Capacitor as power supply circuit

[mrpsychotic]

O.K. all that stuff is nice, but do you have a specific chip in mind that is a solution to the original post. For a 5v boost converter in a dip package, the following criteria are arranged roughly in order:

-Very low minimum voltage for the input
-Efficiency
-Maximum load able to deliver.

[Zero999]

You can get DC-DC converter ICs which output 5V and run down to under 1V, check out the National Semiconductor, Linear and Maxim websites.

Your first two requirements make sense but your last doesn't. What do you mean? Are you driving an LED which uses 20mA or a motor which uses 20A?

[Murphy]

It doesn't sound like this is intended to power any specific device.

To figure out maximum load for the boost converter I guess you could look up the datasheet for the caps and see how fast they're rated to be discharged, then aim around there.

I like searching for parts on Mouser, their web site is quite good.

[mrpsychotic]

Your first two requirements make sense but your last doesn't. What do you mean? Are you driving an LED which uses 20mA or a motor which uses 20A?

Sorry I meant to say:
"Maximum load able to deliver around 250mA."

[scrat]

As I said before, the impedance of the battery and wiring should be negligible compared to the speakers. Suppose you have two front speakers, two back and a dual coil sub in the boot, that's 6 x 4 Ohm loads in parallel, a total impedance of 4/6 = 0.667 Ohms. The maximum theoretical peak current will be 12/0.667 =  18A, the maximum RMS power will be just over 100W and this is at clipping with the alternator running.

Your headlights should consume more than that continuously.

To get more power, you need to connect more speakers in parallel or install a boost converter.

I think that every high power car amp has to have a boost, otherwise its power will be very limited. So current on the 12V rail could be much higher than those 18A. They speak of thousands of rms Watts...
I'm not a car audio nerd, so I can't be sure.

[Zero999]

I think that every high power car amp has to have a boost, otherwise its power will be very limited. So current on the 12V rail could be much higher than those 18A. They speak of thousands of rms Watts...
I'm not a car audio nerd, so I can't be sure.

That's sort of my point.

If the boost converter is well designed, it should have plenty of decoupling capacitors both on the input and the output. The amplifier shouldn't be unstable, even if the voltage does drop a little on the input because the output should be well regulated.

[scrat]

Back on the original topic...

Supercaps are widely used for energy scavenging, especially in those cases where no maintenance is possible. Their life cycles and power (current) are not equaled by batteries.
They are extremely sensitive to overvoltage, so when using more of them in series one should beware of unbalancing, by measuring voltage or providing another means of protection (big assemblies come with integrated protection).

On the energy exploiting side, AFAIK the ICs which can boost from the lowest voltage are the TPS61200 series from TI. They can power up from voltages down to 0.3V (while providing step-down regulation too). This enables charging a battery from a common peltier cell at low temperature difference, for example.

High currents are usually an issue for the charging circuit only, not for the cap, so one could charge at the highest current possible (and available). Even with integrated regulators (converters) the usual feedback can be altered to achieve current regulation from voltage regulation.

[saturation]

I'm not sure what they cost these days, but ultracaps still cost more overall to design as a portable power supply over a LiIon or NiMH battery, to power a device.

For practical devices, I do own a power screwdriver that uses ultracaps which charges in <=1 minute, and it puts out enough power to run the driver for most any practical job around the house.  Given the charge time is short, its very practical in this design, and uses no voltage regulation, the caps are simply tied directly to a DC motor.  From near zero volts, to 5+ V, it will charge in 90 seconds, but the torque is pretty useless once below 2.5V, so for practical purposes the caps as useful down to there only.  Charging is exponential.

http://www.flashcellscrewdriver.com/

The main advantage of ultracaps is that its rated life is not tied to charge cycles like LiIon or NiMH, or aging in 5 years like any Li cell.  The expected lifespan of a cap is 10-20 years, due to deterioration of its electrolyte, and its independent of charge cycles, so use it as much as you can.  I also use it to power a power vac, it gets about 1 minute of use before it dies, but its so easy to charge up thereafter.

Given this device is out of production since 2007, if you bought a 2nd hand unit, unless the motor burns out or gears strip it should last at least to 2014, to 2021.

[sonicj]

"stiffening capacitor" is the term commonly used for large capacitors in parallel with the power supply of car amplifiers.  its intent is to supplement the power supply during brief transients that consume more power than is otherwise available. larger/more batteries and a high output alternator are likely better solutions if the audio system needs have outgrown the car's electrical system capabilities. the topic of many heated discussions over the years...
-sj

[qno]

Generally speaking using a Capacitor as a voltage source is not a good idea when you look at the discharge characteristic.