[time-nuts] Re: Another reason to monitor line frequency :) - My AC measurement project & question

[Lux, Jim]

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.