Re: Idle Curiosity -- 4-20mA Signalling
- From: Spehro Pefhany <speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 06 Oct 2009 15:25:51 -0400
On Tue, 06 Oct 2009 20:07:35 +0100, Paul E Bennett
<Paul_E.Bennett@xxxxxxxxxxxxx> wrote:
Spehro Pefhany wrote:
On Tue, 06 Oct 2009 09:44:56 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:
On Tue, 06 Oct 2009 11:12:08 -0500, Tim Wescott <tim@xxxxxxxxxxxxxxxx>
wrote:
Is there a (IEEE, EIA, whatever) specification for voltage levels in a
4-20mA signaling loop? Or is there some common way of specifying the
amount of power available to a loop-powered device? If it isn't
specified, is there some common usage?
I'm curious as to how much freedom one has to power ones device when one
designs some gizmo that flaps in the breeze on the end of a current loop.
But I'm not designing anything right now.
Danke.
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.
...Jim Thompson
Loop powered systems usually have a worst-case low end of about 3.6mA
+/-. They have a minimum and maximum voltage so you can determine the
maximum supply voltage and calculate the maximum load resistance. They
can go over the 20mA, usually by 10% or more, sometimes as bad as 30mA
total (for example, as a result of sensor failure) so you should allow
for that. The electronics can use more than 4mA but you'd have to use
a SMPS and eat more minimum voltage. Most will work to well under 10V,
so maybe you could get 6 or 8 mA at 3.3V. Often you need to run at
least one power supply with galvanic isolation plus a microcontroller
and perhaps a display off of that power, plus whatever signal
conditioning, sensor excitation, ADC converters and other stuff is
required to get a signal into the micro, meaning the power supply
can't be wasteful (nor are commercial modules usable).
The maximum load resistance limitation usually comes into play when
several loads are connected in series (and perhaps a lot of wire in
the middle). For example, a field-mounted loop-powered indicator
connected in series with the SCADA system input.
Four wire (non-loop-powered) systems don't have the lower end
limitation, but if you want to tell the difference between railed at
the low end and a broken loop you need to avoid going right to zero.
There's a maximum load resistance specification. If you're going
4-wire it's best to galvanically isolate things as much as possible as
the number of ways to f*** things up increases exponentially.
All the systems I've seen don't mind zero resistance load.
I don't know of any standards, but the above are examples of common
usage. You might be able to find some more stringent 4-20mA specs as
the analog part of the HART (hybrid analog/digital) standard.
Likewise I have never come across a specific standard for the 4-20mA loop
systems but I have seen a range of excitation voltages used in circuits that
feature them. Most I dealt with were the teleprinters running on 60VDC
supplied 4-20mA loops.
The 20mA teleprinter loops were digital.
Wikipedia has some useful info.
http://en.wikipedia.org/wiki/Current_loop
.
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