What exactly are dry contacts

[mjc1060]

Hi,
I often hear the term dry contacts with relays. I have heard several explanations that are conflicting can anyone explain this word to me.
MJC1060:?

 

[iwire]

I always understood it to mean not a source of power, just a switch.

 

[Sahib]

In other words these are potential free contacts.

 

[sparkycoog]

Hi,
I often hear the term dry contacts with relays. I have heard several explanations that are conflicting can anyone explain this word to me.
MJC1060:?

In my usual line of work, dry contacts are contact closures. Just think of it like the security system magnets which are normally closed and as soon as it's opened, the connection between the two wires is lost which prompts a relay of some sort to take action. Handy little buggers.

 

[kingpb]

According to IEEE 100 (Dictionary)

"dry contact is a contact through which no direct current flows."

I'd say they could have expounded on that a bit.

A dry contact is where the power comes from an external source. A good example is an MCC, where each bucket has its own control power transformer. To start and stop the motor external to the MCC you would use a dry contact (like a switch) because the control power is in the bucket.

 

[480sparky]

According to IEEE 100 (Dictionary)

"dry contact is a contact through which no direct current flows."
.........

What about alternating current? :lol:


OK, so if no current flows, how does a contact DO anything?

 

[GeorgeB]

I am used to it meaning without a source of power included; specifically, a dry contact will also have both terminals individually available. I do not consider devices with relay contacts where one side of a group (commonly 2, 4, or 8 as used in many PLCs) is common to be dry contacts, although the group could be used as a SINGLE dry contact. Dry contacts may be either NO or NC.

Dry contact is different than "dry circuit rated". Dry circuit contacts are suitable for current flow approaching zero and are frequently used for switching analog voltage signals into a high input impedance device. They may be, and often are, also dry contacts.

 

[mjc1060]

Thank you that was very helpful

 

[Besoeker]

I always understood it to mean not a source of power, just a switch.

That's what would take to mean to.
Sometimes referred to volt free.
Available just as a relay contact for external use.

 

[RichB]

What about alternating current? :lol:


OK, so if no current flows, how does a contact DO anything?

i use them to close a circuit consisting of a coil of wifre inside another coil to change the inductance of the second coil for the signal to a controller for vehicle counts, etc.

 

[GoldDigger]

Dry contact is different than "dry circuit rated". Dry circuit contacts are suitable for current flow approaching zero and are frequently used for switching analog voltage signals into a high input impedance device. They may be, and often are, also dry contacts.

A very good point, and one which is directly relevant to the OP's use case in which there will be very low voltage and current through the contacts.
A reed relay with gold contacts would be one likely way to go for this application.

But that also brings up the question of whether rapid contact bounce will cause a miscount in the OP's detector or whether there is some filtering on the detector input or output.

 

[hbiss]

I'll make it easy for you all. You go to the supply house and purchase a wall switch. Doesn't matter if it's SPST, 3 way, 4 way. You have just purchased a switch with dry contacts.

The term "dry contacts" simply means a switching device whose contacts are not connected to anything thus making them available for whatever the installer wants to do with them.

There is also the term "wet contacts" and that generally refers to a switching device that has one side of it's contacts already connected to a voltage source, and is usually contained within some device or assembly. The switching device thus controls that voltage which is available to the installer. The wet contact connections available to the installer would be the other contact and a return such as ground or neutral. In simplest terms think of a plug-in appliance timer.

-Hal

 

[480sparky]

i use them to close a circuit consisting of a coil of wifre inside another coil to change the inductance of the second coil for the signal to a controller for vehicle counts, etc.

But how is that accomplished without current flow?

 

[GoldDigger]

But how is that accomplished without current flow?

The current that flows in the switched coil is entirely current induced in it by a sensing current applied to the concentric outer loop. Basically current is driven through the outer loop to measure its inductance and Q, and short circuiting the test coil inside the loop changes those parameters.
Just because no external voltage is applied to the inner coil and indeed no external connections are made to it, does not mean that there is no current. A short circuited secondary in an isolation transformer is an example of this.

 

[ggunn]

I'll make it easy for you all. You go to the supply house and purchase a wall switch. Doesn't matter if it's SPST, 3 way, 4 way. You have just purchased a switch with dry contacts.

Does that mean that they are "dry" only until they are installed?

 

[kwired]

One thing I have always taken into consideration that no one has brought up is the fact that (IMO) it has to be a mechanical type of device with definite open or closed status in normal operating conditions. Solid state switching devices I have never considered to be "dry contacts" no matter how they are powered. All solid state devices are subject to "leakage current" that you will not get from a mechanical set of contact points.

 

[RichB]

The current that flows in the switched coil is entirely current induced in it by a sensing current applied to the concentric outer loop. Basically current is driven through the outer loop to measure its inductance and Q, and short circuiting the test coil inside the loop changes those parameters.
Just because no external voltage is applied to the inner coil and indeed no external connections are made to it, does not mean that there is no current. A short circuited secondary in an isolation transformer is an example of this.

Awesome explanation!

Thanks!!

 

[gar]

130919-1108 EDT

What do you want to call mercury relay contacts? I want to call them wet. These exist in both a big pool of mercury, or so called mercury wetted contacts. Both exhibit no contact bounce.

.

 

[tom baker]

Contacts used to be wet, as in a mercury relay. A plunger would close a circuit to a pool of mercury. Lasts forever, no arcs. So a non mercury contact is a dry contact.

 

[kwired]

Just because mercury is a liquid at room temp, does that mean it is wet?