I'd vote for going with a transformer too, but not just any old
transformer - I'll explain later.
You can indeed connect directly to line with an AC divider, and measure
the signal. In fact, you can even build a very broad band probing system
that can go all the way down to DC, and up to RF, by making something
equivalent to a high (1 meg) impedance oscilloscope front end. There are
complications though. Measurement-wise, what would be needed is actually
a differential system, looking between the neutral and line, so two
identical attenuators are required to get and preserve the signals for
subtraction and processing. It is difficult to get the balance and
symmetry needed for good CMRR at higher frequencies, so line frequency
and plenty of its harmonics can be handled OK, for good waveform
fidelity well into audio, but the higher frequency differential and
common-mode junk will blow right through. You would need all sorts of
filtering and clamping to control the signal quality and protect the
instrumentation. The input resistance should be kept high, like 1 meg,
to be safer against faults, and to prevent tripping GFCIs (RCDs). For
instance, if you're looking at 120 VAC on a 1 meg front, the hot side
input current will be around 120 uA RMS, adding only a small amount to
the earth ground loops, compared to a typical 5 mA GFCI trip point.
The big problem with this, as you can see from the comments, is doing it
safely, with properly rated parts, fault protection, and circuit layout
and construction (especially clearance and creepage distances). Anyway,
it can be done, but tends to be a PITA to do it right.
An obvious question is how much fidelity is needed. No matter how good
your measuring system is, the results are only valid at the point you
sample, and only somewhat representative of what you'd see at another
spot - even in your own home. The incoming mains at your load center may
be pretty solid, but every branch circuit will look a little different,
depending on the loads and distance and so on. When appliances turn on
and off, things will change throughout the system, and there will always
be transients, and ubiquitous HF and RF interference from all the
electronic gear in the system. If you look at it with high bandwidth, it
can appear pretty disgusting, but it works for the main purpose of
distributing plenty of power.
So, in order to remain blissfully ignorant of how ugly it may be, and to
rig up something simple, safe, and easy, a transformer is the way to go.
Regardless of the chosen one, some basic protections are usually
desired, like first a small fuse or PTC on the hot side, to protect the
transformer in case you accidentally short the secondary - a high
probability event when designing and experimenting. If the setup is
experimental, and gets connected or changed around a lot, or you're
fooling around on the primary side, it's a good idea to fuse the neutral
connection the same way, so getting a cord reversed or such, won't
reduce the protection function. Next, transient protection like TVSSs or
MOVs can help to protect the parts and measuring equipment. Transformers
are built to handle all this, so don't really need it - it's mostly for
the other stuff. BTW I noticed in your recent post that you were
plugging into a power strip to get some protection. I'd recommend not
bothering with this - it will tend to cause more distortion, and if it's
shared with other loads, you'll be including their effects too, making
it even more removed from the true mains signals. Build the protections
into the unit itself, and you won't have to worry about placing it
anywhere in the system.
Now for transformer selection. First, remember that power transformers
are not built for signal integrity. They are optimized for maximum
cheapness that adequately meets the specs required for power transfer
and packaging. The biggest cost factor is the amount of core and winding
material needed to get the job done, so there are all sorts of
trade-offs involved. The main thing is to use the smallest core
possible, running at the highest flux density possible, along with the
least amount of copper in the windings, to provide the function with
"acceptable" core and winding loss, which typically may be 5-10 percent
of VA rating. Often, a temperature rise spec is provided, indicating the
total real power loss in operation.
The simplest, biggest improvement you can make in signal fidelity, is to
get the flux level down. The easiest way is to use a transformer with
much higher primary voltage rating versus the line signal size. IOW, for
120 VAC, use a transformer with 240 V primary. For a given VA size, this
will give four times the magnetizing inductance, one fourth the
magnetizing current, half the flux level, and one fourth the equivalent
VA rating (if you were to use it as a power transformer), versus running
at rated voltage. This greatly reduces core loss and improves linearity.
The winding losses become very small in this application, since it's a
signal transformer. The load on the source (line) is mostly the
magnetizing current, through the primary resistance. The secondary will
have virtually no load, since the (high impedance) instrumentation is
just looking at the voltage, so hardly any wire loss.
Transformers are naturally bandpass filters, and are already
differential in normal use, and provide good isolation from the line
environment.
There are other options to get even better performance. More on that later.
Ed
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