On 1/22/22 3:30 PM, ed breya wrote:
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.
With voltage dividers, one has capacitor coupling on both of the two
wires, so if you're going into a Arduino or something, it either has
differential inputs (e.g. Teensy 3.1 or newer), or you tie circuit
ground to one of the wires.
However, as you say, you could buy an AC wallwart intended for 220V
service, and use it on 110V. (leaving half the world to buy *two* and
run them in series)
Or, I suspect, one could pick and choose a AC wallwart that has "OK"
band pass characteristics.
The advantage of the wallwart (vs, say, a doorbell transformer) is that
it is already (theoretically) safe to plug into the wall. And if it
disappears in a puff of smoke, you can probably buy another identical
one easily.
It is surprisingly difficult to search for 220V input 12 VAC output
adapters, BTW.. You'll get a lot of DC output adapters. I guess 120V
land would have to get two adapters and run them in series.
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