This looks interesting. I have some questions though:
- Can I directly connect the 6,5V from solars to capacitor (provided that I cut it off before reaching capacitor max voltage)? I always thought that I shouldn't charge the capacitor with more than its maximum voltage. Well, my electronics experience is not really good...
The solar cell has an internal resistance that will prevent the open circuit voltage from appearing across the capacitor as it charges. How long that takes depends on the current capability of the solar cells and the total resistance in the circuit, however the Arduino is capable of responding in a few micro-seconds so this should not be a problem. Just to be on the safe side, do you have links to datasheets or specifications on both the capacitor and the solar cell? That would help. It may be that we need to add a resistor coming from the solar cell.
- Why is there resistor on gate (G)? To prevent frying my Arduino because of high current to G? Shouldn't I have one larger (ie. 22k) resistor connected between G and S?
Theoretically the gate resistor is there to limit the current sourced by the Arduino. Again, I am not sure what size MOSFET you will need, nor how often you will need to turn on and off the MOSFET. I'll have a better idea once you let me know the specs of the capacitor and the solar cell. I do believe it is highly unlikely you will absolutely need one however, I was soundly castigated recently on this site for suggesting that there are cases where they are not needed. So I promised to be a good boy and suggest them so as not to give other people heart trouble. It's only a few cents, and having one certainly will not hurt in this application. Use 75-150 ohms.
- Wouldn't there be a problem with zener? I always thought it would "eat up" all the voltage above its rating.
It is only there as a safety. If for some reason the Arduino fails to shut off the MOSFET, the zener will prevent the capacitor from being over charged. Since the zener is rated at 5.3V and the Arduio will shut off the MOSFET at 5V, it should never come into use under normal conditions.
There are more I'm measuring voltage on solars (source) with Arduino too. This is to prevent Arduino from dying if the voltage drops too much (it's powered from solars) = when the Arduino is low on voltage, it cuts capacitor from source, so it can regain power for running itself.
- How would your circuit behave in the way of measuring source voltage? Wouldn't the source voltage be the same as capacitor voltage when MOSFET is turned on?
There is no need to do this. If the Arduino is using all the solar power and all the capacitor power, disconnecting the capacitor will not help. The capacitor is not like a battery and will only charge if there is excess power available and is only then a load on the circuit. If there is only enough power to keep the Arduino going, the capacitor being connected will no hurt the situation. I think you can skip doing this altogether.
- Wouldn't the zener limit the source voltage if the MOSFET is on and the capacitor is still charging?
Only if the Arduino fails to turn off the MOSFET.
I'm sorry, if these questions look dumb, but I don't know much about electronics, only basics.
Not everyone can be expected to know everything about electronics, and the only way we can learn is to seek knowledge. There are no dumb questions. The only dumb thing is not to ask.
I will try to write how I understand your schematics - Solars have enough power, Arduino is on and it opens the MOSFET, charging begins. Capacitor's voltage goes up, but solars voltage drops, because the capacitor is "hungry" at the first moments. Arduino keeps itself on, because it closes and reopens the MOSFET repeatedly keeping the solars voltage high enough to power Arduino. MOSFET open state gets prolonged as the capacitor voltage rises and at (ie.) 3V MOSFET stays on (because capacitor isn't that "hungry" and solars voltage doesn't drop that much) until capacitor reaches 5V. Then Arduino closes the MOSFET.
Almost. I would power the Arduino from after the MOSFET. From directly across the capacitor. Set the Arduino to turn on the MOSFET when the voltage reaches about 3.5V. This will charge the capacitor and power the Arduino. When the voltage reaches 5V the Arduino switches off the MOSFET and the capacitor runs the Arduino until the voltage drops to 3.5V, then the cycle repeats.
3.5V for the turn on point is only a guess. If that is not appropriate for your application, choose a voltage that is (4V, 4.5V, what ever is best).
Sorry to write it as a story, but I think it's the most understandable description. Is it how it would work? My main fear is from frying the capacitor with too much voltage and from dropping the solars voltage too much (which should be prevented by software) when MOSFET is opened.
I'll need the specs of the capacitor and the solar cell to help much further. It would also help to know the current draw of the Arduno and any other circuitry while it's working. Also helpful would be knowing what other circuitry is being used.