# Back feed on PLC outputs.



## Frank Mc (Nov 7, 2010)

Sparkee said:


> We have a machine that's being built that uses a PLC for auto mode but in manual mode it runs though relays and bypasses the PLC. I was testing it in manual mode and when I selected up mode the arm also went out and it should have just went up. when I looked at the PLC the outputs for up and out are on and when I removed either one of the wires on the selector switch works fine and the arm just goes up.
> 
> In manual the common hot for the PLC outputs is disabled but the outputs for auto and manual are the same. so my question is if one output on a PLC card gets 110 volts can it backfeed to another output and turn it on?
> 
> Thanks.


Hi 
If the PLC output card (assuming relays) doesnt bring each relay common out to a terminal then YES its possible to backfeed 110v...Its quite common to have one common point for several relays.....
HTH
Frank


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## Sparkee (Sep 22, 2009)

Frank Mc said:


> Hi
> If the PLC output card (assuming relays) doesnt bring each relay common out to a terminal then YES its possible to backfeed 110v...Its quite common to have one common point for several relays.....
> HTH
> Frank


Thanks Frank!

So if the common hot to all the outputs is disabled but one output gets 110 volts applied to it can that output back feed to another output that is also in an on state and turn on the valve it is connected too? 

The manual side controlled by relays instead of being run by the PLC is something I have never seen before.


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## lefleuron (May 22, 2010)

Boy,

I would not think its possible. Each output bit should be opto-isolated, and I just dont see a way for this to happen. I could be wrong, or the card could be messed up but then I think it would fault.

I would look for a mistake in manual mode, somewhere in the relay logic first.


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## Frank Mc (Nov 7, 2010)

Sparkee said:


> Thanks Frank!
> 
> So if the common hot to all the outputs is disabled but one output gets 110 volts applied to it can that output back feed to another output that is also in an on state and turn on the valve it is connected too?
> 
> The manual side controlled by relays instead of being run by the PLC is something I have never seen before.


Hi Sparkee

Yes it can backfeed to the common bus if the plc outputs are on ....Do you have an input to state auto/man...??? If so interlock it on all plc outputs so when manual selected ,none can operate....Hope that makes sense...
Frank
PS...Never seen a totally seperate manual setup like that either...


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## Frank Mc (Nov 7, 2010)

lefleuron said:


> Boy,
> 
> I would not think its possible. Each output bit should be opto-isolated, and I just dont see a way for this to happen. I could be wrong, or the card could be messed up but then I think it would fault.
> 
> I would look for a mistake in manual mode, somewhere in the relay logic first.


Hi lefleuron

I think the plc outputs have relays.....The opto isolation is before the relays..

Im thinking that when operating in manual mode some of the plc outputs are also operating ...if so this will allow 110v to back feed to the common terminal ,allowing other devices to operate if their respective plc outputs are on....
HTH
Frank


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## lefleuron (May 22, 2010)

Frank Mc said:


> Hi lefleuron
> 
> I think the plc outputs have relays.....The opto isolation is before the relays..
> 
> ...


 I took it that manual was an isolated system- completely without PLC. I am wondering why there is any voltage being sunk into an output card that is bypassed when in manual.

I may be reading the entire thing wrong.:001_huh:


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## Frank Mc (Nov 7, 2010)

lefleuron said:


> I took it that manual was an isolated system- completely without PLC. I am wondering why there is any voltage being sunk into an output card that is bypassed when in manual.
> 
> I may be reading the entire thing wrong.:001_huh:


I think the 2 systems are paralleled on the outputs ....

>I may be reading the entire thing wrong

Or maybe i am ;-)

Frank


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## Sparkee (Sep 22, 2009)

Frank Mc said:


> I think the 2 systems are paralleled on the outputs ....
> 
> >I may be reading the entire thing wrong
> 
> ...


You are right. The 2 systems are paralleled.


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## Sparkee (Sep 22, 2009)

lefleuron said:


> I took it that manual was an isolated system- completely without PLC. I am wondering why there is any voltage being sunk into an output card that is bypassed when in manual.
> 
> I may be reading the entire thing wrong.:001_huh:


Only auto uses the PLC and manual uses relays. But in auto it controls the relays that operate the solenoid valves and in manual spring return momentary selector switches control the relays that control the solenoid valves.


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## Sparkee (Sep 22, 2009)

Frank Mc said:


> Hi lefleuron
> 
> I think the plc outputs have relays.....The opto isolation is before the relays..
> 
> ...


I think you may be right because some of the outputs are still in their on state even though we are not using the PLC.


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## Frank Mc (Nov 7, 2010)

Sparkee said:


> I think you may be right because some of the outputs are still in their on state even though we are not using the PLC.


Sparkee

You need to have a plc input to say its in manual mode ...then in the plc program have a n/c contact of the plc input on every plc output that can be controlled manually... Also if in auto mode you should remove power from the manual push buttons.....

HTH
Frank


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## MDShunk (Jan 7, 2007)

I've done it on purpose on an old SLC 150-2, a newish SLC-500 relay card, and on PLC-5 relay cards. No problem. Pretty much for the same reasons as you... remote pushbutton stations that can do the same things as the PLC outputs. If you have any inputs to spare, the preferred method would be to put those remote pushbuttons on an input and put them in the logic instead. That's not always as easily done as backfeeding an output.


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## Sparkee (Sep 22, 2009)

Frank Mc said:


> Sparkee
> 
> You need to have a plc input to say its in manual mode ...then in the plc program have a n/c contact of the plc input on every plc output that can be controlled manually... Also if in auto mode you should remove power from the manual push buttons.....
> 
> ...


Thanks! It was wired so in auto the common hot on the pushbuttons has no power.


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## Sparkee (Sep 22, 2009)

MDShunk said:


> I've done it on purpose on an old SLC 150-2, a newish SLC-500 relay card, and on PLC-5 relay cards. No problem. Pretty much for the same reasons as you... remote pushbutton stations that can do the same things as the PLC outputs. If you have any inputs to spare, the preferred method would be to put those remote pushbuttons on an input and put them in the logic instead. That's not always as easily done as backfeeding an output.


I agree.


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