Dry Contacts

[mityeltu]

I am a fairly new graduate and am now working in manufacturing. I am working on a project to interface my PIC project to the main plant. The PIC is, of course, running on 5vdc, and the plant is running a variety of voltages.

The PIC has its own supply, so this is not a problem, but the interface between the plant and the pic is going to require some relays to electricaly isolate the devices.

I keep hearing my boss mention dry contacts.

My research has produced 2 very similar definitions: 1)contacts that carry no current, 2) contacts that are not connected to a power supply.

Here's the question: what EXACTLY are dry contacts? I mean I know they are contacts on a relay, but are they in use but carrying no current? Or are they not in use at all and can therefore be used for ANY purposen (assuming proper ratings for the function)?

 

[TxEngr]

You pretty much have a grasp of the idea. A dry contact is a contact which has no internal power, e.g. it is isolated from the power system of the controlling device. It is necessary for you to provide the power to one side of the contact to power the device being controlled on the other. Dry contacts will have an amperage rating, often 10 A, which tells you the maximum current the device can carry and be expected to operate properly.

Outputs from a plc can be isolated, non-isolated, and dry contact. There is a difference in isolated and dry contact. The isolated outputs use external power to control the device, but are solid state switches. A Dry contact is not solid state but is a true contact closure.

 

[mityeltu]

Thank you for the reply and the boost to my confidence.

Are dry contacts rated differently from "standard" relay contacts? It seems to get fuzzier the more I think about it.

+24vdc ------- { coil 1 } ------- gnd

120vac -------- {contacts} ------------ { coil 2 } ------- adiitional 120vac load

+5vdv -------------------------------------{ contacts } ---- PIC MCU

If this is the circuit I'm really talking about, to me there are no dry contacts as all of them will be carrying current and/or have a potential across them (the pic might be able to be assumed to be od such high impedance that the current is effectively nil - I know this is not really the case)

Coil 2, to me is an interposing relay strictly for electrical isolation of the various levels. My boss is calling the contacts on this relay dry contacts. Is this correct? I don't really see the difference other than the voltage level. I can see that if the relay had additional contacts that were not being used in ANY circuit that they could be considered dry, but it seems to me that if the contacts are in circuit, then they are not dry. Am I missing something?

 

[GeorgeB]

My research has produced 2 very similar definitions: 1)contacts that carry no current, 2) contacts that are not connected to a power supply.

TxEngr gave an excellent answer to the "what are they" question, and it applies to your definition 2.

Your definition 1 isn't quite how I would have completed the dictionary definition. Contacts (relay or other) with "dry circuit ratings" are designed to switch very low currents and voltages. They were used by the phone companies for analog switching before computers. I use them frequently when I have to switch analog signals (+/-5V or +/-10V commonly) into analog inputs on electronic boards with high input impedance.

Take an example ... 0.1V into 50k ... what's that, 2 microamps?

A-B and SquareD used to offer "logic reed" blocks for the industrial pushbutton; haven't looked for a LONG time.

 

[Jraef]

You are over thinking it a bit.

"Dry" just means there is no direct electrical connection between the controlling circuit and the switched circuit.

So in your case, the controlling circuit may be the TTL level voltage in the PIC itself (although not likely) and the switched voltage is the 120VAC. There is a mechanical connection between the control voltage and the switched voltage in the form of a coil, but no electrons are exchanged.

The reason I said "unlikely" is that the +5V TTL circuit doesn't likely have enough power to energize a coil. So you would have your TTL fire an opto, which uses a higher voltage level (i.e. 24VDC) to energize the relay coil, which then switches the 120V. The opto is isolating the +5 from the +24 as far as energy states, but they share the same common, so technically they are not totally isolated (wet circuit?). But the 120VAC going through the contacts of the relay is not sharing anything in common with the control circuit.

 

[Open Neutral]

"Dry" just means there is no direct electrical connection between the controlling circuit and the switched circuit.

That's the story, right there. I've always seen it applied to small-signal, mechanical relay, contacts. But I suppose it MIGHT mean an equally isolated SSR. And that 500A contact rated at 480V is just as "dry" but no one calls it such.

Both are dry, until YOU wet them with external supply.

I suspect the origin of the term is from telcom lore; where "wet" likely referred to the -48VDC lead acid battery supply that ran everything.

