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Powerline Transformer Question

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nrhoades

Mon Jun 30 2014, 02:00PM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

For those of you out there who own a powerline transformer: I'm interested in knowing what the output impedance is (as seen from the house line) for 1kHz to 1MHz, specifically 40kHz.

Anyone care to take a measurement for me?

Dr. Dark Current

Mon Jun 30 2014, 06:02PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I guess it will be large; why do you want to know?

The "standard mains impedance" for approximate calculations here in Europe is R=0.4 ohm, L=600 uH. It varies wildly, depending on how far you are from the transformer, what type of power line is used (cable or overhead) etc.

teravolt

Mon Jun 30 2014, 07:22PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

usually those are iron core and won't run out side of there frequency ratings. you could make a h-bridge and just try a small iron core transformer and see what happens. zilipoper has done stuff like that. put his name in the search column

nrhoades

Mon Jun 30 2014, 08:51PM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

I want to make a circuit that can remember which outlet it is plugged into based on the impedance it sees. I'll be using a lock in amp with averaging so it'll be pretty accurate I think. Just trying to get an idea for what I'll be expecting to measure.

teravolt

Mon Jun 30 2014, 09:55PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

do you plan to detect the current in each socket? you could use ferrite torides to make the measurements

klugesmith

Mon Jun 30 2014, 11:07PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

As Dr. Dark Current said,
the impedance of a mains supply at RF frequencies
can vary widely depending on many details of building wiring.

That presents a challenge for standardized testing of equipment for conducted EMI.
The relevant standards (such as those by FCC and TUV) call for a special filter called a LISN between the mains and the equipment under test. I once made a 50 amp three-phase LISN.

You might find some useful numbers by searching the Internet for articles about LISN (Line Impedance Stabilization Network) applications. Please let us know what you turn up.

Newton Brawn

Tue Jul 01 2014, 01:39AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

nrhoads'
Let us know wath is the nominal voltage of the line Vs and maximum current Im that can be suppied.., and frequency Fs.

nrhoades

Tue Jul 01 2014, 03:18AM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

lets assume that I am only interested in 40 kHz, with a very tight filter around that. I'll use a transformer to couple to the outlet with a blocking capacitor, so the resonant peak is at 40 kHz.

Im assuming that it'll be pretty noisy. Looking for signal max to be about 1Vpk, 100mA max.

I will be filtering out DC (well, 60Hz), so I don't care about the traditional power present at the outlet.

I'll look up LISNs

klugesmith

Tue Jul 01 2014, 04:40AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I found this reference which talks (broadly) about measured impedance of actual power lines (including, presumably, the final transformer):

This one has a chart of impedance looking into the LISN. The 50 ohm value at high frequency might be there for practicality, rather than being representative of typical mains:

I would worry that with a single parametric measurement, outlets on different circuits might have the same reading.
Have you considered measuring |Z| (or vector Z) at more than one frequency?
Or making a TDR set to get a time-domain view of the cable plant?

What frequencies are used by those controllers that use "out of band" signals on power wires?
The choice must have considered transmission characteristics and the noise environment.

Newton Brawn

Tue Jul 01 2014, 03:14PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Assuming that the nominal voltage of your outlet be 115 V
Assuminng that the max current be 15A
Assuming that the freq be 60 Hz,
And
You measure no load voltge at outlet as 117V
and
you add a 10 ohm resistor in the outlet and get 112 volts
and a current of 11.2 A.

The voltage drop will be 117-112 = 5.0volts
the impedance of line (60Hz) will be E/I = 5.0/11.2 = 0.446 ohms.

Assuming that the X/R = 1 at 60Hz
X = 0.315 ohms
R = 0.315 ohms

Now, for 40kHz,
Freq ratio = 40000/60 = 667

Then, for 40kHz,

X= 667 x 0.315 = 210 ohms
R= 1 x 0.315 = 0.315 ohms
Z= 0.315 + J210 = 210 ohms

The exac value of X/R may be determined by a inductor of 10 ohm,s 10A.

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