Acceptable Ranges for a 4-20mA loop Transmitter

  • Thread starter Thomas Anderson
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Thomas Anderson

I'm desinging a generic sinking 4-20mA current loop transmitter and I'm trying to find out it it's ok for my output to sit at 0mA when the
microcontroller is off.
The system uses a PWM output to a low pass filter, into a Burr-Brown XTR115U. Due to the speed of the microprocess the minimum current I
get out ( a 1% duty cycle of the PWM) is about 4 mA. I had been using a biasing resistor so that the system was always at 4mA when there was
power on the loop (even if there was no power to the micro).
Is there any reason why I shouldn't allow this system to sit at 0mA when the loop is powered and the mico is off? The world of the 4-20mA
is new to me, and I want to be sure that 0mA shouldn't screw up any of the equipment that might be connected to this device.

- Thomas

The device supplying the power to the loop should not realy care if it is open circuit (0mA), it would only be a problem if you put a short circuit on the loop. The controller should be smart enough to realise that there is something wrong(or dumb enough to not care).


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Franz Hoffmann

All the PLCs I know won´t mind if you give them 0mA. They will only indicate an error, as they will assume that the wiring is broken. (This is one reason for those live-zero-signals: detecting broken wires.)

Roibert Scott

It is unlikely that sinking 0 mA will cause any problems. Sometimes the presense of the minimum current is taken as an indication that the loop is not broken. Perhaps there are some 4-20 ma receivers out there that will signal a fault if it detects substantially less than 4 ma. How that fault is handled is a system integration issue and I suppose the systems integrator, knowing that your device behaves like this, could simply ignore the fault. I'm not sure I
understand why you find it necessary to cut the current to zero anyway. If the system has enough current capability to run 4-20 during normal operation, why can't it also sit at 4 ma. when the system is idle?

Thomas Anderson

The Burr Brown XTR115U is a sinking current loop transmitter. If my unit is off, but there is power on the loop the XTR115U will hold the
current at 0mA (because there is no input from my system). During "normal" (i.e. my system is powered as well as the current loop) operation
the system will sit at 4mA for a low.

- Thomas
We want the loop to sit at 4ma, just to make sure the loop is live . 0ma is an indication that some problem in the loop (like an open
loop).Its better to clamp at 4ma, in the system you a re designing, so that you are inline with the general engineering practice.

Though 9 out of 10 cases you may not have problem if you keep it at 0ma.

John Hamilton

The unit should not care about 0ma, however if you are using the XTR115 to power your sensor you are also going to power off your sensor as the Vreg cannot derive any voltage from 0 current.