Solenoids - Sink or Source them with a relay output?

Was working on a machine today with a Fanuc 90-30 with some relay cards when I discovered they used the dry contact relay card to sink the hydraulic solenoid valves.

I see no difference/advantage between sinking them or sourcing them with the relay card.

Anyone have a difference of opinion?

There is a difference in semiconductor material properties. Electrons have more mobility than holes, so it is more efficent to sink loads.

Just saw that these are dry contacts. The disadvantage is that you have hot field devices even with an output off. Someone was used to electronics when they built that.

Sourcing outputs requires less power to operate.

sinking outputs require more power to operate.

since this is a ge, and my memory is correct(it's been a good while)

the sinking card type means that the power is supplied from the card and the load sinks the power to ground which is totally opposite if you are used to AB type cards.

if I am incorrect, someone please correct me.

james

We can't answer your question without more information.

Which side, if any, on your power supply is tied to earth. If neither side of your supply is earthed then unless you have an earth fault sensor you have a dodgy system. Search here for 'Tying 24VDC- to Ground' for a lively discussion.

Which most likely failure mode do you prefer, solenoid on or solenoid off.

The most common electrical fault is a cable going to chassis and hopefully to earth. Draw out your two connection methods and think about the consequence of that fault.

Sinking or Sourcing shouldn't be choice that you make off the top of your head, your site should use one method or the other and all machines on site should stick to that system.

sparkie said:

Electrons have more mobility than holes

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It's a good one

BryanG Has the right idea IMHO. As this is NOT a semiconductor circuit any discussion of semiconductor sink/source discussion is mute.

The problem I saw:
1) Very uncommon method - ie. Not a best practice. Unexpected operations or methods are the #1 source of hazards/ No clear advantage to stray from best practice.
As stated by BryanG:
2) Hazard of actuation of the device in the case of short circuit on the return 24VDC common (NOT ground or earth).

I also note that in a perfectly isolated DC power supply system, grounding or earthing either the supply or common should result in no current flow. However due to additions and poor wiring practices, devices connected that mixed their power suppplies with the system power supply, breakdown in isolation insulation or improper grounding, this can be compromised and result in current flow to earth.

Now BryanG also asked which side of my 24VDC is grounded. Which brings up the discussion of grounding DC control power supplies. If you ground the DC power supply, then common is no longer common. If you have local code regulations that state this is necessary, please reveal that. As this is low voltage, I doubt there is any code to follow so then it becomes a design/methodology discussion. How many people ground their DC control power supplies and why would you do that?

Thanks for all your feedback people. Anyone in disagreement?

A pretty authoritative article on 24VDC power supply grounding to the same ground as AC (not).
https://www.ecmweb.com/content/resolving-grounding-issues-switch-mode-power-supplies

In Europe we do have code that says that one side of the output of a transformer or power supply must be earthed. I don't know enough about the US code to comment. I can say that a system left floating is potentially dangerous unless there is also an isolation monitoring system.

The article is interesting but I wouldn't call it authoritative, how can you have two things that are called Ground but are electrically isolated from each other. I can to some extent agree that you could run two earth cables from a panel back to a common ground point, which may reduce the chance of high frequency interference, but the grounds still aren't isolated or separate.

In short, it's possible to design a safe system with true isolation between AC and DC and two separate grounding systems.

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????????

However, in no way should you connect the AC ground to either the positive or negative connection of the 24VDC power supply.

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European regulations state otherwise.

In a 3-phase, 4-wire layout, the sum total current (including all harmonics) flows through the neutral wire — and sometimes the ground

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He doesn't say how the harmonics jump from the Neutral to the Ground.

Imagine the implications when 24VDC analog and digital control signals are referenced with heavy-duty 480VAC loads.

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That would be bad, but a common earth system does not cause this to happen. A floating system with an undetected fault can and does cause this to happen.

Please don't base your decision as whether to earth one side of a power supply based on this article, study some more.

BryanG said:

In Europe we do have code that says that one side of the output of a transformer or power supply must be earthed. I don't know enough about the US code to comment. I can say that a system left floating is potentially dangerous unless there is also an isolation monitoring system.

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Well, at least here in Sweden you dont have to but it is recomended.
When you have a floating DC system you must have 2-pole fuses and ground fault detection though. It is fairly common in process plants etc here.
Even unearthed ac-systems exist and are great when used properly.
Check it-grounded system on google for more info on that.

BS-EN60204-1 Safety of Electrical Machinery

6.4 Protection by the use of PELV
6.4.1 General requirements
b) one side of the circuit or one point of the source of the supply of that circuit shall be connected to the protective bonding circuit.

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If you don't do this then your aren't creating a PELV circuit. You may be creating an SELV circuit but that requires different design considerations and careful separation of high and low voltage circuits and wiring.