Jump to content

As of July 17, 2015, the LabJack forums here at forums.labjack.com are shut down. New registrations, topics, and replies are disabled. All forums are in a read-only state for archive purposes.

Please visit our current forums at labjack.com/forums to view and make new posts. To post on the current forums, use your labjack.com login account. Your old LabJack forums login credentials have been retired. There are no longer separate logins for labjack.com and LabJack forums.


Photo

reference voltage from GRND and DAC0


  • Please log in to reply
5 replies to this topic

#1 autolog

autolog
  • Members
  • 5 posts

Posted 02 April 2013 - 11:00 PM

hello everyone, i am new to this forum as well as labjack. we are a research team working on an automation project. here is a quick overview of what we am working on. we are using labjack to get a reference voltage (0 - 5v) analog from the DAC0 port, and GRND. This ref. voltage is fed to a MOTORCONTROLLER which generated an output voltage from 0-24vdc to run a motor. Using a Laptop, When we use the LJControl Panel , Test option, using a multimeter the output generated across DAC0 and GRND is what we desired. But the GRND of the MOTORCONTROLLER is the Battery (-ve) terminal. so, clearly seen, the GRND from labjack is connected the Battery (-ve) terminal, so that the MOTORCONTROLLER and LABJACK has a common GRND. My question: The MOTORCONTROLLER still says, the ref voltage is not correct. Is there a possibility that the GRND terminal of the laptop is grounded internally and is not given to the USB port where the LABJACK is plugged? there seems to be a grounding issue here. your help will be of great use. hope i can find a solution here. thanks guys

#2 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 03 April 2013 - 08:24 AM

Sounds like you have an issue where you measure DAC0 (which LabJack?) with a DMM. The DMM reads the correct voltage with nothing connected to DAC0, but when you have DAC0 connected to a motor controller the DMM reads a different voltage than you are commanding?

I am having trouble clearly understanding your post. To help I suggest you make a wiring diagram of your setup. One great way to share a wiring diagram is using Google Drawings:

http://support.googl.......4&ctx=topic

Make a drawing and then post the link to the drawing here.

#3 autolog

autolog
  • Members
  • 5 posts

Posted 03 April 2013 - 08:55 PM

@ support team: i have attached the wiring diagram as a pdf with this reply. i am using a labjack U3 for this purpose. controller: gets a reference voltage from labjack, and scales it from 0v to 24vdc output. in the wiring diagram, the controller gets the power supply from 24v battery. and, the labjack gets the power source from a laptop. i have heard that some laptops have their battery pack GRND isolated separately and not the same as the USB port GRND. so, my doubt is, if I ground the LabJack with the Battery -ve terminal, will the controller and the labjack have the same ground? hope i have explained clearly. let me know if further explanation is required. regards

Attached Files



#4 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 03 April 2013 - 09:40 PM

The way you have drawn it looks correct. Since you are not sure if the U3 has a common ground with the controller, you do need the connection from U3 GND to the controller ground.

Are you having problems with this? Or just checking if it is OK?

The only risk in your setup is if the U3 ground is connected to earth ground (through the laptop), and the battery is also connected to earth ground. That would mean have a big ground loop, which could cause problems. In other words, you want the connection from U3 GND to Controller B- to be the only ground connection. If there is another ground connection somehow through the USB then you have a loop.

If you want to be safe and make sure you don't have such a ground loop, add a USB isolator such as the following:

http://microcontroll...roducts_id=3769

#5 autolog

autolog
  • Members
  • 5 posts

Posted 03 April 2013 - 10:11 PM

there is no earth ground since it is going to be a mobile unit. The battery negative terminal is the COMMON for both - controller and labjack. test1 conducted: using labjack control panel, voltage was generated across DAC0 and GRND, and was tested using multimeter, and results were good. test2 conducted: using labjack control panel, voltage was generated across DAC0 and GRND (in this case, the DAC0 was connected to PIN2, and GRND was connected to Battery-ve) to have a common ground. output: the controller gives error message that the input voltage is out of range(meaning that pin 2 is not reading a reference voltage) scenario explained in test 2 may be because the controller and labjack is not having a common ground line. using a USB Isolator (as mentioned earlier) how will it ensure that the ground loop is stopped? my question may be repetitive, but its still confusing for me.. sorry incase i am asking the same thing again and again....

#6 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 04 April 2013 - 12:22 PM

Are you using DAC0 from the U3? Use a DMM to measure DAC0 versus GND at the U3 before you connect DAC0 to PIN2 on the Controller. Confirm that DAC0 measures as expected. Now measure DAC0 versus B-. Should be the same since GND is connected to B-. Now connect DAC0 to PIN2 and repeat the measurement. Does DAC0 still read correctly?

scenario explained in test 2 may be because the controller and labjack is not having a common ground line.

But I thought you did have a common ground? Your diagram shows U3 GND connected to Controller B-. Is B- the common reference for PIN 2?

using a USB Isolator (as mentioned earlier) how will it ensure that the ground loop is stopped?

The ground loop described happens back through the USB connection. If you have an isolator inline, that breaks that ground connection.


0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users