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When we switch on for example 8 output channels (cFP-DO-401), the output voltage of one channels will drop down. The voltage is 10 or 15 V instead of 24 VDC.
The output card will switch on/off small relays from phoenix (PLC-RSC-24DC/21). The input current of these relays is 9mA.
What can be the problem?
We use Labiew 7.1 with Compact fieldpoint (cFP-2000)
This is may be due to insufficient current, just calculate the current as:
Square the current on each channel and add the squares together. If the sum of all the squares is less than or equal to 8 A
2, the total current is within the limit. In the following example, one channel supplies 1.5 A and five channels supply 1 A each:
(1.5 A)2 + 5 × (1 A)2 ≤ 8 A2
When an inductive load, such as a motor or relay, is connected to an output, a large counter-electromotive force may occur at switching time because of the energy stored in the inductive load. This
flyback voltage can damage the output and the power supply. It is best to limit flyback voltages by installing a flyback diode across an inductive load. Typically, you should mount the flyback diode within 18 in. of the load.
The [c]FP-DO-401 contains flyback diodes to prevent excessively high voltage from damaging the module. National Instruments still recommends using an external protection circuit across any inductive load.
