Hiya peoples,
After playing around with a stepper motor and a few big caps I can get an array of high powered led's to light the way home from my shed to to the house which is around 500 metres away. Now using 5 10,000uf caps I could get around 1/2 an hour out of the array but I found a place where I can get some 50 farad caps :shock: :shock: :shock: .
Now I'll be able to make up a simple wind genny to charge up the caps and have a nite light all nite long. Well these cap's will have much more use in my workshop I've got to say to date the biggest cap I can get in Oz is 1 farad.


Anyways here's the link to Coopers that make the caps

http://www.cooperet.com/index.asp


Cheers Bryan1

10mF caps are already the size of coke cans. And you want 50F caps? Firstly, they are gonna cost. Secondly, they are fatal. Why not consider alternative power sources such as car batts? Easy to charge, and my gues is that it's cheaper and more portable than 50F caps.

Originally Posted by checkmate

10mF caps are already the size of coke cans. And you want 50F caps? Firstly, they are gonna cost. Secondly, they are fatal. Why not consider alternative power sources such as car batts? Easy to charge, and my gues is that it's cheaper and more portable than 50F caps.

Eh cheskmate,
These cap's are the size of a AA battery and of the site where I found them they can deliver upto 35 amps of power @ 2.7 volts.
I've requested a few as samples so I'll do some test once I get them but I'm my opnion dey R awesum :twisted:

Cheers Bryan1

I've seen 1F, 2F, and 4F audio caps.

I'd suggest getting a few of those and throwing them in parallel... i've seen this done with 8 1F caps with no problems at all.

How can you get 1/2 hr of light from only 50,000uF?
1) An array (10 LEDs) of high-powered (25mA ea) LEDs (3.5V white) needs a total of 250mA.
2) Charge the 50,000uF cap to 7.0V.
3) Use ten 140 ohm current limiting resistors for the LEDs, so that their current is 25mA ea when the cap is charged to 7.0V.

The LEDs' brightness will begin dropping immediately, and they will turn-off when the cap discharges to 3.5V or a little less.
The cap's voltage will discharge to 3.5V in 5 time-constants, which is 5 X 14 ohms X 50,000uF = 3.5 seconds. You would have useable light for about 2 seconds.
A 1/2 hr has 1800 seconds, doesn't it? :roll:

Lets try a 50F supercap.
1) Can't use white LEDs because the cap's recommended voltage is only 2.5V, so use ten 2.0V red LEDs operating at a starting total current again of 250mA.
2) Charge the cap to 2.5V.
3) Use ten 20 ohm current-limiting resistors for the LEDs, so that their current is again 25mA ea when the cap is charged to 2.5V.

Again, the LEDs' brightness will begin dropping immediately, and they will turn-off when the cap discharges to 2.0V or a little less.
Again, the cap's voltage will discharge to 2.0V in 5 time constants, which is 5 X 2 ohms X 50F = 500 seconds. You would have useable light for about 330 seconds. That's only 5.5 minutes, isn't it? :roll:
You'll have to spend a fortune on enough supercaps for the LEDs to last all night long. :lol:

I know, just use four AA Ni-MH 2500mA/hr rechargeable cells. Their voltage will remain at about 4.8V and they will power ten white LEDs at 25mA ea for about 10 hours (600 minutes) per charge! :lol:

Originally Posted by audioguru

You'll have to spend a fortune on enough supercaps for the LEDs to last all night long. :lol:

I just noticed that Digikey sells these 50F/2.5V PowerStor supercaps for $299.20CAN for 10. I figure that you'll need about 48 of them to power 10 white LEDs for 10 hours through a low-dropout regulator so the result would be the same as using Ni-MH cells.
So $1,436.16CAN worth of supercaps equal $14.00CAN of Ni-MH cells! More cost than 100 times more.

The way I read the problem:
1. I'd assume the LEDs are used in a way that is basically constant current (let's say it's got a transistor drive to sink the current).
2. 50F means that it will have a dV/dT rate of 1 V/50 sec under a 1 amp constant current. Under a constant 250 mA, you'd get 200 sec for every volt before the threshold is reached.
3. If you had them charged to 7V, you'd have 11.7 min until you reach 3.5V.
4. If you used a dc-dc buck converter with a constant current output, the picture is much better since it can output 250mA@3.5V output at only 125mA@7V input. Should last basically 1.5 times as long, basically 17.5 min.

Batteries are a more logical choice.

Yeah, also look at (if you can even see them) solar garden LED lights. They are supposed to shine (very dimmly) all night long. Have you ever seen one with any light after only a few hours?

Have you seen my 6V Ultra-Bright LED Chaser project? It can blind you for weeks on just four AA alkaline cells. The LEDs are blinked for only 15ms, and the whole thingy pauses after a few rounds before starting is chasing again. When I turn up its speed it can be used as a flashlight or nightlight for weeks.
http://www.electronics-lab.com/proje...004/index.html