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

Pump control with analog out and SSR control with digital out of U6 Pro

U6 Pro SSR Solid State Relay Pump Micropump

  • Please log in to reply
3 replies to this topic

#1 ryanGus

ryanGus
  • Members
  • 2 posts

Posted 12 July 2014 - 04:20 PM

I am hoping to switch my data acquisition needs from National Instruments (NI) to LabJack, but I have a few questions/concerns. I am trying to acquire signals from four 0.5-5.5 V output pressure transducers, two 4-20 mA output conductivity transmitters, two 4-20 mA output pH transmitters, and four RTDs with 4-20 mA output transmitters – I assume I can take care of all of these with one U6 Pro and four LJTick-CurrentShunts.
 
I am also looking to control the speed of two pumps that require a 0-5V input signal – I assume I can do this by using the two analog outputs on the same U6 Pro. I also need to switch on and off a solid state relay (SSR) which is connected to an immersion heater – I assume I can do this using one of the digital outputs of the U6 Pro.
 
I am thinking that I can do all of this with one U6 Pro (plus CB15 and CB37 terminal boards) and four LJTick-CurrentShunts. However, I am a little unsure about whether I will be able to control the pumps and the relay. We tried to control a peristaltic pump’s speed with an NI USB-6009 and it didn’t work; I was told it was because the USB-6009 couldn’t put out enough current for its 0-5 V output. I am worried that I will run into the same problem here with controlling the pumps and the SSR.
 
I’m thinking that I could use one of the digital outputs from the U6 Pro to switch the SSR on and off, since the digital outputs put out 3.3 V and the SSR requires at least 3.0 V to switch on/off. However, I’m wondering if I will run into the same problem as I did with the NI USB-6009 where there will not be enough current with that 3.3 V digital output signal. I found this website that says the trigger current for the SSR is 7.5 mA/12 V, but the specifications for the high digital output of the U6 Pro says that the max sourcing of current is 5 mA on EIO/CIO with a typical voltage of 2.4 V. This makes me think that it won’t work because it can’t reach 7.5 mA, but I also don’t feel like I’m fully grasping how the current and voltage of the digital outputs are chosen/varied or if they are always at a fixed voltage and/or current for the high state. Any help with this would be greatly appreciated.
 
I think I might also experience problems using the analog outputs of the U6 Pro to control the speed of the MicroPump EagleDrive pumps. I can’t find a minimum current rating for the EagleDrives and when I called MicroPump the guy said they probably sink about 10 mA for the speed control loop, but that he wasn’t exactly sure. Looking at the specifications for the analog outputs of the U6 Pro, I don’t see a max current rating, but it does look like the higher the current the lower the range of voltages can be put out (0.04 to 4.95 V with no load or 0.225 to 4.775 V at 2.5 mA). Based on this, it looks to me that I will be able control the speed of the pump, but I won’t be able to cover the whole range of 0-5 V, and therefore won’t be able to cover the full range of speeds that the pump can offer. Can someone verify that my understanding of this is right? Will I only be able to output a small fraction of the 0-5 V output range at a higher current around 10 mA? Why does the output voltage range change depending on the current being drawn by the pump for that signal?
 
Thanks in advance.
 


#2 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 14 July 2014 - 09:44 AM

Your signals for 12 analog inputs (with 4 LJTCS) sound fine.

 

 

The SSR is no problem.  The input specs at the link you provided are not real clear, but if we assume it will draw 7.5mA, then if you are using an EIO/CIO line that will result in about 1.35 volts of drop, so with the sinking configuration using VS you will still have about 3.65 volts across the SSR:

 

http://labjack.com/s...trolling-relays

 

If you don't want to think about it, just buy an LJTick-RelayDriver to be sure.

 

 

For the pumps, I gather that they have a different power supply connection and you are just driving a 0-5V signal connection?  I am surprised the signal connection draws so much current (10mA?).

 

There are 2 main effects that cause you to lose voltage on the analog outputs.

