LJTick-CurrentShunt Datasheet
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Datasheet Revision 1.00, March 9, 2007
The LJTick-CurrentShunt (LJTCS) is a signal-conditioning module designed to convert 2 4-20 mA current loop signals into voltage signals that vary from 0.472-2.360 volts. The 4-pin design plugs into the standard AIN/AIN/GND/VS screw terminal block found on newer LabJacks such as the U3, U6, and UE9. The major advantages of the LJTCS, compared to using a simple load resistor, are ease of use, high common-mode range, and lower voltage drop on the 4-20 mA signal.
Figure 1: LJTick-CurrentShunt
Figure 2: LJTick-CurrentShunt With U3
The pins shown on the right side of the LJTCS (Figure 1) connect to the LabJack. The VS/GND pins power the LJTCS, while the OUTA/OUTB pins send the output voltage signal to analog inputs on the LabJack. Following are descriptions of the screw-terminal connections:
SGND: This terminal connects to LabJack ground (GND), with a 750 mA self-resetting thermal fuse in series. Often, the 4-20 mA sensor has its own power supply, and that supply ground needs to be connected to LabJack ground to provide a common reference. SGND allows that to be done, but protects from the risk of the external power supply dumping excess ground current through the LabJack.
VS: This is the same 5 volt output as the VS terminals on the LabJack itself. This is an output terminal, not an input. It can be used to provide 5 volt (nominal) power as needed.
INA+/INA- (or INB+/INB-): The 4-20 mA current loop should be connected to INA+ and INA- such that the current flows into INA+ and out of INA-. Each channel has a 5.9 Ω measurement resistor, and the voltage across that resistor is amplified by x20. So 4 mA gives about 0.472 volts and 20 mA gives about 2.36 volts.
In addition to the 5.9 Ω shunt resistor, there is a self-resetting thermal fuse in series. This has a typical resistance of 18 Ω (min=3.6 and max=50). This does not affect the measurement, but does affect how much voltage is dropped as the 4-20 mA signal passes through the LJTCS.
The common-mode input range of the LJTCS is -8 to +28 volts. That means that the voltage of each of the inputs must be within that range compared to LJTCS ground.
If the 4-20 mA signal does not have a common reference at all with the LJTCS, one needs to be made. One common way of doing this is by connecting ground from the sensor to SGND on the LJTCS. SGND has a fuse in series and then connects to normal GND. The fuse is a 750 mA self-resetting thermal fuse, and prevents other systems from using the LJTCS ground as their power ground.
The following figures show typical connections. Figures 3 & 4 show typical connections with a 3-wire sourcing type sensor. This type of sensor sources the 4-20 mA signal, which is then returned to sensor ground. The common-mode voltage of INA- is 0, while the common-mode voltage of INA+ is equal to the signal loading of the LJTCS (typically 0.48 volts @ 20 mA).
Figure 3: 3-Wire Sourcing Sensor
Figure 4: 3-Wire Sourcing Sensor w/ Ext. Supply
Figures 5 & 6 show typical connections with a 2-wire (loop-powered) sourcing type sensor. This type of sensor sources the 4-20 mA signal, which is then returned to sensor ground. The common-mode voltage of INA- is 0, while the common-mode voltage of INA+ is equal to the signal loading of the LJTCS.
Figure 5: 2-Wire Sourcing Sensor
Figure 6: 2-Wire Sourcing Sensor
Figures 7 & 8 show typical connections with sinking type sensors. This type of sensor sinks the 4-20 mA signal, and thus the common-mode voltage of INA+ is equal to the full supply voltage and the common-mode voltage of INA- is equal to the supply voltage minus the signal loading of the LJTCS.
Figure 7: 3-Wire Sinking Sensor
Figure 8: 2-Wire Sinking Sensor
Specifications:
(1) The 4-20 mA signal passes through a sense resistor, and the voltage across that resistor is amplified to produce the output voltage signal. Thus the response of the LJTick-Current is 118 mV/mA.
(2) The total input impedance is the sum of the sense resistor and the thermal fuse resistance, and is typically 21-27 ohms at room temperature. The voltage dropped across this total impedance by the 4-20 mA signal is the loading.
(3) The voltage from IN+/IN- to GND must stay within the common-mode limits.
Dimensions:
Declaration of Conformity
Manufacturers Name: LabJack Corporation
Manufacturers Address: 3232 S Vance St STE 100, Lakewood, CO 80227 USA
Declares that the product
Product Name: LabJack Tick Current Shunt
Model Number: LJTCS
conforms to the following Product Specifications:
EMC Directive: 89/336/EEC
EN 55011 Class A
EN 61326-1: General Requirements
File attachment:
Comments
#1
I have a sensor that needs 24 VDC +,-10%, it puts out a 4-20 ma signal. in the specs for the LJTick-currentshunt it says the maximum supply voltage is 12V so does that mean I can't uses this device to read my sensor?
#2
No, that is not what it means. The Supply Voltage is the voltage powering the LJTCS via the VS pin. Normally you clamp this in the screw-terminals on a U3/U6/UE9 so it is getting 5 volts.
The applicable spec for the signal is the Common-Mode Range and note #3.
Your sensor will get power from a 24 volt external supply that you provide and you will connect it as shown in Figure 4 or Figure 6 (or less likely Figure 7).
#3
I have a U6 pro can i buy 2 of these LJTCS's and hook up four sensors to the labjack and then see them in lab view? I plan to make 2 loops. Each loop will have its own power supply powering 2 sensors on each loop. Should this work?
#4
Sounds fine. Ideally, though, you would use as few of external power supplies as possible, so can you just use 1 power supply for all 4 sensors? I suggest you email support@labjack.com with the part numbers of your sensors and power supplies so we can take a look. Also, if possible make a drawing in Google Drawings and share it with labjacksupport@gmail.com so we can help further.