Goals and Symptoms

If an analog output is scaled to 4mA-20mA, is it possible to prevent the meter from outputting less than 4mA, when the input drops below the 4mA scale point?
For example: Someone wants to associate real power from 15kW to 100kW to 4 to 20mA. They do not want the output of the analog output to go below 4mA when the power value is below 15kW.

Causes and Fixes

There is no way in hardware to make a 0-20mA output become a 4-20mA output.

In order to do this one forces the 4mA point to become the lowest input possible, while 20mA point remains the full scale value. This is done by
Zero scale = -.25*Full Scale (according to the Digital and Analog Document)
For example:
Full scale = 100kW
Zero Scale = -6.25kW
Thus 4mA output at 15kW, but a -6.25kW input would still result in 0mA output,

Unless…

You use an arithmetic module (or a setpoint module) to prevent the input from going below 4mA point.
S1 = real power value
R1 = input to the analog output module
Formula 1 = IF (S1<15kW, R1=15kW,R1 = S1)
This will force the analog output module to stay at 4mA when the real power goes below 15kW.

It could be used to force the output to 0mA at inputs less than the corresponding 4mA point by:
S1 = real power value
R1 = input to the analog output module
IF (S1<15kW, R1=-6.25kW,R1 = S1)
This results in all real power vales less than 15kW forcing the analog output to 0mA.

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Legacy KB System (APS) Data: RESL200536 V1.0, Originally authored by MiNe on 11/20/2009, Last Edited by MiNe on 11/20/2009
Related ranges: ION8600, ION8800, ION7550/ION7650, ION7300, ION7700, ION7500/ION7600