Wondering what the concerns would be with directly switching loads using discrete SNAP I/O with optical isolation such as the SNAP-ODC5-i or SNAP-ODC5SRC? Is there ever a reason to use interposing relays if SNAP I/O can accommodate necessary switching voltage and current? I know these are only rated to switch 3 amps on the field side. It would seem relays are a likelier source of failure if they are electromechanical vs. the optically-isolated solid state SNAP I/O. The loads in question vary from 5V dry contact logic inputs to 24VDC lamps and a 24VDC signaling horn. Just wondering if there are concerns with certain types of loads and what the best practices are. Thanks.
I think there are probably a few ways to answer this question, and I encourage others to weigh in on this thread with their real world experience…
That said, I want to correct or make clear one important point you bring up… the SNAP modules are rated for 3 amps TOTAL. In other words, if all 4 outputs are switched on together, then each output can switch a total of 750 mA.
Of course, if you can make sure in software that you never turn all the outputs on at once, then any one channel can switch the full 3 amps of the module.
With THAT said, lamps (LED moreso) and horns are generally pretty low current devices and should (within the current limits of the module) be fine to switch directly.
Where you do get issues is with pumps, solenoid and large contactors that have large inrush and back EMF issues.
Lastly, be sure to check out the minimum current specification as well. Often times people switch very low loads and find the SNAP module will turn the load on, but wont turn it off, that’s because of the solid state nature of the SNAP modules, there can be a small leakage current that will keep some loads on.
It will be listed in the spec sheet for the module, but is just something you should be aware of.
(Often times a bleed resistor can be used to put enough load on the SNAP module to have it switch the small load as you expect).
Thank you Beno! All very good information and points you bring up. Especially about the solid state leakage current. This would be the “Off-state leakage” spec of 1mA@60VDC from the datasheet correct?
Off-state Leakage 1 mA @ 60 VDC 1 mA @ 60 VDC
Yup. That’s the one.
And my hopeless memory seems to recall that it can be slightly different leakage for different modules… so its worth checking for your application.
Ok good to know. Looks like the SNAP-ODC5-i or SNAP-ODC5SRC would work fine for the PLC compatible signaling horn we’d be using. It has a max allowed off state leakage of 25mA. Seems minimum load spec of 20mA for these SNAP modules is also a concern though. Some LED lamps may not draw this much current in the “on” state.
