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USB to 4-20ma?


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#1 pproby

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Posted 19 May 2008 - 03:29 PM

Hello! I am looking for a device that will convert a digital signal from the computer to a 4-20ma readout... I saw many of your products however, being new to this, I am not sure if this is what I am really looking for. Thanks!

#2 LabJack Support

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Posted 19 May 2008 - 03:36 PM

So you have some other device that wants a 4-20 mA signal.

 

 

1.  Voltage output through fixed resistance.

 

Many such devices just provide a fixed resistance (250 ohms is typical), and if so you can control them with the voltage outputs on the LabJack or the LJTick-DAC plug-in.  For example, if the load is 250 ohms and you set the voltage outputs to 2.0 volts, you will get a resulting current of 2.0/250 => 8.0 mA.

 

Many devices provide a very small effective resistance, say 1.0 ohms, so you can add a resistor (e.g. 240 ohms) in series that controls the current.

 

 

2.  Add a transistor with resistor.

 

A reasonable way to do your own voltage to 4-20 mA conversion is with the circuit shown in Figure 3 of the following app note:

 

http://www.maxim-ic....s.cfm/an_pk/722

 

 

3.  Commercial-off-the-shelf (COTS) voltage to current converter.

 

A nice COTS solution to convert the voltage outputs from the LabJack into current outputs might be the 5B39-01 or 5B39-03 from Analog Devices ($125):

 

http://www.analog.co...ts/product.html

 

One key advantage is that these are isolated, which is sometimes important in 4-20 mA applications.  If you go with these, you might want some sort of backplane for mounting like the 5B03 or 5B04:

 

http://www.analog.co..._cards/fca.html



#3 pproby

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Posted 19 May 2008 - 04:46 PM

Thanks for the prompt reply! ------ Here is some background info: I am gathering data from a device to the computer using the USB port. After processing the data on the computer and doing some computations, I need to send it out to another device that only accepts 4-20ma inputs. ------ So, what you are suggesting is to use say the U3 to get my data from being digital to being analog. However, the U3 only has voltage outputs. Therefore, I would need another circuit such as the 5B39 to convert the voltage outputs to (4-20ma) current outputs and then finally be able to connect it to my 4-20ma device. I know it seems that I just repeated what you said above but being so new to this I want to be sure that I am understanding this right. So what I am saying is correct right? Much Thanks!!!

#4 pproby

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Posted 20 May 2008 - 06:42 AM

Actually, my client just told me that they can receive 0-5V DC into their analog board. Therefore, I only need the U3 right?

#5 LabJack Support

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Posted 20 May 2008 - 07:21 AM

Yes, although the typical range of the built-in DACs on the U3 is about 40mV to Vs-40mV, so you if you have to be sure you get all the way to zero and all the way to 5.0, then consider adding the LJTick-DAC. See the end of Section 2.7 of the U3 User's Guide which discusses differences between the built-in DACs and the LJTick-DAC.

#6 pproby

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Posted 20 May 2008 - 08:17 AM

Perfect! Few quick questions: Do you guys usually ship out on the same day? The only way for me to get my data from the computer to the DAC is through custom programming using the dll or is that possible with the DAQFactory Express? What are my options with regards to getting my data from the USB computer port to the 0-5VDC analog outputs on the U3? Thanks!

#7 LabJack Support

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Posted 20 May 2008 - 09:02 AM

Yes, orders usually ship same day, and extra effort is made if you select 1-day or 2-day shipping.

DAQFactory Express should be able to do everything you need. Go to daqexpress.com for more info, and start by looking at the quick tutorial and DF-LJ app guide:

http://www.labjack.c...3_downloads.php




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