4-20 Loop 2 Wire vs 3 Wire

[az1]

thanks for the reply.. i'm using Yokogawa Singal Condition Card (GS 33M50H85-40E) Model EA1.. it uses 2 wire system as in attachment.. my gas detector uses 3 wire system which is 24V,GND and 4-20mA signal.. if can't view the attached images, the images can be found at http://www.imagebam.com/image/801eef211082433 and http://www.imagebam.com/image/289494211082630.. thanks..

 

[cbuysse]

I'm just guessing here. I'm not familiar with Yokogawa and that documentation is just about as ambiguous as you can get. It appears to me that the card can supply 24 VDC from A to B and then there's a 250ohm resistsor connected from B to C so that all it's really doing is converting the signal to 1-5VDC. I can't tell for sure. My first attempt would be to connect it this way:
DCS Transmitter
A --- 24V
B --- GND
C --- 4-20mA

If that doesn't work, I'd go with a separate power supply. Connect the (+) and (-) from the power supply to 24V and GND on the transmitter. Also connect the (-) from the power supply to B on the DCS and then connect 4-20mA from the transmitter to C on the DCS. It'll end up looking exactly like the middle diagram on one of the earlier posts.