Re[2]: Measuring AC
I suspect (but don't know for sure) that the accuracy of a transformer is the way it performs, relative to it's spec's, rather than relative to its history. That is, a _given_ transformer may be extremely accurate once calibrated, but the _initial_ accuracy may be difficult to predict. Adding some calibration mechanism on each board can be expensive, but if you want a highly accurate reading of line voltage, you may have to do this anyway. Jonathan Malton S-S Technologies, Inc. Kitchener, Ontario, Canada I'm not so arrogant as to suggest that my opinions are corporate policy __ Reply Separator _ Subject: re: Measuring AC Author: Max mkel...@chekov.corp.es.com at mailway List-Post: emc-pstc@listserv.ieee.org Date:1/22/97 4:10 PM I want to thank everyone for the great information I got in response to my question about measuring AC. One thing I hadn't thought about with my off-the-cuff idea of simply using a voltage divider is the issue of isolating the AC return and the circuit board return. Not isolating these raises a variety of issues that I'm not prepared to deal with. So, I think they need to be isolated. The method suggested by Jonathan Malton strikes me as a really elegant and highly accurate solution to the problem. This involves the use of a voltage controlled oscillator and an optoisolator. Using op-amps for rectifying diodes is also a great idea I think. The problem of providing a separate off-line power supply for the VCO, sort of takes some of the fun out of the idea, but this problem is, nevertheless manageable. I'm also going to review IEC950 and IEC1010 as suggested by Horst Haug. The basic problem with transformers is that it's not possible to get a specification on their accuracy. In talking to Signal Transformer, they give me a verbal specification of +/-2% accuracy (no load). Advanced Components Industries, OTH, provide a verbal specification of +/-10%. The difference might be in the size of the transformers. The Signal transformer is relatively large and relatively expensive. The Advanced Transformer is of the PCB type and is very small (0.6 inches high). The lady at Advanced seems to be very knowledgeable and says that the accuracy is not simply a function of turns ratio, but also depends on the wire and core construction. She claims that they can calculate transformer accuracy simply by looking at the wire and core specifications. Max mkel...@es.com
Re: Re[2]: Measuring AC
% % % I suspect (but don't know for sure) that the accuracy of a transformer is % the way it performs, relative to it's spec's, rather than relative to its % history. That is, a _given_ transformer may be extremely accurate once % calibrated, but the _initial_ accuracy may be difficult to predict. Yes, that's my understanding also. A manufacturer, for instance, could, perhaps buy two separate batches of cores (for use with one batch of wire) and all of the voltages from the transformers made with the first batch would be extremely close together and all of the voltages from the transformer made with the second batch would be extremely close together, but the delta between the two different transformer batches could vary by as much as 20%. With 10V transformers, for instance, batch number 1 might result in voltages of 9V +/- .1% (just guessing) and batch number 2 might result in voltages of 11V +/- .1% % Adding % some calibration mechanism on each board can be expensive, but if you want % a highly accurate reading of line voltage, you may have to do this anyway. Yes, I expect that a calibration procedure would result in extremely accurate measurements, but would require measurement of the output of each transformer. % % Jonathan Malton % S-S Technologies, Inc. % Kitchener, Ontario, Canada % % I'm not so arrogant as to suggest that my opinions are corporate policy % % % Max Kelson mkel...@es.com
