XTR117: trimmer-regulated 4-20 ma driver

matteo ricciutelli

Part Number: XTR117

Dear All,

I am trying to design a simple circuit whose function is the following:

By turning a potentiometer, the circuit shall output a current in between 4-20 ma

To do so I tried to put together a simple design involving XTR117, however after soldering the components and plugging the system, I found no current flowing and a weird voltage on Vreg!

So my hypothesis is that either the xtr117 ic is damaged or my design is plain wrong, and since I am quite unexperiencedI ask you if there are mistakes with my projects:

The principle is the following: when the potentiometer is at one of its ends, the 5V supplied by Reg is connected by a 25K resistor to " I in", the output should be 20ma

When the potentiometer is at the other end, then the diode should clamp at 1 V, which should get an output of 4ma.

Best regards, Matteo.

over 5 years ago

0 kai klaas69 over 5 years ago

Guru 140200 points

Ciao Matteo,

can you show a schematic of your circuit?

Kai

0 matteo ricciutelli over 5 years ago in reply to kai klaas69

Prodigy 30 points

Sorry, It appears it did not upload correctly on this forum, here it is

0 kai klaas69 over 5 years ago in reply to matteo ricciutelli

Guru 140200 points

Hi Matteo,

the 1k pot is too low ohmic. Choose >=2k. Read section "MINIMUM OUTPUT CURRENT" of datasheet of XTR117.

Kai

0 Tamara M Alani over 5 years ago in reply to matteo ricciutelli

TI__Genius 10850 points

Hi Matteo,

Kai is correct. This device has a current gain of 100, so to generate a 4-20mA output signal, we must input a 40-200µA signal. The XTR117 allows quite a bit of flexibility in the design of the input circuitry stage. Some applications require under scale monitoring capability. This allows for the detection of unintended out of range operation of the sensor. By allowing a configurable low end set point, a user-defined under scale point can be set. Here I am showing how to set a 40µA offset to allow for a 4mA output current based on an zero input signal. The total input current fed into the device at a zero input condition is the 40µA offset which is amplified by 100 to yield a 4mA output. Likewise, with a full scale input condition of 160uA along with the 40uA offset current, the total input current of 200µA creates a 20mA signal at the output.

-Tamara

0 matteo ricciutelli over 5 years ago in reply to Tamara M Alani

Prodigy 30 points

Dear tamara and Kai, thank you for your support

However it appeared the fundamental cause was a faulty or damaged xtr117 unit, after having it swapped with a new one, the circuit worked as intended!

Regards, Matteo

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XTR117: trimmer-regulated 4-20 ma driver