Damaged NI-6518 IO Board

[skjomni]

HI,

We are using a NI-6518 IO board for some simple IO. We have used the boards in the past without issue, however we have two boards blow on us in quick succession. The board is wired as per the attached schematic, where the output 2.5 is connected to A1 of a relay coil, and the relay A2 connected to ground. The power supply is a Puls 24V 10A supply, this is connected to VCC.

The first board began to behave strangly, outputs etc were not coming on, when we examined the board we discovered the damage on the IC shown in the attached image. We then ordered another board only for the same thing to occur again with damage to the same IC  again occuring.

We discovered that the relay was damaged, there appeared to be 3A flowing into the coil of the relay. Would the board not limit the current it can provide or is it possible that 3A was pulled through the board by the relay. Also is there an issue with how we have connected the board based on the schematic

Thanks 

[Seth_B.]

Hello skjomni,

The NI 6518 does not have current limiting hardware on board.  The maximum allowable currents are given in the NI 651x Specifications.  3A definitely exceeds the maximum allowable current for this board.  I would strongly recommend adding external current limiting circuitry as well as a flyback diode to protect the digital outputs while switching the inductive load of the coil relay.

[skjomni]

Thanks, i was looking at this article below and it suggested that the board has over protection circuitry:

http://digital.ni.com/public.nsf/allkb/18666283A34B819B8625756D004EEC4D

Looking at the specification it seems to suggest 350mA if using only one port. If using multiple lines per port its 75mA per line.

Does that mean every single output needs to have a resistor and diode connected to the load it is switching?

What we were expecting was :

to connect the 24V supply to the VCC of the board, and connect the outputs to their respective loads. We taught the outputs were only capable of providing 75mA per line (which was fine for the signals we needed to switch). If there was a load connected to the outputs that required excess of 75mA/350mA the board would not be able to switch it.We did assume the board limited the current it could supply on its output channels and not pull excess current through it. Is there an alternative board that can do this?

Thanks

[Seth_B.]

Hello skjomni,

The fuse mentioned in overcurrent protection is intended to protect the inputs against momentary excesses in current.  The fuse is self resetting (in order to prevent the need for RMAs for a one time excess in current) and self resseting fuses can fail if they are left in an over current state for extended periods of time.

Additionally, the flyback currents associated with switching inductive loads such as coil based relays are often quick enough that they can damage devices before fuses can trip.  At minimum, I would strongly recommend flyback diodes to protect the inputs from the inductive load.  Pretty much all boards will have this susceptibility unless they are specifically dedicated relay drivers.