Coming from the harvesting circuit with a diode, we encountered the problem of overcharging. If you overcharge a battery, give it more energy while being fully charged, you destroy it, or even worse (in the case of a lipo battery) – it can get dangerous.
You can regulate the voltage, that is limit it to some preset value by using a voltage regulator like the LM317.
This component looks like a transistor TIP122, but it acts differently. You can give it between 3 and 28 Volt and with a voltage divider you can set the voltage of the output. The energy above this voltage will be given of in heat, so the metal part can get quite hot!
Hw to insert the LM317 in your circuit:
But this all is fine for a constant energy source, but we have a slight problem for a solar cell in our climate conditions with clouds and so on. The LM317 regulates the relation between the input and the output, not the absolute values. When there is a cloud, the solar cell produces less voltage, and the LM317 just lowers the output also.
And you see what happens in the pictures, the regulated 4.28V goes to 0.74V. This is not really what you want…
The solution is to have a relatively low voltage input and to boost this voltage (using a smart setup with an inductor) to a stable input voltage that is fit for your specific energy storage device. For this case the energy harvesting IC’s are developed.
You can use the same LM317 to limit the current. Changing lightvalues not only influence the volatge, but also the current. Rechargeable batteries don’t like to big currents…
The limiting current setup with the LM317 is simpler than the voltage delimiter:
As you need both, voltage and current, you can insert two LM317′s.
Normally, all these circuits require capacitors to stabilize voltages.