4-20ma controls

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

 

[PeteS]

As noted - 4mA is a good 'zero' that is also a valid signal. I use
4-20mA loop devices and zero current implies (quite reasonably) that
the sender is either broken or not connected.

Shorting the leads causes no damage whatever - the current will still
be whatever it was, regardless of where the short occurs, provided it
is beyond the sender, of course. For an industrial situation, this is
perfect.

Another good reason for using this vs. a higher current (besides the
fact that this is easily sufficient) is it gives greater range for a
given output driver. I have used (and seen used) 4-20mA loops for over
1km, where the compliance requirements can exceed 100V, but as this is
simply I*R(loop), a higher current would have a higher compliance (Vout
range) for a given loop.

I will note that many pieces of industrial equipment can actually be
switched between 4-20mA and 0-20mA, which can actually cause a great
deal of grief for remote monitoring if one is not aware of it, and the
switch is on the wrong position.

Cheers

PeteS