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Use XTR105 for potentiometer to 4-20 current converter

Greg.A.
Prodigy 210 points
Other Parts Discussed in Thread: XTR105

Hello,

In the past we have make a board to convert 10K potentiometer to 4-20mA with XTR105.

But if i remake the schematic, it don't work, the output current is not linear to the potentiometer position.

The XTR105 have very light datasheet.

I have mesured the IR1 current, but is less than 800mA and it depend of the potentiometer position.

P1 is for set 20mA to output when we have full range in potentiometer.

I attach my schematic.

The + is connected to 24vDc power supply.

The - is connected to ground through a 500ohms resistor.

Thanks for your help

  • 0
    TI__Guru 62035 points

    Hi Greg,

    I think you're hitting the compliance limit on the XTR105 output current source.  This system follows Ohm's Law and if there's not enough voltage headroom (compliance voltage) then the current source magnitude must decrease. 

    With a +24V supply, you lose 0.7V across D1, and at the maximum 20mA output current the load will have a 10V drop across it (500Ohms * 20mA).  This leaves only 13.3V as the supply voltage for the XTR105.  The current sources have a typical compliance voltage of 2.5V (maximum 3V) from the positive supply.  This reduces the maximum voltage that the current source can handle to 10.8V typical. 

    The 0.8mA IR1 current source through the 4.7k, 10k, and 2.2k require 13.52V to operate which is above the 10.8V that the current sources can regulate to which is why the current source output has to decrease to maintain Ohm's Law.

    Are you sure you didn't use a smaller resistance potentiometer in the past, a smaller load resistor, or a higher supply voltage?  I suspect changing any of those variables should enable the system to function properly.  

  • 0 in reply to Collin Wells
    Prodigy 210 points
    Thanks for the answer.
    I have founded the solution:
    R1 to 0ohms
    R3 to 1.5K.
    Add a resistor of 1K in parallel to the potentiometer (J2a and J2C).
    Thanks