High voltage (120 VAC) on SCXI-1321

[OHP]

I am currently using the SCXI-1321 attached to SCXI-1121.  I would like to measure 120 VAC through this module.  As far as I can tell, it should be able to handle 250 Vrms (and this was confirmed by the NI help staff), however when I try to set the minimum and maximum voltage in SignalExpress or Measurement and Automation, it limits me to +/- 5 V.  How can set the minimum and maximum voltage to a larger value?

The SCXI-1121 jumpers are set to a gain of 1 and the filter frequency is set to 10,000 Hz.  The Measurement and Automation software is configured to match the jumper settings.

Thank you for your help.

[Alex A.]

You will need the NI-1327 attenuator terminal block in order to extend the range of the 1121 to 250Vrms.  There is a knowledgebase here that discusses the issue, and the product page here on Table 3 mentions using the 1327 to extend the range.  The voltage range on your 1121 will still be +/-5V, but your larger voltage scale will be attenuated to fit in that range.

[OHP]

I must admit I am curious why the SCXI-1321 has a label printed on the front of the device saying "High Voltage" and the SCXI-1121 says "250 V Max Channel to Channel" on the front of the device.

Thanks

[Buechsenschuetz]

Channel to channel isolation specs and max. input voltage per channel are two different issues.

The "between channels spec" indicates the difference in voltage measured between any input of ONE channel and any input of a DIFFERENT channel. This spec is important if you measure voltages coming from different voltage sources having NO common ground (GND) connection. Depending on the setup there may be significant voltage between the reference terminals of these voltage sources. This voltage must not exceed the "between channels spec" of the measuring device.

Anyhow I haven't seen any A/D boards yet which handle significantly more than +/- 10V input voltage without a voltage divider.

When using voltage dividers or attenuation resistors, be careful! ALWAYS check whether they are properly connected BEFORE connecting anything to your system, otherwise the input voltage specs may be exceeded. Keep in mind that the "absolute maximum specs" are NO operational specs, i.e. the absolute max voltages should NOT be applied over extended periods, and usually proper operation of the system is not guaranteed if you exceed the operational specs.