Question on 4-20mA in a 4-Wire Transmitter

[SaraXelon]

I can understand 4-20mA signal in a 2-wire loop powered transmitter (transmitter regulate the current in the loop; power supply (say 24VDC) provides the voltage to make that current possible). However, in a 4-wire transmitter, power supply (say 24VDC) is provided to 2 of transmitter terminals and the 4-20mA signal is out from the other two transmitter terminals. I assume some voltage is involved with 4-20mA signal. What could be this voltage? If not, how the 4-20mA signal is possible without any voltage involved in the loop?

 

[IRstuff]

The voltage is ANYWHERE between ~2V to 24V, depending on the series resistance of the all the downstream receivers. This is no different than the so-called "ideal" current sources you encountered in freshman EE class, with the exception that these are "real" current sources, and therefore have compliance limits and series resistances that affect the voltages.

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[danw2]

4-wire 4-20mA outputs usually comes from an AC powered device like a chemical analyzer, a stand-alone PID loop controller, a PLC's analog output card, a position feedback signal from an electric motor actuator; all of which being AC powered devices have one or more internal DC power supplies to power whatever specialized functions are needed.

The 4-20mA output is just one of several functions running from an internal DC power supply. One advantage of a 4-wire output is that it can generally drive more load resistance than a 2-wire loop powered device; which is important when driving, say, two split-ranged valve positioners.

 

[itsmoked]

The 24vdc to the transmitter is the driving voltage for its output. It is likely galvanically isolated from that 24vdc supplied to the transmitter.

galvanically isolated = you can't measure a resistance from the 4-20mA output terminals to the 24vdc power input terminals. (done with a magnetically coupled switching power supply) That's the way I did it when scratch-designing several different transmitters, at least the ones that weren't loop powered.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[danw2]

>It is likely galvanically isolated from that 24vdc supplied to the transmitter.

Well, yeah, an output would be isolated if you designed it, Keith, but lots of other designer don't share your concerns with getting it right. I've seen lots of effort put into the analyzer electronics and little or none on the analog output, which tend to be non-isolated, single ended, ground referenced 4-20's or 0-0.1/1/5/10V outputs. Ripe for common mode ground loop problems.

 

[itsmoked]

Interesting to hear Dan. Seems most of the transmitters I've messed with that weren't loop powered were isolated, but for whatever reason, were crazy expensive top-end units. Probably explains my experiential notion of isolation.

Hired out of school the first thing dumped in my lap was an assignment to design a loop powered microprocessor-based self-code altering humidity transmitter. The prior micros used at the company were 4004 and latest was an 8008 project underway. I used a windowed 8048 and a cross compiler running on a radio shack computer. Boy was it hard to get that transmitter to run happily on only 4mA.

Keith Cress
kcress -

http://www.flaminsystems.com