Connecting to AC signals with a SCC module

[J Bowen]

I have a USB DAQpad 6015 and a SC-2345 signal conditioning unit and I was looking for modules to connect an AC signal (ie to do generator output testing).  Can I just use the SCC-A10 voltage attenuator even though it will cut off 20V?

[Travis G.]

Hello J Bowen,

The module to use really depends on the characteristics of the AC signal you are trying to measure.  Specifically, what is the amplitude and frequency of the AC signal that you are trying to measure?  Is your AC signal around the typical 60 Hz associated with power generation?  The SCC-AI01 module can measure a signal range between +/- 42V and includes a 10kHz lowpass filter.  If your amplitude and frequency fall within these ranges, I would recommend this module.  If you need to measure higher amplitude signals, then the SCC-A10 can attenuate your signal so that it can be acquired by your DAQPad.  Finally, if all your interested in is the frequency of the AC signal, I would recommend the SCC-FV01, which converts a frequency measurement of an AC signal into a DC voltage to be read by your DAQ card.  Let us know some more detailed specs of your signal and we can narrow down which module would be best for you.

Regards,
Travis Gorkin
Applications Engineering
National Instruments
www.ni.com/support

[J Bowen]

I'm doing generator testing so at the least I'd be measureing the frequency (60Hz) and voltage (120V) as different loads are applied. Ideally I'd like to measure RMS voltage and frequency and was hoping there was a module that did that, or atleast allowed me to sample enough data to calculate it in labview (ie capable of reading 170 to -170 peak to peak voltage)

[Travis G.]

Hello J. Bowen,

Unfortunately, there is no single SCC module that can be used to perform the type of attenuation you need to read a 120Vrms signal with the DAQPad 6015E.  Our higher-end line of signal conditioning products, the SCXI series, includes the NI SCXI-1125 module which is ideal for this application, with an input range of +/- 300V.  With upgrading to this system, in addition to the module, you would need an SCXI chassis, an NI SCXI-1327 High-Voltage Attenuator Terminal Block, and an NI SCXI-1600 USB DAQ module, so it would be a relatively serious investment.  Another possible option is to develop your own attenuator or voltage divider circuitry in order to scale that voltage down into something that your existing system could handle.  We do offer the SCC-FT01 Feedthrough Module for SC systems.  This module contains a small breadboard area for designing custom signal conditioning circuitry.  This module does allow direct access to the analog input lines on your DAQPad, so I would recommend being very careful with the high voltages you are working with here.  I would not recommend just using the SCC-A10 Voltage Attenuator because you would be greatly exceeding its operating range with actual peak-to-peak amplitudes of +/- 170 V.  I hope this helps and feel free to reply with any more questions.

Regards,
Travis Gorkin
Applications Engineering
National Instruments
www.ni.com/support