Now, to get REALLY confused, there were 'wetted' contacts. Some relays were in sealed containers with mercury inside as well. The mercury coated the contacts and ensured excellent low contact resistance. I doubt such are still made.

 

[tkb]

Just think of a dry contact as a switch.
They are rated for various voltages and currents.

----||----
This is usually the symbol.

 

[realolman]

That's the story, right there. I've always seen it applied to small-signal, mechanical relay, contacts. But I suppose it MIGHT mean an equally isolated SSR. And that 500A contact rated at 480V is just as "dry" but no one calls it such.

Now, to get REALLY confused, there were 'wetted' contacts. Some relays were in sealed containers with mercury inside as well. The mercury coated the contacts and ensured excellent low contact resistance. I doubt such are still made.

I would not call solid state relays dry contacts.

I think what you stated about the mercury relay is the difference that makes dry relay contacts 'dry"

regular relay or "dry" contacts will also "bounce" or open and close several times on one closure... this can and will be picked up by electronic equipment and can cause several counts on one closure.

IMO dry contacts would refer to the contacts of a regular mechanical relay, as opposed to a solid state relay, and have nothing to do with whether it was powered or not.

 

[zaptd]

Alarm panels have dry contacts. One two or three sometimes. When the panel goes into alarm, I used this relay to close my simple 4v fire smoke loop at a burg panel. This panel could also throw another relay on trouble only and another on a supervisory signal. So in this case, it is "dry" because i am not powering this relay, just using it's function at the FACP. Otherwise I would have to use the a voltage created upon alarm and use it to power the coil on a relay I provide the close my loop. I hope this cleared your confusion,

 

[jeremysterling]

I am also interested in this terminology. My boss refers a lot to "form C" contacts on relays. This is when you have a common and NO/NC poles with a single throw (like a residential three-way light switch).

Would "dry" contacts be the same as "form A"?

 

[zaptd]

I am also interested in this terminology. My boss refers a lot to "form C" contacts on relays. This is when you have a common and NO/NC poles with a single throw (like a residential three-way light switch).

Would "dry" contacts be the same as "form A"?

Yes. Relays have a C,N/O,N/C

 

[david luchini]

Would "dry" contacts be the same as "form A"?

No. "Dry" contact and "Form A" contact are not the same thing.

A "Form A" contact is a normally open contact. A "Form B" contact is a normally closed contact. And A "Form C" contact is, as you mention, a NO and NC with a single throw.

Any of these forms could be "wet" or "dry."

 

[david luchini]

Thank you for the reply and the boost to my confidence.

Are dry contacts rated differently from "standard" relay contacts? It seems to get fuzzier the more I think about it.

+24vdc ------- { coil 1 } ------- gnd

120vac -------- {contacts} ------------ { coil 2 } ------- adiitional 120vac load

+5vdv -------------------------------------{ contacts } ---- PIC MCU

If this is the circuit I'm really talking about, to me there are no dry contacts as all of them will be carrying current and/or have a potential across them (the pic might be able to be assumed to be od such high impedance that the current is effectively nil - I know this is not really the case)

Coil 2, to me is an interposing relay strictly for electrical isolation of the various levels. My boss is calling the contacts on this relay dry contacts. Is this correct? I don't really see the difference other than the voltage level. I can see that if the relay had additional contacts that were not being used in ANY circuit that they could be considered dry, but it seems to me that if the contacts are in circuit, then they are not dry. Am I missing something?

Your boss is right. The contacts are "dry" because you had to provide a separate voltage source to them, 120V to the first, and 5V to the second.

If you change your schematic like so:

+24vdc ------- { coil 1 } ------- gnd
.............|
..............---- {contacts} ------------ 24V load
.............|
..............----{ contacts } ---- 24V load

Then they would be wet, as the voltage source is already applied to them as part of the relay.

 

[benaround]

The term 'dry contacts' is just a generic name, a single pole switch could be a dry contact

Dry contacts can be limit switches, ansul system switches, in relays, in solid state

equipment, in alarm panels, fire alarm panels, used to control dampers, the list goes on and

on. The thing that dry contacts all have in common is, you need to supply the power to

them for whatever is being switched.

 

[Jraef]

I am also interested in this terminology. My boss refers a lot to "form C" contacts on relays. This is when you have a common and NO/NC poles with a single throw (like a residential three-way light switch).

Would "dry" contacts be the same as "form A"?

