Solar Fan v2 – FAIL

March, 2014

The Solar Fan I created last year has been moderately successful at circulating the air in my master closet.  During the heat of the day, the sun energizes the fan and the fan pushes air quietly.  However, the fan will stall when direct sun is interrupted by a cloud.  I’d like to have a bit more consistency in driving the fan.

I saw this very nice Instructable on Google+ from Phenoptix and dug in to learn more about how the solar power was used to charge via a USB port.  They key was a small PCB I found on eBay for less than $4 USD.  The PCB accepts a +5V DC input and charges a battery which drives an output constrained to +5V.  While my selected fan is rated for +12V DC, I’ve confirmed it works fine at +5V albeit a bit slower – sufficient for my needs.  My pair of solar panels are theoretically capable of delivering up to +10V DC at 400mA, but in my experience, this almost never happens – a more typical output voltage is 4-6V, even in direct sunlight.  I tested the PCB to confirm that it has an over-voltage protection circuit to stop charging if the voltage gets above +6V DC – a nice feature that will save my rechargeable battery from exploding.

DSCN0258

Here’s my assumption on how this will work:  

– When the solar panels produce between 4-6V DC, the PCB will charge the single D Cell battery  (it has a nice LED charging indicator so I can confirm)

– The PCB drops the voltage down to 1.2V-ish DC in order to charge the single cell.

– When the D Cell battery has enough charge, the PCB steps up the 1.2V  to 5V DC and drives the fan

– When the D Cell battery has exhausted its charge, the fan will stop

The PCB will work with any standard rechargeable battery that uses 1.5V which includes AAA through D.  I picked a D Cell because it provides the most mAh and should drive the fan longer.  My concern is that it may take longer to charge this battery which may result in a delay of the fan running when its needed most.  I’ve also got a rechargeable AA battery + holder and will substitute if I discover issues.  A final option to consider is to use multiple batteries in parallel to provide longer life.  I will experiment and update this post with the results.

My other concern is that there is current loss at the final step where the PCB steps up the 1.2V DC to 5V DC which may impact the ability to move the fan.  I tested the fan and saw that it only requires about 60mA, so hopefully I’ll be ok.

DSCN0259

Update

Unfortunately, this project goes into the FAIL category.  It failed for two main reasons:

  1. The 5V DC coming out of this circuit is not enough voltage to push my 12V fan.
  2. The PCB only produces a 5V output when the charging circuit is on.  I really wanted this circuit to continue to push the fan after the solar panel stopped producing voltage to keep the fan running longer – no such luck.

So, I’ve reverted back to running the fan directly from the solar panels.  I’ve added a 1000 uF capacitor which does help a bit.

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