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Thread: Dry Contact Basics

  1. #1

    Dry Contact Basics

    Hello,

    I'm having a bit of a hard time getting a truly straight answer on what a dry contact is... I'm under the impression it's switch that closes or opens by some external control source but doesn't have a voltage source itself - it basically just connects to pieces of wire together. So say a control panel is MONITORING a dry contact, what it's doing is trying to pass a current through a circuit and if it successfully passes it's current through the circuit it knows that the contact is closed. Is that correct? So the dry contact is not the source of the current, the thing doing the monitoring is?

    I'm looking for a device (hopefully it's a simply type of relay!) that has it's own power supply and monitors a dry contact with a 48V max signal. Depending on whether the dry contact it is monitoring is closed or open, it closes or opens it's own dry contact rated for 120VAC, that in turn a PLC system will be monitoring. Keep in mind that I'm using "dry contact" assuming that my description above is correct! Can anyone tell me what this type of device might be called?

    Thanks!

  2. #2
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    the contacts on a relay for example are dry. They have no voltage present. You have to provide the voltage to the common side of the contacts. Just like a single pole off/on switch.

  3. #3
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    "Dry contacts'' usually means that it has no voltage applied to the contacts at all from its initiating equipment. This allows a remote piece of equipment or instrumentation to use its own control loop and "loop through" the "dry contacts". So two or more pieces of control equipment are not electricaly common, but only mechanicaly common.

    in addition, you can always create your own "dry contacts'' by adding a relay with the coil voltage being the same as your control voltage. Let the control voltage energize the coil, then use one of the relays contacts to supply a contact closure to a remote location or piece of equipment....

    Thats a crude explanation but hope it helps....
    "Mule"

    Yes, Im stubborn as a mule..that's why,
    I'm a "Has Been" that "Never Was"
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  4. #4
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    A relay will do exctly what you want, but you will need a power supply, and said supply must be compatible with the relay coil. 12VDC, 24VDC and 48VDC coils are popular, and supplies for all those voltages are easy to come by, even DIN rail mounted supplies which whilst relatively expensive, make for a neat, quick and easy installation.

  5. #5
    Thanks guys! I definitely feel a little stupid asking that question... haha!

    Is there such thing as a relay where the coil is also a power supply? For example, the coil itself provides the 24VDC used to monitor a dry contact, and when that contact closes, the current in the circuit activates the coil on the relay then closes the dry contact that belongs to the relay? In the end, it would just save getting an external power supply...

  6. #6
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    Quote Originally Posted by MD88 View Post
    Thanks guys! I definitely feel a little stupid asking that question... haha!

    Is there such thing as a relay where the coil is also a power supply? For example, the coil itself provides the 24VDC used to monitor a dry contact, and when that contact closes, the current in the circuit activates the coil on the relay then closes the dry contact that belongs to the relay? In the end, it would just save getting an external power supply...
    The other post mentioned DIN rail relays, you can also get DIN mounted control transformers to do what you want in conjunction with the relay.....
    "Mule"

    Yes, Im stubborn as a mule..that's why,
    I'm a "Has Been" that "Never Was"
    :D

  7. #7
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    Your usage of the term "dry contact" as a contact only and without a directly associated power source is totally unfamiliar to me.

    However, the following usage shown in Wikipedia would be my expected definition:
    " "Dry" contacts - when switching very low level signals, special contact materials may be needed such as gold-plated contacts "
    The intent of the dry contacts is to prevent degradation of low level signals.

    Also note there are wet contacts. Here I am familiar with mercury wetted contacts, and water or other conductive liquid materials.

    I think it would be quite unusually to find an integral package of a relay and a power source, except for special production products. General purpose relays consist of an actuator and actuated contacts. You add your power source wherever you want.

    In many packaged applications there likely will be more than just a relay and power source. Possibly various semiconductor components to do special things like trigger on a very small and consistent resistance change.

    One example of transformer and AC relay as a package is a Charles F. Warrick liquid level control. The remote (dry switch) is a conductive tank with water in it and an insulated probe projecting into the tank. When the water level touches the probe current flows and the relay pulls in. Really this is a wet contact or switch in this application.

    .

  8. #8
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    "Dry" is a standard term that means without power. So for example a telephone line pair intended for a private extension that is just a pair of copper lines between two places is a dry pair. The obvious opposite is wet, though you dont see the term used much.

    When switching low level signals then yes, special contacts are needed. But 12VDC to control a relay is not a small signal in this context. Audio signals, which are under a volt and have currents that can be in microamps would be an low level signal needing gold plated contacts.

    Another common term for a set of dry contacts is "volt free contacts".

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  10. #10
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    Quote Originally Posted by gar View Post
    090218-2059 EST

    Your usage of the term "dry contact" as a contact only and without a directly associated power source is totally unfamiliar to me.

    However, the following usage shown in Wikipedia would be my expected definition:
    " "Dry" contacts - when switching very low level signals, special contact materials may be needed such as gold-plated contacts "
    The intent of the dry contacts is to prevent degradation of low level signals.

    Also note there are wet contacts. Here I am familiar with mercury wetted contacts, and water or other conductive liquid materials.

    I think it would be quite unusually to find an integral package of a relay and a power source, except for special production products. General purpose relays consist of an actuator and actuated contacts. You add your power source wherever you want.

    In many packaged applications there likely will be more than just a relay and power source. Possibly various semiconductor components to do special things like trigger on a very small and consistent resistance change.

    One example of transformer and AC relay as a package is a Charles F. Warrick liquid level control. The remote (dry switch) is a conductive tank with water in it and an insulated probe projecting into the tank. When the water level touches the probe current flows and the relay pulls in. Really this is a wet contact or switch in this application.

    .
    There you go taking it to the extreme again. You are taking it leterally. When in fact the correct answer and answer this guy needed has already been given.

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