Solar supply question

Thread Starter


Joined Apr 6, 2009
Hey all, Thanks for helping dummies like me...I am building a solar collector and will be powering a small fan with a pv panel controlled by a thermistor voltage divider setup and a comparator. My question is... I am using a voltage regulator to peg the voltage, but it will still leak undervoltage until it reaches 12 volts. There may be times when the sun isn't producing 12 volts, but the temperature is high enough to keep the comparator output high. Will the lower voltage harm the fan, if yes, how do I keep it off until it reaches say.. 11.5 volts? I've played with zeners, but I think i'm missing something.:confused:
The lower voltage shouldn't harm the fan, but to keep it off until the voltage is above 11.5V I would suggest using a transistor that has been biased to not turn on until the desired voltage is reached (there are many ways to approach this). If you already have a comparator and some zener diodes then you are almost there. The zener (11.5V) would be used to regulate an incoming signal on one input of the comparator while the voltage to be 'compared' would be on the other input of the comparator, and the output of the comparator would be connected to drive the fan, once the voltage to be measured is greater than the zener voltage, then the comparator output should go high (depending on how it is connected) to drive the fan.

Anyways, have fun!

Thread Starter


Joined Apr 6, 2009
If I understood you correctly, I put a 11v zener on the inverted input, and then put a 500 kΩ pulldown resistor on the non inverted side to keep that voltage slightly lower until 11 volts is reached, the zener pegs the inverted input, while the non-inverted keeps rising, sending the output high and biasing the transistor. It Worked!!! You are the man.... Thank-you!!!:D
Last edited:


Joined Apr 27, 2009
Second thought: I may have misinterpreted your message, and you are in fact doing something similar to what I describe here, but since I've already typed it up I'll post it anyway.

I'm assuming you're using a 24 cell panel. (Commonly known as a "12 Volt panel")

What (Why) are you using to regulate the voltage for the fan? (Do you regulate the voltage for the fan, or the output from the panel?)
The fan will run at any voltage high enough to start it, and low enough to not melt it.

Ideally, you should rather take steps to ensure that the solar panels' terminal voltage never exceeds 12 Volts - and doesn't get to low either. This voltage is slightly dependent on light intensity, and very dependent on temperature, so it may be a good idea to reduce it a bit to be on the safe side.

This can be done by placing a capacitor across the panel terminals, feed the voltage across the cap into a comparator with hysteresis, and use this output to connect/disconnect a load as needed. This could be the fan directly, or a batterycharger/supercap since you don't need the fan to run until the thermistor gets to warm.