Grossel
Well-Known Member
At a facility, I came over some sensory equipment (ref. device A) that feed 4-20mA to a HMI device (ref. device B). During fault location, I couldn't miss noticing that a 4-20mA output from device A, two terminal feeding into a wire with 2 strands to another room - that measuring voltage between ground to any of those two strands would lead to a zero volt reading, whilst voltage on terminals are according to actual readings (there was never an actually electrical fault anyway).
This tells me that the 4-20mA loop is somehow isolated from ground, maybe due to use of optical isolators (the equipment is like 20 years old and schematic doesn't exist in public).
This leads me to the following question. Let say I've ±12V and ground (rails not isolated from ground) to start with. How can I construct an op-amp based circuit that have 2 channel output - constant current - but in addition to a regular constant-current output, when there is a load (presumably resistive) on the output, each wire should act as isolated from ground (not mention if the output are lifted close to rail voltage - i.e. say it never get lifted beyond ±10V ?
I assume in real world, this is acomplished by using smtp combined with optical isolators for feedback, but I'm curious if this can be accomplished without smtp/optocouplers ?
I've looked at some of the circuits on this page for ideas, but I can't see that the outputs are true isolated from ground.
This tells me that the 4-20mA loop is somehow isolated from ground, maybe due to use of optical isolators (the equipment is like 20 years old and schematic doesn't exist in public).
This leads me to the following question. Let say I've ±12V and ground (rails not isolated from ground) to start with. How can I construct an op-amp based circuit that have 2 channel output - constant current - but in addition to a regular constant-current output, when there is a load (presumably resistive) on the output, each wire should act as isolated from ground (not mention if the output are lifted close to rail voltage - i.e. say it never get lifted beyond ±10V ?
I assume in real world, this is acomplished by using smtp combined with optical isolators for feedback, but I'm curious if this can be accomplished without smtp/optocouplers ?
I've looked at some of the circuits on this page for ideas, but I can't see that the outputs are true isolated from ground.