Voltage *and* current regulation for a solar panel ?


I want to use a solar panel as power source for a circuit. How can I regulate both voltage and current from the panel to have a predictable source ?

Thank you in advance.

Maybe you could explain your requirements in more detail? Voltage regulation is pretty simple as there are a ton of fixed voltage and adjustable voltage regulators available. Current regulation is a little more complex, depending on what you actually mean and need. Maximum adjustable current limiting is one thing, adjustable constant current is more complex if that is what you actually require. Generally you have to explain what loads your are trying to power with your panel to understand what you actually require, as load resistance information is needed when talking about constant current modes of regulation. Also the specifications for your panel are needed as far as maximum voltage and max current ratings.


How can I regulate both voltage and current from the panel to have a predictable source ?

Basically you can't regulate both at the same time, it is against the laws of Physics. You can control only one as the other is fixed by the load.

I understand it is not possible to regulate both current and voltage, but my concern is indeed more to limit the current because of the nature of the panel it might change according to time.

Have you seen this:- http://www.zen22142.zen.co.uk/Circuits/Power/clps.htm

amundsen: I understand it is not possible to regulate both current and voltage, but my concern is indeed more to limit the current because of the nature of the panel it might change according to time.

Well a panel may decrease in it's maximum current capacity in time, especially if you don't clean it off once in awhile, however I can't think of how it could increase it's maximum current capacity in time unless you happen to move to a location that has more sunlight? Why wouldn't a simple fuse be a good current limiter? Keep in mind that the load generally determines how much maximum current will flow not the panel even when the panel is producing it's maximum voltage.

Again the better you can explain your requirements and application, the better the quality of the possible answers might be.


I like Lefty's answer. Solar panels are by nature current limiting. I use solar for my routers, and they are 135w solar panels. That is about 7.5A at 17 volts. That same panel also produces 7.5A at 14 volts. It produces about 7.5A at 9 volts, and about 8A short circuit. That is the case with these PWM chargers. http://www.solar-electric.com/stpwmchco.html

If you want more current, then consider a MPPT charger. It keeps the panel output voltage at the correct level for maximum amperage output. http://www.solar-electric.com/admpchco.html

Right, the panel has its own limits. But how does one know the limit for current and voltage when the specs only tell about the maximum power (2.5 W in my case) ? Of course P = UI but if you got P how do you get U and I ?

Also, if I don't give requirements it's because I have a panel and I am just trying to figure out the kind of funny circuit I could build using it...

You must determine the voltage rating for that panel. For example, my panels are rated at 135w at 17 volts, but they do not produce that if connected directly to a discharged battery. In my case, if the battery voltage dropped to 10 volts, then the output of the panel would be 7.5A * 10V = 75 watts

That is not even close to 135 watts, is it?

edit: As a comparison to a MPPT charger, that charger would keep the output voltage of the panel at 17 volts. Since the output is 10 volts, then the charge current would be about 13.5 amps.

Also, more than the limit of the panel, the real limit might come from the lighting conditions. Measuring the output of the panel with a multimeter, I got a change from 10 mA to 20 mA with a small change in the panel position with a low belgian late afternoon sunlight. Therefore I suppose it would be better to connect the panel to an accumulator to smooth the output a bit (and possibly use the accumulated power even at night).

Good idea on the smoothing. I use an 85AH deep cycle marine battery to smooth that power out. And it is smooth. But I use about 1A average current on my devices.

I'm about to find out how smooth. Tropical storm Isaac is headed here. The day due to arrive here is Tuesday. HA! I think maybe Wednesday. If you see Jim Cantore from the Weather Channel in Destin, Florida on Monday, that is a bad sign for me.

Its not normal practise to use a Solar panel by itself to power electronics. You usually need a battery which the Solar panel charges, and the battery provides the power to run the electronics. As other posters have indicated, solar panels are current sources and the current produced is proportional to the light falling on the panel. The best arrangement is Solar Panel > MPPT charger > battery > electronics to be powered. You can also use Solar Panel > PWM charger > battery > electronics to be powered, but this loses around 30% of the Solar panels capacity.

On my first solar powered (SLA charger in order to measure the battery … I took a measurement of the battery voltage and then I shut down the charger for 5 seconds and made a second measurement… This was done every time I used the device which drew about 100 mA for 100 mS. Doing this provided a charge method that got 3 years use from a Yuasa NP1.2-12 battery as I could tailor the charge to the battery. While I understand the MMPT method… it was done for another reason. The design used (Not my choice) amorphous silicon solar panels. The sputtered silicon type. They as a class have a different life cycle than the preferred crystalline type… they loose efficiency as they are exposed to the sun and can loose as much (the ones I was forced to use) as 30% of their capacity. Today there are solar panels in production that do “wear” out over time. It’s easy to tell them apart… they are an amorphous brown color not the pretty blue/purple color of crystalline type… Nor do they look like crystals. This was done before there were efficient one package, inexpensive buck/boost mode switchers that could be processor controlled…