Hi,
I have a setup with a breadboard Arduino that runs 24/7 in sleep, waiting for a pin interrupt. It is powered by two 1 Farad 5.5v capacitors that are charged by a solar panel. The solar panel will output 7v (but very low current) in good sunlight, but that is obviously too high for the Arduino or the capacitors.
I currently have a 5.1v zener diode regulating the voltage from the panel, but I don't think it is very good at 5v regulation due to small reverse current flow even below 5.1v. This tends to add up to reducing the capacitors' voltage to about 4.7v, since the solar panel doesn't output much current.
Is there a more efficient way to regulate to 5v when dealing with very low current sources?
Thanks
A silicon diode is about .7volts therefore use 3 1N4148 diodes.
7 - 2.1 = 4.9 volts which would work.
What current is needed?
Well I could do that, but the voltage from the panel will fluctuate over the day so it will end up less at the beginning and end of the day. The current for charging the capacitors is any amount, and the Arduino uses probably less than 1mA. I've heard about low-dropout regulators. Will one of those work?
Hi, LarryD's idea will work very well.
Place the 3 diodes in series, then connect anode end to the positive of the caps and the cathode to the negative of the caps.
The diode chain will only conduct when the panel puts out a voltage higher then 4.9V.
It is doing the same job as the zener but much lower leakage.
The zener regulates in reverse bias, this series regulates in forward bias when the sum of the forward bias voltages is exceeded.
By the way do you have a blocking diode to prevent the caps discharging back through the panels at night?
Tom..... 
Oh right. I thought he meant using the voltage drop of a diode to reduce the voltage. I'll try what he said when I can get the components. Yes I do have a blocking diode so actually, the input voltage will be a bit lower than 7. Anyway, in the meantime, could one use a low dropout regulator to have similar effect (just for general knowledge).
Thanks
Hi, might work, you may have to add some capacitance to the input side of the regulator, haven't had a play with them unfortunately.
Tom.....