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I think your approach is flawed. When you say charges and discharges into a motor, do you mean repeatedly starting up the motor to spin for a sec and then letting it stop while you charge the cap again?This is extremely wasteful of power. Most if not all of the power you stored in the cap will be wasted just getting the motor to start spinning. DC motors waste a lot of power during start-up to overcome momentum and draw close to their stall current for a a good portion of a second. A better design would be to charge up a larger bank of capacitors to a high voltage using a boost regulator from the solar panel, and then running the motor through a buck regulator to step the voltage down to the motors operating voltage. What motor are you using? Hom much power/energy will it need to lift the weight?
...we have to build a solar powered "crane" that runs off a 1.5V 400mA solar panel...
Some rediculousness for thought: http://www.goldmine-elec-products.com/prodinfo.asp?number=G179303000F Cap at 2.7v
buck-boost (or any dc/dc converter for that matter) doesn't increase or decrease power (measured in watts) but simply changes the voltage and inversely the amperage.
How long can the solar panel be exposed to light before your crane must do work? The idea of using a small bank of capacitors doesn't actually sound like a bad idea. Are you able to also use a battery/charging circuit?
I will have to take a harder look (I'm at work right now), but 3lbs is an awful lot for a 1.5V motor. Have you done any math for power required to lift 3lbs, and energy required to move the 3lbs the distance you want?
ore food for thought - What about counter weighting the crane as well? At 1.5lbs to lift 3lbs, you would only ever be lifting 1.5lbs in either direction (if 3lbs is your imposed limit).
The problem with using a counter-weight in this case is that our score is based on how heavy our crane is vs. how much it can lift. So any points we would gain by using a counter-weight would be lost because it would make the crane heavier.
Remember that capacitors discharge power linearly (right?) but that voltage is not a linear function over time
you actually have a small window when you will have a usable voltage. (that part I know is true)
With all other variables being constant, does putting multiple capacitors in parallel affect the charge and discharge rate?
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