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A pratical/theoretical question.. is a charge controller always necessary?

Example: A ventilation fan needs to run when the sun is out.

Can I directly connect a 12v solar panel to a 12v fan?

Will it just run in proportion to sunlight?

Will it burn out if too much/little sun?

More generally, to what degree can one build a super simple solar system?

Fridge, fan, and other devices, each with their own simple PV panel?

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    \$\begingroup\$ You can but you may have issues with the inrush required to get the motor spinning, depending on the quality of the fan and the size. Many solar panels don't provide a whole lot of wattage, even at 12V output. For example, a 300 square inch panel provides about 20 watts @ 12V, or around 240mA of current. \$\endgroup\$ – Ron Beyer Jun 28 '17 at 3:49
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    \$\begingroup\$ When you don't mind killing the battery. (NB 20W@12V is a lot more than 240mA) \$\endgroup\$ – Brian Drummond Jun 28 '17 at 6:41
  • \$\begingroup\$ When you don't mind wasting energy. \$\endgroup\$ – Voltage Spike Jun 28 '17 at 6:44
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You don't need a charge controller when you don't have batteries. When you don't need to store energy, you don't need batteries. Charge controllers are only for charging batteries. You may need a voltage controller when you don't have batteries. If you want constant voltage for anything, you need a voltage converter. The voltage from a solar panel varies with the intensity of the sun light and the current drawn by connected loads. If the fan or pump is larger than 100 watts or so, you might have difficulty with getting it started. However the solar panel should not have a problem working into a short circuit. However, if there is not enough voltage to get the motor moving, it will sit there stalled with enough current flowing to potentially overheat it. You may want something to prevent the motor from remaining connected if the speed is too slow.

You can size a solar panel to a fan or centrifugal pump motor and let the speed of the fan or pump vary as the sunlight varies. You can not do that with a fridge. Even if you are willing to let the fridge temperature increase overnight, you need constant voltage because the motor and refrigerator compressor will not perform well with changing voltage.

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Given the concerns raised in other answers about the starting / stall current, you could consider charging a large capacitor until the voltage gets high enough to start the motor reliably - say 10 V, for example. A voltage threshold trigger would then connect the fan.

Similarly, you should disconnect the fan before the voltage gets so low that it would stall. For this your threshold trigger would need some hysteresis.

At dawn the fan would run in pulses which get longer as the light level increases until it is bright enough to run continuously.

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