Designing a supercapacitor circuit


Old 05-18-10, 02:05 AM
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Designing a supercapacitor circuit

i, i'm given a project assignment of building a supercapacitor which can store roughly 700V to 1000V.
Now i'm designing a circuit (discharging) which can help me light up a few LEDs using Voltage charge stored in the
supercapacitor. The purpose of the circuit is to show that the supercapacitor can Light up the LEDS for a certain period of
time.. I would like to know how to design a circuit which can control the rate of discharge of the supercapacitor.
what are the components required to step down and control the high voltage in order not to destroy the LEDs...
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Old 05-21-10, 06:40 PM
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over 700 volts? why?

most super caps are around 3 to 5 volts.
Old 05-22-10, 04:51 AM
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Never trying what you propose, I can only guess. Assuming you are in the 3 to 5v range to be safe I'd suggest that you simply connect the LEDs through a resistance. The proper value is determined by trial and error, perhaps starting with a few Kohms, working down to obtaining normal brightness. You might blow an LED in the process until you get it right. Is this a school project?
Old 05-22-10, 06:12 AM
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As telcom guy asked, "why 700 volts?". Bringing that high of a voltage down to the LED level risks losing the energy of 698 volts just to light one light. You could string MANY LEDs in series like xmas tree lights, but that still has its problems as well.

What is your source of energy?
What do you have for capacitors?
And how many LEDs do you want?

Old 05-26-10, 03:31 AM
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What you are probably looking for is called a "buck boost LED driver" and most of the major chip makers are jockying for win, place, or show.

Keep in mind that when you string N capacitors of the same size in series, the capacitance falls to 1/N. So if you string 100 10F 5V supercaps in series, you end up with a 100,000 uF capacitor. Not so super anymore. Of course your array will ultimately store the same (respectable) amount of energy as hooking them all in parallel, but 500V is out of the range of most buck regulators, let alone buck-boost.

Of course the buck-boost drivers have a minimum V below which they just don't work anymore, leaving a bit of charge left at the bottom of the barrel. So the design challenge is to find the driver with essentially the best minimum and maximum input voltage ratio and design your series-parallel capacitor array to suit. Essentially, but not quite. Closer to the truth, to achieve the highest practical efficiency, the energy left in the caps at the low end must be calculated and subtracted from the energy available at the high end to determine net energy delivered, and the efficiency curves of the driver must be considered, as well as the efficiency of the LEDs. Look up the calculus concept of integration if you're not familiar with it, and if you can't import the mfgr. data into a program to do it, don't have a planimeter, and don't want to do it by hand, an old engineer's trick is to print the curve, cut it out with a pair of scissors, and weigh it in a sensitive scale. Of course to properly compare across drivers you must be careful to "zoom" and make other adjustments (essentially scale and offset), see "How to Lie with Statistics."

If you're determined to stick with HV, I'd suggest you start by doing some research into how off-the-shelf 120V or 240V LED bulbs work. I'm not familiar with that but I've noticed a lot of them flicker, which leads me to believe they are relying on an AC waveform. Probably a dead end for you. The latest high end LED bulbs may be different, IE no flicker, I haven't observed them carefully enough to know.

Good luck with your project!
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