Accuracy is likely secondary. I expect that most applications are good with 1.5 digits (aka 95% accurate or so). You don't want to blow the entirety of this 5% allowable error on one micro-spec of one component, but I don't expect that most people especially need lots of accuracy here.

The issue is that any circuit with 1 to 5 amps of current is a serious amount of power, meaning power efficiency is likely one of the top priorities.

A 5-Amp circuit with a 0.01 Ohm sense resistor wastes 250mW on the resistor alone, likely more than the entirety of your microcontroller!! You can actually run an entire Linux capable microprocessor + Low-power DRAM off of that kind of power!!

Dropping down to 0.0001 Ohms uses 1/100th the power or 2.5mW. which is likely a more reasonable cost.

That depends on what "most applications" means. I remember a paint program I saw in high school that some kids had written to draw sprites and backgrounds for their video games. The documentation explained that it could only edit 320×200×256 images, but that that should be adequate for "most projects". Depends on the context!

In the contexts I'm thinking of, I would think that, if your load is drawing 5 amps of current at 3.3 volts, which is 16.5 watts, an extra 0.25 watts in the 10mΩ current shunt is not likely to be a big problem. And if it's 5 amps at 48 volts or 240 volts, it's even less of a problem, relatively speaking. I guess you're thinking of different contexts, contexts where the power-measurement system is paid for from a different budget than the load, but I can't figure out what they are.

You're sniffing out the fact that I didn't have an exact application in mind when I wrote my earlier posts, lol. But yes, you are correct on this front.

The more I post on this subject, the more I'm "backwardsly-targetting" a solar-powered MPPT circuit.

Maximum Power Point Tracking circuits improve your solar-panel's efficiency by changing the current (through the use of a buck-boost converter, changing the voltage-and-current downstream). Or maybe you have excess current sunk ionto a battery of some kind. Either way, you have some kind of configurable-load and can therefore maximize the solar panel's Voltage/Current curve characteristics to seemingly magic energy out of nothingness.

If it costs you 250mW to just *sense* the current and run the calculations, it becomes much harder to justify the small gains of any MPPT circuitry.

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But yes, I'm changing the target application to suit my argument style. Apologies on that but I think you can forgive me on this!! The point of MPPT is to magic more energy out of nearly nothingness so efficiency is of great concern here!

Oh, I see! And you might very plausibly be getting 5 amps at 0.7 volts or something there, if you're controlling a single solar cell? (I might be misunderstanding how they work.) If you're controlling a whole 200-watt panel it's less of an issue because usually they have several cells in series to get a more convenient output voltage.

I feel like a car's transmission or a bike derailleur may be a good analogy to explain it to people, though an MPPT tracker is a ratcheting CVT.

I'll just explain solar cells really quick then. The gist is that solar-cells are nearly constant-voltage, until you draw too much current and then suddenly they drop to useless-levels of voltages. The scary part is that the clouds change this point severely.

So a 60W Solar Panel might be 12V 5-Amps in the best case scenario (directly pointed at the sun during clear skies). If you draw 5.1-Amps, suddenly the voltage drops all the way to 0.7V or some other nearly useless level.

What is annoying about solar panels is that this point changes depending on temperature, shadows, and other conditions. As the sun sets and hits the solar panel at a shallower angle with dimmer afternoon light, it might drop to 12V 2-Amps, and of course at night it will drop to 12V 0.01Amps or less.

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A MPPT circuit "tracks" this point where you can obtain the maximum amps at the maximum voltage (or really: the maximum volts * amps), and changes the current draw of the effective circuit to "find" this point.

It requires some kind of power-sink (ie: hooking it up to the wall power and assuming wall-power can take infinite amounts of energy). Or more likely, a lead-acid battery or LiFePo4 charging circuit that can "store the extra energy dump".

Oh, I did understand them at that level. I can confirm that that's a pretty good explanation of solar cells and MPPT circuits.

What I meant was that I wasn't confident that I was remembering the open-circuit voltage of a silicon solar cell, and in fact I had it wrong—it's not 0.7 volts, but 0.5 to 0.6 volts. And I couldn't remember if the diode junction was forward-biased or reverse-biased in normal operation. It's reverse-biased—the photocurrent goes the opposite direction from normal diode current. Now that I think about it, maybe that's why shade on one cell knocks out a whole string.

One quibble, though: if you maintain 12 volts in the usual direction across a series of 10 monocrystalline silicon solar cells at night you are just going to lose a subthreshold leakage current through them, because you're forward-biasing the pn junctions in the cells, just not quite by enough to turn them on. They'll emit a little bit of infrared light, a feature used to analyze solar panel failures (so-called "EL testing"). Illuminance at night is at best a million times dimmer than sunlight https://en.wikipedia.org/wiki/Orders_of_magnitude_(illuminan... and that subthreshold dark current is not going to be a million times lower than your normal current. Even under indoor lighting, which is only about 2000 times dimmer than direct sunlight, monocrystalline silicon PV cells will consume power rather than producing it.

Because of the intermittency you're describing, I suspect that thermal energy storage of various kinds (sensible heat, phase change materials, or especially TCES) is going to be important for the wide adoption of solar power, because according to my notes lead-acid batteries store about 20kJ/US$ and LFP a bit less, while industrial calcium chloride costs about US$300 per tonne (US$272/tonne according to https://derctuo.github.io/notes/desiccant-climate-control.ht...) and can absorb about its own mass of water from the air, liberating the water's enthalpy of vaporization, providing TCES.

I believe the heat thus stored is 408kJ/kg (see linked notes) which works out to 1500kJ/US$ at that price, roughly 1% of the cost of the same energy storage capacity in a battery. And, depending on the desiccant, it's plausible that you could reduce that by another order of magnitude, or two orders of magnitude for industrial installations. You can probably get by with impure calcium chloride or as-mined carnallite, for example.

> I remember a paint program I saw in high school that some kids had written to draw sprites and backgrounds for their video games. The documentation explained that it could only edit 320×200×256 images

Sudden memories of https://en.wikipedia.org/wiki/Autodesk_Animator , which was commercial software with exactly those limits (due to inheriting them from VGA). Despite the limited resolution it had a spectacular array of features.