Modify a standard LM2577S boost converter module to be used as a fixed power point converter using the available solar energy efficiently.
These cheap eBay boost converters are based on the LM2577S-ADJ boost converter chip from National Semiconductor. Using them to convert a low voltage solar energy panel results in a input voltage drop when too much current is drawn on the output. Power (watts) is calculated as Voltage x Current. When the input voltage drops, the input power drops.
We are going to modify a typical application circuit schematic of a boost converter module to make more efficient use of the available solar energy. In some way we must convince the boost converter chip to stop using the solar energy for a while, that gives a chance for the input voltage to rise.
In the above schematic you can see a modification of the original circuit. Resistor R4 is added in series with the feedback pin. That gives the fixed power point add-on circuit a chance to influence the behavior if the boost converter. Specifically, the feedback voltage is raised when the input voltage is too low. This makes the boost converter think that the output voltage is too high and stop switching (thus allowing the input voltage to rise).
The add-on circuit is really simple. When power is applied to the boost converter diode D2 switches off the LM2577. After a while the input voltage rises to (lets say) 5V and transistor T1 starts conducting, this negates the effect of D2 and the boost converter starts working. When the input voltage drops again, T1 stops conducting allowing D2 to switch off the LM2577 again. This feedback system allows the input voltage to settle on a fixed value, hence this circuit is called a fixed power point converter.
Modify the boost converter, and lift pin 2 of the LM2577S of the PCB and solder R4 (4K7) in series. Check if the module still works as a normal boost converter. Build the rest of the fixed power point module on a piece of board.
How to tune this circuit for maximum use of the solar energy?
First, disconnect diode D2 so the add-on circuit has no effect and apply power to the input of the boost converter. Use the trimmer potentiometer of the boost convert to set the initial output voltage. Lets say 12V. Reconnect diode D2.
Measure the the open voltage of your solar panel (by disconnecting it from any circuit and connect it only to your multimeter). This voltage should be equal of less than the output voltage of the circuit. And of course, do this measurement in the full sun. Measuring the open voltage in your dark electronics cave does not make sense. Open a window, or go outside.
Lets pretend you measured 4.8 volts. That would be a solar panel made up of 8 solar energy cells. The input voltage of the fixed power point converter should be 80% of the measured open voltage. In this case 3.84 volts is a good starting point to make maximum use of the available solar energy. (lookup ‘solar panel power curve’ and have a read)
Trimmer potentiometer R8 can be used to tune at which input voltage the boost module start working.
Now comes the tricky part. You need to connect some load to the output which loads the circuit just enough to make the input voltage drop, but not enough that it short circuits the output. A good choice would be a few power leds in series so that the voltage needed to power the leds is higher than the open voltage of your solar panel, but less voltage than the output voltage of the boost converter. Make sure that the power leds are more powerful than your solar panel. In the case of 4.8V, 2 white power leds with 2.7V forward voltage in series will do just fine. This loads the output to the maximum current available, but is not a short circuit. In case your output voltage is about 12V, you could also use a few meters of a 12V led strip. A bad choice (for now) is anything resistive. If you have a good feeling on how the circuits works, a small DC electric motor would also work. If you load the axle (with your fingers) than it would present a load for the circuit.
OK, you found some way to load the circuit but not short circuit it. Connect your multimeter to the output of the solar panel. Start adjusting trimmer R8, look at your multimeter. At some point you should see that turning the trimmer has an effect on the input voltage. Trim it to 80% of the open voltage of your solar panel.
Now it is tuned. You can connect any load to the output. The load will get the maximum solar power available up to the output voltage set by the boost converter.
Having trouble tuning the circuit? If your solar panel is too powerful, try adding a 100 ohms power resistor (or low voltage Christmas tree lamp) in series with the output. In that way the circuit has less power to work with, the tuning will be easier. After tuning, remove the resistor.
Also, if your have a bench power supply, you can use that also to tune the circuit. Set the voltage to the open voltage of your solar panel (for example 4.8V), and set the current to a maximum of 30mA.