 

  - The output op-amp is a rail-to-rail type, and powered by VS/GND.  The ability of the amp to go to the rails decreases with increasing current.

 

  - There is 50 ohms of output impedance on each DAC line which is part of the overvoltage/ESD protection circuit.  So if you draw 10 mA, you will lose 0.5V due to that.

 

 

To get substantially more current, you can add an op-amp to the output:

 

http://labjack.com/s...s-guide/2.7.1.1



#3 ryanGus

ryanGus
  • Members
  • 2 posts

Posted 14 July 2014 - 07:01 PM

For the pumps, I gather that they have a different power supply connection and you are just driving a 0-5V signal connection?  I am surprised the signal connection draws so much current (10mA?).

Yes, the pumps do have their own power supply and I am just driving a 0-5 V signal to control the speed. The guy from Micropump didn't seem too sure what current the pumps draw, but that was more of his best guess.

 

 

To get substantially more current, you can add an op-amp to the output:

 

http://labjack.com/s...s-guide/2.7.1.1

Are you suggesting to increase the current of the output signal to the pumps? From what I can tell, I would want a lower current so that I have less voltage drop and can cover a larger portion of the 0-5 V range. Are you saying that I could use a non-inverting op-amp to increase the voltage to the pump so that I can cover the entire 0-5 V range?

 

I understand how you got the 0.5 V voltage drop for 10 mA and 50 ohms, but does that resulting 0.5 V voltage drop mean that I will only be able to use a range of 0-4.5 V, or 0.5-5 V, or something else?

 

How does one determine what current an instrument will draw? Does that current vary depending on something?

 

Thank you for your help!



#4 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 15 July 2014 - 09:34 AM

Yes, the pumps do have their own power supply and I am just driving a 0-5 V signal to control the speed. The guy from Micropump didn't seem too sure what current the pumps draw, but that was more of his best guess.

 

How does one determine what current an instrument will draw? Does that current vary depending on something?

 

Since it has a separate power supply and this is just a control input, I would expect the current to be microamps, unless it is driving some sort of analog isolator perhaps?  The fact that the NI device could not drive it, though, suggests that maybe the control input does require substantial current.  You can send us a pump if you want us to check it out.  Contact [email protected] and we will issue an RMA if you want to do that.

 

You could probably measure this yourself.  You need a variable 0-5V voltage source that can provide plenty of current, and a way to measure current (e.g. a DMM that can measure to at least 0.1mA or the voltage source itself might tell you how much current is being drawn).  Make sure the voltage source is not current limited (within reason, say >100mA), then set it to different voltages and measure the current drawn by the load.

 

 

 

Are you suggesting to increase the current of the output signal to the pumps? From what I can tell, I would want a lower current so that I have less voltage drop and can cover a larger portion of the 0-5 V range. 

 

The control input circuit of the pump has a certain requirement for current at a particular voltage.  For example, perhaps if you give it 4.0V, it needs to draw 10mA.  If the voltage source cannot provide 10mA at 4.0V, the source will droop until you reach a balance of what current the source can provide at what voltage, and what the current the load requires at what voltage.

 

If the load requires more current, you need to add a buffer (non-inverting op-amp) to the DAC output so that it can provide more current without so much voltage droop.

 

 

I understand how you got the 0.5 V voltage drop for 10 mA and 50 ohms, but does that resulting 0.5 V voltage drop mean that I will only be able to use a range of 0-4.5 V, or 0.5-5 V, or something else?

 

 

Based on just the DAC source impedance, and the assumption that the load draws 10mA at any voltage, you would set the DAC to 0.5-5.0V, and at the load would get 0-4.5V.  Setting the DAC to 0-0.5V would provide <10mA to the load.

 

When you include the limits of the op-amp to drive substantial current near the rails, I would expect your actual range of voltage you can get at the load to be 0-0.5V minimum to perhaps 4.0V maximum.  I will try to test later for a better estimate.




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users