Power supply for analog inputs

[Tim Chavis]

Can anyone tell me how i should power my analog inputs on the 888 model i will be using all 8 inputs. The question i have is would it be ok to power the A/I with the same power supply i'm using to power the plc, the transmitters will be 2-wire transmitters. Im asking this because sometimes you have isolation problems, and I havent found anything in the manuals except how to wire up a pot. to manipulate the inputs,  Thanks Tim

[support]

The T100MD888+ automatically uses the PLC's power supply to provide reference voltage and supply current to its analog i/o so there is no need to connect external power supply to the analog i/os.

I am not sure if your sensors has isolated power supply for the transmitter, or whether the "-" terminal of the two-wire transmitter is in common with its negative power rail. If it does then you can use the same power supply for the PLC as for the sensor transmitter.

If not, then you have to check for isolation issue. If you are using 4-20mA two-wire transmitter, then it needs to be connected to a 250 Ohm resistor to turn into 1 to 5V. The "-" terminal of the two wire-transmitter has to be connected to the PLC' 0V power rail. You need to check whether if this would create a short circuit on the transmitter circuitry. Use a meter to measure the voltage between the "-" terminal of the transmitter and its power supply to see if there is any active voltage. There should not be a constant DC voltage between them otherwise you will need to use a different power supply for the transmitter so that there will not be a ground fault.

[Tim Chavis]

I'll be using 4-20 ma, 2-wire, loop powered transmitters, there will be 8 of these transmitters coming into 1 PLC , what i was wanting to know is can i use the PLC's power supply to power all 8 transmitters, and would this cause a problem for the transmitters, these are not  4-wire transmitters, they do not require a seperate P.S. at the transmitter. If powered this way there will be 8 250 ohm resistors in seiries with 8  transmitters but all 8 resistors and transmitters will be in parallel with the PLC's power supply, the transmitters will control the current across the resistor's which will provide the 1-5v drop, this is what i'm not sure about .  If it would be easier to scetch somthing in MSpaint or somthing please feel free to send it to me at tchavis@cei.net ......these transmitters are $550 dollors apeace, so before I buy these things I would like to know for sure if they will work, if i have to buy separate P.S. for each transmitter that would cost another $400 .....isolators are about $200 apeace so you can see where I'm going with this
thanks alot    Tim Chavis

[support]

I think you are referring to the following wirings. Yes, it should work: ?



The picture show wiring of 1 sensor but it can be extended to all 8 sensors.

What kind of sensors are you using? $400 per module is pretty steep. If you are measuring thermocouple or microVolt signal you may like to check out the I-7018 which can connect directly to thermocouple. ?For more info on I-70xx, please visit:

http://www.tri-plc.com/i7000.htm

[Tim Chavis]

The transmitters are Dissolved Oygen sensor's with      4-20 transmitters in the sensor itself, you can buy the sensor with just the mV output (0 to 33 mV)but the sensor will be a long way from the plc so you need the 4-20, AnalogDevices.com offers a sigle DIP packedge that  has a mV input and a 4-20 or 0-20 ma output and can be loop powered , they have pins for setting the gain and  zero  , that can save you $200 bucks per sensor.  You should look into this more, you offer great plc's, but the signal conditioning required to interface to your inputs/outputs is somtimes more expensive than the PLC itself.  With the experiance and knowledge that you guys have you could design, build and offer these products to compliment all your other great products the cost would'nt be that high if you used 3rd party products like the AD693 from analog devices.  And Yes that is what i was refering to in your drawing, Thanks for going through the trouble of doing that it was very helpfull. Thanks again  Tim Chavis