# capacitors used to power LED's

Discussion in 'Homework Help' started by dwaynerayburn, Dec 10, 2008.

1. ### dwaynerayburn Thread Starter New Member

Dec 10, 2008
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I want to use a 12VDC generator to charge a capacitor. Then use this capacitor to power 3 LED's(3.0V at 25Milliamps).
What size Capacitor?
What type Capacitor?
How long will these LED's burn with the correct sized capacitor?
The goal in mind is a 3 hour duration of service.

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Uhhhh.... don't think so.

Figure as a rough estimate 1 RC time contant. You want this TC to last 3 hours, or 10,800 seconds. So lets say their in series, and have a 20V power supply at 20ma. After 1 TC it would discharge to around 6.4 Volts. The R in this RC is 1000Ω. 10800/1000 = 10.8 Farads. These numbers are extremely rough, but they point the way to the problems.

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Thinking about it, there are other ways. Caps aren't very good at this job mainly because they don't regulate voltage, by definition. There are very good batteries out there that can soak current as fast as a cap.

Assuming you can get the needed value, you could equalize the discharge brightness of the LEDs with a constant current sink.

4. ### mik3 Senior Member

Feb 4, 2008
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I do not think you can find such large capacitors. Use a 12V lead acid battery.

Dec 11, 2008
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6. ### Wendy Moderator

Mar 24, 2008
20,772
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YES! For \$325 you too can power a couple of LEDs!

7. ### dwaynerayburn Thread Starter New Member

Dec 10, 2008
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Scarecrow could you guide or direct me in the sizing of the Cap.
Maybe also with the correct LED.s and a hand at charging the Cap,s
using a 12vdc small generator??

8. ### dwaynerayburn Thread Starter New Member

Dec 10, 2008
4
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Just to use this site is an Honor.
Is there a website usable For Electronic and practical uses???
I am wanting to build a circuit board from scratch, so i can use a 12vdc generator
and charge a capacitor without damaging CAP or myself.
All ideas, I am Greatly Thankful.

9. ### dwaynerayburn Thread Starter New Member

Dec 10, 2008
4
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The Info shared is very helpful, could i possiblily call on your knowledge as
The emblem with the G at center!
My Grandmother is 66. How Old is your Grandmother? Brother?

10. ### ScareCrow New Member

Dec 11, 2008
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You are correct. That's why I said they were big and expensive. But he wants to use caps and the ones I directed him to fit the calculations you performed. I think a couple of 7 amp hour lead acids and a inexpensive charger would be cheaper and much longer lasting. But I also think working with large capacitors can get real interesting and sometimes even surprising in a blinding flash kind of way.

11. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
We all help each other around here, so feel free. I don't use PMs to answer questions, though they can be used to point out a public post. You'll find everyone around here is pretty helpful, and I'm not the sharpest pencil in the cup (just one of the most verbose) with lots of experience.

Yep, if you look you can almost see the G in mine. Alas, Hiram, I knew him well.

12. ### ScareCrow New Member

Dec 11, 2008
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Honestly I don't think you should be using such large size capacitor if your new to them. They do represent an arc flash and arc burn hazard. A 10 farad capacitor accidentally shorted could produce a dangerous arc that can do a bit of damage including starting fires. I would suggest experimenting with some smaller 1000uf to 2000uf capacitors and some regular red LEDs and some resistors of various sizes from about 500 ohms to about 1,000 ohms and see what you can do with them. What I just listed wouldn't cost more than about \$15. Divide the the performance you want by the results you get and I think you'll find Bill_Marsden is right about the very high cost.

Last edited: Dec 12, 2008
13. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You could instead use a single 1.5v AA cell, a CMOS 555 timer with a small capacitor, a few resistors, an inductor and a transistor to power several LEDs in series for most of a week.

Scroll to about the middle of the attached PDF file.

The attached circuit wouldn't work well if you tried to power it from a capacitor. Capacitor voltage drops off very rapidly, where a battery holds it's voltage level pretty constant until it's nearly discharged.

• ###### Design Ideas - 6598371.pdf
File size:
331.9 KB
Views:
26
Last edited: Dec 12, 2008
14. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
So why are you so hot to use capacitors, when they cost so much. You could have a high quality battery system for 5% of the cost.