

There are two items to be calculated for the design of the I/O Power Supply. The first is the power supply capacity, and the second is the voltage drop in the power supply.

During the calculation of these items, using the NX-Series Data Reference Manual (Manual number W525) is advantageous. This manual can be found at the following link:

NX-Series Data Reference Manual (W525)

Power Supply Capacity

For each NX unit on the PLC rack, the current consumption must be determined. Once these values are known, they can be summed into a total.

For NX input cards, the following is an example of the information that the Data Reference Manual provides:

From this table, we require the following items:

• Current consumption from I/O power supply (mA).
• Input Current (mA).

From the system configuration, we will need to know:

• Number of inputs used.
• Current consumption of connected input devices.

Once this information is known, the current consumption of the NX input card can be determined using the following formula:

Current Consumption = Current consumption from I/O power supply + (Input current * Number of inputs used) + Total current consumption of connected input devices.

For NX output cards, the following is an example of the information that the Data Reference Manual provides:

From this table, we require the following item:

• Current consumption from I/O power supply (mA).

From the system configuration, we will need to know:

• Total load current of connected loads.
• Total current consumption of connected output devices.

Once this information is known, the current consumption of the NX output card can be determined using the following formula:

Current Consumption = Current consumption from I/O power supply + Total load current of connected loads + Total current consumption of connected output devices.

For Mixed IO NX cards, calculate for both inputs and outputs.

Note that certain NX cards that are not specifically I/O cards, such as the NX-PF card itself, will consume some current from the I/O Power Supply. For example, from the below table it can be seen that the NX-PF0630 consumes 10mA.

To determine the total current consumption of the NX cards on the PLC rack, sum the previously calculated totals together. This can then be compared to the existing maximum current of I/O power supply for the connect NX-PF card. If the maximum value is greater than the calculated value, the system has an appropriate power supply capacity.

If the maximum value is less than the calculated value, the system requires additional NX-PF cards. When using multiple NX-PF cards in a single rack, perform the above calculation only for the NX units from an NX-PF card to the next NX-PF card.

Voltage Drop

A voltage drop occurs in the CPU units and NX units due to the contact resistance at the connection points between the units. As such, the I/O Power Supply must be designed such that the voltage specifications of the NX units and connected external devices are maintained.

The voltage drop per NX unit depends on the Total Current Consumption, according to the below table:

To design the I/O Power Supply, the voltage drop for the NX card farthest from an NX-PF card must be calculated. To do this, the voltage drop per unit from the above table is used with the number of units that the power passes through, as per the following equation:

I/O Power Supply Voltage at position x = I/O Power supply voltage at the I/O Power Supply Terminals - (Voltage drop per unit * Number of units passed through(x-1)

Once this value is calculated, compare it to the Rated Voltage characteristic of the specific NX unit. This value is presented in the datasheet for the NX unit.

If the calculated value is within the range given for the Rated Voltage of the specific NX unit, the I/O Power Supply is properly designed. Otherwise, consider adding additional NX-PF cards, or rearranging the NX units such that the units with higher voltage requirements are closer to the NX-PF card.