Re: gEDA-user: Transformer as voltage transducer?

[Randall Nortman <geda-lists@wonderclown.org>]

Thanks for all the feedback so far.  You have softened some of my
paranoia, but also raised some other issues I hadn't thought about --
in particular the fact that transformer response is
frequency-dependent, and anything too far from the 60Hz a power
transformer is meant to transmit will be attenuated -- i.e., spikes,
dips, and switching noise.  Right?

On Sun, May 27, 2007 at 01:31:31AM -0400, Darryl Gibson wrote:
[...]
> Without knowing what the application is, it's environment, signal
> source, etc... it is difficult to make specific recommendations.
[...]

The application is a power analyzer to show me instantaneous and
average true power, apparent power, power factor, RMS voltage, and
line frequency, plus kWh accumulated over time.  There are some great
chips from Analog Devices now that integrate all this into one package
with a digital output -- I just need to supply the current and voltage
transducers.  (I know you can get cheap devices like the "Kill-A-Watt"
now to monitor plug loads, but I'm wanting to watch hard-wired
appliances like HVAC, plus the actual main service for the whole
house.  And of course I want it to feed data straight to a PC and log
it over time, which the Kill-A-Watt can't do.)

The environment is a metal box in a garage, right next to the breaker
panel.  It will be cold in the winter and hot in the summer, but
nothing that will seriously affect electronics, so long as my power
supply is designed for it.  Signal source will be CTs (current
transformers -- donuts) put over the mains lines inside the breaker
panel, plus whatever I settle on as voltage transducers.  The voltage
transducers will most likely sit outside the breaker panel, just plug
into a regular three-prong outlet.  I'm not sure how else I could do
it without violating code in a big way.  Well, if I use transformers,
I could hard-wire them in, but I think I'd still be doing that outside
the breaker panel.  And yes, I will have a certified electrician to
handle everything happening inside the breaker panel, like slipping
the donuts over the mains lines.  Of course, the "downside" of that is
that he damned sure isn't going to violate code for me.

So now I have to figure out if I really care about those >60Hz spikes,
dips, and noise.  I am thinking not, so transformers should be just
fine.  Then again, the reference designs for these Analog Devices
chips just put a resistor divider between hot and neutral and feed it
straight in without isolation.  They use neutral as DC circuit ground
and derive power from the hot line, so the whole circuit is tied to
mains without isolation.  If I did that, I would of course have to
make sure that the digital interface between my measurement system and
the PC that captures the data is isolated, which is pretty easy.  Any
thoughts on that setup?


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