How would you wire this?

This is intrinsically safe barrier, which is giving me a hard time. I wired an input on sinking plc card (Koyo) to terminal #2.
I have 3 of these barriers. Now, when 1 sensor closes, all 3 inputs turn ON. There is only 3 terminals on the barrier and additional screw for GND. This screw touches DIN rail, which is separately grounded. PLC card is connected to 24VDC-
Any ideas?

I see you're using a Gems single-channel barrier...

I'm betting you have terminal 2 going to the PLC input. Since this connects to the DIN rail, terminal 2 on all barriers are now connected. You want to PLC input connected to terminal 1, with the grounded side of the power supply (0V) connected to terminal 2. The +24VDC of the power supply goes to the PLC input common.

Note that this requires a SOURCING input card. You stated that you have a SINKING input card. If you're switching the +24VDC, then your +24VDC winds up getting grounded when any of the sensors turn on. If your power supply's 0VDC is grounded, it would blow the fuse when any sensor turns on, so I'm guessing your supply is floating?

Unless you can swap out your input card, you might want to just use a opto-coupler to interface the barrier to your PLC.

I have only used a zener barrier once in my career, and it was a dual-channel, so you should probably wait for a few other guys to chime in on this... :nodi:



-Eric

Eric, how would the PLC input change if its connected to the power supply output? The circuit doesn't look right to me. -Jeff

jstolaruk said:

Eric, how would the PLC input change if its connected to the power supply output? The circuit doesn't look right to me. -Jeff

Click to expand...

The diagram from Gems that awalinski posted is lousy. If that 'box' on the right was only a power supply, it would short out when the sensor completed the circuit...

I think what they intended to mean was this:

   SENSOR    +-------------+        +----------+

+----] [-----| 3         1 |--------| - LOAD + |------> +24VDC

|            |   BARRIER   |        +----------+

+------------| 2         2 |---+----------------------> 0VDC

             +-------------+   |

                               V (Ground)

I think awalinski has it currently wired like this:

   SENSOR    +-------------+

+----] [-----| 3         1 |--------------------------> +24VDC

|            |             |

|            |   BARRIER   |              PLC

|            |             |       +--------------+

+------------| 2         2 |---+---| INPUT    COM |---> 0VDC

             +-------------+   |   +--------------+

                               |

                               V (Ground)

And I think this works without shorting out because his power supply does not have the 0VDC grounded. The +24VDC winds up being referenced (i.e. connected) to ground when any of his I.S. sensors turn on. Since all the barriers share this common ground, they all 'see' +24VDC on terminal 2 when any ONE sensor turns on. The PLC inputs are also connected to terminal 2, so all inputs turn on when this +24V is present.



-Eric

okay, thanks, that looks reasonable. Though I would suspect with zener diodes in the barrier it would be sensitive to the current through the load (or the lack of current). Wouldn't it better to use a optically coupled device?

I found THIS page which gives a much better description of I.S. wiring to PLCs (though the pictures are kinda small). Notice that a sourcing (0VDC switching) input card only requires a single channel barrier (figure 3), while a sinking (+24VDC switching) input card requires a dual-channel barrier (figure 2).



-Eric

Eric,this barrier was chosen for me by the distributor knowing my control design. I gues he was just a salesmen. Do you think grounding the -24VDC side instead of grounding terminal 2 would help?
Your circuit #2 is exactly right. This is the way I have it wired.

I don't think there's any way to use a single-channel barrier with your sinking input card. Your choices are:

1.) Use a sourcing input card
2.) Use a dual-channel barrier (your 'salesman' should take back your single-channel barrier, no questions asked, since he was wrong)
3.) Add a second barrier and wire as shown in Gems' lower diagram on their 'Typical Wiring Diagram' page.
4.) Add a solid-state relay between the barrier and your PLC to separate the circuits.



-Eric