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Simple voltage cutoff schematic for solar charger, any tips?
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Psi:
Thanks for the circuit.

Yeah, it's an option, but i think just disconnecting the panel has more advantages.

Most likely using a comparator setup with schmitt trigger to control the fet.
Need enough hysteresis so when you cut off charging the voltage doesnt drop enough to turn it back on again
snarkysparky:
https://www.digikey.com/en/products/detail/microchip-technology/ATTINY412-SSNR/8594943

Do the MCU route.  It's handy when you determine you need more function from the circuit after building.
NiHaoMike:

--- Quote from: mikerj on April 19, 2022, 03:52:54 pm ---Once it's switched off it wouldn't switch on again during the day if the battery voltage drops, though it would (probably) reset overnight.  Maybe that's ok for the OP?

--- End quote ---
Connect the load on the same side of the MOSFET as the solar panel, then it will supply the load from the panel once full charge is reached.
Zero999:

--- Quote from: NiHaoMike on April 21, 2022, 12:32:55 pm ---
--- Quote from: mikerj on April 19, 2022, 03:52:54 pm ---Once it's switched off it wouldn't switch on again during the day if the battery voltage drops, though it would (probably) reset overnight.  Maybe that's ok for the OP?

--- End quote ---
Connect the load on the same side of the MOSFET as the solar panel, then it will supply the load from the panel once full charge is reached.

--- End quote ---
Exactly. When it gets dark, the MOSFET's body diode will conduct, allowing the battery to power the load.


--- Quote from: snarkysparky on April 21, 2022, 11:58:40 am ---
https://www.digikey.com/en/products/detail/microchip-technology/ATTINY412-SSNR/8594943

Do the MCU route.  It's handy when you determine you need more function from the circuit after building.

--- End quote ---

--- Quote from: Psi on April 18, 2022, 09:36:02 pm ---
--- Quote from: Zero999 on April 18, 2022, 04:11:22 pm ---
--- Quote from: Psi on April 18, 2022, 10:42:50 am ---The more I think about it the more i like the MCU option.  That was until I looked at stock levels of sub $1 MCUs at digikey.

--- End quote ---
How many do you need?

--- End quote ---

Not that many. Maybe 1000 a year, but I like to design products using a MCU that digikey/mouser stock at least 30k of, which is getting very hard of late.
I just really hate having to redesign a product every few months to use a new MCU, its starting to get annoying.


--- End quote ---


--- Quote from: Psi on April 21, 2022, 11:29:18 am ---Thanks for the circuit.

Yeah, it's an option, but i think just disconnecting the panel has more advantages.
--- End quote ---
Which is exactly what the circuit I posted does.

Do you need to switch the positive side of the panel for any reason?
Psi:

--- Quote from: Zero999 on April 21, 2022, 12:37:31 pm ---
--- Quote from: Psi on April 21, 2022, 11:29:18 am ---Thanks for the circuit.

Yeah, it's an option, but i think just disconnecting the panel has more advantages.
--- End quote ---
Which is exactly what the circuit I posted does.

Do you need to switch the positive side of the panel for any reason?

--- End quote ---

Sorry, i got confused and thought your circuit was one that shorted out the panel.

Yeah, i've been thinking about this and doing some testing but the main problem with the comparator + nfet idea seems to be that it will need quite a bit of hysteresis added.
Which is doable, it just makes it more complicated as it probably needs a second opamp to buffer the input so the hysteresis feedback can be added to the comparator without needing a low impedance threshold source that needlessly uses up the battery.
Without 1-2V of hysteresis it will oscillate due to the voltage falling as soon as charging stops which turns charging on again.
That oscillation will likely settle into a state where the mosfet is conducting 'just' enough to dissipate the unwanted voltage as heat, which is back to the initial problem we're trying to solve.

The MCU idea will work great, and is simple.
I also like that idea of using a 555 timer that nctnico suggested, maybe somewhat jokingly.
Have the voltage cutoff threshold trigger 555 to do a 30sec fet disconnection period repeatedly.  Technically it is still charging just a very short pulse every 30sec which is just long enough to retrigger the 555. But i think that is fine. It solves all the problems I can think of, only downside is it needs more parts to setup the 555 compared to using a MCU.
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