What size resistor to convert 4-20ma signal to voltage on a CB-50LP

[UTKEngineer]

 I guess the subject says it all.  I'm trying to import the flow measurement from a F1700 series Micromotion flowmeter into LabView.  This requires the conversion of a 4-20mA signal into a voltage signal.  The student before me did this using a resistor across the + and - terminals.  I can't find the correct resistor size and was hoping someone might be able to clue me in.

Thx

[AnalogKid2DigitalMan]

A 250 ohm resistor will yield voltages between 1 and 5 V for a .004 to .020 A current.

Ohms law, V=IR, V=volts, I=amps, R=ohms. So 4mA (0.004A)*250ohms = 1V

Likewise 20mA (0.020)*250 = 5V.

Hope this assists

[UTKEngineer]

That would work if I could induce a total resistance of 250 ohms.  However, there is inherent resistance in the circuitry of the CB-50LB board.  So, I can't simply use ohms law.  If I recall correctly (and I may not), the board is configured so that a single resistor of x ohms connected between your AI and AGRND will give you the correct conversion.  However, I can't find the correct value of x either in the manuals or in my predecessors notes.

[K C]

Hi,

If I am right, the CB-50 is a connector block without any component's like resistors.

The Analog input of your board has a high impedance much higher then the resistor you need for the current to voltage conversion.

As the Kid suggested a value of 250E is normal. Mostly the range is 50 to 250 Ohms. The current source will drive the correct current regardless of what the resistor value is. Within in the range of course.

[JB]

I fully agree ! 5*

[UTKEngineer]

The CB-50 is just a connector block w/o any other components and the material I have found does suggest a very high impedence across the terminal blocks.  It's also coincidental that you say 50 - 250 ohms as, as best I recall, the resistor used previously was a 50 ohm.

I'll buy that any resistor in that range will convert to a voltage signal, but I have one remaining concern.  The CB-50 looks for a 0-10V signal by default and I am certain that that is the range we were measuring previously.  So, will any resistor in that range convert a 4-20 mA signal to a 0-10V signal?

Also, I'm sorry to keep adding constraints to the problem.  I would just go figure this out w/ trial and error but we recently changed the setting on our flow sensor and were going to use the DataAcq to verify it was reading correctly only to realize we're missing the resistors.  So, I REALLY appreciate the replies.

[AnalogKid2DigitalMan]

500 ohms will get you 10V from 20mA, 2V from 4mA.

"Resistance is futile, you shall be assimilated"

[K C]

The 500 ohm resistor will do, if you can find it. 499 ohm is a normal value. It depends on the accuracy you want. You can also use two 1k resistors in parallel.

But check if you device can drive 500 ohm's

And I don't want to be assimilated 

Message Edited by K C on 06-13-2007 07:55 PM

[The Chuck]

A better way to do it (if you have time and room) is to use a RCV420 IC. It will give you a 0-5 signal output from the 4-20. That is a 20% better resolution.

[JB]

The Chuck a écrit :

A better way to do it (if you have time and room) is to use a RCV420 IC. It will give you a 0-5 signal output from the 4-20. That is a 20% better resolution.

---

Have you noticed that this is a more than 3 years old post ?

Anyway, having used the RCV420 in many applications, I fully agree and also recommend it. Its sense resistance is 75 ohms.