XTR115: Pin3 to Pin4 Resistance value Tolerances.

Thanks for your response. 

Here I further elaborate on the problems we are experiencing.

Failure mode: The unit(s) randomly stop generating the (4-20mA) loop current.

In our returned products lab, we have elected to measure the resistance value between pin3 and pin4 in order to determine the health status of the XTR115. It is a parameter that can be accessed while the component is in circuit.

Our findings so far show the resistance to change( 0 ohms to Mega ohms). The change can be permanent or momentary.

In cases where the change is permanent:- The loop current becomes disproportionate to input or disabled.

In cases where the change is momentary:-

After a series of testing (power-cycling), the measured value returns and stabilizes at 31ohms. 

These units have been returned for dropping out (No loop current, while everything else is on).

In our tests as manufacturer, the units are giving correct loop current.

In your response, please include advice on  the following.

• Can we rely on measuring the Pin3 and Pin4 resistance to determine the health state of the XTR115. 
• Is the resistance quoted for  Pin3 and Pin4 coming from a passive or active component.
• What could cause the XTR115 to momentary stop producing loop current. The stop can be a few hours. And at times the unit has to be power-cycled to re-establish the loop current.

Thank you Kai and Zak.

We are setting up a series of tests that take into your valuable advice. I will update you with our findings.

In the meantime,..

Please advise the expected time to failure of the XTR115 when it is run in the following mode

• Continuously ON, 24hours a day, 365days a year.
• loop current averaging 9mA.
• Ambient temperature 60 to 85 Degrees Celsius.

Thank you for this advice.

We indeed have functional earth connected to Pin3. Our Engineering team is going to avail me some schematics to share with you. 

May I highlight to you that failures are being reported after 12months of product being in service.

There are some further points that we need to clarified:

From Zak Kaye 

…..maintaining the proper ratio between R1 and R2. You may see a shift in this value of +/-20%, but the ratio will be very accurate.

• Question. What is the target value for this ratio? We have taken measurements from a brand new XTR115, R2=29.3ohms, R1=2708ohms. R1/R2=92.3. What values should we expect when read from our PCB? This can help us detect faulty soldering.

…..The momentary case is much stranger. If your loop supply dips heavily then the loop may be out of compliance for the XTR. Are you using an external transistor or relying on the internal transistor? If you use the internal transistor or an external transistor placed too close to the XTR then this could be a thermal issue. Understanding the conditions in which this occurs can help eliminate this as a variable. Does this typically manifest at higher output current levels?

• Please Elaborate on the term "Thermal Issue". At what thermal point is the product affected and to what effect?
• In our design, an external Transistor (MJD47TH from ON Semiconductors) is employed. The emitter (of the Transistor) is approximately 7mm away from the XTR115. Is this distance adequate?.
• The module is then potted with epoxy. Does this contribute to the thermal issue?

Hi Victor,

you need functional earth being connected to pin 3? Oops!!! Not good!! This is the cause for the failures! If possible, connect functional earth to pin 4 and not to pin 3. If not possible, change the circuit! Connecting functional earth to pin 3 will damage the chip again and again!!

Kai

Thanks

Thanks once gain for your advice. Here is a top level block diagram of our xtr115. In your advice and comments please further elaborate on how and why the xtr115 is impeded by Earth on Pin3.

Hi Victor,

even if the 24VDC supply is labelled "isolated" there's always stray capacitance between the 0VDC terminal and mains voltage! This can be the interwinding capacitance between the primary and secondary winding of mains transformer or the interwinding capacitance of the switch-mode-supply's transformer. This stray capacitance to mains voltage can be up to 10nF very easily. And if there's a surge of some kV on mains voltage, and you have dozens of them every day, a current is running through this interwinding capacitance and through the connection between pin 4 and pin 3 of XTR115 to the earth terminal of your PSU block!

This will kill the XTR115 sooner or later!

Kai

Thanks once again. Currently our engineering team have very little roam to change the design.

A minor change to the deign that has been accepted is illustrated below. A resistor (R3) is placed between Pin 3 and 0V(earth). It is hoped the resistor would provide some kind of isolation of Pin 3 from earth.  We have done some tests and managed to get correct output loop current. Do you foresee any problems with this proposed change.

Hi Victor,

at first sight, this seem to be a good solution. The current between pin 3 and pin 4 is limited by the inserted 6k8 resistor and should no longer destroy the chip.

But, unfortunately in the situation of a surge it will kill the XTR115 now due to a massive overvoltage between pin 3 and pin 2 of XTR115. This can be seen from the following picture:

Even if only 1nF stray capacitance (interwinding capacitance) to the mains voltage is assumed in the 24V supply on the right, a massive overvoltage will occur at your 6k8 resistor during a surge.

A surge is an overvoltage pulse of about 1.2µs rise time and about 100µs fall time. In the CE standards it is coupled between the mains voltage and earth. e.g., has a peak voltage of 2kV then and is current limited by 12R. In the following you will see a simplified surge analysis:

It can clearly be seen that for a brief period nearly the full peak voltage is dropping across your 6k8 resistor, while the voltage at pin 2 is clamped to about 4.7V. This will cause a massive overvoltage between pin 3 and pin2 of XTR115.

Another disadvantage of the 6k8 resistor is that every mains noise is directly coupled into the signal path now, because of the voltage drop across the 6k8 resistor.

So, I would strongly recommend not to insert such a resistor. I would try a diode clamp between pin3 and pin 4 as shown in the following picture:

Using 1N4001 diodes for the clamp should be adequate. The needed current rating can be estimated from the voltage change rate of surge pulse by the help of the formula C = Q / U = dQ/dt / dU/dt:

I = dQ / dt = C x dU / dt = 1nF x 2kV / 1.2µs = 1.67A

With a stray capacitance of 10nF the diodes must withstand a current 16.7A. So, the 1N4001 should work. The 1N4001 has the advantage of a very small junction capacitance which can be useful because the diode clamp is sitting in the feedback loop of XTR115 between pin 3 and pin 4.

Eventually, an additional diode clamp made of two antiparallel BAV99 cross pin 2 and pin 3 can be helpful. 

If this scheme does not help, you need to redesign your circuit. Please keep in mind that the XTR115 is just not designed to have any connection to outer world at pin 2 and pin 3. All the input circuitry is expected to be supplied by the internally generated supply voltage coming out of pin 8, only.

Kai