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Circuit that can divert power based on power produced by a solar panel?

amd7000

Jan 24, 2014
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Hi everyone. :)

I'm new here and have a basic understanding when it comes to circuit building (Cheap hobby stuff) so sorry if this doesn't make any sense!

I've bought a 40 watt solar panel that I would like to power fans for a wood store to aid drying time. I don't really want to use a charge controller and battery with the solar panel, I would just like to run the fans in the daytime directly from the solar.

The problem is that I've only got one 12v 40w fan and so when its cloudy/dull the fan wont turn. Could you advise me on how to build, or find a circuit that would divert the minimal power generated when its cloudy to a smaller fan(maybe multiple pc fans) that would be able to operate at a lower wattage?

Thanks Andy
 

Harald Kapp

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Welcome on the forum.

Figure 2 here shows a circuit that responds to light to control lighting. You can adpat it to your needs.
This circuit use 15V, it should be able to run from 12V, too. Decouple it from the solar panel via a diode and an electrolytic capacitor for a reasonable stable supply. Or even better add a voltage regulator e.g. 9V.

Replace the LED at the output by a relay with double throw contact (changeover contact). Connect the big fan to the contact that is closed in bright light, connect the small fans to the contact that is closed in low light.
 
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(*steve*)

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The problem is that I've only got one 12v 40w fan and so when its cloudy/dull the fan wont turn. Could you advise me on how to build, or find a circuit that would divert the minimal power generated when its cloudy to a smaller fan(maybe multiple pc fans) that would be able to operate at a lower wattage?

There's another possible option, which I'll mention here (a) so I don't forget it, and (b) because it might generate some ideas from other people. I don't think it will be immediately useful to you.

If the option Harald gave you doesn't seem like the right thing, then post again and maybe we can develop this a little more.

This is basically a big joule thief :)

The solar panel's rating of 40W only applies in the sunniest conditions, with a cool panel, and with an ideal load. In other words, almost never.

Let's assume the fan requires 40W (that's over 3A at 12V)

The panel's voltage will droop to a very low value if you try to power this fan in less than optimum conditions. As you have noticed in some cases it doesn't turn at all. However, even on overcast days, the open circuit voltage of the panel probably exceeds 12V. Also, the fan, once turning, will probably continue to turn even if the voltage falls substantially (perhaps to 8V).

A simple circuit consists of a large bank of capacitors across the solar panel and a circuit which will apply power to the fan when the voltage across the capacitors exceeds 12V, and turns it off again when it drops below 8V

A circuit like this would essentially allow the fan to operate in short bursts in very overcast conditions, rising to something closer to continuous operation in full sun.

However, the larger fan, operating in very short bursts may not be very effective in overcast conditions.

Perhaps, if you had 3 nominally 12V fans, rated at (say) 12W each, you could build three of these circuits, one switching at 12 and 8 volts, then next at 13 and 9 volts, and the last at 14 and 10V.

This would have the first fan operating intermittently at low light levels until it ran continuously, at which point the second fan would start to operate intermittently, until the light levels are sufficient to operate it continuously, and then finally the last fan would operate.

For safety, you could build a 4th circuit which would dump power into another load (perhaps a resistor) if the voltage increased above 15V where the fans are possibly in danger of overheating) and turning off again at perhaps 12V. This would be very rarely used I expect.
 

amd7000

Jan 24, 2014
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I like that idea! Could I use a few caps from old motherboards and power supplies? With not much else? Thanks for the explanation. I would like to salvage as much as I can from the old stuff I have laying around. I'm stingy. :)

At the moment the panel is struggling to generate 1w. Is that normal? It's really dull, rainy and cloudy though.
 
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(*steve*)

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I like that idea! Could I use a few caps from old motherboards and power supplies? With not much else? Thanks for the explanation. I would like to salvage as much as I can from the old stuff I have laying around. I'm stingy. :)

You might be able to. You want them rated for the open circuit voltage of the panel (or higher). That may be 18 to 24V. And you want as many as you can find connected in parallel

At the moment the panel is struggling to generate 1w. Is that normal? It's really dull, rainy and cloudy though.

Yeah, pretty much.

If you were running a circuit like I suggest, a 10W fan would be running about 1% of the time (not a lot at all really).
 

amd7000

Jan 24, 2014
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so I could just use capacitors in series and when they're fully charged they would run a small fan for a small amount of time without any other components?

I've only found some 16v caps and the max output voltage of the solar panel is 17v, would they be o.k for at least a while?
 

(*steve*)

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so I could just use capacitors in series and when they're fully charged they would run a small fan for a small amount of time without any other components?

No, you would need to let them charge in parallel (I did say parallel for a reason), and then connect the fan.

If you had 100,000uF of capacitance (that's a lot) and the fan draws 1A (so about a 12W fan -- smaller than you have), the voltage would drop by about 10V per second from those capacitors. If you charged them up to 16V and allowed them to discharge until the voltage dropped to 8V, it would take less than a second. i.e. the fan would run for less than a second.

Again, using very rough calculations, it would take about 15 minutes to charge the capacitors up again (assuming the panel is producing 0.1W).

I've only found some 16v caps and the max output voltage of the solar panel is 17v, would they be o.k for at least a while?

Probably. Long term it wouldn't do them any good. It would be fine if you never allowed the voltage to rise above 16V -- connecting the fan would fix that :)
 

amd7000

Jan 24, 2014
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Ooops, meant to say parallel. :) Thanks for your time. appreciate it.
 
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