Help me find the right capacitor Size

Thread Starter

Ree

Joined Oct 9, 2013
5
So a little background on the project I'm working on. My brother is going to Tanzania in a month and I want to send him with something that can be they can use to read/do other activities at night. So, I'm using a solar panel to charge up capacitor than then power LED's. I'm using capacitors because I want something that's going to last for a really long time. Anyway, currently I'm using 7 , 5V 1 farad capacitors to power 4 LEDs (for a little over an hour). I'd love to light 10 or more 3V LED's with one capacitor (for simplicity), but I don't know what size capacitor to buy. Any suggestions would be greatly appreciated and feel free to ask question.
 

MrChips

Joined Oct 2, 2009
22,107
Have you calculated how much energy is required to power an LED for 1 hour?

A rechargeable battery might be a better choice.
 

Thread Starter

Ree

Joined Oct 9, 2013
5
Have you calculated how much energy is required to power an LED for 1 hour?

A rechargeable battery might be a better choice.

I need around 2000 Joules. Which would be very easy to store in a battery, but like I said, I want this to be able to last for a long time (Plus I just think it'd be cool to work with capacitors). So, a battery would work better, but fail after 1000ish charge cycles.

With 2000 Joules, something like a 5v, 160F capacitor would work (ultra capacitor really) but I'm wondering how you calculate optimal capacitance/Voltage for the circuit.
 

crutschow

Joined Mar 14, 2008
25,680
It depends upon how long you want the LEDs to light and how much voltage drop you can tolerate from the capacitor. For best efficiency you could use a switching constant-current output regulator to power the LEDs from the capacitor. With that you don't need a power wasting resistor in series with the LEDs and the voltage drop on the capacitor will not affect the LED brightness.
 

BobTPH

Joined Jun 5, 2013
2,601
If your 10 LEDs are 20ma each that is 200ma. A 1 Farad capacitor will lose one volt every 5 seconds at that rate. So a 5V 160F capacitor will lose 2 volts (making it too low to light the LEDS) after 160*5*2 seconds = 28 minutes. At that point you have extracted 64% of the energy, so using a buck-boost converter could not give you another 14 minutes.

An 18650 4000mAH LiIon battery would keep going for 20 hours with the same load.

Take you pick.

Bob
 

Thread Starter

Ree

Joined Oct 9, 2013
5
The joule thief seems like an interesting addition, allowing me to drain the capacitors much closer to fully empty. A 18650 4000mAH Li Ion battery is sweet, but like I said, i want to work with a capacitor. I'm looking for the LED's to light for about an hour, so something like a 5V 160F capacitor would work, but so would a 4V, 250F capacitor. Which one would work better? Or if I use a joule thief, would a 3V 450F work even better...
 

Thread Starter

Ree

Joined Oct 9, 2013
5
So I need around 2000 joules. Since I can't use less than 3V (unless I have a joule thief), I'm going to want something with a higher voltage and lower capacitance. The Solar cells I have produce around 9V, so I would want a capacitor that was around 50F, and 9V (Using Joules=1/2*C*V^2). But, since I can't use the joules below 3V (.5*3^2*50=225), I need another 225 Joules. So a 9V capacitor with 55F. Is this right? The most efficient capacitor size would be 9Volts, 55 Farads.
 

#12

Joined Nov 30, 2010
18,210
No sarcasm intended. That was the best I could do for finding appropriate capacitors in only 1 hour. I expect there are better prices somewhere, but there is a limit to how many hours a person can spend on an impractical method when practical methods are jumping at the chance to be used.
 

BobTPH

Joined Jun 5, 2013
2,601
To be fair you can capacitors in that range on Ebay for about $20. But I have no knowledge of whether these are any good or not.

Bob
 

wayneh

Joined Sep 9, 2010
16,401
We all think batteries are a better solution, except the OP wants to ensure very long life.

My point is to stick with batteries and put the money you save (compared to capacitors) in the bank. Buy more batteries only when you need them. This lowers the initial capital investment in exchange for an increased operating cost, but I'm virtually certain that the overall long run costs will be lower using batteries.
 
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