TPS62125 Harnessing wasted energy in a 4- to 20mA current-loop system schematic

Is this the paper you refer to? www.ti.com/.../slyt488.pdf

Using the EVM, connect Vin to the level shifter/165 Ohm and connect GND to ground in the diagram. Vout goes to power whatever you want to power (INA138, etc.). Note that the transmitter is powered by a different supply voltage.

Also, you will likely need to widen the enable hysteresis on the EVM, increase the input capacitance, and add a zener diode as described on pages 2 and 3.

Thank you.  Yes, that is the paper I was referring to. 

I have a power supply for the transmitter. 

I will try your suggestions and let you know how it goes.

Thank you!

Hi Chris,

I guess I'm still missing something.

"Using the EVM, connect Vin to the level shifter/165 Ohm" Should I connect Vin for the supply to the + side of the sense resistor? as indicated in Figure 2 of the document?

With 4 mA flowing through the current loop, I only have 1V across that (using a 250Ω sense resistor) (only 0.66V with the 165Ω load) I believe the TPS62125 needs a minimum of 4 Volts to regulate?

Any thoughts please? Thank you.

Well, the voltage across your sense resistor isn't what powers the TPS62125. That's the signal you're trying to measure. The current loop should develop a much greater voltage across the resistor and TPS62125 and this is what powers the IC.

Hi Chris..

When you say the current loop should develop a much greater voltage across the resistor, are you referring to the sense resistor? or another resistor?

When I connect the TPS62125 as your diagram indicates, it breaks the current loop, and I no longer get my 4-20ma current, and the TPS62125 circuit powers fine.

Per your previous suggestion, I should connect Vin of the TPS62125 to the - side of the sense resistor?

I'm sorry, but what am I missing? I've tried several various configurations to try to get this to work..

I've loaded the TPS62125 power supply, so that it's always drawing current, that kept the loop active, but not over the whole 4-20 mA range.

I've tried another resistor in series with the sense resistor, to ensure current flow in the loop, but could not sustain enough voltage to keep the TPS62125 circuit active over the full 4-20mA range. I'm using a 12V power supply for my transmitter. I've also tried it up to 18V with no luck for the whole range.

Thanks

Brian

I meant the (sense resistor + TPS62125). That block completes the loop on the receiver side. The output signal of the XTR111 should connect to Vin and then tie the grounds of the EVMs together. Remove any load on the TPS62125 and add a zener diode on the input from Vin to GND.

Send a waveform of the output voltage of the XTR11, Vin, Vout, and EN of the TPS62125. Also, remove JP1 on the TPS62125 EVM.

When I connect the TPS62125 EVM like that, it interrupts the current flow through the current loop.

Is the zener diode meant to protect the circuit ( from voltages greater than 15.6 V, which is my understanding) or is it required to complete the loop.

Could you please sketch up a schematic and post it?   Perhaps I'm not understanding your written directions.  

I'll post some sketches, soon, of the topologies I've tried, and the issues with each.

Figure 60 in the D/S shows a schematic when powered from a current source. I did something similar here.

The diode should just be needed for protection. If the lop is broken, the loop voltage should go up to the power supply voltage. This would be more than enough to power the TPS62125.

Please send some waveforms of what you're seeing.