Re: gEDA-user: Transformer as voltage transducer?

[John Doty <jpd@wispertel.net>]

On May 27, 2007, at 6:02 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?

No sensing technology will accurately handle DC-gamma rays. Note that  
the classic Elihu Thomson electromechanical power meter still used by  
power companies doesn't have a lot of bandwidth.

>
> 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.)

You could get a Kill-a-Watt, take it apart, see what the sensor is.  
If you can extract the voltage sensor you could even use it, but you  
might need a bigger current sensor for large loads.

>
> 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.

Moisture and rodents.

>   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 CTs have similar limitations to voltage transformers. Probably OK  
for this job, though.

>   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?

Your PC will be grounded to the "U" ground. You could put *two*  
attenuators to that, one on hot, one on neutral, feed a differential  
amplifier whose ground reference is the U ground, and you'll have an  
analog signal in the PC's potential world. Power the diff amp from  
the PC (or other grounded supply). Leakage <1 mA isn't going to be  
trouble, so just use attenuators with enough resistance up front.

On the other hand, if the PC isn't near the sensor box, you might  
want an isolated digital interface anyway. How much lightning do you  
have where you are?

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@noqsi.com




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