Make a Solar Power supply

Solar power is a finicky animal.  Yes It's just DC power but it's wildly fluctuating DC power.   Depending on the day, clouds, shadows, etc..  a solar panel can output 20Volts to 5 Volts and everything in between in the matter of seconds.   Plus solar panels are kind of wierd.  you have two voltages and two amperages to think about.   Open circuit and max power.    Max power is where you want to calculate things as I assume with solar power you want as much power you can generate.  But if your load drops off, things will change.   So treating a solar panel like a battery just does not work.

You can buy solar chargers,  But they are designed to charge a battery,  some times you just want a power supply that is somewhat stable, and in my case that is exactly what I want.  a 13.5 volt power supply that replicates a car's electrical system (no your car is not 12 volts, when it's running it's 13.5 or higher.  Go ahead and grab your voltmeter and check.) for a QRP Ham radio portable station that I like to bring camping.   I dont want to carry a battery with me as the radio has a built in battery pack,  but I do want to run off of free solar power during the day as well as charge that internal battery pack.  So I decided to build a very simple regulator power supply to stick to the back of a small solar panel I have.

I bought off of ebay a couple of years ago, several 10 watt small solar panels for $20 each,  I was going to set them up to run a couple of LED lamps in a shed but instead decided this project will be more fun and useful.    They were sold as 12 volt 10 watt panels,  but solar panels dont work that way.  These panels are actually 18 volt 10 Watt peak power panels that have an open circuit voltage of 24 volts.  so depending on the power draw the voltage will change.  These numbers are also under perfect conditions on a clear day.  Reality is that you never ever get perfect conditions.    Add to the whole mix that I want to power a portable HF radio and I need to be careful as to what kind of voltage regulation I want to use.    A switching supply is highly efficient but will generate a TON of RF noise, so I need to use something that is quiet and lives in the DC world.  Linear Regulators are where I needed to go, specifically the good old 7812 regulator from the 78xx series of linear regulators.    These regulators have been around for ever and are available in a 5V 9V 12V and 15V flavors.   But what if I want a different voltage?  well you can do some tricks to make them adjustable, but I like to simply put a couple of diodes on the ground leg to "lift" the ground voltage and fool the regulator.   Diodes do not conduct electricity perfectly, they have some loss, and typically you get from 0.3 to 0.7 volts lost.  I want high power diodes so I am looking at 0.7 volts lost.  Putting two on the ground leg will give me 1.4 volts.   This will lift the 7812 regulator up to 13.4 volts or 13.5 after all losses and rounding. 

But, the 7812 regulator NEEDS 2.5Volts of power higher than the output.  so a 7812 needs 14.5V at a minimum to work properly, adding another 1.5 with the diodes that brings us up to 16V needed at a minimum from the solar panel to work.   Luckily the panel I have runs at 18V at peak power draw so I have 2 volts of safety margin.  I can already hear everyone out there screaming about my choice in voltage regulator,  Yes they waste power.  These old 78XX regulators can waste up to 50% of the power IF the supply voltage is too high.   I am going to be 2 volts higher than what the regulator wants so this should work just fine and only lose about 25% of the power to heat.   I could get away with 1Amp diodes, but I like to over design for safety.  I will never really see 1 amp of power through them unless things go sideways, but it can get close to a half amp if I do not have a load connected to the regulated side.  

Each 7812 an handle about 1.3 amps,  I have two so I can handle more than the 2Amps needed by my radio when it is transmitting at full power.  Again I like a safety margin in my designs. I might want to add a second device, or even use it to charge a couple of cellphones when not used for my radio.  building in some extra capacity means you can use your projects for more tasks in the future without a redesign.  For example, staying at 13.5 volts I can actually use this to charge a lead acid battery, because it's voltage is less than the 13.8/14.0 Volts that a typical lead acid wants for full charge it can not overcharge the battery if left connected.  Granted it will not have the ability to charge a dead car battery, but a very small 12V sealed gel battery will work just fine to even out a partly cloudy day for my radio.  So I do have the option of upgrading the system with a small lead acid or even a LiFePo4 battery that is very tolerant to charging in this way.  Battery charging is it's own animal,  I will not be covering it with this circuit.