Solar Attic Vent Question

[dsnyder]

I have four 12 volt vent fans running to cool my attic. They are hooked up to a deep cycle battery that is charged by 110 watts of solar panels.

For reasons that don't really matter here, I need a switch in the circuit to turn off the fans when it gets dark and energize the circuit when it gets light.

Obviously this is the opposite of most applications for photo switches as they turn things on when it gets dark.

I'd like to avoid a lot of relays ect... since I don't want to waste any of my solar power.

The system runs between 10 and 14 volts depending on how bright the sun is and the charge condition of the battery.

Basically I'm looking for an inline switch that allow current flow when it is light and disconnects when it is dark that can handle up to 14 volts and perhaps 2 or 3 amps.

Suggestions?

 

[Resqueline]

A small solar panel all by itself, only/directly supplying the fans and nothing else?

 

[dsnyder]

A small solar panel all by itself, only/directly supplying the fans and nothing else?

There are 8 small solar panels, 6 are 15 watt and 2 are 10 watt. They go into a charge controller and then to a deep cycle battery. The 12 volts then goes from the battery to the fans. I have a car stereo and some 12 volt decorative lights that I turn on occassionally that are in the loop but the primary purpose of the system is to run the attic fans when it is hot.

 

[duke37]

Here is a simple circuit that could work. You will need a light dependent resistor, the ORP12 has been around for many years and is still sold. The 4093 is of the CMOS family.

The resistor should be set so that the voltage across the resistor is about 6V at an intermediate light level. There will be a large hysteresis and if this is too large, a slightly more complicated circuit using an op-amp or comparator could be used.

Many N-channel fets can be found which will pass a few amps.

 

[Resqueline]

Sounds like you really should have had a differential temperature controlled fan switch instead, but here's a very simple circuit that turns on the fans with the sun.
The MOSFET could be any low-voltage N-channel type in a TO-220 case, mounted on a small heatsink.
R1 & R2 could for example be 100k & 22k respectively, or replaced with a single 10k-100k potentiometer to set the turn-on point.
The fans will be turned on & off relatively gradually, but I don't think this'll cause any problems.