Multifunction DAQ

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Reading dry relay contacts using PXI-6225 multifunction DAQ card

Is the PXI-6225 (or any PXI card for that matter) inherently capable of reading dry relay contacts (open or closed) without any external pull-up resistors.  I would love to be able to just wire the contacts directly to the digital inputs on the card and read them in Labview.  Is this possible or am I stuck with using external pull-ups?
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Message 1 of 10


If it's just the state of the relay you want to read you can use digital-out to digital-in.

But, be sure that the relay contact work at low current

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Message 2 of 10
Digital out to digital in?  I'm not sure what you mean by that.  And yes, all I need is the state of the relay.  The relay is controlled external to my application, I just need to read the current state (open or closed).
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Message 3 of 10

Make one digital output high and feed that output voltage to your contacts. Wire the other side of your contact to the digital inputs.

Inputs have a weak pull-down so a open contact is a '0' at the input and a closed contact a '1'

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Message 4 of 10
Ahh, I thought that's what you might mean.  I'll give it a try.  Thanks.
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Message 5 of 10
Still be carefull if the relay or not for small signals. Otherwise the current can be too low   Smiley Surprised
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Message 6 of 10
I assume you mean if the resistance between the contacts in the relay is too high, the current will be too low and won't be able to be used for reading an open/closed state.  Is that what you mean?
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Message 7 of 10

No, if you have a relay that is specified for currents like 5A they can have a minimal current for the contact to work. Below that minimal current the manufacturer won't guarantee that the contact will conduct.

This way of testing the current will be lower then 1mA.  Or add a resistor at the input to get more current through the contacts.

Again if you have small relays it should not be a problem. Check the datasheet of the relay.

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Message 8 of 10
Well, it will be different relays everytime.  The system I'm developing this application for will be traveling to various customer test sites.  I don't think the current issue will be a problem though.  I am currently reading the state of the relay with a small resistor (can't remember the value) pulled up to 10V and fed into analog inputs, so i'm sure the relays do not have large current requirements.
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Message 9 of 10

OK, just mentioned it to be sure.

Good luck with your project. Funny at this moment I am also working on a test for a relay module. Smiley Happy

Using a PXI-6251.

Message Edited by K C on 05-23-2007 04:33 PM

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Message 10 of 10