for a school project I have to do something that needs the environment to work, my plans are to make a solar powered mini fan, now my question. How can I make that the fan ( 6v dc motor) keeps working for little while even after I block the solar panel. Do I simply put a capacitor? If yes, how do I know wich capacitor( values) should I get?. Thanks for the help
A large capacitor would work, yes. Just get something rated for more than six volts (I'd recommend more than twelve, a wide margin is a good thing to have!) and a large capacitance; the larger, the longer it will continue to run, assuming the panel was able to fully charge the capacitor before it was blocked. If you want it to run for a very long time, you might want to look at electric double-layer capacitors, also known as supercapacitors, which can have very high capacitance.
Correct - a capacitor would work. But you also need a diode to stop current going back into the solar panel when the solar panel is covered up. Some panels already have this though. Either way, you can add one, like this circuit, as explained here.
I'd start with a 100,000uF capacitor or thereabouts. Just make sure that whatever capacitor you choose has a voltage rating greater than 6v, and that you connect it the right way round or it'll explode on you. (Not hugely dangerous, but certainly messy). 100,000uF = 100mF = 0.1F.
Or use a supercapacitor (aka double layer or EDLC). These have a far higher capacitance, but finding one with a high enough voltage rating could be hard. Instead, you'll need to connect them in series. Say you have five 100F super capacitors in series, where each is rated to 2.7v. The total capacitance will be reduced (to 20F) but voltage rating increased to a total of 2.7x5 = 13.5v. But if you put caps in series for extra voltage like this, you must add extra leeway, say 50% minimum, since voltage will not be totally evenly spread between them, and some may end up with a voltage higher than others. So I'd use a minimum of 3x 2.7v caps, but 4 to be safe.
Remember that the higher the capacitance, the longer the fan will keep going - but also the longer it will take for the fan to start going, since the capacitor will steal current from the fan, until the charge in the cap has reached enough for a high enough voltage to drive the fan.
Approximate figures: Assume the fan uses about 50mA at 6v. A 1 Farad capacitor drops 1v per second when 1 amp is drawn. So at 50mA, 1 Farad capacitor would last about 20 seconds per volt dropped. So if the fan continues to use 50mA until it goes down to 3v, then 1F capacitor would drive the fan for 60 seconds. Similarly, if your fan draws 100mA, a 10,000 micro Farad capacitor would last 3 seconds.
Say you have 4 x 100F capacitors in series. The capacitance will be 25F. As above, say you motor draws 50mA. It would last about 25 x 3 / 0.05 = 1500s. Or say your fan draws 500mA, it would last 150 seconds.
As for how fast the capacitor takes to charge, let's say you use the setup above with a 10W, 6v solar panel. In full sunlight it will deliver about 10 / 6 = 1.666A. To charge a 25F capacitor bank to 3v, given a supply current if 1.666A, it will take 25 x 3 / 1.666 = 45 seconds. But the closer the power rating (in watts) your motor is to your solar panel, the longer it will take to charge the capacitor, so the longer the delay for the fan to start.