Re: 4-20mA Analouge input

On Feb 18, 3:23 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@xxxxxxxxxxx> wrote:
Lefty wrote:
On Feb 18, 2:44 pm, "AJ" <itwasm...@xxxxxxxxxxx> wrote:

Hi,

I have been asked to make a 4-20mA input and im not 100% how to go about
this. Im sure there is a standard im meant to be following but im not sure
what it is. Can anyone help point me in the right direction? Im imagining
a sense resistor and an amplifier???

Best regards

AJ

4-20 dc ma is a standard industrial process control interface signal
between field sensors (pressure,temp,flow,etc) and process control
electronics in the control house. This 'loop' is generally powered
from a 24vdc power supply. The advantage, of course, is that there is
no error induced because the length (resistance) of the signal wire
pair that can be hundreds to a few thousands feet long if needed. The
loop voltage also 'powers' the field sensor device. Most input
circuits just wire the 4-20 in series with a 250 ohm resistor to
measure the 1-5vdc voltage drop using a diff input voltage amplifier
to further process or convert it with a A/D convertor. The reason the
4ma = 0% level (live zero) is to have a means of detecting an open
loop condition and of course to have a small amount of power for the
field device to consume independent of the measurement value. Works
very well and has been in use for decades.

Lefty

and remember, that in cases where you have multiple devices connected
in series, each device will attempt to maintain 20 ma on the TX in the
loop as an off condition so that the other devices in the loop can lift
the loop to 4 ma so that a change in condition can be seen at the
receiver. This is mostly for SERIAL communications.
Just thought i would through that in!

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.http://webpages.charter.net/jamie_5- Hide quoted text -

- Show quoted text -


I'm not sure at all what you are describing but it's not the
industrial standard instrumentation 4-20ma interface. This is strickly
a DC analog measurement standard and not a digital communication
interface. There can be digital communications superimposed by
injecting a small ac fsk signal onto the DC measurement (HART
protocol) but the basic 4-20ma standard is to support one field device
hardwired to a central control point and was in use well before
digital electronics came onto the industrial control world.

Lefty


.

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