On May 25, 2007, at 7:21 PM, Randall Nortman wrote: > I would like to measure mains voltage (110-240V, 60Hz), but I want my > measurement circuit to be isolated from the voltage being measured. I > am going to be sampling the waveform at a high sample rate (relative > to the 60Hz waveform being measured) and comparing that with the > current on the same lines with (near-)simultaneous measurement. So it > is important to me that the voltage waveform not be distorted or > phase-shifted, and the voltage I see should be related to the source > voltage by a simple linear ratio. > > Seems like a simple transformer will do it, in theory. But I'm not > sure about how they will work in the real world. Do transformers > distort the voltage waveform or phase?
Some. Cores are nonlinear, there are ohmic losses, eddy current losses, magnetization inductance, leakage inductance, capacitance, ... For a stiff voltage drive and a light load, you'll probably find the distortion minimal. But you haven't stated a quantitative requirement and transformer manufacturers generally don't write specs in a way you can relate to here, so you'll want to to get a transformer and make some measurements, I suppose. > What happens if I put a load > on the secondary of more than a few nA? For a small power transformer you'll probably find the difference between no load and 1 mA difficult to measure. > What non-linearities are > there? Mainly the nonlinear hysteresis curve of the core. > What sort of transformers are going to give me the best > response? Hard to say. Again, you haven't stated quantitative requirements, and the usual specs won't tell you anyway. So, I suggest just getting a cheap power transformer, trying it (look at the AC in and out on a scope, say), and if it isn't good enough come back here with details of the problem. > > Or if the transformer idea is awful, any other ideas for isolated > voltage transducers? A transformer is probably fine. Alternatively, google "isolation amplifier". These are good for sensing current (through a small resistor) too. John Doty Noqsi Aerospace, Ltd. http://www.noqsi.com/ [EMAIL PROTECTED] _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
