I want to use a 1.5 F super capacitor as the output capacitor in an audio range power supply

coinmaster

Joined Dec 24, 2015
502
I want to use a 1.5 F super capacitor as the output capacitor in an audio range power supply regulator but I have a question about the cycle limits of supercapacitors.
Usually capacitors can be expected to last forever at low temperature but I guess supercapacitors have maximum charge/discharge cycles.
Let's say my wall voltage frequency is 50hz and I have a continuous sinewave of 10hz being pulsed from the capacitor into a load that draws 2 amps.
1.5F at 2 amps will last somewhere around 30 seconds in my power supply but the lowest expected frequency being 10hz that means it will only draw current in 100ms cycles and it will be charged every 20ms, so in reality it will lose almost no charge in between rectifier cycles.
Does this mean that it have to go through enough tiny discharges to amount to the equivalent of the maximum complete charge/discharge rating of the capacitor or does it mean it won't even count as a single charge/discharge?

Last edited:

Papabravo

Joined Feb 24, 2006
14,205
Why exactly do you think using a supercapacitor in a power supply is a good idea? It is not what they were designed for. They have a larger than normal ESR. If the only tool you have is a hammer; does everything start to look like a nail?

EM Fields

Joined Jun 8, 2016
583
I want to use a 1.5 F super capacitor as the output capacitor in an audio range power supply regulator but I have a question about the cycle limits of supercapacitors.
Usually capacitors can be expected to last forever at low temperature but I guess supercapacitors have maximum charge/discharge cycles.
Let's say my wall voltage frequency is 50hz and I have a continuous sinewave of 10hz being pulsed from the capacitor into a load that draws 2 amps.
1.5F at 2 amps will last somewhere around 30 seconds in my power supply but the lowest expected frequency being 10hz that means it will only draw current in 100ms cycles and it will be charged every 20ms, so in reality it will lose almost no charge in between rectifier cycles.
Does this mean that it have to go through enough tiny discharges to amount to the equivalent of the maximum complete charge/discharge rating of the capacitor or does it mean it won't even count as a single charge/discharge?
Generally, the main reservoir capacitor is placed right after the rectifier(s) of a linear supply and is sized to make sure that the input voltage to the regulator never drops below the regulator's dropout voltage. That is, it becomes the current source for the load - through the regulator- and must supply the load's current, twice each cycle, when the rectified AC waveform dips below the load's requirements.

Also, putting a huge capacitor in parallel with the output of the regulator isn't usually a good idea because when the input voltage to the regulator dips below the voltage on the output of the regulator, nasty things can happen since linear regulators don't usually expect their output voltages to be higher than their input voltages..

It seems you have smallish demands since your load only needs 2 amperes, so if you can post text about what you're trying to do and a schematic of how you plan to do it, that would help a lot in getting you the information you need.

coinmaster

Joined Dec 24, 2015
502
I appreciate the help but I really only need an answer to my original question.

Papabravo

Joined Feb 24, 2006
14,205
I appreciate the help but I really only need an answer to my original question.
I think a charge discharge cycle means charge to 90% of maximum and discharge to 10% of maximum. I don't think anything less will contribute to or detract from, in a meaningful way, the life of the product. Is that what you were looking for?

When people ask about doing things which are either silly or potentially dangerous we have an obligation to warn them. If you ask a question on this forum you don't get to dictate the terms of the response. You gotta take what you get. You don't have to like it and you can vote with your feet, but that's the way it is.

coinmaster

Joined Dec 24, 2015
502
I think a charge discharge cycle means charge to 90% of maximum and discharge to 10% of maximum. I don't think anything less will contribute to or detract from, in a meaningful way, the life of the product. Is that what you were looking for?
Yes that was exactly what I was looking for, thanks.
As to your earlier comments, the ESR is a total of 90 miliohms with the series connection and the filter caps are at a sufficiently high value to avoid dropping voltage. The output cap will handle all of the AC current if the value is high enough.

Joined Feb 20, 2016
3,591
coinmaster. I agree with with the previous answers, the super cap is not really the way to go as an output capacitor. Saying that, I too agree with you comment as to wanting an answer to your original question because sometimes I too want to try a different way to do things.
But it will work better in almost every way to have a good reservoir capacitor and regulator for an audio power supply. You did not give any voltage ratings at all for your power supply. The volts on the super cap will sag pretty rapidly as it is discharged. Are you looking for a "UPS" type of unction? It may work better to have your super cap before the regulator but you will probably run into voltage limit problems. high voltage super caps cost! It may help for some more info on your application as what you are actually doing, and what are the full power requirements. I have a couple of power drills that have super caps in stead of batteries and they are fast charging. Very handy, but both are now dead and will not hold a charge anymore. So I'd advise giving super caps a miss if you are after long life.

Duracell2, It sounds like your power supply is current limiting and the volt meter is either before the current limit circuitry or is not a real volt meter but an indication of what the voltage is to be set too.
It is not a real good idea to charge a super cap of more that 2.7V unless you have some way to automatically disconnect it at full charge. Doing it manually s ok I s'pose as long as you don't get distracted. If you do, you may hear a loud noise to attract your attention again

ronv

Joined Nov 12, 2008
3,770
I want to use a 1.5 F super capacitor as the output capacitor in an audio range power supply regulator but I have a question about the cycle limits of supercapacitors.
Usually capacitors can be expected to last forever at low temperature but I guess supercapacitors have maximum charge/discharge cycles.
Let's say my wall voltage frequency is 50hz and I have a continuous sinewave of 10hz being pulsed from the capacitor into a load that draws 2 amps.
1.5F at 2 amps will last somewhere around 30 seconds in my power supply but the lowest expected frequency being 10hz that means it will only draw current in 100ms cycles and it will be charged every 20ms, so in reality it will lose almost no charge in between rectifier cycles.
Does this mean that it have to go through enough tiny discharges to amount to the equivalent of the maximum complete charge/discharge rating of the capacitor or does it mean it won't even count as a single charge/discharge?
Don't forget your diodes when they have to charge it for the first few cycles.