Re: 4-20mA Analouge input
- From: "Lefty" <llah@xxxxxxxxxxxxx>
- Date: 18 Feb 2007 15:03:36 -0800
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
.
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