incorrect current with scc 68

[Jo-Jo]

I am using a PXI 6251 with an scc 68, I have installed a 178 Ohm reistor accross the pins in order to make a differential measure of current.  The current displayed in max is the current accross the resistor (I verified it by taking a voltage measure across the resistor and appling Ohms law), but it is not the total current being put out by my current source, as there is obvisouly current flow through the scc 68.  I am trying to read a 4-20mA signal, but the reading in MAX is much reduced.  How do I read the total current put out by the current source?  It doesn't do me any good to know the current through the shunt reistor, as I don't know what the internal reistance of the scc 68 is.

Thanks,
Benjamin

[JB]

Which type of module are you using to measure the current ? I would recommend to use a SCC-CI20 which includes a high precision onboard resistor of 249R.

[Jo-Jo]

Alright, I got it.  I had the thing connected incorrectly.  I was trying to make things more difficult than they needed to be.

~Benjamin

[Mark_E]

Hi Benjamin,

Are you trying to measure the current through an external current source or the current from your SCC-68?

When you said that the “current displayed in max is the current accross the resistor…but it is not the total current being put out by my current source,” were you referring to the small amount of current that will go through the measuring device?

I don’t know which device you’re using, but they should have a precision resistor of a known value and tolerance and you can calculate any uncertainty in the specifications for that device.

Feel free to describe in more detail your application and setup if we’ve misunderstood.

This article is more particular to lower measurement ranges than mA, but the material can be useful as well.