Relay getting stuck closed 1796-OW16 module

[sh281986]

I have a relay output card that keeps having issues but with different relays. Every couple of weeks or so, an output relay will energize but will not de-energize. I'm not really given enough time to troubleshoot the machine so I just put another module in to keep it running for a few more weeks until the issue inevitably happens again. The OTE in the program shows that it should not be energized but it is. The output terminal shows 24VDC and that reading never goes to zero when the output signal should be off. I checked the incoming voltage and its around 24vdc. All of the connections are tight as well. Does anyone have any advice for me on why the relays keep failing? Thanks.

 

[Steve Bailey]

I think the first thing to check would be the current draw of the loads controlled by the outputs, both the holding current and the inrush. If you're switching inductive loads, you'll improve contact life by adding surge suppressors. Relay contact outputs are generally rated by holding current capacity and don't have as much margin for inductive 'kick' as solid state outputs.

 

[sh281986]

The relays energizes 24vdc coils which opens a solenoid valve. Would these require surge suppression?

 

[Steve Bailey]

I'm going to assume you mean 1769-OW16 which are rated for 2 amps continuous at 24 VDC. The coil on a solenoid valve is definitely an inductive load and needs suppression using an appropriately sized diode.

 

[sh281986]

Ok. So I need to measure the output current and see if it is going over 2 amps. Thank you.

 

[Steve Bailey]

you should really look up the rating of the solenoid coil and based on that select a flyback diode to install in parallel with the coil.
https://en.wikipedia.org/wiki/Flyback_diode

 

[sh281986]

I was looking at the manual for that module and the schematic shows that it has a diode already in the circuit. PG. 109.
https://literature.rockwellautomation.com/idc/groups/literature/documents/td/1769-td006_-en-p.pdf
The coil is rated at 25 Watts for 24VDC.

 

[Saffa]

The diode shown on the page in that manual is across the cards internal relay coil, not the output terminals.

25W solenoid is a big boy. I would personally use an external interposing relay to drive the solenoid and fit that with a flyback diode across its contacts, as Steve Bailey said.

I normally only deal with 8W solenoids at 24VDC that cycle probably once every 10-30 seconds. We would destroy 6A relay contacts within 9-12 months. Replaced them all with solid state relays and fitted flyback diodes, no issues for the last 3 years.

 

[Steve Bailey]

That diode is across the relay coil on the module. It is not in the load circuit.

 

[Skidood]

We have used little varistors for years with no problems on 10W 110VAC coils for solenoid valves.

 

[James Mcquade]

i see two issues that you have.
1 the relay outputs have a lifespan, typically 600,000.
how often is the output cycled?

2. you need to put a reverse bias diode in the load side of the output to keep the reverse voltage from basically arcing and welding the contacts.
a 25 watt / 24 volt solenoid draws 1.04 amps once it is energized in a steady state. BUT, in my experience, the surge current can be 6-10 times that or 6.25 amps if it were 6 times the steady current.

you might try a solid state output card if they make them, and also add the reverse bias diode.
regards,
james

 

[Rob...]

As above, I would recommend a small interposing relay installed. They're a couple of $ a piece and far cheaper an easier to replace if they end up burnt out than an output card.

Secondly to monitor that the output isn't being driven on an off in rapid succession. Chattering an output on an inductive load will shorten the life span dramatically.

 

[JesperMP]

The relays energizes 24vdc coils which opens a solenoid valve. Would these require surge suppression?

Yep. These can be some of the worst. Look at the size of the solenoid. If it is big, it will be a large induction.
Just be used to add surge suppression to any large coil, that be relay, contactor, valve or whatever.

 

[JesperMP]

2. you need to put a reverse bias diode in the load side of the output to keep the reverse voltage from basically arcing and welding the contacts.
a 25 watt / 24 volt solenoid draws 1.04 amps once it is energized in a steady state. BUT, in my experience, the surge current can be 6-10 times that or 6.25 amps if it were 6 times the steady current.

I have never seen a socalled reverse bias diode on a PLC output.

Also, it is more the induction that is the problem, not only the current value itself. Think of the current flowing as a wheel spinning and the induction being the mass of the wheel. A light wheel can be stopped by a person grabbing the wheel with the fingers whereas a heavy flywheel can shear the teeth of gears if you try to stop it instantly.

A 'freewheeling' diode for DC coils is like a directional ratchet clutch. The current will flow until the energy as dissipated in the same a wheel would spin until stopping by internal friction. So that can mean that the turning off will take longer time.

An RC snubber is like sticking a rubber plug in the spokes of a wheel. The stopping will be more violent, but essentially instantaneous and less violent than just stopping directly. Works with both AC and DC.

A varistor is like a friction clutch. Somewhere between the two above. Works with both AC and DC.

 

[dwoodlock]

Any reason he couldn't add small interface relays between the output and coil?

This is typically what I would do although depending on the number of inductive loads the relays could be more expensive.