Relay board?

[chuggins143]

Ok, this may or may not be the best place to post this, but does NI have a relay board similar to...

http://www.mccdaq.com/usb-data-acquisition/USB-ERB24.aspx

I've done a product search, but found nothing.  It's looks like to me that NI uses the term "switch" to refer to a relay and the only "switch" hardware I can find are PCI cards and such... but nothing stand alone.  MCC is a subsidiary of NI so I thought they might have a crossover.  If anyone knows of a similar unit I'd really appreciate to hear about it. 

Thanks!

Chad

[AnalogKid2DigitalMan]

Do you intend to use LabVIEW? If so the ERB24 does come with LabVIEW drivers so programming should be fairly straightforward.

-AK2DM

[chuggins143]

I'm already using this unit with LV2009... I was just looking for alternatives.  Possibly a unit with more relays.

Thanks,

C

[Wayne.C]

NI makes I/O modules for cRIO, cDAQ and cFP that use electromechanical relays.  Due to the poor design of the web pages they can be difficult to discover.  To achieve similar functionality you could take an 8 slot cDAQ USB chassis and populate with 8 C-series 9485 modules to get 64 channels.  This will be expensive compared to your current solution.  How many more relays do you think you will need? 

[chuggins143]

Wow.  That'd be considerably more expensive than my current solution.  I can buy two of the MCC units for just what a 4-slot cDAQ unit would cost not including the 9485's.  I guess I'll stick with my current solution.  

The reason I'm looking for a NI option is that I have a test stand built using a cDAQ with a 9225 & 9239 for measuring my electrical and power data, in addition to that I'm using a MCC USB-1208LS (virtually identical to a NI USB-6009) to measure my analog inputs and the relay board to run my actuators.  The issue I'm running into is that my signal from the 1208-LS keeps dropping from my USB hub when my relays are being actuated.  I can watch the connectivity light on the hub and it'll blink as the relays are switching... not the light for the relay board, but rather the light for the 1208 board.  The lights for the cDAQ and the relay boards remain stable.  The light for the 1208 board blinks mostly in sync with the switching relays and will eventually not come back on which means the connection has been dropped.  I've tried different hubs, a different 1208 module, different USB cables, plugged it into different slots in the hubs... all to no avail.  My next solution is to do away with the cheap daq module all together and just throw a 9263 and a 9205 into the cDAQ unit and be done with it.  I'm guessing there's some interference going on between the two... odd thing is I'm using the same module and relay board combo with several other control systems with no issues.  The only difference between the two systems is the other is not running LabVIEW.... it's running VB.Net with the old MCC SoftWIRE plugin. 

Anywho, thanks for the info guys/gals!  I do appreciate it!

Later,

Chad

[Wayne.C]

Chad,

do you have snubbers installed across relay coils?

[chuggins143]

Snubbers??  Never heard of that...  unless you're talking about the (IIRC) diode that's put across the pins to negate the spike when the coil de-energizes.  I vaguely remember reading about that somewhere when I was looking at building my own relay boards. This relay board came as a manufactured unit so I'd assume they've already got that feature in there.   As I mentioned before, I'm using this same relay board and DAQ (1208LS) module with other systems without issue.  I'm thinking it's some other issue that may be in LabVIEW since the other systems are not running LabVIEW.  Now that I think about it, this also could be an issue with the relay board... I might try a different relay assembly and see if that solves the issue.

C

[AnalogKid2DigitalMan]

Bummer you have to register on MC's site to get manuals. I would presume the coils have diodes already.

What type of loads are the relay switching, aand at what voltages/currents?

-AK2DM

[chuggins143]

I'm pretty sure they'd have the diodes already, but I'll check... I have the manuals around here somewhere.

My loads aren't much... 24VAC, <1A.  Mostly I'm actuating damper actuators to adjust static pressure on my air flow... this is where the issues are occurring.  Anything that's carrying high voltage (120/240/277) I'm using another contact relay to switch.

[Wayne.C]

As you mentioned, whenever a coil is de-energized, a spike is created.  That spike creates 2 problems.  One is a reverse or elevated voltage that can damage the driver circuitry.  The second is EMI created due to the high dvdt of the transient.  The proper snubber installed directly across the coils will take care of both.  Typically for a DC coil a reverse biased diode is used although it actually decreases the life of the relay contacts.  A combination of a zener diode and a rectifier diode controls the reverse voltage and does not affect contact life.  For an AC coil I prefer an RC snubber.  I have also seen transzorbs and MOV's used as well but my experience has been that the RC snubbers do a better job of snubbing the EMI.  Often snubbers are installed across relay contacts to protect them and reduce arcing.  People that work with PLC's seem to like MOV's for this.  If you are controlling inductive loads with the 1208LS, you still need to install snubbers accross the coils of those devices as well.  If you don't you are creating large amount of EMI that can cause havoc with other devices in your system.