SLC Analog Signal Conditioning

I am trying to make a pressure transducer work with my SLC but I don't want to fry my analog card. I am powering the transducer with 24V DC. The problem is the maximum input voltage for my 1747-NIO4V with a 4 to 20ma current loop is 7.5V. Does anyone have a quick fix for dropping a 24V current loop down to the 7.5 volts the SLC uses??

Just do it!

If your transducer is able to output a 4-20ma current signal the NIO4V will accept it ok. The loop voltage is really irrelevant. If you have been looking at the output from your transducer as an open loop voltage, that will fool you. When you connect the + and - terminals to the analog input module, the voltage across the input will be fine for the NIO4V. Power to the transducer being 24VDC does not mean the output current loop will operate at that voltage. If you want to see what the NIO4V will see, take a 250 ohm resistor and connect it to the current output from your transducer as it the resistor was the module. The voltage across the resistor will be in the range of 1 - 5 VDC. Be sure to not have that resistor installed when you actually connect to the module, it already has one inside.

The transducer is a 2 wire setup so the 24V supply makes up the current loop.




24V+ --------------- + trans trans- -------- + PLC in

24V- ------------------------------------------- - PLC in


Sorry I didn't clarify in the beginning

Just go for it

This does not change anything. The open loop voltage may be 24VDC but that voltage does not appear when you connect it to the PLC input. As I suggested earlier, try just putting a 250 ohm resistor across your sensor output leads and see what the voltage is. You might also want to put an ammeter in series at the same time to watch the loop current. If you can cause the sensor input to change you will see a corresponding change in the loop current and the voltage across the resistor will vary between 1 and 5 volts. Try it and see.

As Randy said, if the trasducer is a 4-20ma out, it will be fine.

Well I just had to see for my self.
Got the resistors this morning and all the readings were great.
I will wire it up at lunch.

Thanks for all the help

Tim