Idle Curiosity -- 4-20mA Signalling

That's really a system spec. The limit is the power supply voltage. 40V
is about the most you'll find, and 24V most likely. The drop across
other devices that may be in seried tells you how much you have left. At
20ma. At 4 ma, you have 1/5th of that. Google will turn up a lot of
stuff like
http://www.bapihvac.com/CatalogPDFs/I_App_Notes/Understanding_Current_Loops.pdf

Jerry
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I'm curious about a "standard" as well.  I _think_ the device
electronics must simply consume less than 4mA, so that "zero signal
input" can cause a draw of exactly 4mA??

This one...

http://analog-innovations.com/SED/SpehroLoopProtect2.pdf

dates back 8 years, in answer to an original post by Spehro.
        
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Don't know either - I think I would make it less then the SELV voltage
limit in any case?

--

John Devereux

Tim Wescott wrote:

You'd have to go by the compliance range given in the datasheets:

http://www.cla-val.com/pdfs/E-X117D.pdf
http://www.aamatrix.com/datasheets/AF-FtR_DS.pdf

The standard would be ANSI/ISA S50.1 which I don't have but parts are
outlined here:

http://www.dataforth.com/catalog/pdf/an104.pdf

As Lasse hinted you'd have to obtain the compliance ranges of all the
devices in the system. After all, it won't do you any good if the whole
thing complies to the standard but doesn't work reliably.



But, as the scouts say, be prepared :-)

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Tim Wescott wrote:

   I would say an excellent question; another way is to ask what is the
voltage compliance.
   If one wanted to be really bizarre and have a large voltage
compliance, then the open circuit voltage could be up to 100KV which
would allow a small break in the circuit to be "ignored".

On Tue, 06 Oct 2009 21:29:30 -0700, Robert Baer


This would require a quite large loop current also.

While the high open loop voltage may allow sparks to occur across air
gaps, the created ionization does not last long, unless sufficient
power is supplied to compensate for the radiation losses.  

Of course with long wires and large stray capacitances that would
create a resonant circuit, so this would be a nice spark gap
transmitter and as such, could be used for wireless communication :-).

Paul