1734-OB8 with inductive loads

the_msp

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I am using AB point IO, specifically 1734-OB8, to drive a DC motor (I.n 1.22A @ 24V DC) via a set of relays; Phoenix Contact PLC-BSC-24DC/21.

Question: do I need to add any additional components to protect the PLC output card? Frequency of switching will be low, duration of motor running will be generally between 2 and 15 seconds.
 
Are you driving the coil of the relay with the OB8? If so AB usually recommends that you put a Diode on the coil of the relay.

In practice I normally don't add the diode if I am driving a simple relay.
 
Yes OB8 -> Coil, then relay contacts handle the motor current.

Are there relays you can get that have a diode built in? Only using this model of relay as it was a spare. Not that it's a big job to add 4 diodes, just for future reference if I was driving lots of relays.

Thanks
 
(I know for that relay socket you can get SSR's, but is there an electro mechanical relay, with built in... is 'surge suppression' the right phrase..?)
 
Phoenix contact makes a relay base that has built in diodes.

But I believe that these are more for suppression of the inductive load through the contacts of the relay itself and not for the relay coil.
(At least I dont think they contain both, its been a while since I held one in my hand)

Your OP was sugesting more for protection of the PLC output.

Anyway if you are interested you can look up:

Phoenix contact order # 2941374 (DEK-OV-24DC/24/DC/3)

It has been a while since I used these, but you can look up and see what the exact spec is, might be what you are looking for.

We use these on the solenoids that clutch in/out our main engines to the reduction gear box.


BCS
 
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Another thing to keep in mind:
If you have a Solid State Output like the OB8 does it can "leak" a little current. That leak current can cause a issue if the output is wired to small loads like PLC inputs. In that case a Suppressor is then used to increase the load on the output.

In my experience the leak current from the OB8 is so low that it wont cause most relays to activate. If it does you can order AB relays with internal suppressor to take care of that issue.
 
Ok, just got a moment to look these up and refresh my memory on them again.

Looks like they are OptoCouplers with freewheel diodes across the coil, and have built in suppression across the solid state output. So best of both worlds.

Part number I listed is for 24vdc coil and 24vdc output 3 amp max. (Price around $100 each)

Also looks to be avalible in 10 amp output and A/C versions also.

BCS
 
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I just looked up the Spec's for the OB8 module. I don't see a leak current specified so I wouldn't worry too much about needing a bleed resistor on the Relay.

The diode on the Relay Coil helps control voltage spikes caused by the coil's collapsing magnetic field (like the ignition coil on a old car). The only time I have had to use a Diode was when I was driving a 24VDC solenoid directly off the output. I don't think a small relay coil will cause any issues at all. But if you want the most protection a relay like BCS posted would do the trick.
 
Or if the $$$ are coming out of your pocket just use a general purpose diode across a standard relay coil if you are worried about your plc output. (cathode to +)
(This will be way cheaper)

We only use the ones I showed you because they are going to a critical system and the Optocoupler coil is being driven off of a Microprocessor, so if that output fails I cannot just change the output card (because there isnt one) or switch to a different output. (It is a propriatary system and we have no access to the programing)

BCS
 

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