Minimum Voltage to drive 4-20ma 4 wire current input

[chuah]

Hi,

May i know the how to know the minimum voltage to drive a analog input card. I'm connected to 4-20mA channel for 4 wire, supply voltage from transmitter is 14.4V. From the S7, It not able to detect the current feedback from the transmitter.

Please Help. thanks.

 

[danw]

There is no pat answer for your question as to how much voltage a 2 wire transmitter needs to operate. Each manufacturer specifies how much voltage a transmitter requires.

But only in a shop or laboratory does a transmitter connect only to a power supply; in the real world additional voltage beyond the minimum is needed to drive the total loop resistance.

Mr. Rosemount says, "the total resistance load is the sum of the resistance of the signal leads and the load resistance of the controller, indicator and related pieces. Note that the resistance of intrinisic safety barriers, if used, must be included."

Frequently, a graph showing required voltage vs loop resistance is published, as shown below, or an equation for resistance and voltage.

In some cases, a minimum voltage is specified, leaving the user to calculate any additional required voltage for additional loop loads.

Check the transmitter's documentation and have some idea of how much input load resistance, wiring resistance and any other loads there are in the loop.

 

[shooter]

depending on input card, some work with 200 mV some work with 5 or 10 volts.

 

[danw]

I missed the words "analog input card" and mentally substituted 2 wire transmitter. Oh well, Shooter caught it.

There are a number of analog input modules for the S7, but one, the Analog input module SM 331; AI 8 x 16 bit; (order number 6ES7331-7NF00-0AB0) uses 250 ohm resistors, as this drawing from the S7-300 Automation System Module Data Manual (6ES7398-8FA10-8BA0) [page 297] shows:

If one Siemens card uses 250 ohms, they all likely do, but you can check for your specific card.

250 ohms drops 1.0V at 4.0mA.

Things like wire break detection trigger at lower mA values.

But a valid 4-20mA signal should have a minimum of 1.0Vdc across the analog input's (+) and (-)

Have you done the jumpering necessary to put the resistor into the input circuit?