Ive seen a lot of industrial controllers such as plcs and they all seem
to use a signal of 4-20 ma in proportion to relay sensor information to
the controller.
My question is .....why 4-20 ma.....why not 1-10 ma or 1-50 ma... what
is the reasoning between using 4-20 milliamps
thanks
4..40 ma is actually the common range for many industrial units, that is
why when you get a little current source hand held, you will noticed
that most of them will at least 40 ma's
the one i use will do up to 100 ma, but i have never used it that high.
its a nice tool if you need to calibrate a device to work with in a
range of (ma) window far from the actual controller that drives it.
steam/plumber fitters and the like must make sure that the valves
for example actuate properly for the described range.
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currents sourcing is nice because you can assure that you are getting
above some levels of current ambient current caused by adjacent wires in
raceways and the like but, this only applies to small levels of course.
current loops are also used in close loops type of communication
devices so that all devices on the line can monitor current activity so
that it can talk to the other units on the same wire or not collide with
current traffic.
these kind of devices normally need to have one unit as the current
source, the source current device normally tries to maintain the set
current point was devices get added on the line.
most devices use an optical type coupling or a LED or transistor,
depending on the direction of flow. each device creates a lost of
voltage that the current source supplies. at some given point, the
current source will not be able to maintain it as devices are added in
the loop of course, this is where repeaters come into play!
depending on the manf and device, the number of devices can really
get up there, for example RedLion type of serial display of a type i
have in mind usings 20 ma current loop supplied from an 18 volt source,
these units can address up to 100 0..99, but since each device and lose
and average of 0.7 volts in the TX state and 1.7 in the RX state, it
kind of limits the number of devices you can actually have on the loop.
enough of that.
...