Is running Solenoids off an AB Micrologix Relay output bad practice?

jane101 · Nov 9, 2015

Hey guys,
Quick question. Got a little control panel to build for a hydraulic press. There's about 12 solenoid valves being controlled. From what I can see they are within the guidelines of what the output can handle. Just wondering if it's bad practice (someone had suggested it might be as they are prone to fail?) and suggested digital outputs and 12 interposing relays. Other than driving a run light that's all the little PLC is controlling. What are your thoughts?

adisharr · Nov 9, 2015

I don't use any relay output cards. I usually need something that I or maintenance can replace quickly. If they don't have an FET output card that can handle the solenoids, I would use either some small card relays or another type they already use in the plant.

jane101 · Nov 9, 2015

It's a stand alone unit (20 inputs 12 outputs). Not much other automation going in the plant (not much of a plant). Thinking of a micrologix 1200. Why is it that you're leary of relay outputs? Are they really that prone to fail?

adisharr · Nov 10, 2015

jane101 said:
It's a stand alone unit (20 inputs 12 outputs). Not much other automation going in the plant (not much of a plant). Thinking of a micrologix 1200. Why is it that you're leary of relay outputs? Are they really that prone to fail?

Depending on who is supporting this (even though it's a small system) it's much easier for someone to plug in a new relay versus replacing an output card. I don't know much about the Micrologix 1200 models though. I don't know if the relays in the cpu are socketed or not. That would make it easier but you would still have to disassemble it.

For ease of troubleshooting, you can swap a couple of relays in literally 10 seconds and rule that out. If you go the relay route, just make sure the sockets have diodes across the coils.

James Mcquade · Nov 10, 2015

jane101,

In my opinion, it would be better to go with a ml1100 or ml1400, you can do online editing.

in regards to the relay outputs, they typically begin to fail about 500,000 to 1 million cycles. They are only rates for 1 million cycles to start with. Also, the inrush current needs to be considered. I would use a triac output card for ac coils. use the dc equivalent for dc coils. I would also use surge arrestor terminal blocks as well.

regards,
james

jane101 · Nov 10, 2015

Totally see your point from a troubleshooting perspective and I agree. Relays are cheap.
However the 1200's run about $900. They are fairly standard with the 16 point relay out. Some modules have 7FETs and 8 relays meaning I would have to get an expansion module for more FETs. At a cost of roughly $300. Plus extra relays at a cost of $100. Plus the extra time it takes to wire (say one hour at $100) and the added time to draw into the schematics (autocad is not my strong suit).
Solenoids probably only run 50-100 times a day (very slow process) intermittently, throughout the year. Seems like alot of extra parts to fail, for something that is relatively easy and cheap to replace. Wish I knew the lifespans on those things.

jane101 · Nov 10, 2015

Oh hey thanks for the lifespan James!

BobB · Nov 10, 2015

Use transistor outputs to drive relays - make sure the relays have flywheel diodes across the coils. Much easier to change a relay in the middle of the night than a PLC - and much cheaper. Think about best practice and not 'as cheap as' - you only get what you pay for. I stopped using relay cards in the 1990s - you get a short across a solenoid and the relay output goes bang - rep[lace PLC - if an external relay only a relay - then 'as cheap as' is not. I hate it when people do not design to best practice!!!!! The extra cost to do it properly is chicken feed if a relay output blows up - replace the PLC - none in stock - have to go and get one - loss of production - all of a sudden 'cheap as' is not!!! Do it right the first time.

briancr · Nov 10, 2015

+1 BobB

just the cowboy · Nov 11, 2015

prewired relay boards for that purpose.

You can used one of these modules that has 16 relays on a din mounted board
http://www.automationdirect.com/adc...es/DirectLogic_205_PLC_ZIPLinks/ZL-RRL16-24-2

And connect it to the PLC with this cable

http://www.automationdirect.com/adc.../Connector_Cables_(Complete_List)/ZL-CBL24-1P
Over and done with $12 for a relay replacement.

jdbrandt · Nov 11, 2015

BobB said:
Use transistor outputs to drive relays - make sure the relays have flywheel diodes across the coils. Much easier to change a relay in the middle of the night than a PLC - and much cheaper. Think about best practice and not 'as cheap as' - you only get what you pay for. I stopped using relay cards in the 1990s - you get a short across a solenoid and the relay output goes bang - rep[lace PLC - if an external relay only a relay - then 'as cheap as' is not. I hate it when people do not design to best practice!!!!! The extra cost to do it properly is chicken feed if a relay output blows up - replace the PLC - none in stock - have to go and get one - loss of production - all of a sudden 'cheap as' is not!!! Do it right the first time.

And there is a hard lesson to be learned ... there is a difference between a relay output and a contact output card.
One has a relay contact with surge protection wrapped around it.
And that surge protection does/can have leakage current associated with it.

The way you can tell is to look at the specifation of the module, and see if it calls out leakage...if it does, you have a snubber circuit.

As other posters have commented, the use of a true relay contact output card is , in most cases, just not a good practice, if only due to wear and tear.

It is very common to see these type of cards in aged systems with one or more channels marked "bad", and software patches used to re-assign channels, along with the (hopefully) red-lined schematic diagrams.

Firejo · Nov 12, 2015

I’d use a MicroLogix 1100 so that you can connect via Ethernet and you can use free software to access it. As to the relay outputs, interposing relays is always the best way to go (in my opinion) if for no other reason than what has already been said, the relay’s can be replaced quickly and without having to program a new PLC. Relay’s will be the first point of failure (normally).

V0N_hydro · Nov 16, 2015

to be clear the diode or other means of surge suppression has to go across the solenoid coil not just the coil of the relay the PLC is controlling. Otherwise your interposing relay contacts won't be lasting anywhere near their rated lifespan.

Is running Solenoids off an AB Micrologix Relay output bad practice?

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