Potential Free vs Powered Contacts

Potential free when the receiving/controlled device is supplying the power; powered otherwise.

We dont follow this procedure. We give IO list to system vendor at the start of the project.

I am not describing a procedure. I'm making a statement of fact.

reply was meant for post by PWSlack

Then it's in the wrong place.

If the System Integrator is doing the schematic drawings then the answer is "when the system integrator deems it appropriate to do so to effect correct circuit design".

The information will be contained in the Wiring Schematic Drawings that are being produced in able to prepare the I/O lists.

This is a design function.

Powered contacts are useful if the device being controlled needs some voltage input to perform its function. The contacts are just "switches". They either have power already applied on one side of the switch, or the device to be "switched" has power already on it, and this is to make it go, or stop, as the case might be or not.

As others have said powered contacts can be useful, but in general volt-free are preferred because they can connect to any receiving device (subject to voltage and current limits, naturally). Can be changeover contacts, often useful. Powered contacts only give a specific voltage. Also you can use volt-free as powered by supplying volts to one connection, but not vice-versa.

What's best in your case depends on the details of the kit.

I have a feeling, that you are not understanding the basics.

A potential free contact (which is actually two contacts), is for example the contacts of a relay, that are "controlled" by some circuit as when they are either open or closed, but are only physically mounted on say the PCB or in the same box, but do not have any electrical connection to the voltages/currents of the controller circuit.

The usual reasons for having them is often due to keeping contrl circuitry from causing "interfernce" in the "controlled" circuit.

See here:-

You can see clearly that the transistor circuit, on the left, has only a "magnetic" effect on the state of the two potential free contacts on the right. The dotted line shows this magnetism in a basic way.....

I hope this helps.

Nice picture, but I would assume OP knows the difference, or he would have asked that, rather than just when to use one or the other. And you only show volt-free, not how powered contacts go (not that it's complicated) ��

What you referred to as "flywheel" diode, is what us Texans call "flyback" diode. Now which is it in fact? Can it also be a capacitor series resistor (as some have told me)?

Inquiring minds want to know, good friend...

It's often called a flyback (or sometimes catching or clamping) diode, among other names, over here. Maybe Andy's been in Germany too long!

The advantage of a diode over RC networks here is that it makes sure that the top end of the transistor can't go above (supply rail + 1 diode drop).

Quite right.

I just found that on the IoT, I did not notice the slight error, but we all know what it does, and the original question was not about that side anyway....

"Transient suppressing" or "clamping" diode.

The former for reports, the latter for the shop.

Florida speak.

Thanks for the Florida speak. That beats California speak any day, in my book!

Thanks Andy for the explanation. However I do understand how potential free contacts work. For few signals I don't understand whether to use pot free or powered contacts, may be from where its getting powered isn't clear. Will discuss specific examples.

Msn,

Where I am, potential-free contacts are often called "dry contacts". They can be in the form of relay contacts, optically-isolated transistors or triacs or similar electronic contacts, mechanical switches (such as float or pressure), and similar. Powered contacts often are in the form of 2-wire (or 3-wire) proximity switches or photo-eyes, and draw their operating power as a parasitic load on the circuit. In this form they have an off-state leakage of a few milliamps and need a minimum voltage to function. Depending on the device they are wired to, this parasitic leakage can be sensed as an ON condition, so careful design is needed to ensure proper voltage to the equipment as well as its sensing parameters.

In the design of equipment, you have to pay attention to proper separation of signals so you do not have inductive or capacitive interactions giving false results. Also you have to be aware of code and safety rules that require separation of power and signaling circuits.

I don't have a link or document for you, but I suggest you look for the application details typically provided in the printed or on-line literature of many manufacturers. In this, you will get a discussion of when to use one type of sensor and when to use another.

One other thing--operating the sensing circuits on AC has a significant limit on speed of sensing the contact's change of state because of the 50 or 60 Hz frequency with the circuit's voltage going through zero and to a peak at 100 or 120 times each second.

--JMM

Some One other important point:

1. Do not use a solid state relay meant for 110V AC to switch DC voltage. Sometimes the lack of a zero crossing means that it will not "turn off".

I have heard a different meaning for "dry contacts". If I have a relay interrupting power to something (even if it is just another relay coil) I can use normal relay contacts (appropriate for load). If relay is interrupting very low power (like a logic input or an audio signal) I must use a relay with dry contacts (usually a reed relay) because normal relay contacts depend on power to remove slight oxide coating. If I use a normal relay in very low power circuit it often fails after a few months for this reason.

query about MCC related signal.

When I am giving Digital Output from DCS to MCC to start a motor, actually I am energizing a relay in interposing relay panel. So this Digital Output is powered.

Whereas when I am getting Run indication then MCC is energizing relay and its potential free contact for me.

Is my understanding correct or not?

You know, the run confirm contact on an MCC motor contactor might be potential free, but I am not sure if all of them are. I am sure it does transition from high impedance to low impedance upon starting the motor.

Is the (powered) Digital Output from the DCS to the MCC relay the voltage that the relay requires? If not and you have the option of volt-free contacts, you can supply one contact with the correct voltage and wire the other to the relay (see my #8). Are the DCS digital outputs powered or volt-free, or can you choose (or specify to the supplier)? What does the circuit diagram indicate?

Whereas when I am getting Run indication then MCC is energizing relay and its potential free contact The way I see it is you need to supply one terminal of the volt-free with voltage suitable for the DCS digital input, and wire the other back to the DCS digital input.

this is the way I m going to consider contacts.

Maybe.

Your first statement regarding energizing a relay appears to be correct, that is a powered output circuit. It is not, however a powered relay contact circuit.

Your second statement is an example of an un-powered (dry) relay contact, where you must provide the power to create a action.