More than 20 mA applied to AB input card

cirtcele

Member
C
Join Date
Apr 2007
Location
Illinois
Posts
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Today while tracking down a 4-20 mA loop problem i came across an input with 42 mA. The channel light for this input was flashing but the device connected to the loop seemed to be working properly. I should mention that this is a SLC 500
analog input card. The loop is powered by an old Ficher Porter unit that supplies the current and has calibration tags indicating that 42mA is correct for full output. My question is the light flashing because of the overrange current level or is the input damaged.
 
Measure the resistance of the input with source disconnected. it should be 250 or 500 Ohms. measure another channel to verify. If the resistance is okay, the resistor probably survived. You could pull the card and look to see if the resistor is burned. Electronics beyond the resistor should be able to handle the overvoltage. It expected a maximum of 5 or 10 volts, and got a bit over double. The IC should be able to handle 30-35 volts.
Put a 250 or 500 Ohm resistor in series to cut range down to 21 mA, then rescale your input. Alternatively, if you have a spare channel, you could wire it across 2 input channels, splitting the current between each one.
 
Last edited:
keithkyll said:
Measure the resistance of the input with source disconnected. it should be 250 or 500 Ohms. measure another channel to verify. If the resistance is okay, the resistor probably survived. You could pull the card and look to see if the resistor is burned. Electronics beyond the resistor should be able to handle the overvoltage. It expected a maximum of 5 or 10 volts, and got a bit over double. The IC should be able to handle 30-35 volts.
Put a 250 or 500 Ohm resistor in series to cut range down to 21 mA, then rescale your input. Alternatively, if you have a spare channel, you could wire it across 2 input channels, splitting the current between each one.
Click to expand...

If this is current loop output, he needs a resistor in parallel, not series. A current loop output will output 42mA no matter the resistance (within other limitations, of course).

Brian
 
This is a current loop and like i stated it appears to be working correctly. The input is used to display gate position on a scada system. I ran into this about 10 minutes before the end of my shift so i did'nt get a chance to dig to deep. The input was showing a value of 20499 and the operator said that the gate was showing fully opened on scada.I thought that this much current on a 4-20 mA input might make the input indicator flash. I'll look at the output file tomorrow and see if i can change the input range.
 
cirtcele said:
This is a current loop and like i stated it appears to be working correctly. The input is used to display gate position on a scada system. I ran into this about 10 minutes before the end of my shift so i did'nt get a chance to dig to deep. The input was showing a value of 20499 and the operator said that the gate was showing fully opened on scada.I thought that this much current on a 4-20 mA input might make the input indicator flash. I'll look at the output file tomorrow and see if i can change the input range.
Click to expand...

If you provide the catalog number of the Analog Input Card as Mickey previously requested, we can help you with the flashing indicator. I don't think this arrangement is going to last in the long term. The Burden Resistor in the Analog Card is dissipating over 4 times what it was designed for. Your best bet is to recalibrate the output of the Fischer Porter unit to a proper 4-20 mA.

Stu....
 
cirtcele said:
analog input card. The loop is powered by an old Ficher Porter unit that supplies the current and has calibration tags indicating that 42mA is correct for full output .
Click to expand...

It must be an old 10 - 50 mA transmitter. The input resistor for that range is 100 Ohms to give 1 - 5 Volts. You could just put another resistor in parallel to give 100 Ohms.
 
Hart Meter

If you have a Hart meter you could just loop test it, also if there's a problem with the device it may be set to fail high.
 
The 8-channel analog input modules for SLC-500 (1746-NI8, for example) have an LED for each channel that blinks when the channel is overrange, underrange, or has a configuration error.

The 1746-NI8 module user manual is here:
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1746-um020_-en-p.pdf

If the problem is a configuration error, Bit 15 of the Channel Status word will be =1. The Channel Status words are available if you're in "Class 3" mode and there are 8 data words plus 8 status words.

The spec sheet says 30 mA is the maximum current input, but I doubt the module is damaged. I'm not sure how it could give you an effective reading of an input that ranges all the way to 40 mA.
 

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