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
