DC Ripple Measurements

[CraigAO]

Looking for help to see if it is possible to add a function to my software to measure the ripple voltage on the output of a dc generator (~28 Vdc) using my existing hardware. I am currently reading the generator output using a NI-9221 module in a cDAQ chassis.

I have the ability to measure this externally with an oscilloscope but am looking to see if there is a way to read/measure peak and peak to peak ripple measurements to add some convenience for my end operators.

Currently running in LabVIEW 2013 with access to the Electrical Power Suite if that helps.

[Gregory]

Do you know what timescale you are looking at, and how many channels you are using at once? It looks like the 9221 supports 8 ch and 800kS/s. The sample rate is usually for all channels, so divide by the number of channels you are using. At the full 800kS/s that puts 1.25us between samples.

It is a 12-bit device so the resolution is about 60V / 2049 = 29mV

So you just have to decide if the features you are looking for will be resolved with the 1.25us timing and 29mV resolution, which you should be able to figure out from your oscilloscope trace.

[cstorey]

What does the ripple look like?  Magnitude and frequency?  Sporatic/Noise or periodic?  If it fits within the measurement specs of the 9221 you should be able to look at it.

I'm not sure what your application is, but you could split your signal, sending it into the NI-9221 directly and the other through a DC block (capacitor(s)).  Measure the magnitude of DC voltage on the first and the ripple on the second.  This way you could see the ripple better as you can use a lower scale (uV or mV rather than >20V).

Craig

[CraigAO]

The operating speed for the generator, for this test point, is 8000 rpm or about 533 Hz. Since the dc output is a rectified from the ac in the generator, the waveform has a typical sine wave look to it. The range we look to measure is 4 Vp-p. This is on only one channel (three other channels being used for other voltage readings). These aren't fine measurements, so reading within a tenth of a volt is acceptable.

[RTSLVU]

Granted it's been a while but isn't measuring AC ripple as simple as measuring the the same point but in ACV mode (rms) instead of DCV, then taking the Vrms measurement and multiplying it by 1.414 to get Vp-p?