adccommunitymod (AutomationDirect) asked a question.
Created Date: September 10,2010
Created By: bartb102
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I 've run into a situation with a 4-20ma analog transmitter that can only drive 200 ohms. F2-04AD-1 module has a 249 ohm resistor in the loop so I can get only 17.4ma from the transmitter, not 20ma, at maximum. Easy & best but expensive solution is to buy signal isolators that have lower value input resistors. Second option is to put 1000 ohm resistor across input (parallel built in 250 ohm), thus get 200 ohm load & 20ma from transmitter. I lose input voltage (20ma @ 200 ohm = 4V = 3071 counts), but acceptable. BUT at 4ma, I get only 0.8 V, so minimum measurable transmitter output is 5ma (5ma @ 200 ohm = 1V = 0 counts). Combined I lose quite a bit of resolution. Transmitter is too smart- at maximum it knows it isn't putting out 20ma and won't calibrate correctly. 1. Is there any way to re-calibrate the F2-04AD-1 card so that 0.8V = 0 counts & 4V = 4095 counts? 2. Are there any low cost signal isolators available? Ones I 've seen are $248 each. Input load must be 200 ohms or less. AD's are low cost but 250 ohms. Thanks, Bart
Created Date: September 13,2010
Created by: FACTSTech
Bart,
1. Is there any way to re-calibrate the F2-04AD-1 card so that 0.8V = 0 counts & 4V = 4095 counts?
No
2. Are there any low cost signal isolators available?
Yes, however we may be able to use FC-11 but need more information about the transmitter output open circuit voltage.
The burdening voltage must be subtracted from the theorical output voltage when driving the maximum current (i.e. 20mA).
Using Ohm's Law - open circuit voltage - burdening voltage>= 4V(20mA)
Created Date: September 14,2010
Created by: JimHoward
Bart,
Most analog transmitters have a supply voltage vs. maximum loop load graph. This data would typically allow you to provide a higher supply voltage to the 4-20ma analog transmitter, and it in turn can drive a higher resistance load.
But, if that is not possible, have you considered NOT using the F2-04AD-1 4-20ma analog input module, and instead use a F2-04AD-2 voltage analog input module? With a precision resistor of 200 ohms across the input of the F2-04AD-2, you will get 0.8 VDC (at 4 mA) and 4.0 VDC (at 20 mA) as you mentioned above.
Scaling math in the PLC completes the process.
Created Date: September 15,2010
Created by: bartb102
The circuit is being powered by 24V, & the manufacturer specifies 0.2k ohm maximum load. So, no luck there.
Yes, the voltage input module would work, but I was trying to use what I have. I'm out of slots & the other two sensors work fine on the ma card.
Luckily I found a dual signal isolator stuck in the back of a parts bin that seems to be functioning ok.
Created Date: September 15,2010
Created by: FACTSTech
Bartb102
AD's are low cost but 250 ohms. (FC-11 $89)
The FC-11 does not have 250 resistor, it is loop powered (dynamic)
If you follow the instructions of my post, the FC-11 will work with your transmitter and the output of the FC-11 will connect directly to the F2-04AD-1 input.
Created Date: September 15,2010
Created by: bartb102
You are correct. Next time I'll know better.
Created Date: September 10,2010
Created by: bartb102
I 've run into a situation with a 4-20ma analog transmitter that can only drive 200 ohms. F2-04AD-1 module has a 249 ohm resistor in the loop so I can get only 17.4ma from the transmitter, not 20ma, at maximum. Easy & best but expensive solution is to buy signal isolators that have lower value input resistors.
Second option is to put 1000 ohm resistor across input (parallel built in 250 ohm), thus get 200 ohm load & 20ma from transmitter. I lose input voltage (20ma @ 200 ohm = 4V = 3071 counts), but acceptable. BUT at 4ma, I get only 0.8 V, so minimum measurable transmitter output is 5ma (5ma @ 200 ohm = 1V = 0 counts). Combined I lose quite a bit of resolution.
Transmitter is too smart- at maximum it knows it isn't putting out 20ma and won't calibrate correctly.
1. Is there any way to re-calibrate the F2-04AD-1 card so that 0.8V = 0 counts & 4V = 4095 counts?
2. Are there any low cost signal isolators available? Ones I 've seen are $248 each. Input load must be 200 ohms or less. AD's are low cost but 250 ohms.
Thanks,
Bart