USB to 4-20ma?
Posted 19 May 2008 - 03:29 PM
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.
With either of the above techniques, you must consider that the analog outputs on the U3/U6/UE9/T7 have 50 ohms of source impedance. For example, if the load resistance is 250 ohms, you would actually use 300 ohms in your I=V/R equation to estimate the resulting current. For better accuracy, use analog output to measure the actual output voltage of the DAC and adjust as needed.
These techniques might have problems near the limits. The actual typical output range of the DAC on the U3 is 0.04 to 4.95 volts. The minimum just above 0.0 volts means that you will likely not be able to get the current all the way to 0.0 mA. The maximum is further dependent on the VS power supply from USB and could limit the ability to get the current all the way to 20.0 mA.
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:
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):
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:
Posted 19 May 2008 - 04:46 PM
Posted 20 May 2008 - 06:42 AM
Posted 20 May 2008 - 07:21 AM
Posted 20 May 2008 - 08:17 AM
Posted 20 May 2008 - 09:02 AM
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:
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