Using a MicroLogix 1100 with 4-20 mA loop powered field device

Yes. It does not matter. I use this set up all the time using moeller smart relays (which only accept 0-10 VDC) with loop powered Magnatrol 4-20 transmitters.

You don't want more load on the device powering the transmitter than rated but 500 OHMs is OK and actually requested as a minimum resistance in many loop powered systems.

This will give you 2 VDC to 10 VDC not 0-10 as you are starting at 4 MA not 0MA.
 
That's great! So no risk to fry my input if I'm using 24V loop supply? And also, where is the 500 ohm resistor in the circuit? I never used this trick before so I don't know where it needs be installed.
 
Take + source to transmitter.

Take signal back from transmitter to one side of resister. This is the same point you will read your + volts at.

Take the other side of the resister to - source. This is also the same point you will get your - from for your volt reading.

You are reading the voltage drop accross the resistor.
 
Outstanding! I was considering using a converter such as Weidmuller P/N 8445040000 which I guess would be a more "elegant" manner of doing it and cheaper than adding an expansion card, but I think I'm going to use the resistor technique to give it a shot. Thanks a million mr Robert!
 
If the resistor were to open then the transmitter may put a much higher voltage across it in an attempt to transmit 4-20ma. Whether this would damage the im=nput depends on its protection.
 
For the greatest accuracy measure the resistor with an accurate ohm meter and then calculate the actual voltage drop you will get across it at 20mA and 4mA and use those figures in your scaling. 500 ohm resistors are rarely exactly 500 ohms.
 
If the resistor were to open then the transmitter may put a much higher voltage across it in an attempt to transmit 4-20ma. Whether this would damage the im=nput depends on its protection.

No different than a wire break. Damage unlikely.

500 ohm resistors are rarely exactly 500 ohms.

Correct. Look for .1% or less tolerance rating. I assumed you allready had one. I order mine from: http://www.workaci.com/accessories/resistor.htm

I use the 499 OHM.
 
Alaric said:
For the greatest accuracy measure the resistor with an accurate ohm meter and then calculate the actual voltage drop you will get across it at 20mA and 4mA and use those figures in your scaling. 500 ohm resistors are rarely exactly 500 ohms.

Actually I'll use the register value in the PLC to do my scaling. I'm also assuming that being from 2-10V instead of 0-10V reduces my resolution by 20%, but this is a level transmitter and I don't think it will be much of a problem.
 

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