Solid State Relays


Thread Starter

Mike Weickert

I have an Allen Bradley ControlLogix 5550 system with a 1746-OB32 (PNP Sourcing) output card. Allen Bradley currently does not offer a sinking output card for this platform. The signal is used to synchronize external devices to master. The signal is roughly 3 Hz, 18 hours/day, or 194,400 pulses per day which would quickly exceed the life expectancy of dry contact relays I am familiar with. The last requirement is that the solid state relay be optically isolated. Any information would be greatly appreciated.

Steve Myres, PE

You can probably use a (much cheaper) pull down resistor instead of a solid state relay. Connect the output of the PLC directly to the input of the other devices, and connect a resistor between this wire and the grounded leg of the DC. You will need to look at the threshold voltages involved, as well as the input and output impedences, but 2.2Kohm and 4.7Kohm are frequently used for this purpose, so that may give you a good ballpark figure. Remember that this also inverts the logic of the signal (when the PLC output goes true or high, the input on the other device is now false), so you may need to invert the PLC output. If you just have a recurring, heartbeat type signal, this probably will not matter. Steve Myres [email protected]

Eric M. Klintworth

Phoenix Contact, Weidmuller, and Enterlec are three vendors that come immediately to mind. They all make DIN rail mount versions of optoisolators. There are many others, of course. Eric M. Klintworth Columbus, Ohio
Mike Seems like the most direct approach would be to take the sourcing output and run it through a dc/dc solid-state relay. We do this all the time using the OMRON I/O terminal series but any number of vendors make suitable devices. Depending on the power requirements, the G3TA-IDZR02S would probably do it. Plug into a P7TF-05 socket for din rail mtg. Response time is 0.5mS and the life would be essentially infinite. Bill Marsh Integrated Controls