I'm having trouble understanding how measurement ranges, source limits, and source ranges all relate. I'm trying to figure out how to best to set up our unit for an application, which is basically this:

We are trying to observe a solenoid's self-heating behavior under various currents. We set up the 2461 to source current, then read the resistance as the part heats up. Once it reaches a trigger point (either a max heat or the resistance stabilizes), we then allow it to cool until repeating with another current level. We're using a LabVIEW program sending SCPI commands to the SMU.

What we seem to be experiencing is that even though we explicitly set SOUR:CURR:VLIM to some reasonably high value, we appear to be hitting a voltage limit equal to the number we set for DIG:VOLT:RANG. So in hotter tests, the resistance rises to a point where the voltage exceeds the DIG:VOLT:RANG, the unit can no longer hold the constant current, and it seems to switch to constant voltage. The manual describes some interplay between measurement ranges and source limits and ranges, but I'm having a hard time understanding exactly how. I don't think we want to just set DIG:VOLT:RANG to 24V or something and lose resolution at lower voltages, but I don't know if I can predict what a good value would be. We're currently setting DIG:VOLT:RANG to the calculated starting voltage + 20%, i.e. we expect the voltage to rise no more than 20%.

So I guess my questions are:

  • Can someone confirm that DIG:VOLT:RANG can become the V limit while sourcing current, and perhaps even why or how?
  • Is there a smarter way to handle this other than to just try and determine a better measurement range? We don't want to use auto range due to the time it takes.
  • Is the problem that we are not actually specifying SOUR:CURR:RANG? Or even a SOUR:VOLT:RANG?

I can provide a full list of the commands being communicated, but here is an example with what I believe are the relevant ones in order:

:SOUR:VOLT:READ:BACK ON
:SOUR:CURR:READ:BACK ON
DIG:FUNC "VOLT"
SOUR:FUNC CURR
DIG:VOLT:RANG 9.035928
DIG:VOLT:SRATE 10000
DIG:VOLT:RSEN
DIG:VOLT:UNIT OHM
SOUR:CURR 0.464811
SOUR:CURR:VLIM 12.000000

Voltage quickly rose to about 9.036 where it stopped, and current began falling as if switching from CC to CV.

I hope that's all clear and thorough enough. Apologies if this is obvious, but the manual isn't quite clear. It mentions that these various settings are related, but sometimes stops there, declining to describe how or when one affects another.