Re: gEDA-user: Transformer as voltage transducer?

[Steven Michalske <smichalske@gmail.com>]

FTDI chips have a optically isolated mode of operation and a USB  
interface  You could even run this over some multimode fiber to  
really isolate and make remote mounting really easy.

you can filter out the high frequency spikes easily with a low pass  
filter,  you want to have a low pass filter anyhow to prevent  
aliasing on your device that is sampling.

as for voltage monitoring, I wouldn't worry much about using the  
resistors.  put in a fast acting fuse at a reasonable low current.

If you get the one that supports the di/dt Rogowski coil like the  
ADE7753 then you won't have to disconnect the mains.  Some thing that  
the electrician probably won't do with out pulling the meter.

Steve

On May 27, 2007, at 8:02 AM, Randall Nortman wrote:

> 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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