Author Topic: Simple voltage cutoff schematic for solar charger, any tips?  (Read 4568 times)

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Online Zero999

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Re: Simple voltage cutoff schematic for solar charger, any tips?
« Reply #25 on: April 23, 2022, 07:26:49 am »
Thanks for the circuit.

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

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

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.

It should disconnect the battery nice and cleanly, without the need for hysteresis. When the MOSFET starts to turn off, the voltage on the solar panel will rise, causing the voltage on the TL431's reference to increase further. This positive feedback results in latching action, disconnecting the solar panel from the battery until the of the day, when it's reset. A night the battery powers the load via the MOSFET's body diode. It might become unstable, when resetting, but I doubt that much hysteresis is required. You really need to test any solution with a real battery, load and panels.

Try a feedback resistor between M1's drain and the TL431 reference? That should provide positive feedback, as the TL431 is an inverting amplifier and so is the MOSFET.

You'll probably need another resistor between the TL431's output and the MOSFET's gate. I forgot the minimum voltage across a real TL431 will be around 2V, which could exceed its threshold voltage.
« Last Edit: April 23, 2022, 08:02:33 am by Zero999 »
 

Online Zero999

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Re: Simple voltage cutoff schematic for solar charger, any tips?
« Reply #26 on: April 24, 2022, 02:34:09 pm »
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.
How many do you need?

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.

The MCU idea will work great, and is simple.

How about the ATTINY10? Digi-Key has >50k in stock and they're cheap. The only downside is it only has 4 IO pins, but that should be enough for this application. A thermistor could be used to measure the battery temperature and perhaps a look-up table could be used to simplify calculating the cut-off voltage?

https://www.digikey.co.uk/en/products/detail/microchip-technology/ATTINY10-TSHR/2051008
 

Offline PsiTopic starter

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Re: Simple voltage cutoff schematic for solar charger, any tips?
« Reply #27 on: April 28, 2022, 02:23:50 pm »
The constraints put on me for this project have been changed.
I can now locate the circuit somewhere else and so burning off the excess voltage as heat is doable.  I'll probably just go with an LM317 or similar and a PCB large enough to heatsink it.

Thanks for all who helped and put forward ideas.
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Offline PsiTopic starter

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Re: Simple voltage cutoff schematic for solar charger, any tips?
« Reply #28 on: April 29, 2022, 05:25:45 am »
Out of intertest,  D1 is only there for protection from the solar panel being connected backwards, correct?
It's not doing anything special other than that?


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Online Zero999

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Re: Simple voltage cutoff schematic for solar charger, any tips?
« Reply #29 on: April 29, 2022, 06:46:26 am »
I've used that circuit before, in the same application and it works.

Yes, D1 can be removed, if you can guarantee the solar panel won't be connected backwards. How much current can the solar cell produce? If it's >1A, use a larger diode, as the LM317 might not current limit until 2.2A.

I would also recommend adding a 33V zener diode across U1. I had problems with the LM317 failing, which I put down to lighting strikes and the zener fixed it. Obviously it doesn't protect against a direct strike, but it will suppress shot spikes, capacitively or inductively coupled from a nearby strike.

I thought you wanted to vary the battery voltage, depending on the temperature? You could make part of R1 or R2 a thermistor.
 

Offline PsiTopic starter

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Re: Simple voltage cutoff schematic for solar charger, any tips?
« Reply #30 on: April 29, 2022, 09:14:17 pm »
I've used that circuit before, in the same application and it works.

Yes, D1 can be removed, if you can guarantee the solar panel won't be connected backwards. How much current can the solar cell produce? If it's >1A, use a larger diode, as the LM317 might not current limit until 2.2A.

I would also recommend adding a 33V zener diode across U1. I had problems with the LM317 failing, which I put down to lighting strikes and the zener fixed it. Obviously it doesn't protect against a direct strike, but it will suppress shot spikes, capacitively or inductively coupled from a nearby strike.

I thought you wanted to vary the battery voltage, depending on the temperature? You could make part of R1 or R2 a thermistor.

Panel is only 18V 350mA, so no problems spec'ing the diodes.

My currently design includes a 36V TVS diode on the battery side to handle any spikes.
However I do wonder if that maybe better located between INPUT and OUTPUT, since ADJ to OUTPUT is kind of current limited due to the divider resistors.
« Last Edit: April 29, 2022, 09:25:32 pm by Psi »
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Online Zero999

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Re: Simple voltage cutoff schematic for solar charger, any tips?
« Reply #31 on: April 29, 2022, 09:23:56 pm »
My currently design includes a 36V TVS diode on the battery side to handle any spikes, so should be fine.
That's no good. The battery doesn't need any protection. It's the LM317 which needs protecting against spikes.
 

Offline PsiTopic starter

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Re: Simple voltage cutoff schematic for solar charger, any tips?
« Reply #32 on: April 29, 2022, 09:26:20 pm »
My currently design includes a 36V TVS diode on the battery side to handle any spikes, so should be fine.
That's no good. The battery doesn't need any protection. It's the LM317 which needs protecting against spikes.

yeah, you're right.  Thinking about it more i agree.  Needs to be between input and output.
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