Any set of contacts that is not already otherwise powered is a dry set of contacts. The corm, NO, NC or Form C, is not related. If you take a Form C set of contacts and tie the Common to the same source voltage as the control circuit, it is no longer considered "Dry".

And this is a relative term as well. If, let's say, I have a 24VDC control circuit and I am switching two sets of dry contacts, once I apply a 120VAC power source to one set it is no longer "dry" in relation to the other set; it is now part of a control circuit, just a different control circuit.

Form has to do with the operational state, dry has to do with the energy state. Apples and oranges.

 

[IMM_Doctor]

My guess is the term "dry" contacts started out as slang or laymen speak, and was used so frequently, that it is used by manufacturers and in data sheets as well.

Europeans use the term "Potential-Free" contacts, (Voltage-Free).

A realay is a great device for communicating digital logic status between two pieces of equipment, and having no actual electrical connection between them. The binary logic is conveyed through the use of mecahnical movement.
The source machine applies it's power to the coil of a realy turning it into an electromagnet, the electromagnet then moves an armature (contacts) from one position to the other. The slave machine provides it's own interrogation source voltage to the potential free contacts to determine if the contacts on the relay are open or closed. Ones, and zeros can be conveyed, yet there is no electrical continuity between the two isolated voltage systems of the two seperate machines.

In industry, this is very common having a robot next to some stand alone automaton equipment. There is typically a bank of relays at some point between the robot and the machine. Half of the relays will be controlled by the source machine, the other half of the relays will be contolled by the robots control system. Then each machine interrogates the other machines status via the DRY (potential-free) contacts of the other machines relays. These relays are also know as signal INTERPOSING relays.

There is a solid stated equivilent of this device, it is called an "Opto-Isolater". It is basically a Light Emmiting Diode, and a Light Activated Transistor, closely coupled into one smal package. The sorce machine illuminates the the LED which stimulates the gate of transistor, and causes electrical continuity of the ISOLATED interrogated transistor. Again two machines with different voltage systems without any electrical continiuity between the two. Binary signal information is sent via LIGHT.

 

[dbuckley]

I suspect the origin of the term is from telcom lore; where "wet" likely referred to the -48VDC lead acid battery supply that ran everything.

Exactly correct.

Normal telephone lines (POTS) are wet, as they have the "central battery" (often abbreviated to CB) powering them.

If you want to put an extension to a building's private exchange (PABX, PBX, PMBX, whatever terminology you like) in another building, the telephone company will supply a "dry pair" for you, which is literally just a twisted pair copper path.

Dry pairs have lots of other uses, including low cost inter-building data.

UK readers will recognize a dry pair as a BT EPS9 circuit. Or two of them as an EPS8.

 

[mityeltu]

I think I get it. Thank you for all the useful/helpful information. Let me see now if my explanation makes sense to you guys:

If I have a set of "wet" contacts, when I remove the driven circuit, I will still have a potential difference between the contacts.

ex:
120vac---------------------||---------------- driven circuit
48vdc -------------------{coil}-------------- gnd

When the driven circuit is removed I will still show 120vac on the contacts, right? So these would be "wet" contacts (I have seen some of the Hg contacts in the past, but I do not believe this is what my boss is talking about).

With dry contacts, the current *through* the contacts is supplied by the circuit *on* those contacts. Not sure if my "drawing" will look like what I mean.

ex:
|--------------- circuit with power ---------|
--------------------------||--------------------

48vdc ----------------{coil}---------------gnd

Now, when the powered circuit is removed, whatever potential difference existed while the circuit was connected is also removed leaving "dry" contacts.

Is this a decent explanation of what we are talking about here?

Thanks again.

 

[rcwilson]

You got it!

"Dry" means I can connect my device to your contact without goofing up either system. (Assuming contact amp and voltage ratings are sufficient.)

 

[realolman]

If I have a set of "wet" contacts, when I remove the driven circuit, I will still have a potential difference between the contacts.

ex:
120vac---------------------||---------------- driven circuit
48vdc -------------------{coil}-------------- gnd

When the driven circuit is removed I will still show 120vac on the contacts, right? So these would be "wet" contacts (I have seen some of the Hg contacts in the past, but I do not believe this is what my boss is talking about).

.

Why don't you ask him what he is talking about?

This explanation certainly would not be what I meant by dry contacts, and although it may be phone company slang, I have never heard of wet contacts, except for mercury wetted contacts.

I think everyone should actually try to limit their use of jargon.