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PXI 2567 (64 Ch Relay Driver) - Zero Voltage

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Hi all,

 

I have a PXI-2567 board I will be using to drive some SP2T switches. I will be using the 12V pin from the card to supply the power to the switches. I tested the 12V pin (pin 19) and ch0 (pin 40) with a multimeter and to my surprise it read approximately 9 V. I then tried closing the channel (ch0->com0) through NI-Switch and when I took another reading it said approximately 12V. How can I set the voltage to 0 V instead of 9V when the ch0 is set to open? If possible I would like to make it a default setting on all the pins.

 

Any help resolving this issue is greatly appreciated. Please let me know if further information is required.

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Have you ever gotten this card to display 0 volts? I'll dig into some documentation tomorrow. 

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Hi BrandonC1, no I have not. I always check voltages before plugging stuff into test sets, so I tested the pins before hooking up my prototype switch board. I immediately saw 9V and went through double checking my setup and everything else. I'm a novice when it comes to NI equipment, so it is probably a setting somewhere I haven't seen (at least I hope). Please let me know if you do find anything. Thank you for the help.
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Solution
Accepted by topic author Anadyr009

Anadyr,

I spoke with a coworker and what it seems to come down to is the fact that we don't spec our equipment when nothing is connected to it. We're confident that if you were to connect your relays everything would function as intended. If you'd really like to do a manual test I can try to see what kind of resistor setup you would need to do so, but what you are experiencing is expected behavior. 

 

Brandon Castro 

Applications Engineer 

National Instruments 

http://www.ni.com/support 

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Thank you for the reply Brandon. After testing everything does function as expected. I just couldn't find any information anywhere on the voltage I would see when the channels are unpaired and this led me to question if I had to tweak something before continuing.

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I totally understand! I had to brush up on my relay driver fundamentals to get to the bottom of why it was doing that. I'm glad everything is in working order!

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Hardware explanation: Each channel on the PXI-2567 is an open drain FET (the source is tied to ground).  If you don't connect anything to the channel, then the channel line simply floats.  The 9 volt measurement you observed is meaningless... when the channel is turned off, the FET floats to whatever it is pulled to.  With nothing connected to the channel, the "9 volts" is probably the result of charge injection from your voltage measuring device (purely speculation).

 

For sure, though, the "9 volt" measurement you observed is meaningless unless you tie some resistance to a relatively low ESR source (or ground).  For example, you could tie a 10 kOhm resistor from the channel to ground and then you'd certainly measure 0 volts when the channel was off.  Or you could tie that same resistor to +12 V and then you'd measure +12 V when the channel was off.  Note the resistor is not actually required, but is just to illustrate a point.

 

In actual use, the key point on the 2567 is that when a particular channel is off, then the voltage on either side of the relay coil - in relation to system ground - will measure whatever the positive voltage source driving the relay is set to (e.g. +12 V).  Since the voltage is the same on both sides of the relay coil, there is no voltage difference across the relay coil, so no current flows through the relay coil and thus the relay stays off.

 

When you turn the channel on, the FET pulls the connected side of the relay coil to ground via the low resistance FET (which was high resistance before), causing current to flow through the coil, and thus connecting the coil.

 

Hence, you should only see 0 volts on the PXI-2567 channel when that channel is on.

-John Sullivan
Problem Solver
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