Solar Startup Still Tricky

I have modified a second MP1584 buck converter module so that it would not activate until input voltage surpasses 13V, comfortably above the output voltage of 3.3V. I want to connect input pins to a solar panel so the associated components (ESP8266 WiFi microcontroller and INA219 voltage/current sensor) would be powered exclusively by the sun.

First is a test run with my bench power supply. Gradually increasing supply voltage starting from zero volts. Thanks to the modification, there were no odd behavior or sounds of a MP1584 trying to work with too low of an input voltage, which is great. As I increased voltage past the ~13V threshold, I saw the blue LED of my ESP8266 blink and it booted up as planned. This time, it was able to find INA219 on I2C bus, which is further than I got before.

Feeling optimistic, I connected this circuit to my solar panel at night and hoped I would wake up to find the system running. Sadly I woke up to disappointment, as there were no logged messages from the ESP8266. Probing the circuit with my volt meter, I confirmed a 3.3V supply voltage was present, but for whatever reason the ESP8266 failed to boot that morning. I manually disconnected and reconnected the circuit board, and this time ESP8266 booted up fine (now it has full daylight power) and started reporting values measured by INA219.

I don’t know what happened at sunrise. I hypothesize that when the solar panel output voltage rose past 13V, it has still yet to produce enough power to successfully start an ESP8266. So when MP1584 activated, it could supply 3.3V but not enough amperage to supply an ESP8266 through its boot process, putting it in a glitched state that was neither on nor off and stuck there until I power cycled the system. [UPDATE: Further experimentation found this hypothesis was correct, the panel would reach operating voltage well before generating appreciable power.]

I didn’t have my oscilloscope set up to capture the startup waveform to confirm or disprove this hypothesis. It’s clear there are additional subtleties I don’t know about starting up a circuit on solar power. Do I want to invest the time to learn and experiment with this problem domain? After thinking it over for a bit I decided “nah” and abandoned the idea of running everything exclusively on solar power. I’ll retreat to what I know and incorporate batteries into the system instead. Starting simple with household alkaline AA batteries.

7 thoughts on “Solar Startup Still Tricky

  1. Two suggestions for you:

    BULK CAPACITANCE
    If you add a substantial capacitor between the PV and your circuit, that capacitor itself is a load on the PV DC output, and the PV output cannot raise as easily as it will when there is effectively no load. Consequently, you won’t have an apparent 13V supply that blinks out the instant you add some load, and the capacitor provides a power buffer for your circuit – say when a cloud rolls by, or even your startup load is a bit much, but the stable load after that is lower. So, with a say, 3300µF capacitor of sufficient voltage rating on the output of the PV, as the PV starts to come up in the morning, you will not have 13V (or whatever) on the output until that cap has charged to 13V. With E=(C * V²)/2 that means you’d have 0.278850 Joules, or a bit over a quarter of a watt for 1 second, which doesn’t sound like much, but you’re bucking that to 3v3, so that 13V charge is going to last longer – and if your PV is outputting more than 13V (which I suspect it is), as that PV voltage climbs, that capacitor is more and more stored charge – at 24V it would store nearly a full joule (1 watt-second). If you had access to a seemingly endless supply of industrial rated high capacity capacitors (as I do), things get fun and you make little capacitor banks, I run 33mF (0.033 Farad) 100V cap banks off of some PV in front of battery chargers. Note, I do that right on the output of the PV, BEFORE any bucking/regulating circuitry.

    VOLTAGE SUPERVISOR or SUPERVISORY RESET
    There are ICs such as the Maxim MAX809TEUR+T which are intended to hold a processor in RESET state until the voltage is above a certain level. Such devices ensure that if the supply drops below a preset voltage, the RESET will be asserted and held for a minimum specified time, which helps to ensure your device actually RESETS (setting the processor to a known state) rather than entering some self-imposed funk.

    Voltage supervisors are available in multiple preset voltage thresholds, and also configurations (the ‘809’ in the proffered part is a Push-Pull _RESET output, but there’s also Open Drain _RESET (803) and push-Pull RESET. They’re small (SOT23-3 or SC70), and require no external components, though since they draw so little current, there’s the potential of using say, a voltage divider on the input to take a fixed threshold supervisor IC and give it some adjustment, so a 3V IC you might have picked up a strip of actually keeps your mcu in reset until you have 5V supply, which might be more appropriate for driving some ancillary gear (for instance).

    https://www.ti.com/lit/eb/slyy167/slyy167.pdf is a worthwhile doc to browse on the topic.

