4-20mA loop energy harvester

Question

I'm designing a board which is powered from 4-20 mA Hart loop. Only two wire.

Actually I had 19~20V due to the 250ohm resistor of the master side.

I would like to power two electronic stages 12V@4mA and 5V@50mA.

I was trying to use a buck converter to generate 12V in series with an LDO for the 5V to power a microcontroller, sensor and wireless solution.

By simulating the power stages, I saw that the input current is much higher than 20mA. By adding a bulk capacitor, it had been reduced but still > 20mA. I'm worried that will cause problems. I tried to use some dc dc converter like the TSM2405S from traco power and i was able to power the rest of the electronic but it was not a good solution because i got less than 5v in output because the input source is less than 24v. but it works and i did not lose the hart communication On the board there will not be any variation of the current. i need only the hart signal flying on the 4-20 mA loop

Is the architecture of the power stage good, or should I change the topology? What is the best way to get power from a 4-20mA loop?

Answer 1

Normally if you are getting power from a 4-20mA loop you can only count on about 3.6mA, since the signal can be anywhere from 4-20mA nominally, and you want to allow it to go under and over-range since those are also important- so with (say) 12V drop you'd have about 43mW. Even with a 24V drop you'd have less than 100mW. That requires some care if you need galvanic isolation since there's not a lot of room for quiescent current (and it's generally considered rude to drop too much voltage since the user might want to put other things in series).

If your output signal is related to the input signal (for example, a npn-inverting isolator) you can, of course, have more output current for more input current.