We are using 4-20mA control for a PWM SSR as part of a coffee roaster control system. We've been looking at the XTR116 for driving the 4-20mA, but we are having trouble getting even a very simple test circuit working. Can anyone with experience with the XTR116 take a look at this and let us know what we might be doing wrong?

With the circuit shown, driven by an 18V power supply, we see 5V on pin 8 and 4.1V on pin 1, so it seems the XTR itself is functioning correctly. Putting the ammeter in series with pin 2, we see current move steadily from 0 to 0.2mA, which is what we would expect. However, no matter what we do, the current in the output circuit seems to be stuck at 1.6mA.

 

Putting the ammeter in series with pin 2

Presumably you have it in series with pin 4 if you want to measure the output current?
What resistance does the output load have?

 

I've used the XTR116 in that circuit, as shown; it does work. All I can think of is check your wiring/soldering.

 

Presumably you have it in series with pin 4 if you want to measure the output current?
What resistance does the output load have?

Hi Alec - yes, in series with pin 4 to measure output, in series with pin 2 to measure input. For our test circuit there is no load on the output (other than the ammeter). Could that be an issue?

 

I've used the XTR116 in that circuit, as shown; it does work. All I can think of is check your wiring/soldering.

Thank you. It's certainly possible, although we've tried three XTRs and swapped out the transistor etc. Bad batch of XTRs, conceivably?

 

Thank you. It's certainly possible, although we've tried three XTRs and swapped out the transistor etc. Bad batch of XTRs, conceivably?

I rather doubt it, as I've never seen a bad batch.