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Well, since I was already going to install a regulated DC wall-wart in the panel to power the circuit, I had this idea. If instead I use an unregulated AC wall wart, I can bring in lowish voltage AC into my enclosure. I can take the unregulated AC and provide my own regulated DC from it using a half bridge rectifier and voltage regulator. And I can also measure the AC using a voltage divider.
But, hmm, how would I handle the part of the wave that goes below reference ground on the micro-controller? I'm also assuming that unregulated AV wall-warts don't vary in characteristics over time. Is this safe to assume?
You'd need to use a transformer exclusively for measuring, as (dynamically) loading it will throw off the results. Make sure the loading of the measuring transformer (passive voltage divider) is light and static.
Even if the load is just the MCU capturing analog data, you think that would be 'dynamic' enough to vary measurements?
Now I just need to get an ac output wall wart...
What about the negative half though?If uneven loaded, it may look different.
Quote from: Soeren on November 09, 2010, 01:19:59 AMWhat about the negative half though?If uneven loaded, it may look different.I'm monitoring the sine source with a an oscilloscope, both halfs look good. If anything the negative half looks 'more round' because there's no slight distortion when the diode begins conducting again. More importantly, the peaks are roughly the same voltage.
That's now. ??It strikes me a bit like if examining whether humans can give birth - to make it easier, we examine only the male population, since both kinds have the same number of arms and legs anyway
I'm getting an error of +/-.5V measured off the mains. That's an error of .9%
The 1M resistors in the outputs of the op-amps is going to A/D inputs (as far as I read it) and I doubt the input impedance of the A/D inputs is large enough that it doesn't matter (a good rule is 20 times up, so they'd need to be 20MOhm).
A less crucial thing, but a good rule to follow, is that when using potentiometers as a variable resistors, always connect the wiper to the end with the lowest impedance (to supply). That would be ground in your case.
It's best to balance the impedance at both inputs of an op-amp (to improve CMRR), but it's not easy when one is variable of course. The resistor at the non-inv. input should equal the parallel combination of the trimmer and the feedback resistor (R4 and R5), so if the trimmer ends on say 50k, R1 and R3 should be (50k//100k) = 33k.
I'd make C3 and C7 larger, like 22µF, or at least 'scope the outputs - AC coupled at a high sensitivity - to check for possible tendencies to oscillate. Strike a few load changes to try and provoke it into bad behaviour, while keeping an eye on the 'scope - This is best done with a second person helping, as it's hard to probe, read 'scope, make changes, avoid electrocution and make notes all at once.
Is the uneven input capacitance (+/-) because of lower demands of the negative line, or is it to get some kind of supply sequencing?
Why 100A and 200A ranges? Regular mains fuses wouldn't allow anything like that through anyway.
ADC impedance is 100MOhm. So should R6/R7 be 1/20th that?
QuoteA less crucial thing, but a good rule to follow, is that when using potentiometers as a variable resistors, always connect the wiper to the end with the lowest impedance (to supply). That would be ground in your case.Is this a "shorting out to case while turning screw" thing, or something else?
I've been monitoring both the AC in and the DC regulated out. Haven't noticed anything, but i'll look closer. Why add more capacitance after the voltage regulators? I thought the low value was needed after the regulator just to filter out HF noise.
Not sure how it works over there, but here the minimum residential service is 100 Amps, as is mine. My "mains fuse" is a 100amp breaker. Most housing residences over 1000-2000 sq ft typically have a 200 amp service.
No, it's a case of "less is more".The 20x up is just the minimum to not load the output driving it and by this introduce errors 100x up is even better - as long as you're sure it is that high.
More a "large" piece of metal acts as a receiving antenna thing.
It's both to reduce noise and to keep it from oscillating - hence the "try to provoke it by sudden load changes". If you see ringing at a load change, add capacitance (preferably low ESR).
In DK, we have 10A, 13A or 16A each group (at 230V) and 16A to 25A is normal in 480V (2 or 3 phase) groups - but you can have as many groups as you like/can afford. That's just for residential use of course.The master fuses in the leads into a home is way larger of course, but it's out of reach for the consumer.
Had an issue with one of the current channels. If the current channel was 180 deg out of phase with the voltage, the signal after the OpAmp was destroyed. Decreasing the resistor value between the op Amp output and ADC input from 1M to 10k fixed the issue. I suspect that when the adjacent ADC pin connected to the feed AC voltage through a voltage divider goes into the below ground cutoff region of the wave, it changes the impedance of all adjacent input pins, thus destroying the signal into the ADC pin watching the current channel. Lowering the 'source impedance' fixes the issue.