Capacitor as power supply circuit?

I have seen those heavily discounted maxwell ultracapacitors on electronic goldmine lately, and with the holidays around the corner I plan on having my hands on one soon.

The two possibilities i might get are 1 3000F 2.7V capacitor, or 2 2600F 2.5V capacitors which can make 5200F 2.5V or 1300F 5V.

(A side question: capacitors cannot really be chained in a long series like resistors can without running into problems, right? Isn't there a problem with the energy not being distributed evenly?)

My main question is what circuit or ic I can use to turn this large capacitance into a useful power supply. Because of its discharge curve, If I wanted to run something that needed 2 volts, it would stop running although there is still plenty of energy. The other issue is the inherent low voltage of ultracapacitors in general. I hear this is because the activated carbon used to achieve such high capacitance has a low breakdown voltage and there isn't much they can do about it. If I wanted to create a power supply that delivered a constant 5 volts from a capactior with lower voltage, how would I go about doing this?Thanks!

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You could make a switching powersupply that will suck energy from the cap at a constant rate, and convert it into 5V. It would be pretty easy, if you keep the bank at 2.5V.
seanroberts (author)  steveastrouk7 years ago
Could you elaborate?

You might not have been saying something different but I took it as you can step up some voltage (in this case 2.5) to 5v.

The thing is I want to use the energy from the capacitor until it is discharged, or as near as possible. Having it only boost to 5v if the bank is at 2.5 v or higher makes it so that all the energy the capacitor could deliver at a voltage lower than 2.5 volts would be unusable.

It seems to me I would need to use some sort of boost converter that could take any voltage under 5v (there is probably some floor voltage like .7 or something) and boost it to 5 volts. Here is the weird part of that though: if the output voltage and current load were constant, say 5v and 100mA; then as the input voltage decreased, the input current load would need to increase to provide the right amount of power. if the capacitor was at 2.5 volts it would (ideally) draw 200ma, then when it dropped to 1 volt it would draw 500ma.

2 ? about above, does a boost converter with a variable input and a fixed output even exist? Would a capacitor supply whatever current the circuit needed, or does it have some function over time like voltage of capacitors?
No, providing the input is less than the output, a boost switcher will provide a steady 5V output.

You're quite right, the input current increases automatically, to maintain energy balance.

Of course variable input PSUs exist ! That's rather the point of having a switcher. They can't go right down, unless they start at a high-ish voltage - the lowest starting conventional switchers I've seen start at no less than 0.8V, but I don't know how far they would suck down once started, since I've never aimed to extract maximum energy from the source like you are. Some experimentation is required here.

seanroberts (author)  steveastrouk7 years ago
"You're quite right, the input current increases automatically, to maintain energy balance."

Sorry, I am still a little confused, what you said about the current, does it mean:
The input of the converter will require more current to maintain the constant output current, or the input will require more current and the capacitor will supply this increased current.

I think you mean the latter and the whole system will work.

Can anyone provide the link to a chip that outputs 5v in a dip package (with preferably a low minimum input voltage and high efficiency.) 
The latter.

What current do you want ?

seanroberts (author)  steveastrouk7 years ago
For the output I have a minimum of 100mA but preferably 250mA and probably a maximum that I would even need for anything is 500mA. So if a chip can provide 250mA and 5V n the output that would be ideal.
Well I'd've said Maxim MAX1674,1675 or 1676, but if you need to be in DIP packages, you can only get away with a MAX856

They'll run down to 0.8V input.

You know a power supply like this is sold on the street in the form of a tool designed for powering cell phones from one or two AA batteries. Usually it has the words "emergency" and "charge" in its description, like "emergency phone charger", or something like that. It does what you are talking about. It is a switching power supply proving output of 5 VDC at a few hundred milliamps or so, and it runs from an input supply of approximately 1 to 3 volts. Some examples:

For those who really want to build it from scratch, I think someone did an 'ible on that.!---Small-battery-powered-USB-charger/
rickharris7 years ago
You have identified the main issues with caps as PSU - Short term only if any level of current is drawn - Can run an LED for up to 8 hours on one though.

Some people are running small indoor RC aircraft on them to get fast charge rates and put up with relatively short discharge times.