A solar supercap powered Moteino (15Farad charged by BQ25504)

[joelucid]

What's the application? 100ms listen window is very short - I think you can do longer in most cases.

[WhiteHare]

I think initially a solar node for controlling/powering blinds would be a good application.  I've read that response times under 100ms give the impression of being "instant," which is the most satisfying.

[WhiteHare]

The leakage rates on supercaps appears to be significant, relatively speaking.  For instance, on one of my 20F supercaps (http://www.vishay.com/docs/28421/220edlcenycap.pdf), the datasheet says the leakage rate after holding peak charge for a half hour is 6ma, whereas after holding peak charge for 72 hours, it becomes 75ua. 

However, from my post above, the average current consumed by the receiver node is 40ua.

Now, granted, the true figure of merit would be the supercap's self-discharge rate.  However, as previously noted, that number is harder to come by.

[WhiteHare]

I like the simple approach without integrated charger. But one thing that needs to be figured out is how to prevent the moteino from starting up until VCC exceeds 1.8V. Otherwise you'll never make it since both the radio and the 328p draw quite a bit while trying to start up. That's where the "power good" pin of the charger comes in handy.

Maybe this?  https://www.onsemi.com/pub/Collateral/NCP300-D.PDF

[Edit: I'm guessing that an ncp301 1.8v voltage detector in combination with a load switch will do the business.  Likewise, a similar 2.7v voltage detector could be used for charge termination on the supercap.]

[WhiteHare]

Closer inspection of the above datasheet revealed that the chip appears to issue a reset pulse rather than a sustained signal when the voltage is in the sweet spot.  What I want is the latter, so I ordered some chips made by MicroChip and STM instead:

https://www.digikey.com/product-detail/en/microchip-technology/MCP121T-195I-TT/MCP121T-195I-TTCT-ND/704607
https://www.digikey.com/product-detail/en/microchip-technology/MCP131T-195I-TT/MCP131T-195I-TTCT-ND/704609
https://www.digikey.com/product-detail/en/stmicroelectronics/STM1061N27WX6F/497-4954-1-ND/1003425
https://www.digikey.com/product-detail/en/microchip-technology/MCP1316T-27LE-OT/MCP1316T-27LE-OTCT-ND/6244599

To carry meaningful current, it would appear that they'll need to be married to something like a load switch or a P-FET.

[WhiteHare]

I received the above voltage detectors, and they work as expected.  I also received an NCP301, and it works fine too.  I like it because it has a lower quiescent current than some of them, and it also seems to have a wider hysteresis.  On a rising voltage the RESET pin switches ON (and stays ON) at 1.88v, and on a falling voltage it switches OFF at 1.79v.  Therefore, you'll need a supercap whose effective voltage (i.e. also accounting for ESR) doesn't fall by more than that amount during Moteino startup, or else you may risk getting some failed starts.

[WhiteHare]

I connected the NCP301 to a load switch (see attached photo), and voilà!  It prevents the Moteino from being booted until the voltage on the supercap reaches the threshold voltage (in this case, 1.88v).

For voltages between 0.4v and 1.88v, it doesn't appear to pass any voltage through to the Moteino.  For voltages below 0.4v, it does appear to pass the input voltage through.  Ideally, it would not, as it serves no useful purpose and it does create a needless current drain.  Not sure what solutions there might be for that, but I'd welcome any suggestions.

[perky]

Nice, I may well use one of them on my next project..

[WhiteHare]

Great!

As for the overvoltage protection, which would be kicked on by a 2.7v detector, I'm hoping this p-fet will do the business of disengaging the solar panel from the supercap when a 2.7v rated supercap reaches 2.7v of charge:  https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage/SSM3J338R,LF/SSM3J338RLFCT-ND/5810258

[WhiteHare]

There are versions of ideal diodes that require a supply and I wanted it to work down to 1.6V and couldn't find any that went that low, so hence the discrete version.