    Good luck with your project.

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    1. This is great information, thank you! I don’t have a huge supply of capacitors as you do, or the cool supervisor ICs you mentioned. Hopefully I’ll come across a batch of surplus or something. Anyway, I’ll keep this info in mind and I hope I can try it out the next time I have ambition for an exclusively-solar project.

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      1. Well, I work for a power electronics manufacturer (in the PV sector), so there’s all manner of interesting parts from dev and test operations that get e-wasted (our devices have multiple high-value caps, so 5 minutes with a clipper in a lab e-waste bin yields me a bag of several dozen caps – fortunately with a fair bit of lead length still on them). That sadly (though, to my own benefit), includes sample parts different engineers will order for experimentation, and then dispose of the unused parts (still in cut tape and antistat bags) because the experiment wasn’t fruitful for the particular scenario. Also parts no longer applicable due to discontinued models.

        I’m also aware of some parts in certain devices which have utility to me, and aren’t too much of a bother to remove (or cost enough to make it worthwhile to deal with), so I keep an eye out for them, or connect with the FA (Failure Analysis) Techs that deal with certain devices and ask if they’ll set some of those aside for me, and then I can scavenge the part from them, then e-waste the device carcass (which is where it would have gone originally).

        I order plenty of new parts from distributors for my personal use, but I’ve a lot of interesting parts I probably wouldn’t have gone out and purchased on my own just to have, or if I had, would be really stingy with using. That gives the freedom to experiment with some parts where I might not normally have dropped the money on it just to have a strip of them handy. Within the past week, I scored a bunch of ACS722, which are a galvanically isolated current sense IC that supports AC or DC (and has a linear voltage output signal, not digital), which is a near US$5 a piece proposition. Spin a small PCB and tinker at no real cost.

        For your particular situation, just one capacitor should do though, and a few 100’s of µF would probably suffice (I note you’re using 220µF/25V on your switcher) – it’s just the potential for highly variable input to your switcher that begs for more capacitance. So, either parallel a few of those on the switcher INPUT side, or root around in your electronics debris. Large capacitors tend to be the domain of power equipment, and at the consumer level, a radio amplifier, television, or desktop computer power supply are your most likely candidates. Watch the voltage rating.

        Speaking of which, your MP1584 mods – I saw 25V caps being used there – is your PV never outputting above ~ 24V (is there a regulator BEFORE your switcher?). If it goes above, you risk damaging the caps (as well as the MP1584, depending upon its ratings – IIRC, it’s rated for up to 28Vin, with an abs max of 30V). From another image I saw on your blog, it looks as if the MP1584 is being used to power just your monitor device. Are you only ever really checking the PV output when the battery charge controller is attached as a load?

        This here is an example of one of the cap banks, two of these wired in series gets me 100V, but this represents a LOT more capacitance than your circuit needs:

        If you want to step up your PV project work, check CraigsList of something similar in your area, or locate some PV installers, and inquire about used PV modules (aka “Solar Panels”). For not a lot, you can get the used modules they pull off of someone’s house when upgrading them to newer more efficient modules. A 200W (= over a kWh a day average) module can be had for US$40 or less. A few months ago, I picked up 22 such modules, delivered for US$700, which is perhaps more than you want for experimenting, but the stuff is cheap in the used market (and I don’t live in a cheap region).

        Beh, sorry for flooding your blog. If you want to contact me offline, I’m fine with it.

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      2. I envy your access to hardware for experimentation, I definitely don’t have anything like that!

        My cheapo panel specification list open-circuit voltage maximum of 24V but I’ve measured only 22V in bright sunlight. And yes, it’s always connected to a load which keeps voltage even lower.

        As you can tell from my mistakes, I’m still on the “crawl” phase of crawl/walk/run with PV projects. Making mistakes with tens of watts before I risk mistakes with hundreds or thousands of watts. If/when I become confident enough to play with more power, those retired surplus panels indeed look very tempting.

        Thank you for humoring beginners.

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      3. FTR, I meant “Scale up” (as in more power) not “Step up”. Poor word selection on my part.

        BTW, I neglected to mention that if you want input buffer capacitance for your project you should have a diode (a low-Vf Schottky would be good) on the input, anode on the +V wire from your PV, cathode towards + on your capacitor(s). – (really, GND) from the PV needs no diode. This will prevent whatever other loads on your PV from sapping the capacitive reserve from your project.

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