4-20mA current source help.

Hi every body.
I have some problems Easidew On-line Hygrometer
now I used a Easidew On-line Hygrometer to meansuring humidity of air ,
It have a analog output 4-20mA current source over the entire the dew-point range.
now I want to use this analog output to connect with plc s7-400 via module AI of ET200s 4-20mA. but it not compatible and module fault.

If somebody know it ,please help me.


thank you very much

best regard,

Current source implies that this is a 4-wire device and the power for the 4-20 mA loop is coming from the transmitter. This must be wired differently than a two-wire device like a pressure transmitter, but if wired properly it should be compatible with any analog input. I don't have the specifics of the Siemens module handy, but if you talk to your local distributor he should be able to provide you with the wiring diagram for a four-wire transmitter.

The Hygrometer manual, http://www.kahn.com/hygrometers/manuals/easidewonlinemanual.pdf page 5, shows only three connections for the sensor:

-Red +16V nominal power
-Green 4 to 20 mA source
-Blue & shield ground

I think that in this case "sourcing" means the opposite of "sinking". In other words, current comes out of the signal wire (green). If you have an input device that has a common GND connection instead of a common + connection, this is fine. However, if your input device has a common +, with current sinking inputs, this will not work for you. Some devices, such as http://www.icpdas-usa.com/products.php?PID=268 , have differential inputs, and will work with sourcing or sinking output devices. I was not able to find the Siemens documentation to figure out what type of input device your ET200s is.

Thanks everybody very much.I used module et200s 6ES7 134- 4GB10 -0AB0
(4 WIRES) to connect analog output 4-20mA current source on rear panel ok.but it reduced currents (2mA) in comparision with before.therefor display value on plc different on Easidew On-line Hygrometer (local display panel).
I dont understand.

Very often the outputs from inexpensive instruments are not isolated from their power supply or ground so you get more than one path for the output. It's easy to check this with a multimeter, just see if you get a circuit from the output. If this is the case you may need to put the instrument on it's own ungrounded power supply.
Roy

red put 24 or 16 volts on it
green goes to input
blue goes to zero AND a wire from the card also to zero.