Dry Contact

[mhester]

I am using LabView 7.1 and have an application where I need to send outputs to a device which monitors dry contacts. How can I go from LabView to dry contacts? I am not familiar with dry contacts, so information on them would be helpful also. Thanks.

[AnalogKid2DigitalMan]

Dry contacts typically infer relays (not the mercury type, these are 'wetted' contacts). Could you be more specific. You state you need to send outputs to a device that monitors dry contacts. What type of inputs are needed by this device?.

If you mean that you need LabView to control sets of dry contacts, then this scenario might entail a digital I/O card that drives relays or relay modules. I believe some National hardware cards even include dry contact outputs.

But before I get ahead too far, please provide more information to clarify what is needed.

[tbob]

You will need hardware to monitor the dry contacts. Labview can then communicate with the hardware. Typically, a DIO (Digital Input Output) board is used. One side of the contact is tied to ground. The other side is connected to a DIO input. When the contact is open, the DIO will report a hi logic level (1/On/True) to Labview. This is because the DIO usually has a pull up resistor on its input which pulls up the voltage to 5v. When the contact is closed, the input is grounded and the DIO reports a lo logic level (0/Off/False). The logic is reversed, but you could put an inverter ("Not" function) to switch the logic in Labview. DIO's are very cheap nowadays. You can buy a USB connected DIO for about $100, or a PCI DIO for a couple of hundred. There are many DAQ/DIO examples that come with Labview.

[spaceman_spif]

Analog Kid is right. A "dry contact" refers to a set of metal contacts that are usually open to prevent a circuit, and then something will trigger them to close, completing a circuit. A "dry contact" usually means that the metal contacts themselves don't really care which direction the electricity flows nor do they care if the electricity is AC or DC. (Note that I said "usually"!) Some contacts are designed for specific electrical flow direction, type of current, etc. A dry contact is a general, all-purpose contact.

Do you want to monitor the current coming out of the dry contacts? Is that what you want to look at? Or do you want to monitor the coil of the relay? That will determine the type of input card you need.

Most relays have what's called an "auxilliary contact" which is an extra contact that closes when the relay is activated. These are useful, for example, if your relay is controlled by a 120 VAC coil, and the contacts allow 480 VAC current to flow, but you only have a 24 VDC input card. You can wire 24 VDC into that aux. contact and use that as your input instead of having to find another input card for 120 VAC or 480 VAC input.

If this is stuff you already knew and it came across condescending, sorry!

[mhester]

I need to use LabView to write to dry contacts so that the other device can read their states. I am still trying to get more info on the other device; I have only been told dry contacts for now. I will look into a NI card for this application. Thanks for the help.

[tbob]

If you want to control the contacts, you need to know the voltage rating of the relay coil. Then you could use a DIO (configured for digital out) and a driver circuit to control the relay with Labview. Post the coil voltage and I could describe a driver to use.

[AnalogKid2DigitalMan]

Didn't look too deep and no input on specs, but something like an NI PCI-6503 24 bit digital I/O in conjunction with an NI ER8/16 electromechanical relay accessory may be worth looking into as an NI solution.