On/Auto/Off - how to?

Well I'm trying to save some input and the way I see it, I need at least 2 input per switch to be able to have an On/Auto/Off option.

Am I missing something?

Depends on what each switch does. If any one of your switches not being in auto pauses a sequence, wire all the auto's in series - then you have one input which tells you "all switches are in auto" or "one of the switches is not in auto".

If your "on" state is a "hand override"... can you hardwire the switch to what it's controlling instead of having the PLC relay this state?

Well I'm using a GE 90 Micro, 24v input, 120V relay output.

The reason for the ON is overide, Auto beeing controled by the PLC. I'm using small ON/OFF/ON switch, and I would have prefered not to have 2 different voltage on the same switch.

I could put 24v on one side and 120 on the other...

the_msp said:

Depends on what each switch does.

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This is true. In most of the cases that I work with, the HOA switches are connected to motors. The Auto position is the only one wired to a PLC input. The Hand and Off positions are hardwired into the starter circuit. This has the added benefit of allowing manual control to work even in the event of a PLC failure.

You would need DPDT's for that - not SPDT's (for 2 voltages). Or use a SPDT, and use the 24V to signal the PLC for auto, and pull in a relay for the 120V override?

I have DTDP switch (ON/OFF/ON toggle) and all output of the PLC are wired to 120 relay to switch on small powers.

I'm building this PLC to automate my Reef Aquarium. A nice little project.

I'll see if I can hardwire 120v to one side of each switch and an send a signal to the relay. The only problem I see with this is the switch would have been better as ON/ON/OFF, On/Auto/Off.

jimbo3123 said:

This is true. In most of the cases that I work with, the HOA switches are connected to motors. The Auto position is the only one wired to a PLC input. The Hand and Off positions are hardwired into the starter circuit. This has the added benefit of allowing manual control to work even in the event of a PLC failure.

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I burn up 2 inputs per HOA just so the PLC knows the position of the switch. In your example the PLC doesn't know whether it's "off" or "hand".

Why do you need the HOA, what is your goal? Perhaps there is another way of doing it.
Does the PLC have, to have feedback? What happens if it's ON, does it matter to the PLC?

muusic_man said:

I burn up 2 inputs per HOA just so the PLC knows the position of the switch. In your example the PLC doesn't know whether it's "off" or "hand".

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Sure it does. If it's not in auto, the PLC does not enable the RUN output. If the motor is running and the switch is not in Auto, then it must be in hand mode.

jimbo3123 said:

Sure it does. If it's not in auto, the PLC does not enable the RUN output. If the motor is running and the switch is not in Auto, then it must be in hand mode.

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Disagree.
That logic does not hold water. In an industrial environment never assume that kind of thing. In the OP's case for his home project it might be ok.

Mickey said:

Disagree.
That logic does not hold water. In an industrial environment never assume that kind of thing. In the OP's case for his home project it might be ok.

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Sure it does. The only cases in which it could fail are a hardware failure or a change to the hardwire circuit. Neither of those cases can ever be programmed around. Unnecessarily complicating your design with extra inputs will only increase the probability of problems.

Obviously this is one of those cases were the implementation is key. The purpose and requirements for the device connected to the HOA switch should influence how its control circuit is designed. However, the simplest design is usually the best.

We call them Hand-Off-Auto switches. I've done them two basic ways, both require a contact for Auto and Hand.

First way is all PLC, no bypass function. Wire Auto and Hand as inputs to the PLC. Auto means the normal logic is in control. Hand means run the pump, now. No Auto and no Hand is Off. Safety interlocks active in both modes, process interlocks may be active in Auto only. Used to manually pump out tanks below low level interlocks etc. Just did a project like this with all the H-O-As on ASi, worked pretty nice.

Second way is having the Hand mode independant of the PLC. I'm seeing less of a demand for this, but it's still out there. Here the Hand contact is wired in parallel with the PLC output. In Hand it runs regardless of the state of the PLC. A second Hand contact may be wired into the PLC to report status on the screen. Auto works the same as the first way.

In the case of your project, you can have all 24vdc on the switch. Wire the Hand contact in parallel with the PLC output, it will pull in the same relay that the output does to control the 120vac pump.

Edit: I see I misread your PLC output. Still, it's a good point in favor of using 24vdc output cards with separate relays to keep your 120vac wiring separate.

On the subject of "do you have to wire the Hand contact as an input"... If you are going to indicate the position of the switch on the screen, then IMO YES. You could fudge this indication as Jimbo3123 suggests, but his way relies on motor running feedback which we may not have for a 120vac pump. In an industrial situation, motor running feedback is common. However, without feedback on the Hand position, there is no way to report a failure of a pump in Hand. The indication is flakey too because the Hand indication only works if the pump is running and not in Auto. If it's in Hand and not running, it says the switch is Off, which isn't helpful if the pump is tripped out.

I've got a GE 90 micro PLC with 21 input 24v, 16 output 120v. I'll use probably 16 input alone for control so I can't double the 16 number to 32 for H/O/A.

I guess I'll wire a 24v to the input and a 120v to the relay on the switch.

We just use this small interface relay with an AOM switch built in and also has a feedback contact (13,14) in case we need it. We use it all the time instead of a normal interface relay and we don't have to use the AOM switch in the case the PLC stops working. It is a PLC-RSC-24UC/1/S/H from Phoenix Contact.

jimbo3123 said:

Sure it does. The only cases in which it could fail are a hardware failure or a change to the hardwire circuit. Neither of those cases can ever be programmed around. Unnecessarily complicating your design with extra inputs will only increase the probability of problems.

Obviously this is one of those cases were the implementation is key. The purpose and requirements for the device connected to the HOA switch should influence how its control circuit is designed. However, the simplest design is usually the best.

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Notice right off the bat you said "the only cases". When I program, I do so with those "cases" in mind and try to do everything to limit "the only cases". I always want to know the position of that switch for equipment and personnel safety. We all do things differently, and this is my method. That's all I'm saying.