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.
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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