# Designing a SuperCapacitor Circuit

Discussion in 'General Electronics Discussion' started by magicasd, May 18, 2010.

1. ### magicasd

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May 18, 2010
hi, 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...

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Who has given you this assignment. It sounds quite bizarre. Why not use rechargeable batteries, for instance?

For maximum efficiency, you would probably look at a switchmode constant current source, but I'm still boggling at the thought of a 1000V supercapacitor.

3. ### magicasd

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May 18, 2010
help

we are GIven this supercapacitor to run a circuit so a certain period of time. so i dont really wish to use batts or anything. I jus wanna know how to step down the high voltage and light the LEDs for a period of time. i need suggestions. e.g stepdown transformers , regulatars

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Please check the ratings on your super capacitors and get back to me.

I can't imagine for one second that they are rated for anything like 700 volts. I'd expect something more like 5 volts.

And this is for a class assignment, right?

5. ### NickS

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Apr 6, 2010
I second that. I have worked with super caps and some reach as high as 5v but most are less and you can't operate right at their max voltage rating either so that is even further away.

Any way I am betting you end up looking for a boost circuit instead of a buck. If you do manage to find those high voltage supercaps let me know because I would love to have a couple.

6. ### magicasd

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May 18, 2010
is it that hard to believe a 700-1000v supercap exist.

just want to find a way how to properly discharge it just to show it is actually discharging. ie LEDs

7. ### 55pilot

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Feb 23, 2010
Not at all. It is not hard to believe, it is virtually impossible that super-capacitors can exist much past 5V. The 5.5V ones are a rarity. Most are around 2.5V.

Now a bank of supercapacitors that can be charged to 700V is a possibility but it is something well beyond the capabilities of a hobbyist. In fact, it is an EXTREMELY specialized field.

---55p

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Do you want a safe means to discharge a high voltage capacitor? That's a different and far easier objective.

9. ### magicasd

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May 18, 2010
Sorry for being inactive and thx for the inputs. I do want to know what is this easier objective that steve mentioned.

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
To build a device to safely discharge a high voltage capacitor, do the following:

take 8 1n4001 diodes, 2 red LEDs and a resistor (more on the resistor later)

Join 4 diodes up in series, and connect a LED in parallel with that (so that the anode of the LED is at the same end as the anode of the first 1n4001. Make a second one of these exactly the same.

Now connect these in parallel, but pointing in opposite directions.

At one end of this connect your pointy probe, ant the other end connect the resistor, then to that a ground lead.

Insulate everything really well.

With the ground lead connected to ground (or one side of the capacitor) touch the other capacitor lead with the probe and one LED will glow, getting dimmer and dimmer until... it goes out. The capacitor is now discharged (or your resistor has gone open circuit)

The resistor needs some discussion. It is best to limit the voltage across the resistor to around 200V max, and also to limit its power dissipation. If you have capacitors charged to more than 200V then place several capacitors in series.

Pick a total resistance so that no more than about 200mA will flow. If you calculate the wattage (say at 1000V) you'll find it's really huge. (200W) you may want to reduce the current. You can get away with resistors rated for about 1/5 the wattage since the total power dissipated is actually quite low (it falls exponentially). In use, you'll never even feel the resistors get warm. I use 5W resistors.

Incidentally, you can tell if a resistor goes open circuit because the LED will go off very rapidly, rather than tailing off to ever decreasing brightness.

If you use the probe for excessive voltages, the resistor may become open circuit, but the capacitors will still discharge! but lower voltage capacitors won't. The high voltages (say >1kV per resistor) set up arcs in the resistors that can bridge gaps!

I built mine in a small plastic case, for really high voltages I'd use a long pvc pipe with a string of many resistors.

11. ### florinanghel

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Jun 14, 2010
magicasd, supercapacitors have high capacitance, but low voltage. A regular capacitor, with lower capacitance, can store 1kV with no problem. But that's not a supercapacitor anymore.

12. ### ChosunOne

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Jun 20, 2010
magicasd, charging _anything_ to 700-1000 volts sounds weird, let alone using supercaps for it. That's more like a carbon-arc lighting demo than an LED demo.
Are you sure there isn't a typo or misunderstanding involved? Like maybe your teacher wanted you to charge to charge to 1000 microfarads at a voltage more suited to LEDs?

13. ### NickS

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Apr 6, 2010
Perhaps schools are resorting to new methods for thinning the heard.

14. ### davennModerator

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Sep 5, 2009
just more prime fodder for the "Darwin Awards"

I agree with ChosunOne's comments ... there must be some misunderstanding in what the project is about and what is required.

Dave