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Running relays off I/O port

annuelloannuello Junior MemberPosts: 294
Gday all,

What would people say is the "recommended practice" when using the I/O port to drive relays, particularly with a view to the NI-700? My intent is to use one I/O channel to control a relay which in turn switches mains on/off.

Some things that I am pondering are:
1. Driving the relay coil from the AMX supply rather than an additional power supply.
2. Is a protection diode (across the relay coil) required to prevent back-EMF from upsetting the I/O port on the NetLinx controller, or does it already have such protection built in?
3. Opto-isolating or secondary "low current" relay? Is that overkill?
4. My relay is 12VDC, 160ohms => 75mA. The NI-700 I/O port can provide up to 200mA. Is that spec for the whole 4 channels? Would that limit me to 50mA per channel? I'll be driving one relay and a 20mA PIR off the I/O port.

Roger McLean


  • annuelloannuello Junior Member Posts: 294
    Extron relay box (IPA T RLY4) on NI-700

    Gday all,

    Just revisiting running relays off the IO ports. Has anyone actually used the above mentioned Extron box? I'm pondering the following problem...

    The Extron box is designed with a common Gnd in mind. Presumably, that means that inputs 1-4 are driven high to activate the relays. I presume that the extron box has built in diodes for each relay coil, to prevent back EMF.

    When using the AMX IO ports to drive a relay, we use a common high, and drive each line low (current sink). If the box was connected to an AMX with the Extron Gnd connected to the AMX 12v, the diodes would effectively be around the wrong way and would provide no protection to the AMX. Worse still, the diodes would privide a direct short between the AMX 12v and the I/O pins.

    While the NI-700 spec says that it can cope with 12v on the I/O lines, my test bed NI-3100 doesn't seem to be able to..... :( Either that or I have messed up the whole NI device somehow. Something to test tomorrow...

    Roger McLean
  • annuelloannuello Junior Member Posts: 294

    Well, it seems as though I only damaged the individual I/O port on the NI-3000 device. At first there appeared to be a lot of things not working with it, but either there is a self-healing mechanism in the NI-3000, or I was initially flustered and not doing my tests methodically. :o

    Glad I didn't order 100 of those Extron boxes... Actually, I didn't purchase any. If someone has one, can you please confim/deny the presence of an internal protection diode?

    It would be nice to know whether or not we should be putting in a protection diode on relays driven by the I/O ports. (Any clues, AMX hardware guys/gals?) If the diode is not required, then it reduces the risk of silly people (like me) from connecting the diode backwards. If the diode is required, then it would be useful to have a little example schematic for us to follow. (Not that it is hard to figure out, but one picture is 1000 words.)

    Roger McLean
  • hanshans Junior Member Posts: 2
    Hi Roger,

    Just solder a IN4148 (or similar) across the relay coil. The coil terminal that has the diode cathode ( the stripe ) goes to your +12v. The other coil terminal goes to your IO channel. You should be able to use the AMX supply ( if you don't have a lot on it already ). If you're using a seperate supply just be sure to common the grounds(-ve).

    I hope this makes sense. I can do a drawing if you really need it.

  • jneffjneff Junior Member Posts: 25

    I cannot remember the relay board we used and the classrooms we put them in. We needed spdt relays. I am pretty sure it has six relays on one board. It had a removable resistor on the board for 12 or 24 volt operation and we used an external power supply- wall wort type to power them. We put an inline fuse 200?ma to protect the IO from the trigger input and I believe the trigger required somewhere around 20ma. We have smartboards and classroom computers. When a instructor brings in their laptop and routes the laptop video to the smartboard, the spdt relays switch the RS-232 port to the laptop.

    I think we ordered the boards from smarthomes.com amd they were around $20.00. We installed them two or three years ago and have no problems.
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