Down to what voltage does your discrete version work?  What happens below that voltage if, say, the light were blocked to the solar panel?  I suppose if the voltage went down low enough, it wouldn't matter much if it lost its diode-like properties, as there wouldn't be much stored charge to be lost.

[WhiteHare]

By the way, for anyone else who might be interested, I just now stumbled across this youtube video by TI about their SM74611:

https://www.youtube.com/watch?v=fp1DB30WEpU

[pjeran]

For over-voltage protection, could you just use a LDO?

My little panel outputs 5.5V in full sun, it is a cheap one from a solar motion light.  I have used a 3.3V LDO and a Si diode for reverse current protection.  That puts 2.7-2.6 at the cap.  I have been running this for a couple of days and it takes about 2 hours to fully charge a 25F cap from about 2V to 2.7V and with a full charge the cap only discharges 0.2-0.3 V over night.

I am looking at incorporating the voltage detector mentioned above in case the cap discharges below 1.8 V.  Now I pre-charge the cap to get the system running and then sleep for an hour at a time when the voltage drops below 1.9V, but it is sub-optimal.  I am also looking at the DS and notice that there is a way of increasing the hysteresis, I might do that to power down at 1.8V and not come out of reset until I get to 2V to make sure that the start-up does not cause a drop below 1.8V.

BTW - great work here - I was playing with using LiPo, but this is far simpler and I don't have to worry about environmental issues for charging.

Paul

[WhiteHare]

For over-voltage protection, could you just use a LDO?

Good idea.  And since a Moteino comes with an LDO, you can just relocate it without necessarily having to buy a new part.

My little panel outputs 5.5V in full sun, it is a cheap one from a solar motion light.  I have used a 3.3V LDO and a Si diode for reverse current protection. 

Nice and thrifty.  I like it.

Thanks for your post!

[Edit: By the way, your post reminds me of another idea, which is using the ATMEGA328's ADC to check the supercap voltage as it's rising on the high end and then invoke a charge termination when it has risen to the supercap's charge limit.  All it would take is a p-fet and a free pin on the Moteino.  So far I've resisted the idea, if only because it doesn't seem quite as fail-safe as a pure hardware solution.  However, what do other people here think about it?  Perhaps I'm being overly conservative.  I suppose an argument could even be made for doing BOTH, as that would provide additional protection against possible overcharge.  Or would that be too conservative to be worth the bother?  It's a judgment call either way.  I'd be curious what opinions others here have about the matter.]

[WhiteHare]

I am also looking at the DS and notice that there is a way of increasing the hysteresis, I might do that to power down at 1.8V and not come out of reset until I get to 2V to make sure that the start-up does not cause a drop below 1.8V.

Yes, I think this is another good idea.  The defaults don't leave a lot of margin for error, and who knows how these things might behave as they age.  I hadn't realized that the hysteresis was adjustable, but now that you've mentioned it, I think I've found the part of the DS that you're referring to (page 18 of the NCP301 DS).  I found that I had to use a pull-up resistor anyway (I somewhat arbitrarily picked 10K), and so I can further refine the hysteresis a bit more by adding just one more resistor.

In retrospect, I should have ordered 2.6v voltage detectors to handle the maximum charge termination, because the way the hysteresis adjusts, the threshold on the falling voltage is fixed (well, at least with the NCP301 that's how it works).

Thinking now about the pros/cons of LDO vs voltage detector, I'm still slightly leaning toward the voltage detector.  I may be wrong about this, but the rate of voltage accumulation seems to slow down a bit if the charging voltage is reduced (as it is with an LDO).  Now, that may or may not matter--and if you're able to reliably charge in just 2 hours it probably doesn't matter--but it is maybe a slight advantage for the voltage detector approach if charging in low light conditions.  I'm not completely sure though.  On the other hand, the LDO is just one part, whereas with the voltage detector approach three or four parts are needed (vd + pfet + one or two resistors).  Those, then, are some of the possible tradeoffs that come to mind.

[WhiteHare]

I'm thinking now that another simple way to ensure the charge on the supercap never exceeds 2.7v is to simply put a 2.7v zener diode across it.