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How to turn input voltage to ground with U3-LV?


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6 replies to this topic

#1 AstroClimber

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Posted 21 October 2013 - 07:34 AM

Hi,

 

I have this device with an LED is on when the readout terminal of the LED is +5V. In order to turn the LED off, I need to ground that +5V terminal. I wonder what's the best way to do it with the LabJack U3? Should I use one of the FIO?

Since the ground of the device is not the same ground as the U3's GND terminal, what can I try to accomplish what I want to achieve?

 

Thanks



#2 LabJack Support

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Posted 21 October 2013 - 08:21 AM

Sounds like need to control a contact closure input.  Do they provide any specs?  They should tell the value of the pull-up resistor that is holding the input high, and/or how much current you must sink to pull the input low.  They might also tell you the low threshold voltage (e.g. <0.8 volts).

 

Where does the device get power?  Is the common/ground of the contact closure input, the same as AC mains ground, or is there any other possible connection to U3 ground?

 

 

Related topic, although it is about the U12:

 

https://forums.labja...p?showtopic=596



#3 AstroClimber

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Posted 21 October 2013 - 12:13 PM

Thanks for the reply,

 

The device is a homebrew instrument in our lab by my advisor. The current required is pretty low as far as I know. The device get power from an external PS. The ground of the device is the same as the ground of power supply. So is it possible to set the U3 to have the same ground with that power supply?

 

As I looked closer to the LabJack spec, since all the digital I/O only provide 3.3 V, the document of U3 suggests that I will need an open-collector to have Digital output of 5V. Do you think that will solve my problem? Just connect that LED's anode to the open collector, have the FIO of the U3 to toggle between high and low out (3.3V and GND, respectively).

 

Cheers,



#4 LabJack Support

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Posted 21 October 2013 - 02:52 PM

No much to go off.  You might try a simple test with a resistor to simulate an FIOx terminal.  Take a resistor around 560 ohms, and connect it from the device terminal in question to device ground.  Does that do what you need?

 

To try it with the U3 connect FIO4 to the device terminal in question and U3 SGND to device ground.  Set FIO4 to input to act like an open switch, and set FIO4 to output-low to act like a closed switch to ground.  If the voltage on the terminal does not drop low enough, you could try an EIO line rather than an FIO, and could even put multiple EIO in parallel.



#5 AstroClimber

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Posted 21 October 2013 - 04:59 PM

Hi,

 

I got it to do what I want with the help of the open-connector/buffer IC S7407N, as suggested in the Digital IO documentation of the U3.

 

Thanks!



#6 AstroClimber

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Posted 21 October 2013 - 05:21 PM

Just one more question,
 

Should I connect the Device's ground to SGND of the U3 unit instead of the GND since the device is not neccessary sharing the same ground as the USB ground?

 



#7 LabJack Support

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Posted 22 October 2013 - 10:39 AM

Either are likely fine, but I would try SGND.  It seems better for this case.




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