Current limit on PLC 5000

[DLK 0]

I need help has anyone else come across this?

Recently we converted a PLC 5 to PLC 5000 at the Burns Harbor site. During loop testing we were unable exceed 11.8 mA from one of the channels. This resulted in the valve connected to the output not being able to open more than 44%. When we removed the standard 250 ohm resister in the loop we could achieve the required 20 mA. 　

The conversion requires the use of an islolated analog output module "OF6CI". This module uses power from the rack power supply and not the separate 24VDC power supply most modules use. This apparently reduces the power available to drive the attached device. We found that the standard circuit with 250 ohm resister worked fine with all devices except the METSO 724 series Smart Hart positioner. I believe this specific METSO positioner consumes too much power for the isolated module output to power.　

This condition needs to be considered when future conversions are done. The resister is not really needed with modern output modules so I don't perceive any problem with removing it when this specific problem is encountered.　

Please pass this email on to anyone you feel would benefit from it.

Note that these are not my findings and I am confused by this as all other devices on this card work fine any thoughts would be a great help!

[PLCMentor.com 60]

I see that nobody has commented on this so I will shoot.  First I have not experienced this with the 5000 series controllers.  I did experience this on one of my first projects that involved a PLC5 rack with lots of analog I/O.  The PLC5 supplied its analog outputs via the rack power supply and I had too many cards in the rack for the power supply specified.  Not a pleasant moment!  Second I am curious about the "standard" 250 ohm resistor.  I assume from the text that the resistor is in series with the device.  All of the transmitters and other loop powered devices that I have used have sufficient load built into the device to not require an external resistor.  Of course having too much resistance on the loop will cause problems as will adding to many devices on to the loop (ie, adding too much resistance).  I do agree that the resistor is not necessary with modern output modules or (I will add) field devices. 

[Joe E. 259]

The only time I've had to add an external resistor is in parallel with the device to convert the mA signal to voltage.  That's on an older Eurotherm chart recorder.