solar input power source: 100mA 7.2V vs 200mA 5V ?

[tcc8]

Hi,
I want to power my arduino running low powered sensors(DHT, etc) with a solar input power source.

What are the trade offs of using 100mA 7.2V vs 200mA 5V?
eg. http://sra-solder.com/product.php/7019/139/monocrystalline_solar_cell
http://sra-solder.com/product.php/7021/139/monocrystalline_solar_cell


I understand that the input range for arduino is 5-10V, what will using the 7.2V over 5V mean? and does having more current just mean being able to power more sensors?

Thanks

[aj88keys]

Which arduino are you using? If your arduino uses 5v logic level (uno, mega 2560, leonardo, etc), the 5v solar panel will likely not be able to keep it stable. I would lean toward the 7.2v panel as it can power the arduino through its regulator. If you need more current, you could always add another panel in parallel to power more sensors.

If you are running a 3.3v arduino(nano, due), you should be ok with either panel; the 5v panel's higher current would allow you more sensors, and the 7.2v panel would be able to run the system in lower light.

[tcc8]

I'm using the UNO.

Thanks, I'll get the 7.2V one to be safe.

Is stability problem because the 5v panel is rated at 5V but likely to be 4.9V?

[CrossRoads]

You can run from 5V just fine.
Connect your solar panel to 5V and Gnd. (on the power header - not the barrel jack connector).
Put a diode from 5V (cathode) to Vin (anode) to avoid reverse biasing the 5V regulator and damaging it.
Don't get it backwards, you will damage stuff up if swap the 5V and Gnd pins.

[aj88keys]

My only concern with running the arduino directly off the panel would be that the panel could output more than 5v in bright sunlight. I have a few panels rated at 1.5 and 3 volts, and in direct sunlight, they will often output up to 2.5 or 4 volts, respectively. Whether or not an extra volt would cause damage or not, I couldn't say, but I personally would prefer to go through the regulator to be safe. The stability issue that I was talking about would be from passing the 5v panel through the regulator (connecting it to V_in), which would drop the voltage down a volt or two.

[CrossRoads]

Could always put a 5.1V Zener across the panel output to be safe.
Or use the 7.2V, 200mA panel if you wanted more currrent. SC20072
http://sra-solder.com/product.php?xProd=7022

[Robin2]

Solar panels only produce the output in the advertising material if they are aimed directly at the sun in bright cloudless conditions.

Unless you can be sure of maintaining these conditions it would be wise to assume the solar panel can only produce about 25% of its advertised output.

I would have a suitable lead-acid battery (perhaps 6v) in the circuit which would be charged by the solar panel and could provide power in low light. You should check whether the solar panel has a diode to prevent the battery discharging into the panel.

...R

[MarkT]

Solar panels only produce the output in the advertising material if they are aimed directly at the sun in bright cloudless conditions.

Unless you can be sure of maintaining these conditions it would be wise to assume the solar panel can only produce about 25% of its advertised output.

No, its about 5 to 15% in overcast conditions, but its not that simple.  Voltage and current are not independent for a PV cell,
so as less light falls on the panel you get less current and less voltage.

In practice this means for powering a 7V circuit you need a panel that can produce at least 9--10V open-circuit in full sun,
or more like 10--11V full sun in order to produce best current at 7V in low light.

Typical PV panels are series-connected arrays, and you get about 0.55V usuable voltage per cell
in sun and 0.45V per cell in bright overcast conditions (with good current).  For use with a 12V battery
panels have 36 cells (ie 20V good sun, 16V in poor light - you need about 15V to power a battery charger
circuit and another volt for the blocking diode to prevent the battery discharging through the panel at night).

One solution is to use a power-point tracking circuit (effective a buck-boost converter that compensates
for the light conditions to run the panel at its sweet spot and produce the same output voltage.  These
cost a bit more but can be combined with battery charger circuit to good effect.

[Robin2]

Mark, I don't disagree with your more pessimistic estimate of solar panel output. I was probably being a bit optimistic in case the OP was in a sunnier place than the UK.

I have yet to see any data that justifies the cost of an MPPT controller in preference to buying a bigger solar panel, and I think I have looked fairly widely. (I use solar panels to charge my boat batteries and I'm glad I resisted the temptation to buy an MPPT controller).

However it seems to me the OPs requirement is for a very simple and modest system. If the output of the solar panel is equivalent to a trickle charge for a lead acid battery it's unlikely that any form of charge regulator is needed.

...R